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Unformatted text preview: M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 325 CHAPTER SEVEN DEFLECTION OF SYMMETRIC BEAMS
Learning Objective
1. Learn to formulate and solve the boundaryvalue problem for the deflection of a beam at any point.
_______________________________________________ Greg Louganis, the American often considered the greatest diver of all time, has won four Olympic gold medals, one silver
medal, and five world championship gold medals. He won both the springboard and platform diving competitions in the 1984
and 1988 Olympic games. In his incredible execution, Louganis and all divers (Figure 7.1a) makes use of the behavior of the
diving board. The flexibility of the springboard, for example, depends on its thin aluminum design, with the roller support
adjusted to give just the right unsupported length. In contrast, a bridge (Figure 7.1b) must be stiff enough so that it does not
vibrate too much as the traffic goes over it. The stiffness in a bridge is obtained by using steel girders with a high area moment
of inertias and by adjusting the distance between the supports. In each case, to account for the right amount of flexibility or
stiffness in beam design, we need to determine the beam deflection, which is the topic of this chapter
(a) Figure 7.1 (b) Examples of beam: (a) flexibility of diving board; and (b) stiffness of steel girders. We can obtain the deflection of a beam by integrating either a secondorder or a fourthorder differential equation. The
differential equation, together with all the conditions necessary to solve for the integration constants, is called a boundaryvalue problem. The solution of the boundaryvalue problem gives the deflection at any location x along the length of the
beam. Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 7.1 SECONDORDER BOUNDARYVALUE PROBLEM Chapter 6 considered the symmetric bending of beams. We found that if we can find the deflection in the y direction of one
point on the cross section, then we know the deflection of all points on the cross section. In other words, the deflection at a
cross section is independent of the y and z coordinates. However, the deflection can be a function of x, as shown in Figure 7.2.
The deflected curve represented by v(x) is called the elastic curve.
dv
dx y v(x)
v x
z Figure 7.2 Beam deflection.
January, 2010 p(x)
x M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 326 The deflection function v(x) can be found by integrating Equation (6.11) twice, provided we can find the internal moment as
a function of x, as we did in Section 6.3. Equation (6.11), this secondorder differential equation is rewritten for convenience:
2 dv
M z = EI zz 2
dx (7.1) The two integration constants generated from Equation (7.12.a) are determined from boundary conditions, as discussed
next, in Section 7.1.1.
As one moves across the beam, the applied load may change, resulting in different functions of x that represent the internal
moment Mz. In such cases there are as many differential equations as there are functions representing the moment Mz. Each additional differential equation generates additional integration constants. These additional integration constants are determined
from continuity (compatibility) equations, obtained by considering the point where the functional representation of the moment
changes character. The continuity conditions will be discussed in Section 7.1.2. The mathematical statement listing all the differential equations and all the conditions necessary for solving for v(x) is called the boundaryvalue problem for the beam deflection. 7.1.1 Boundary Conditions The integration of Equation (7.1) will result in v and dv/dx. Thus, we are seeking conditions on v or dv/dx. Figure 7.3 shows
three types of support and the associated boundary conditions.
Note that for a secondorder differential equation we need two boundary conditions. If on one end there is only one boundary condition, as in Figure 7.3b or c, then the remaining boundary condition must come from another location. Doubts about a
boundary condition at a support can often be resolved by drawing an approximate deformed shape of the beam. x
A A v(xA)
v
dv (x )
v
dx A 0 v
v(xA) 0 A
v
dv (x )
dx A 0 0 (a) (b) (c) Figure 7.3 Boundary conditions for secondorder differential equations. (a) Builtin end. (b) Simple support. (c) Smooth slot. 7.1.2 Continuity Conditions Suppose that because of change in the applied loading, the internal moment Mz in a beam is represented by one function to the
left of xj and another function to the right of xj. Then there are two secondorder differential equations, and integration will Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm produce two different displacement functions, one for each side of xj, together, these will contain a total of four integration
constants. Two of these four integration constants can be determined from the boundary conditions, as discussed in Section
7.1.1. The remaining two constants will have to be determined from conditions at xj. Figure 7.4 shows that a discontinuous
displacement at xj implies a broken beam, and a discontinuous slope at xj implies that a beam is kinked at xj.
Assuming that the beam neither breaks nor kinks, then the displacement functions must satisfy the following conditions: v1 ( xj ) = v2 ( xj ) (7.2.a) d v1
d v2
 ( x j ) =  ( x j )
dx
dx (7.2.b) where v1 and v2 are the displacement functions to the left and right of xj. The conditions given by Equations (7.2) are the continuity conditions, also known as compatibility conditions or matching conditions. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams v(x) (a) v2(x) v1(x) 7 v(x) (b) Discontinuous
Slope Discontinuous
Displacement v2(x)
v1(x) x x
xj xj Figure 7.4 (a) Broken beam. (b) Kinked beam. 327 • Example 7.1 demonstrates the formulation and solution of a boundaryvalue problem with one secondorder differential
equation and the associated boundary conditions. • Example 7.2 demonstrates the formulation and solution of a boundaryvalue problem with two secondorder differential
equations, the associated boundary conditions, and the continuity conditions. • Example 7.3 demonstrates the formulation only of a boundaryvalue problem with multiple secondorder differential
equations, the associated boundary conditions, and the continuity conditions. • Example 7.4 demonstrates the formulation and solution of a boundaryvalue problem with variable area moment of
inertia, that is, Izz is a function of x. EXAMPLE 7.1
A beam has a linearly varying distributed load, as shown in Figure 7.5. Determine: (a) The equation of the elastic curve in terms of E, I,
w, L, and x. (b) The maximum intensity of the distributed load if the maximum deflection is to be limited to 20 mm. Use E = 200 GPa,
I = 600 (106) mm4, and L = 8 m.
y wx L w (N m) x Figure 7.5 Beam and loading in Example 7.1. L (m) PLAN
(a) We can make an imaginary cut at an arbitrary location x and draw the freebody diagram. Using equilibrium equations, the moment
Mz can be written as a function of x. By integrating Equation (7.1) and using the boundary conditions that deflection and slope at x = L
are zero, we can find v(x). (b) The maximum deflection for this problem will occur at the free end and can be found by substituting x = 0
in the v(x) expression. By requiring that v max ≤ 0.02 m , we can find wmax. Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm S O L U T IO N
(a) Figure 7.6 shows the freebody diagram of the right part after making an imaginary cut at some location x. Internal moment and shear
forces are drawn according to the sign convention discussed in Section 6.2.6. The distributed force is replaced by an equivalent force,
and the internal moment is found by equilibrium of moment about point O.
2
3
1 wx x
1 wx
 3
M z = –   ⎛ ⎞ = –  2L⎝⎠
6L Mz
Vy (E1)
1 wx 2
2L wx L
Mz
x (m)
(a) Vy x3 Figure 7.6 Freebody diagram in Example 7.1. (a) Imaginary cut on original beam. (b) Statically equivalent diagram.
1 wx 2 January, 2010 (b) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 328 We substitute Equation (E1) into Equation (7.1) and note the zero slope and deflection at the builtin end. The boundaryvalue problem
can then be stated as follows:
• Differential equation:
2 3 1 wx
dv
E I zz  = –  2
6L
dx (E2) v(L) = 0 (E3) dv
(L) = 0
dx (E4) • Boundary conditions: Equation (E2) can be integrated to obtain
4 dv
1 wx = –   + c 1
dx
24 L
Substituting x = L in Equation (E5) and using Equation (E4) gives the constant c1: (E5) EI zz 4 3 1 wL
–   + c 1 = 0
24 L
Substituting Equation (E6) into Equation (E5) we obtain wL
c 1 = 24 or 4 (E6) 3 dv
1 wx wL
EI zz  = –   + 24
dx
24 L (E7) Equation (E7) can be integrated to obtain
5 3 1 wx wL
EI zz v = –   +  x + c 2
24
120 L
Substituting x = L in Equation (E8) and using Equation (E3) gives the constant c2:
5 (E8) 3 4 wL
1  wL wL
or
c 2 = – –   +  L + c 2 = 0
120 L
24
30
The deflection expression can be obtained by substituting Equation (E9) into Equation (E8) and simplifying. (E9) w
4
5
5
ANS. v ( x ) = –  ( x – 5 L x + 4 L )
120 EI zz L Dimension check: We note that all terms in the parentheses have the dimension of length to the power of five, that is, O(L5). Thus the
answer is dimensionally homogeneous. But we can also check whether the lefthand side and any one term of the righthand side has the
same dimension,
F
w → O ⎛  ⎞
⎝L ⎠ x → O(L) F
E → O ⎛  ⎞
⎝ L 2⎠ 5 4 I zz → O ( L ) 5
(F ⁄ L)L
⎛
wx
⎞
 → O ⎜  ⎟ → O ( L ) → checks
2
4
EI zz L
⎝ (F ⁄ L )O(L )L ⎠ v → O(L) (b) By inspection it can be seen that the maximum deflection for this problem will occur at the free end. Substituting x = 0 in the deflec4 tion expression, we obtain v max = – wL ⁄ 30 EI zz . The minus sign indicates that the deflection is in the negative y direction, as expected.
Substituting the given values of the variables and requiring that the magnitude of the deflection be less than 0.02 m, we obtain
4 4 w max ( 8 m )
w max L
v max =  =  ≤ 0.02 m
9
2
–6
4
30 EI zz
30 [ 200 ( 10 N/m ) ] [ 600 ( 10 ) m ] or 3 w max ≤ 17.58 ( 10 ) N/m Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm ANS. (E10)
w max = 17.5 kN/m COMMENTS
1. From calculus we know that the maximum of a function occurs at the point where the slope of the function is zero. But the slope at
x = L, where the deflection is maximum, is not zero. This is because v(x) is a monotonic function— that is, a continuously increasing
(or decreasing) function. For monotonic functions the maximum (or minimum) always occurs at the end of the interval. We intuitively recognized the function’s monotonic character when we stated that the maximum deflection occurs at the free end.
2. If the dimension check showed that some term did not have the proper dimension, then we would backtrack, check each equation for
dimensional homogeneity, and identify the error. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 329 EXAMPLE 7.2
For the beam and loading shown in Figure 7.7, determine: (a) the equation of the elastic curve in terms of E, I, L, P, and x; (b) the maximum deflection in the beam.
P y
2PL C A
x Figure 7.7 Beam and loading in Example 7.2. B L L PLAN
(a) The internal moment due to the load P at B will be represented by different functions in AB and BC, which can be found by making
imaginary cuts and drawing freebody diagrams. We can write the two differential equations using Equation (7.1), the two boundary conditions of zero deflection at A and C, and the two continuity conditions at B. The boundaryvalue problem can be solved to obtain the
elastic curve. (b) In each section we can set the slope to zero and find the roots of the equation that will give the location of zero slope.
We can substitute the location values in the elastic curve equation derived in part (a) to determine the maximum deflection in the beam. S O L U T IO N
(a) The freebody diagram of the entire beam can be drawn, and the reaction at A found as R A = 3 P ⁄ 2 upward, and the reaction at C
found as R C = P ⁄ 2 downward. Figure 7.8 shows the free body diagrams after imaginary cuts have been made and then internal shear
force and bending moment drawn according to our sign convention. (a) 2PL M1 O1 A V1 x
RA (b) P
A B O2
V2 L 3P 2 Figure 7.8 Free body diagrams in Example 7.2 after imaginary cut in (a) AB (b) BC. M2 2PL x
RA 3P 2 By equilibrium of moments in Figure 7.8a and b we obtain the internal moments
M 1 + 2 PL – R A x = 0 or 3
M 1 =  Px – 2 PL
2 (E1) 3
(E2)
M 2 =  Px – 2 PL – P ( x – L )
2
Check: The internal moment must be continuous at B, since there is no external point moment at B. Substituting x = L in Equations (E1)
and (E2), we find M1 = M2 at x = L.
M 2 + 2 PL – R A x + P ( x – L ) = 0 or The boundaryvalue problem can be stated using Equation (7.1), (E1), and (E2), the zero deflection at points A and C, and the continuity
conditions at B as follows:
• Differential equations:
2 d v1
3
EI zz  =  Px – 2 PL ,
2
2
dx 0≤x<L (E3) 2 d v2
3
EI zz  =  Px – 2 PL – P ( x – L ) ,
2
2
dx L ≤ x < 2L (E4) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm • Boundary conditions:
v1 ( 0 ) = 0 (E5) v2 ( 2 L ) = 0 (E6) v1 ( L ) = v2 ( L ) (E7) d v1
d v2
 ( L ) =  ( L )
dx
dx (E8) d v1
32
EI zz  =  Px – 2 PLx + c 1
dx
4 (E9) d v2
P
32
2
EI zz  =  Px – 2 PLx –  ( x – L ) + c 2
dx
2
4 (E10) • Continuity conditions: Integrating Equations (E3) and (E4) we obtain January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 330 Substituting x = L in Equations (E9) and (E10) and using Equation (E8), we obtain
32
3
 PL – 2 PL 2 + c 1 =  PL 2 – 2 PL 2 – 0 + c 2
or
c1 = c2
4
4
Substituting Equation (E11) into Equation (E10) and integrating Equations (E9) and (E10), we obtain (E11) 13
2
EI zz v 1 =  Px – PLx + c 1 x + c 3
4 (E12) 13
2P
3
EI zz v 2 =  Px – PLx –  ( x – L ) + c 1 x + c 4
4
6
Substituting x = L in Equations (E12) and (E13) and using Equation (E7), we obtain
13
1
 PL – PL 3 + c 1 L + c 3 =  PL 3 – PL 3 – 0 + c 1 L + c 4
4
4
Substituting x = 0 in Equation (E12) and using Equation (E5), we obtain or (E13) (E14) c3 = c4 c3 = 0 (E15) c4 = 0 (E16) From Equation (E14),
Substituting x = 2L and Equation (E16) into Equation (E13) and using Equation (E6), we obtain
1
13 2
 P ( 2 L ) 3 – PL ( 2 L ) 2 – P ( L ) 3 + c 1 ( 2 L ) = 0
or
c 1 =  PL
4
12
6
Substituting Equations (E15), (E16), and (E17) into Equations (E12) and (E13) and simplifying, we obtain the answer:
P
3
2
2
ANS. v 1 ( x ) =  ( 3 x – 12 Lx + 13 L x )
12 EI zz 0≤x<L P
3
2
2
3
ANS. v 2 ( x ) =  [ 3 x – 12 Lx + 13 L x – 2 ( x – L ) ]
12 EI zz (E17) (E18) L ≤ x < 2L (E19) Dimension check: All terms in parentheses are dimensionally homogeneous, as all have the dimensions of length cubed. But we can also
check whether the lefthand side and any one term of the righthand side have the same dimension:
P → O(F) x → O(L) F
E → O ⎛  ⎞
⎝ L 2⎠ 4 I zz → O ( L ) 3
⎛ FL 3 ⎞
Px
 → O ⎜ ⎟ → O ( L ) → checks
2
4
EI zz
⎝ (F ⁄ L )L ⎠ v → O(L) (b) Let d v 1 ⁄ dx be zero at x = x1. Differentiating Equation (E18), we obtain
P
 ( 9 x 2 – 24 Lx 1 + 13 L 2 ) = 0
1
12 EI zz or 2 2 (E20) 9 x 1 – 24 Lx 1 + 13 L = 0 The roots of the quadratic equation are x 1 = 1.91 L and x 1 = 0.756 L . . The admissible root is x 1 = 0.756 L . , since Equation (E18), and
hence Equation (E20), are valid only in the range from 0 to L. Substituting this root into Equation (E18), we obtain
3 P
0.355 PL
3
2
2
v 1 ( 0.756 L ) =  ( 3 × 0.756 L – 12 L × 0.756 L + 13 L × 0.756 L ) = 12 EI zz
EI zz (E21) To find the maximum deflection in BC, assume dv2 / dx to be zero at x = x2. Differentiating Equation (E19) we obtain
P
 [ 9 x 2 – 24 Lx 2 + 13 L 2 – 6 ( x 2 – L ) 2 ] = 0
2
12 EI zz or 2 2 (E22) 3 x 2 – 12 Lx 2 + 7 L = 0 The roots of the quadratic equations in Equation (E22) are x 2 = 0.709 L and x 2 = 3.29 L . Both roots are outside the range of L to 2L Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm and hence are inadmissible. Thus in this problem the slope is zero only at 0.756L, and the maximum deflection is given by Equation
(E21).
3 ANS. 0.355 PL
v max = EI zz COMMENT
1. When we made the imaginary cut in BC, we took the left part for drawing the freebody diagram. Had we taken the right part, we
would have obtained the moment expression M 2 = ( Px ⁄ 2 ) – PL , which is the simplified form of Equation (E21). We can start with
this moment expression and obtain our results from integration and the conditions as shown. The values of the integration constants
will be different, and there will be slightly more algebra, but the final result will be the same. The form of the moment expression
used in the example made use of the observation that the continuity conditions are at x = L and the terms in powers of (x – L) will be
zero. This form results in less algebra and simplified relations for the constants, as given by Equations (E11) and (E14). January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 331 EXAMPLE 7.3
Write the boundaryvalue problem for solving the deflection at any point x of the beam shown in Figure 7.9. Do not integrate or solve.
y
A Figure 7.9 Beam and loading in Example 7.3. 4 kN/m 5 kN
x B 2.0 m D 12 kN m
C
5 kN m
1.0 m 3.0 m PLAN
The moment expressions in each interval were found in Example 6.10. The differential equations can be written by substituting these
moment expressions into Equation (7.1). We can also write the zerodeflection conditions at points A and D and the continuity conditions
at points B and C to complete the boundaryvalue problem statement. SOLUTION
From the free body diagram of the entire beam the reactions at A and D in Example 6.10 were found to be R A = 0 and R D = 7 kN .
Figure 7.10 shows the free body diagrams used in Example 6.10 to obtain the internal moments
52
M 1 = ⎛  x ⎞ kN ⋅ m
⎝2 ⎠ 0 ≤x<2 m M 2 = ( 5 x – 12 ) kN ⋅ m 2 m<x<3 m 2 M 3 = ( – 2 x + 17 x – 30 ) kN ⋅ m (a) RA (b) 0
O1 A
5x
xm Figure 7.10 x
2 m RA Mz A Vy B
1.0 m (E2) 3 m<x≤6 m 1.0 m
10 kN
xm (E3) (c) 5 kN 0 (E1) O2
12 kN m Vy 4(6 Vy Mz
Mz x) kN O3
(6 x) 2 m
(6 x) m RD 7 kN Free body diagrams in Example 7.3 after imaginary cut in (a) AB (b) BC (c) CD. The boundary value problem can be written as described below.
• Differential equations:
2 d v1
52
EI zz  = ⎛  x ⎞ kN ⋅ m
2
⎝2 ⎠
dx 0 ≤x<2 m (E4) 2 d v2
EI zz  = ( 5 x – 12 ) kN ⋅ m
2
dx
EI zz d 2v 3
dx 2 2 = ( – 2 x + 17 x – 30 ) kN ⋅ m 2 m<x<3 m 3 m<x≤6 m (E5) (E6) • Boundary conditions:
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm v1 ( 0 ) = 0 (E7) v3 ( 6 ) = 0 (E8) v1 ( 2 ) = v2 ( 2 ) (E9) d v1
d v2
 ( 2 ) =  ( 2 )
dx
dx (E10) v2 ( 3 ) = v3 ( 3 ) (E11) d v2
d v3
 ( 3 ) =  ( 3 )
dx
dx (E12) • Continuity conditions: January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 332 COMMENTS
1. Equations (E4), (E5), and (E6) are three differential equations of order 2. Integrating these three differential equations would result in
six integration constants. We have two boundary conditions and four continuity conditions. A properly formulated boundaryvalue
problems will always have exactly the right number of conditions needed to solve a problem.
2. In Example 7.3 there were two differential equations and the resulting algebra was tedious. This example has three differential equations, which will make the algebra even more tedious. Fortunately there is a method, discussed in Section 7.4*, which reduces the
algebra. This discontinuity method introduces functions that will let us write all three differential equations as a single equation and
implicitly satisfy the continuity conditions during integration. EXAMPLE 7.4
A cantilever beam with variable width b(x) is shown in Figure 7.11. Determine the maximum deflection in terms of P, bL, t, L, and E. (b) P
x t bL b(x) P bL b(x) x L
(a) (c) P
t x
L Figure 7.11 (a) Geometry of variablewidth beam in Example 7.4. (b) Top view. (c) Front view. PLAN
The area moment of inertia and the bending moment can also be found of x and substituted into Equation (7.1) to obtain the differential
equation. The zero deflection and slope at x = L are the boundary conditions necessary to solve the boundaryvalue problem for the elastic curve. The maximum deflection will be at x = 0 and can be found from the equation of the elastic curve. S O L U T IO N
Noting that b(x) is a linear function of x that passes through the origin and has a value of bL at x = L, we obtain b(x) = bLx /L and the area
moment of inertia as
3 3
bL t
b(x)t
I zz =  = ⎛  ⎞ x
⎝ 12 L ⎠
12 (E1) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Figure 7.12 shows the free body diagram after an imaginary cut can be made at some location x. By equilibrium of moment at about O,
we obtain the internal moment,
(E2) M z = – Px P Mz
O Figure 7.12 Freebody diagram in Example 7.4. x Vy The boundaryvalue problem can be written as follows:
• Differential equation:
2
Mz
– Px
dv
12 PL
 =  =  = – 2
3
3
EI zz
E ( b L t ⁄ 12 L ) x
dx
Eb L t January, 2010 (E3) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 333 • Boundary conditions:
v(L) = 0 (E4) dv
(L) = 0
dx (E5) dv
12 PL
= –  x + c 1
3
dx
Eb L t (E6) Integrating Equation (E3) we obtain Substituting Equation (E6) into Equation (E5), we obtain
12 PL
0 = –  L + c 1
3
Eb L t 2 12 PL
c 1 = 3
Eb L t or (E7) Substituting Equation (E7) into Equation (E6) and integrating, we obtain
2 12 PL
12 PL x2
v = –  ⎛ ⎞ +  x + c 2
3
3
Eb L t ⎝ 2 ⎠ Eb L t (E8) Substituting Equation (E8) into Equation (E4), we obtain
2 2 12 PL
12 PL L
0 = –  ⎛  ⎞ +  L + c 2
3
3
Eb L t ⎝ 2 ⎠ Eb L t 3 6 PL
c 2 = – 3
Eb L t or (E9) The maximum deflection will occur at the free end. Substituting x = 0 into Equation (E8) and using Equation (E9) we obtain the maximum deflection.
3 6 PL
v max = – 3
Eb L t ANS. (E10) COMMENTS
1. The beam taper must be gradual given the limitation on the theory described in Section 6.2.
2. We can calculate the maximum bending normal stress in any section by substituting y = t/2 and Equations (E1) and (E2) into Equation
(6.12), to obtain
t⁄2
6 PL
σ max = – Px  = (E11)
3
2
( b L t ⁄ 12 L ) x
bL t
3. Equation (E11) shows that the maximum bending normal stress is a constant throughout the beam. Such constantstrength beams are
used in many designs where reduction in weight is a serious consideration. One such design is elaborated in comment 3.
4. In a leaf spring (see page 334), each leaf is considered an independent beam that bends about its own neutral axis because there is no
restriction to sliding (see Problem 6.20). The variablewidth beam is designed for constant strength, and bL is found using Equation
(E11). The width bL is then divided into n parts, as shown in Figure 7.13a. Except for the main leaf A, all other leaf dimensions are
found by taking the onehalf leaf width on either side of the main leaf. In the assembled spring, the distance in each leaf from the
applied load P is the same as in the original variablewidth beam shown in Figure 7.11. Hence each leaf has the same allowable
strength at all points. If b is the width of each leaf and L is the total length of the spring, so that L = L/2, Equations (E10) and (E11)
can be rewritten as
3 3 PL
δ = 3
4 nEb t 3 PL
σ max = 2
nb t C
B bL
bL
bL
bL A bL n B
C bL
bL
bL
bL N
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm (7.3) N Figure 7.13 Explanation of leaf spring design.
5. The results can be used in design as in Example 3.6. January, 2010 (a) 2n
2n
2n
2n 2n
2n
2n
2n P L t A
B
C
N N C (b) B A ¯
b bL n M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 334 MoM In Action: Leaf Springs
When it comes to leaf springs, necessity was the mother of invention. Humans realized very early the mechanical
advantage of a spring force from bending. For example, most early civilizations had longbows. However, thongs and ropes
lose much of their elasticity in dampness and rain. Metals do not, and in 200 B.C.E. Philo of Byzantium proposed using
bronze springiness as a source of power. By the early sixteenth century springpowered clocks attained an accuracy of one
minute a day—far better than the weightdriven clocks seen in the towers of Renaissance Italy. The discovery revolutionized navigation, enabling world exploration and European colonial power.
Around the same time, overland travel drove a different kind of spring development. Wagons and carriages felt
every bump in the road, and the solution was the first suspension system: leather straps attached to four posts of a chassis
suspended the carriage body and isolated it from the chassis. For all its advantages, however, the system did not prevent
forward and backward sway, and the high center of gravity left the carriage susceptible to rollover. The problems were significantly reduced by the introduction of cart springs (Figure 7.14a) or what we call leaf springs today. Edouard Phillips
(1821–1889) developed the theory of leaf springs (see Example 7.4) while studying the spring suspension in freight trains.
It was one of the first applications of the mechanics of materials to engineering design problems.
(a)
(b) (b) Figure 7.14 Leaf springs in (a) cart; (b) conventional vehicles. We still use the term suspension system today, although cars, trucks, and railways are all supported on springs
rather than suspended. To increase bending rigidity, leaf springs have a curve (Figure 7.14b). When the curve is elliptical,
the springs are referred to as semielliptical springs. The Ford Model T had a nonelliptical curve, but with the Corvette
leaf spring design reached its zenith. Unlike the traditional longitudinal mounting of one spring per wheel, the spring in a
Corvette was mounted transversely. This eliminated one leaf spring and significantly reduced the tendency to rollover. A
double wishbone design allowed for independent articulation of each wheel. The onepiece fiberglass material practically Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm eliminated fatigue failure, reducing the weight by two thirds compared to steel springs.
With the growth of front wheel drive in the 1970s, automobiles turned instead to coil springs, which require less
space and provide each wheel with independent suspension. However, leaf springs continue to be used in trucks and railways, to distribute their heavy loads over larger spans.
Both coil and leaf spring systems are part of a passive suspension system, which involve a tradeoff between comfort, control, handling, and safety. Those factors are driving newer design systems called active suspension, in which the
amount of spring force is externally controlled. It took 400 years for leaf springs to reach their zenith, but need has no
zenith, and necessity is still the mother of invention in suspension design. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 335 PROBLEM SET 7.1
Secondorder boundaryvalue problems
7.1 For the beam shown in Figure P7.1, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
y P
x A Figure P7.1 L 7.2 For the beam shown in Figure P7.2, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
y
w
x Figure P7.2 A L2 L2 7.3 For the beam shown in Figure P7.3, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
y PL
x Figure P7.3 A L2 L2 7.4 For the beam shown in Figure P7.4, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
y
x A Figure P7.4 L 7.5 For the beam shown in Figure P7.5, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
y w
x A
L Figure P7.5 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 7.6 For the beam shown in Figure P7.6, determine in terms of w, P, L, E, and I (a) the equation of the elastic curve; (b) the deflection of
the beam at point A.
P PL A
L Figure P7.6 7.7 The cantilever beam in Figure P7.7 is acted upon by a distributed bending moment m per unit length. Determine (a) the elastic curve
in terms of m, E, I, L, and x; (b) the deflection at x = L. y m
Figure P7.7
L January, 2010 x M. Vable 7.8 Mechanics of Materials: Deflection of Symmetric Beams w
x Figure P7.8 A
L2 L For the beam shown in Figure P7.9 determine the deflection at point A in terms of P, L, E, and I.
y PL x Figure P7.9 7.10 A
L L For the beam and loading shown in Figure P7.10, determine the deflection at point A in terms of P, L, E, and I.
y P
x A Figure P7.10 7.11 L L/2 For the beam shown and loading in Figure P7.11, determine the deflection at point A in terms of w, L, E, and I.
y w
x A Figure P7.11 7.12 336 For the beam shown in Figure P7.8 determine the deflection at point A in terms of w, P, L, E, and I.
y 7.9 7 L/2 L For the beam and loading shown in Figure P7.11, determine the deflection at point A in terms of w, L, E, and I . y w
A x Figure P7.12 L L/2 7.13 In Table P7.13, v1 and v2 represents the deflection in segment AB and BC. For the beam shown in Figure P7.2, identify all the conditions from Table P7.13 needed to solve for the deflection v(x) at any point on the beam.
TABLE P7.13 Potential Boundary and Continuity Conditions
w A (a) v 1 ( 0 ) = 0
B 1 x 2 2L (i) v 1 ( L ) = v 2 ( L ) (b) v 1 ( L ) = 0 C (e) v 2 ( 2 L ) = 0
(f) v 2 ( 3 L ) = 0 (j) v 1 ( 2 L ) = v 2 ( 2 L ) (c) v 2 ( L ) = 0 L (d) v 1 ( 2 L ) = 0 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Figure P7.13 (g) dv 1
(0) = 0
dx (k) dv 2
dv 1
(L) =
(L)
dx
dx (h) dv 2
(3L) = 0
dx (l) dv 2
dv 1
(2L) =
(2L)
dx
dx 7.14 In Table P7.13, v1 and v2 represents the deflection in segment AB and BC. For the beam shown in Figure P7.14, identify all the conditions from Table P7.13 needed to solve for the deflection v(x) at any point on the beam.
w A Figure P7.14 January, 2010 x L 1 2 B 2L C M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 337 7.15 In Table P7.13, v1 and v2 represents the deflection in segment AB and BC. For the beam shown in Figure P7.15, identify all the conditions from Table P7.13 needed to solve for the deflection v(x) at any point on the beam.
w wL x1 A 2 B C Figure P7.15
L 2L 7.16 In Table P7.13, v1 and v2 represents the deflection in segment AB and BC. For the beam shown in Figure P7.14, identify all the conditions from Table P7.13 needed to solve for the deflection v(x) at any point on the beam.
w
A wL 1 x B C 2 Figure P7.16
L 2L 7.17 For the beam and loading shown in Figure P7.17, determine in terms of w, L, E, and I (a) the equation of the elastic curve; (b) the
deflection at x = L.
wL y
x A Figure P7.17 B C
2L L 7.18 For the beam and loading shown in Figure P7.18, determine in terms of w, L, E, and I (a) the equation of the elastic curve; (b) the
deflection at x = L.
y w wL2
x A L Figure P7.18 B C
L 7.19 For the beam and loading shown in Figure P7.19 determine in terms of w, L, E, and I (a) the equation of the elastic curve; (b) the
deflection at x = L .
w y
x Figure P7.19 L L Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 7.20 For the beam and loading shown in Figure P7.20 determine in terms of w, L, E, and I (a) the equation of the elastic curve; (b) the
deflection at x = L.
w y
x Figure P7.20 L L 7.21 A simply supported beam in Figure P7.21 is acted upon by a distributed bending moment m per unit length. Determine (a) the elastic
curve in terms of m, E, I, L, and x; (b) deflection at x =L. y
m
Figure P7.21 January, 2010 L x
L M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 338 7.22 A diver weighing 200 lb stands at the edge of the diving board as shown in Figure 7.22. The diving board cross section is 16 in.x
1 in. and has a modulus of elasticity of 1500 ksi. Determine the maximum deflection in the diving board.
y y
x A B 56 in. 64 in. C 1 in. z
16 in. Figure P7.22 7.23 For the beam and loading shown in Figure P7.23, write the boundaryvalue problem for determining the deflection of the beam at any
point x. Assume EI is constant. Do not integrate or solve.
wL
x A Figure P7.23 B
w C
wL2 L L D
2L 7.24 For the beam shown in Figure P7.24, write the boundaryvalue problem for determining the deflection of the beam at any point x.
Assume EI is constant. Do not integrate or solve.
y
A w
2wL2 wL2 xB
L Figure P7.24 D C 2L L
wL Variable area moment of inertia
7.25 A cantilever beam with variable depth h(x) and constant width b is shown in Figure P7.25. The beam is to have a constant strength
σ. In terms of b, L, E, x, and σ, determine (a) the variation of h(x); (b) the maximum deflection.
P
x b h(x) Figure P7.25 L 7.26 A cantilever tapered circular beam with variable radius R(x) is shown in Figure P7.26.The beam is to have a constant strength σ. In
terms of L, E, x, and σ , determine (a) the variation of R(x); (b) the maximum deflection.
P Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm R (x) x Figure P7.26 L 7.27 For the tapered beam shown in Figure P7.27, determine the maximum bending normal stress and the maximum deflection in terms
of E, w, b, h0, and L.
w h0 Figure P7.27 January, 2010 b
L M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 339 7.28 For the tapered circular beam shown in Figure P7.28, determine the maximum bending normal stress and the maximum deflection in
terms of E, P, d0, and L.
P
2d0 d0 Figure P7.28 L 7.29 The 2in. × 8in. wooden beam of rectangular cross section shown in Figure P7.29 is braced at the support using 2in. × 1in.
wooden pieces. The modulus of elasticity of wood is 2000 ksi. Determine the maximum bending normal stress and the maximum deflection.
P = 800 lbs
1 in.
8 in.
1 in. Figure P7.29 6 ft 3 ft 7.30 A 2 in. x 8 in. wooden rectangular crosssection beam is braced the near the load point using 2 in. x 1 in. wooden pieces as shown in
Figure P7.30. The load is applied at the mid point of the beam. The modulus of elasticity of wood is 2,000 ksi. Determine the maximum
bending normal stress and the maximum deflection. (Hint: Use symmetry about mid point to reduce calculations)
P = 1600 lb
y 1 in. A B x Figure P7.30 6 ft C E D 8 in. 1 in.
6 ft 6 ft Stretch Yourself
7.31 To reduce weight of a metal beam the flanges are made of steel E = 200 GPa and the web of aluminum E = 70 GPa as shown
in Figure P7.31. Determine the maximum deflection of the beam.
y y 5 kN
Steel
Aluminum x 10 mm z
10 mm y
Steel
Aluminum Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 7.2 70 mm 2m Figure P7.31 10 mm Steel 10 mm 70 mm FOURTHORDER BOUNDARYVALUE PROBLEM We were able to solve for the deflection of a beam in Section 7.1 using secondorder differential equations because we could
find Mz as a function of x. In statically indeterminate beams, the internal moment determined from static equilibrium will contain some unknown reactions in the moment expression. Also, if the distributed load py is not uniform or linear but a more
complicated function, then finding the internal moment Mz as a function of x may be difficult. In either case it may be preferable to start from an alternative equation. We can substitute Equation (7.1) into Equation (6.17), (that is, into
d M z ⁄ dx = – V y ) and then substitute the result into Equation (E6.18), (that is, d V y ⁄ dx = – p y ) to obtain
2
d ⎛
d v⎞
V y = –  ⎜ EI zz 2 ⎟
dx ⎝
dx ⎠ January, 2010 (7.4) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 340 2 2
dv
d⎛
EI zz  ⎞ = p y
2
2⎝
dx ⎠
dx (7.5) If the bending rigidity EIzz is constant, then it can be taken outside the differentiation. However, if the beam is tapered, then Izz
is a function of x, and the form given in Equations (7.4) and (7.5) must be used. 7 .2.3 Boundary Conditions The deflection v(x) can be obtained by integrating Equation (7.5), but the fourthorder differential equation will generate four
integration constants. To determine these constants, four boundary conditions are needed. The integration of Equation (7.5)
will yield Vy of Equation (7.4), which on integration would yield Mz of Equation (7.1), which on integration would in turn
yield v and d v ⁄ dx . Thus boundary conditions could be imposed on any of the four quantities v, d v ⁄ dx , Mz, and Vy.
To understand how these conditions are determined, we generalize a principle discussed in statics for determining the reaction force and/or moments. Recall how we determine reaction forces and moments at the supports in drawing freebody diagrams: • If a point cannot move in a given direction, then a reaction force opposite to the direction acts at that support point.
• If a line cannot rotate about an axis in a given direction, then a reaction moment opposite to the direction acts at that support.
y p
Vy(0) MA
x B A Vy(L) A B
x L Mz(0) Mz(L) x RA
(a) (b) (c) Figure 7.15 Example demonstrating grouping of boundary conditions. Consider the cantilever beam with an arbitrarily varying distributed load shown in Figure 7.15a. We make an imaginary cut
very close to the support at A (at an infinitesimal distance Δx) and draw the freebody diagram as shown in Figure 7.15b. The
internal shear force and the internal moment are drawn according to our sign convention. Notice that the distributed force is not
shown because as Δx goes to zero, the contribution of the distributed force will drop out from the equilibrium equations. By
equilibrium we obtain V y ( 0 ) = – R A and M z ( 0 ) = – M A . Thus if a point cannot move—that is, the deflection v is zero at a
point—then the shear force is not known, because the reaction force is not known. Similarly if a line cannot rotate around an axis
passing through a point, d v ⁄ dx = 0 , and the internal moment is not known because the reaction moment is not known.
The reverse is equally true. Consider the freebody diagram constructed after making an imaginary cut at an infinitesimal
distance from end B, as shown Figure 7.15c. By equilibrium we obtain V y ( L ) = 0 and M z ( L ) = 0. However, the free end can
deflect and rotate by any amount dictated by the loading. Thus, when we specify a value of shear force, then we cannot specify
displacement. And when we specify a value of internal moment at a point, then we cannot specify rotation. We can thus place the Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm quantities v, d v ⁄ dx , Mz, and Vy: • Group 1: At a boundary point either the deflection v can be specified or the internal shear force Vy can be specified, but
not both. • Group 2: At a boundary point either the slope d v ⁄ dx can be specified or the internal bending moment Mz can be specified, but not both.
Two conditions are specified at each end of the beam, generating four boundary conditions. One condition is chosen from
each group. Stated succinctly, the boundary conditions at each end of the beam are • Group 1: v or Vy and • Group 2:
January, 2010 dv
dx or (7.6)
Mz M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 341 From Figure 7.15c we concluded that the shear force and the bending moment at the free end were zero. This conclusion can
be reached by inspection without drawing a freebody diagram. If at the end there were a point force or a point moment, then
clearly the magnitude of the shear force would equal the point force, and the magnitude of the internal moment would equal
the point moment. Again, we can reach this conclusion without drawing a freebody diagram. But to get the correct sign of Vy
and Mz we need a freebody diagram, with the internal quantities drawn according to our sign convention. We address the
issue in Section 7.2.5. 7.2.4 Continuity and Jump Conditions Suppose there is a point force or a point moment at xj, or that the distributed force is given by different functions on the left
and right of xj. Then, again, the displacement will be represented by different functions on the left and right of xj.
Thus we have two fourthorder differential equations, and their integration constants will require eight conditions: • Four conditions are the boundary conditions discussed in Section 7.2.3.
• Two additional conditions are the continuity conditions at xj discussed in Section 7.1.2.
• The remaining two conditions are the equilibrium equations on Vy and Mz at xj.
The equilibrium conditions on Vy and Mz at xj are jump conditions due to a point force or a point moment to be discussed in
the next section. 7.2.5 Use of Template in Boundary Conditions or Jump Conditions We discussed the concept of templates in drawing shear–moment diagrams in Section 6.4.2. Here we discuss it in determining
the boundary conditions on Vy and Mz and jumps in these internal quantities due to a point force or a point moment.
Recall that a template is a small segment of a beam on which a point moment Mj and a point force Fj are drawn (Δ x tends
to zero in Figure 7.16). Fj and Mj could be applied or reactive forces and moments and their directions are arbitrary. The ends
at +Δ x and –Δ x represent the imaginary cut just to the left and just to the right of the point forces and point moments. The internal shear force and the internal bending moment on these imaginary cuts are drawn according to our sign convention, as discussed in Section 6.2.6. Writing the equilibrium equations for this 2Δ x segment of the beam, we obtain the template equations, V2 ( xj ) – V1 ( xj ) = –Fj (7.7.a) M2 ( xj ) – M1 ( xj ) = Mj (7.7.b)
V1(xj) Mj
xj
M1(xj) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Figure 7.16 Template at xj. x V2(xj) x M2(xj) Fj The moment equation does not contain the moment due to the forces because these moments will go to zero as Δ x goes to
zero.
If the point force on the beam is in the direction of Fj shown on the template, the template equation for force is used as
given. If the point force on the beam is opposite to the direction of Fj shown on the template, then the template equation is used
by changing the sign of Fj. The template equation for the moment is used in a similar fashion. • If xj is a left boundary point, then there is no beam left of xj. Hence V1 and M1 are zero and we obtain the boundary conditions on Vy and Mz from V2 and M2. • If xj is a right boundary point, then there is no beam right of xj. Hence V2 and M2 are zero and we obtain the boundary
conditions on Vy and Mz from V1 and M1. • If xj is in between the ends of the beam, then the jump in shear force and internal moment is calculated using the template equations. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 342 An alternative to the templates is to draw freebody diagrams after making imaginary cuts at an infinitesimal distance from
the point force and writing equilibrium equations. Example 7.5 demonstrates the use of the template. Examples 7.6 and 7.7 demonstrate the use of freebody diagrams to determine the boundary conditions or the jump in internal quantities. EXAMPLE 7.5
The bending rigidity of the beam shown in Figure 7.17 is 135 (106) lbs·in.2, and the displacements of the beam in segments AB (v1) and BC
(v2) are as given below. Determine (a) the reactions at the left wall at A; (b) the reaction force at B and the applied moment MB.
3 2 v 1 = 5 ( x – 20 x ) 10
3 –6 in. 2 v 2 = 10 ( x – 30 x + 200 x ) 10 –6 MB 20 in. ≤ x ≤ 40 in. in. Some loading y 0 ≤ x ≤ 20 in. x B C D A Figure 7.17 Beam in Example 7.5. 20 in 20 in 40 in PLAN
By differentiating the given displacement functions and using Equations (7.1) and (7.4), we can find the bending moment Mz and the
shear force Vy in segments AB and BC. (a) Using the template in Figure 7.16, we can find the reactions at A from the values of Vy and Mz
at x = 0. (b) Using the template in Figure 7.16, we can find the reaction force and the applied moment at B from the values of Vy and Mz
before and after x = 20 in. S O L U T IO N
The shear force calculation requires the third derivative of the displacement functions. The functions v1 and v2 can be differentiated three
times:
d v1
 = 5 ( 3 x 2 – 40 x ) ( 10 –6 )
dx (E1) 2 d v1
 = 5 ( 6 x – 40 ) ( 10 –6 ) in. –1
2
dx (E2) d v1
 = ( 5 ) ( 6 ) ( 10 –6 ) = 30 ( 10 –6 ) in. –2
3
dx (E3) d v2
 = 10 ( 3 x 2 – 60 x + 200 ) ( 10 –6 )
dx (E4) 3 2 d v2
 = 10 ( 6 x – 60 ) ( 10 –6 ) in. –1
2
dx (E5) 3 d v2
 = ( 10 ) ( 6 ) ( 10 –6 ) = 60 ( 10 –6 ) in. –2
3
dx
From Equations (7.1), (E2), and (E5), the internal moment is
M z = EI zz
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 1 d2v 1
dx 2 6 2 (E6) –6 –1 (E7) –6 –1 (E8) = [ 135 ( 10 ) lbs.in. ] [ 5 ( 6 x – 40 ) ( 10 ) in. ] = 675 ( 6 x – 40 ) in.·lbs d2v 2 6 2 = [ 135 ( 10 ) lbs.in. ] [ 10 ( 6 x – 60 ) ( 10 ) in. ] = 1350 ( 6 x – 60 ) in.·lbs
2
dx
From Equations (7.4), (E3), and (E6), the shear force is
M z = EI zz
2 3 d v1
6
2
–2
V y 1 = EI zz  = [ 135 ( 10 ) lbs.in. ] [ 30 ( 10 –6 ) in. ] = 4050 lbs
3
dx (E9) 3 d v2
6
2
–6
–2
V y2 = EI zz  = [ 135 ( 10 ) lbs.in. ] [ 60 ( 10 ) in. ] = 8100 lbs
3
dx
The internal moment and shear force at A can be found by substituting x = 0 into Equations (E7) and (E9),
M z1 ( 0 ) = 675 ( – 40 ) = – 27,000 in.·lbs V y1 ( 0 ) = 4050 l bs (E10) (E11) The internal moment and shear force just before and after B can be found by substituting x = 20 into Equations (E7) through (E10),
January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams M z1 ( 20 ) = 675 [ ( 6 ) ( 20 ) – 40 ] = 54,000 in.·lbs 7 343 M z2 ( 20 ) = 1350 [ ( 6 ) ( 20 ) – 60 ] = 81,000 in.·lbs
V y ( 20 ) = 8100 lbs V y ( 20 ) = 4050 lbs
1 (E12)
(E13) 2 Figure 7.18 shows the freebody diagram of the entire beam. It also shows the template of Figure 7.16 for convenience.
Some loading
RA
A
MA Figure 7.18 MB V1(xj) Mj
xj RD B C D M1(xj) MD RB x V2(xj)
M2(xj) x
Fj Freebody diagram of entire beam in Example 7.5. If we compare the reaction force at A to Fj and the reaction moment to Mj in Figure 7.16, we obtain Fj = −RA and Mj = −MA. As point A is
the left end of the beam, V1(xj) and M1(xj) are zero on the template, and V 2 ( x j ) = V y1 ( 0 ) and M 2 ( x j ) = M z1 ( 0 ) . From the template
equation we obtain R A = V y 1 ( 0 ) and M A = – M z1 ( 0 ) . Substituting Equation (E11), we obtain
ANS. R A = 4050 lbs M A = 27,000 in. · lbs If we compare the reaction force at B to Fj and the applied moment to Mj in Figure 7.16, we obtain Fj = RB and Mj = MB. Substituting for
xj = 20 in. and using Equations (E12) through (E13), we obtain RB and MB,
R B = V 1 ( 20 ) – V y2 ( 20 ) = 4050 lbs – 8100 lbs = – 4500 lbs (E14) M B = M z2 ( 20 ) – M z1 ( 20 ) = 81,000 in.·lbs – 54,000 in.·lbs = 27, 000 in.·lbs (E15) ANS. M B = 27, 000 in.·lbs R B = – 4500 lbs COMMENTS
1. An alternative to the use of the template is to draw a freebody diagram after making imaginary cuts at an infinitesimal distance from
the point forces, as shown in Figure 7.19. The internal forces and moments must be drawn according to our sign convention. By writing equilibrium equations the required quantities can be found.
RA
MA
RA
MA Vy1(0)
A Vy1(20) MB Vy2(20) Mz1(0) Vy1(0)
Mz1(0) B
Mz1(20)
RB (a) Figure 7.19 Alternative to template. Mz2(20) Vy2(20)
Mz2(20) Vy1(20) MB Vy2(20) Vy1(20)
Mz1(20) RB
MB (b) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 2. The freebody diagram of the entire beam in Figure 7.18 is not necessary. From the template equations, the force Fj and the moment
Mj with the correct signs can be found. If Fj and Mj are positive, then RB and MB will be in the direction shown on the template. If
these quantities are negative, then the direction is opposite.
3. This problem demonstrates how we (i) determine the conditions on shear force and bending moment and (ii) relating these internal
quantities to the reaction forces and moments. The same basic principles apply when the displacement functions have to be determined first, as we see next. EXAMPLE 7.6
In terms of E, I, w, L, and x, determine (a) the elastic curve; (b) the reaction force at A in Figure 7.20.
y w
x A Figure 7.20 Beam and loading in Example 7.6. B
L M ET H O D 1 P L AN : F O U R T H  O R D E R D I F F E R E N T I A L E Q U A T I O N
(a) Noting that the distributed force is in the negative y direction, we can substitute py = −w in Equation (7.5) and write the fourthorder
differential equation. The two boundary conditions at A are zero deflection and zero moment, and the two boundary conditions at B are
zero deflection and zero slope. We can solve the boundaryvalue problem and obtain the elastic curve. (b) We can draw a freebody diaJanuary, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 344 gram after making an imaginary cut just to the right of A and relate the reaction force to the shear force. We can find the shear force at
point A by substituting x = L in the solution obtained in part (a). S O L U T IO N
(a) The boundaryvalue problem statement can be written below following the description in the Plan.
• Differential equation:
2
2
d v⎞
d⎛
⎜ EI zz 2 ⎟ = – w
2
dx ⎠
dx ⎝ (E1) v(0) = 0 (E2) dv
(0) = 0
dx (E3) v(L) = 0 (E4) • Boundary conditions: 2 dv
EI zz  ( L ) = 0
2
dx (E5) d ⎛ EI d v ⎞ = – w x + c
⎜
⎟
1
d x ⎝ zz d x 2 ⎠ (E6) Integrating Equation (E1) twice,
2 2 2 wx
= –  + c 1 x + c 2
2
dx
Substituting Equation (E7) into Equation (E5), we obtain
EI zz dv (E7) 2 2 wL c 1 L + c 2 = 2 (E8) Integrating Equation (E7), we obtain
3 2 dv
wx
x
= –  + c 1  + c 2 x + c 3
dx
6
2
Substituting Equation (E9) into Equation (E3), we obtain (E9) EI zz (E10) c3 = 0 Substituting Equation (E10) and integrating Equation (E9), we obtain
4 3 2 wx
x
x
EI zz v = –  + c 1  + c 2  + c 4
24
6
2
Substituting Equation (E11) into Equation (E2), we obtain (E11)
(E12) c4 = 0 Substituting Equations (E12) and (E11) into Equation (E4), we obtain
3 2 4
c1 L c2 L
wL
 +  = 6
2
24
Solving Equations (E8) and (E13) simultaneously, we obtain (E13) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 2 5 wL
wL
c 1 = c 2 = – 8
8
Substituting Equations (E12) and (E14) into Equation (E11) and simplifying, we obtain the elastic curve, ANS. w
4
3
22
v ( x ) = –  ( 2 x – 5 Lx + 3 L x )
48 EI zz (E14) (E15) Dimension check: Note that all terms in parentheses on the righthand side of Equation (E15) have the dimension of length to the power
of 4, or O(L4). Thus Equation (E15) is dimensionally homogeneous. But we can also check whether the lefthand side and any one term
of the righthand side have the same dimension:
F
w → O ⎛  ⎞
⎝ L⎠ January, 2010 x → O(L) F
E → O ⎛  ⎞
⎝ L2 ⎠ 4 I zz → O ( L ) v → O(L) 4
⎛ ( F ⁄ L ) L4 ⎞
wx
 → O ⎜  ⎟ → O ( L ) → checks
2
4
EI zz
⎝(F ⁄ L )O(L ) ⎠ M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 345 (b) We make an imaginary cut just to the right of point A (at an infinitesimal distance) and draw the freebody diagram of the left part
using the sign convention in Section 6.2.6, as shown in Figure 7.21. By force equilibrium in the y direction, we can relate the shear force
at A to the reaction force at A,
RA = VA = Vy ( L ) (E16)
VA
A Figure 7.21 Infinitesimal equilibrium element at A in Example 7.6.
From Equations (7.4), (E6), and (E14), the shear force is
Vy ( x ) = – RA MA x d v⎞
5 wL
d⎛
⎜ EI zz 2 ⎟ = wx – 8
dx⎝
dx ⎠
2 (E17) Substituting Equation (E17) into Equation (E16), we obtain the reaction at A.
3 wL
R A = 8 ANS. M ET H O D 2 P L AN : S E C O N D  O R D E R D I F F E R E N T I A L E Q U A T I O N
We can make an imaginary cut at some arbitrary location x and use the left part to draw the freebody diagram. The moment expression
will contain the reaction force at A as an unknown. The secondorder differential equation, Equation (7.1), would generate two integration constants, leading to a total of three unknowns. We need three conditions: the displacement at A is zero, and the displacement and
slope at B are both zero. Solving the boundaryvalue problem, we can obtain the elastic curve and the unknown reaction force at A. S O L U T IO N
We make an imaginary cut at a distance x from the right wall and take the left part of length L − x to draw the freebody diagram using
the sign convention for internal quantities discussed in Section 6.2.6 as shown in Figure 7.22.
(a) w Mz w(L (b) x)
Mz
O O
L Figure 7.22 Freebody diagram in Example 7.6. x L Vy
RA RA x L 2 x
2 Vy Balancing the moment at point O, we obtain the moment expression,
2 ( L – x )w2
2
or
M z = R A ( L – x ) –  ( L + x – 2 Lx )
M z – R A ( L – x ) + w  = 0
2
2
Substituting into Equation (7.1) and writing the boundary conditions, we obtain the following boundaryvalue problem: (E1) • Differential equation:
2 dv
w2
2
EI zz  = R A ( L – x )   ( L + x – 2 Lx )
2
2
dx (E2) v(0) = 0 (E3) dv
 ( 0 ) = 0
dx (E4) v(L) = 0 (E5) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm • Boundary conditions: Integrating Equation (E2), we obtain
2 3 dv
w2
x
x
2
= R A ⎛ Lx –  ⎞ –  ⎛ L x +  – Lx ⎞ + c 1
⎠
⎝
dx
2 ⎠ 2⎝
3
Substituting Equation (E6) into Equation (E4), we obtain
EI zz (E6)
(E7) c1 = 0 Substituting Equation (E7) and integrating Equation (E6), we obtain
2 3 22 4 3 wLx
x
Lx
Lx x
EI zz v = R A ⎛  –  ⎞ –  ⎛  +  –  ⎞ + c 2
⎝2
6 ⎠ 2⎝ 2
12
3⎠
Substituting Equation (E8) into Equation (E3), we obtain
c2 = 0 Substituting Equations (E8) and (E9) into Equation (E5), we obtain
January, 2010 (E8)
(E9) M. Vable Mechanics of Materials: Deflection of Symmetric Beams
3 3 4 4 7 346 4 LL
LL
3 wL
wL
R A ⎛  –  ⎞ –  ⎛  +  –  ⎞ = 0
or
ANS. R A = ⎝ 2 6 ⎠ 2 ⎝ 2 12 3 ⎠
8
Substituting Equations (E9) and (E10) into Equation (E8) and simplifying, we obtain v(x). ANS. (E10) w4
3
22
v ( x ) = –  ( 2 x – 5 Lx + 3 L x )
48 EI zz (E11) COMMENTS
1. Method 2 has less algebra than Method 1 and should be used whenever possible.
2. Suppose that in drawing the freebody diagram for calculating the internal moment, we had taken the righthand part. Then we would
have two unknowns rather than one—the wall reaction force and moment in the expression for moment. In such a case we would
have to eliminate one of the unknowns using the static equilibrium equation for the entire beam. In other words, in statically indeterminate problems, the internal moment should contain a number of unknown reactions equal to the degree of static redundancy.
3. The moment boundary condition given by in Method 1 is implicitly satisfied. We can confirm this by substituting x = L in Equation
(E1). EXAMPLE 7.7
A light pole is subjected to a wind pressure that varies as a quadratic function, as shown in Figure 7.23. In terms of E, I, w, L, and x,
determine (a) the deflection at the top of the pole; (b) the ground reactions.
w B ⎛ x2 ⎞
w ⎜  ⎟
⎝ L 2⎠ L x
y Figure 7.23 Beam and loading in Example 7.7. A PLAN
(a) Finding the moment as a function of x by static equilibrium is difficult for this statically determinate problem. We can use the fourthorder differential equation, Equation (7.5). We have four boundary conditions: the deflection and slope at A are zero, and the moment
and shear force at B are zero. We can then solve the boundaryvalue problem and determine the elastic curve. By substituting x = L in the
elastic curve equation, we can obtain the deflection at the top of the pole. (b) By making an imaginary cut just above point A, we can
relate the internal shear force and the internal moment at point A to the reactions at A. By substituting x = 0 in the moment and shear
force expressions, we can obtain the shear force and moment values at point A. S O L U T IO N
The boundaryvalue problem below can be written as described in the Plan. Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm • Differential equation:
2
2
⎛ x2⎞
d v⎞
d⎛
⎜ EI zz  ⎟ = – w ⎜  ⎟
2
2
2
⎝L ⎠
dx ⎠
dx ⎝ (E1) v(0) = 0 (E2) dv
(0) = 0
dx (E3) • Boundary conditions: 2 dv
EI zz 2
dx =0 2
d ⎛ EI d v⎞
⎜ zz  ⎟
2
dx⎝
dx ⎠ January, 2010 (E4) x=L =0
x=L (E5) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 347 Integrating Equation (E1), we obtain
dv
d ⎛ EI ⎞ =  wx  + c
⎟
⎜
1
2
d x ⎝ zz dx 2 ⎠
3L
2 3 (E6) Substituting Equation (E6) into Equation (E5), we obtain
wL
c 1 = 3
Substituting Equation (E7) into Equation (E6) and integrating, we obtain
2 (E7) 4 dv
wx  wL
EI zz  =   +  x + c 2
2
2
3
dx
12 L
Substituting Equation (E8) into Equation (E4), we obtain (E8) 2 wL
c 2 = – 4
Substituting Equation (E9) into Equation (E8) and integrating, we obtain
5 2 (E9)
2 dv
wx  wLx wL x
EI zz  =   +  –  + c 3
2
dx
4
6
60 L
Substituting Equation (E10) into Equation (E3), we obtain (E10) c3 = 0 (E11) Substituting Equation (E11) into Equation (E10) and integrating, we obtain
6 3 22 dv
wLx wL x
wx
EI zz  =   +  –  + c 4
2
18
8
dx
360 L
Substituting Equation (E12) into Equation (E2), we obtain (E12) (E13) c4 = 0 Substituting Equation (E13) into Equation (E12) and simplifying, we obtain
w 6
33
42
v ( x ) = –  ( x – 20 L x + 45 L x )
2
360 EI zz L (E14) Dimension check: Note that all terms in parentheses on the righthand side of Equation (E14) have the dimension of length to the power
of 6, or, O(L6). Thus Equation (E14) is dimensionally homogeneous. But we can also check whether the lefthand side and any one term
of the righthand side have the same dimension:
F
w → O ⎛  ⎞
⎝L ⎠ x → O(L) F
E → O ⎛  ⎞
⎝ L 2⎠ 4 I zz → O ( L ) v → O(L) 6
⎛ ( F ⁄ L ) L6 ⎞
wx
 → O ⎜  ⎟ → O ( L ) → checks
2
2
42
⎝ (F ⁄ L )L L ⎠
EI zz L (a) Substituting x = L into , we obtain the deflection at the top of the pole.
4 ANS. 13 wL
v ( L ) = – 180 EI zz (b) We make an imaginary cut just above point A ( Δ x → 0 ) and take the bottom part to draw the freebody diagram shown in Figure
7.24. By equilibrium of forces and moments, we can relate the reaction force RA and the reaction moment MA to the internal shear force
and the internal bending moment at point A, Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Mz ( 0 ) = –MA Vy ( 0 ) = –R A (E15)
Mz(0) Vy(0)
A Figure 7.24 Free body diagram of an infinitesimal element at A in Example 7.7. RA Δx MA Substituting Equations (E7) and (E6) into Equation (7.4) and Equations (E9) and (E8) into Equation (7.1), we can obtain the shear force
and bending moment expressions,
4 2 wx
wL
wL
M z ( x ) =   +  x – 2
4
3
12 L January, 2010 (E16) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 348 3 wx  wL
V y ( x ) =  – 2
3
3L
Substituting Equations (E16) and (E17) into Equation (E15), we obtain the reaction force and the reaction moment. ANS. (E17) 2 wL
R A = 3 wL
M A = 4 COMMENTS
1. The directions of RA and MA can be checked by inspection, as these are the directions necessary for equilibrium of the externally distributed force.
2. The freebody diagram in Figure 7.24, the reaction force RA and the reaction moment MA can be drawn in any direction, but the internal quantities Vy and Mz must be drawn according to the sign convention in Section 6.2.6. Irrespective of the direction in which RA
and MA are drawn, the final answer will be as given. The sign in the equilibrium equations, Equation (E15), will account for the
assumed directions of the reactions. PROBLEM SET 7.2
Fourthorder boundaryvalue problems
7.32 3 2 The displacement in the y direction in segment AB, shown in Figure P7.32, was found to be v ( x ) = ( 20 x – 40 x )10 –6 in. If the bending rigidity is 135 × 106 lb·in.2, determine the reaction force and the reaction moment at the wall at A.
Some loading y
A
x
20 in Figure P7.32 7.33 B
60 in
F
4 3 In Figure P7.33, the displacement in the y direction in section AB, is given by v 1 ( x ) = – 3 ( x – 20 x ) ( 10 –6 ) in. and in BC by
2 v 2 ( x ) = – 8 ( x – 100 x + 1600 ) ( 10 –3 ) in. . If the bending rigidity is 135 × 106 lb·in.2, determine: (a) the reaction force at B and the applied moment MB; (b) the reactions at the wall at A.
y Some loading
MB
Bx C A Figure P7.33 7.34 60 in For the beam shown in Figure P7.34, determine the elastic curve and the reaction(s) at A in terms of E, I, P, w, and x. Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm y P (kN)
x Figure P7.34 7.35 20 in A L (m) L (m) For the beam shown in Figure P7.35, determine the elastic curve and the reaction(s) at A in terms of E, I, P, w, and x.
y Figure P7.35 January, 2010 w (kN/m)
x A
L (m) M. Vable 7.36 Mechanics of Materials: Deflection of Symmetric Beams 7 349 For the beam shown in Figure P7.36, determine the slope at x = L and the reaction moment at the left wall in terms of E, I, w, and L.
y x2
L2 w1 A
x B Figure P7.36 7.37 L For the beam shown in Figure P7.37, determine the deflection and the moment reaction at x = L in terms of E, I, w, and L.
y w A
x Figure P7.37 7.38 B
L For the beam shown in Figure P7.38, determine the deflection and the slope at x = L in terms of E, I, w, and L.
w y x2
L2
wL2 x B A Figure P7.38 7.39 L wL For the beam and loading shown in Figure P7.39, determine the deflection and slope at x = L in terms of E, I, w, and L.
πx
w cos 2L y A x B Figure P7.39 L 7.40 For the beam and loading shown in Figure P7.40, determine the slope at x = L and the reaction moment at the left wall in terms of E,
I, w, and L.
πx
w cos 2L y A x B Figure P7.40 7.41 L For the beam and loading shown in Figure P7.41, determine the maximum deflection in terms of E, I, w, and L.
w sin π x
L Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm y
x Figure P7.41 7.42 L For the beam and loading shown in Figure P7.42, determine the maximum deflection in terms of E, I, w, and L.
w sin π x
L y
x Figure P7.42
January, 2010 L M. Vable 7.43 Mechanics of Materials: Deflection of Symmetric Beams 7 350 A cantilever beam under uniform load has a spring with a stiffness k attached to it at point A, as shown in Figure P7.43. The spring
3 constant in terms of stiffness of the beam is written as k = α EI ⁄ L , where α is a proportionality factor. Determine the compression of the
spring in terms of α, w, E, I, and L.
y w
x A
L k
Figure P7.43 7.44 A linear spring that has a spring constant K is attached at the end of a beam, as shown in Figure P7.44. In terms of w, E, I, L, and K,
write the boundaryvalue problem but do not integrate or solve.
w sin( x 2L) (lb ft)
w (lb ft)
x M B
wL2 (ft lb) C D A
L Figure P7.44 L
F K L wL Historical problems
7.45 The beam and loading shown in Figure P7.45 was the first statically indeterminate beam for which a solution was obtained by
Navier. Verify that Navier’s solution for the reaction at A is given by the equation below.
y P
2 Pa ( 3 L – a )
R A = 3
2L C
x B Figure P7.45 A a where L = a + b b 7.46 Jacob Bernoulli incorrectly assumed that the neutral axis was tangent to the concave side of the curve in Figure P7.46 and obtained
the equation given below. In the equation R is the radius of curvature of the beam at any location x. Derive this equation based on Bernoulli’s
assumption and show that it is incorrect by a factor of 4. (Hint: Follow the process in Section 6.1 and take the moment about point B.)
D C Figure P7.46 3 Ebh 1
 ⎛  ⎞ = Px
3 ⎝ R⎠ h dx
A
R B b x
P O Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 7.47 Clebsch considered a beam loaded by several concentrated forces Pi placed at a location xi, as shown in Figure P7.47. He obtained
the secondorder differential equation between the concentrated forces. By integration he obtained the slope and deflection as given and
concluded that all Ci’s are equal and all Di’s are equal. Show that his conclusion is correct. For x i ≤ x ≤ x i + 1 ,
i 2 i j =1 P1
x0 j =1 Pi P2
x 0
x1
x2 Figure P7.47 January, 2010 xi
R 2 2
( x – xi )
dv
x
EI  = R  – ∑ P i  + C i
dx
2
2 dv
EI  = R x – ∑ P j ( x – x j )
2
dx i 3 3
( x – xi )
x
EI v = R  – ∑ P i  + C i x + D i
6
6
j =1 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 351 Stretch Yourself
7.48 A beam resting on an elastic foundation has a distributed spring force that depends on the deflections at a point acting as shown in Figure P7.48. Show that the differential equation governing the deflection of the beam is given by Equation (7.8), where k is the foundation
modulus, that is, spring constant per unit length.
p
Mz Mz 2
2
d v⎞
d⎛
⎜ EI zz ⎟ + k v = p
2
2
dx ⎠
dx ⎝ dMz dx
Vy Vy
(k dx)v (7.8) dVy Figure P7.48 Elastic foundation effect. 7.49 To account for shear, the assumption of planes remaining perpendicular to the axis of the beam (Assumption 3 in Section 6.2) is
dropped, and it is assumed that the plane rotates by the angle ψ from the vertical. This yields the following displacement equations:
u = –y ψ ( x ) v = v(x) The rest of the derivation1 is as before. Show that the following equations apply:
dv
d
GA ⎛ – ψ ⎞
⎝ dx
⎠
dx dv
d ⎛ EI dψ ⎞
= – GA ⎛ – ψ⎞
⎝dx
⎠
d x ⎝ zz d x ⎠ = –p (7.9) where A is the crosssectional area and G is the shear modulus of elasticity. Beams governed by these equations are called Timoshenko beams. 7.50 Figure P7.50 shows a differential element of a beam that is free to vibrate, where ρ is the material density, A is the crosssectional
2 2 area, and ∂ v ⁄ ∂ t is the linear acceleration. Show that the dynamic equilibrium is given by Equation (7.10).
Mz Vy Figure P7.50 7.51 Mz Vy x A Mz Vy 2 v
t2 x 2 4 ∂v
∂v
 + c 2  = 0
2
4
∂t
∂x where c = EI zz ⁄ ρ A . (7.10) x Dynamic equilibrium. Show by substitution that the following solution satisfies Equation (7.10):
v ( x, t ) = G ( x ) H ( t ) G ( x ) = A cos ω x + B sin ω x + C cosh ω x + D sinh ω x 2 2 H ( t ) = E cos ( c ω ) t + D sin ( c ω ) t 7.52 Show by substitution that the following deflection solution satisfies the fourth order boundary value problem of the cantilever beam
shown in Figure P7.52.
x 1
v ( x ) =  R A x 3 + 3 M A x 2 + ∫ ( x – x 1 ) 3 p ( x 1 ) dx 1
6 EI 0 x ∫0 L where R A = – ∫ p ( x 1 ) dx 1 and M A =
0 L ∫ 0 x1 p ( x1 ) dx1 . (7.11) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Computer problems
7.53 Table P7.53 shows the value of distributed load at several point along the axis of a 10 ft long rectangular beam. Determine the slope
and deflection at the free end using. Use modulus of elasticity as 2000 ksi.
TABLE P7.53 Data in Problem 7.53
x
(ft) p(x)
(lb/ft) x
(ft) p(x)
(lb/ft) 0 275 6 377 1 348 7 316 2 398 8 233 3 426 9 128 4 432 10 0 5 416 Use Equations (2.12a) and (2.12d) to get εxx and γxy. Use Hooke’s law, the static equivalency equations [Equations (6.1) and (6.13)], and the
equilibrium equations [Equations (6.17) and (6.18)]. 1 January, 2010 M. Vable 7.54 Mechanics of Materials: Deflection of Symmetric Beams 7 352 For the beam and loading given in Problem 7.53, determine the slope and deflection at the free end in the following manner. First
2 represent the distributed load by p ( x ) = a + bx + cx and, using the data in Table P7.53, determine constants a, b, and c by the leastsquares
method. Then using fourthorder differential equations solve the boundaryvalue problem. Use the modulus of elasticity as 2000 ksi. 7.55 Table P7.55 shows the measured radii of a solid tapered circular beam at several points along the axis, as shown in Figure P7.55. The
beam is made of aluminum (E = 28 GPa) and has a length of 1.5 m. Determine the slope and deflection at point B.
TABLE P7.55 Data for Problem 7.55
P
R (x) A 25 kN x
(m)
0.0 B R(x)
(mm)
100.6 x
(m)
0.8 R(x)
(mm)
60.1 0.9 60.3 82.6 1.0 59.1 0.3 79.6 1.1 54.0 0.4 75.9 1.2 54.8 0.5 68.8 1.3 54.1 0.6 Figure P7.55 92.7 0.2 x 0.1 68.0 1.4 49.4 65.9 1.5 50.6 0.7 7.56 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Let the radius of the tapered beam in Problem 7.55 be represented by the equation R ( x ) = a + bx . Using the data in Table P7.55,
determine constants a and b by the leastsquares method and then find the slope and deflection at point B by analytical integration. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 353 MoM in Action: Skyscrapers
A skyscraper can be a monument to the builder’s pride or a literal monument, designed to attract tourists and tenants to the city’s, the country’s, or the world’s tallest building. When John Roscoe, in 1930, wanted a taller building than
Walter Chrysler’s, he pushed for his own, completed just one year after the Chrysler building More than 40 years later, his
Empire State Building (Figure 7.25a). was still the world’s tallest building, at 1250 ft. Even today, it is surely the most
famous skyscraper ever. New construction is also driven by the same social forces as those behind the boom in Chicago,
New York, and London at the end of 19th century. Businesses then and now want to be near a city’s commercial center
and emerging economies are seeing a movement of the population from villages to cities. As this edition goes to press, the
tallest building is Taipei 101 in Taiwan (Figure 7.25b). Built in 2003, it stands 1671 feet tall.
(a) (b) (c) Figure 7.25 (a) Empire State in New York. (b) Taipei 101 in Taipei, Taiwan. (c) Joint construction. Social forces, then, have pushed skyscrapers higher and higher, but technological advances have made that
possible. Early highrise buildings had a pyramid design: the building crosssection decreased with height to avoid
excessive stresses at the bottom. The height of these building was limited by the strength of masonry materials and by the
difficulty of getting water to higher stories. Besides, renters did not want to climb too many stairs! With the advent of steel
beams, reinforced concrete, glass, electric water pumps, and elevators, however, the human imagination was freed to build
tall. If one thinks of a highrise building as an axial column, then skyscrapers are like cantilevered beams subject to
bending loads in the wind. A proper variation of both axial and bending rigidity with height is important in design.
Skyscrapers must be strong enough to withstand hurricane winds in excess of 140 mph. With an increase in height, too, the
bearing stresses at the base increase, often requiring digging deep to bedrock.
In addition to the stresses, the deflection of a skyscraper increases with height. By welding and bolting the Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm horizontal girders to steel columns, the rigidity of the joint (Figure 7.25c) is increased, which helps reduce the sway.
Skyscraper designs often have columns on the outer perimeter, which are connected to the central core columns. The outer
columns act like flanges to resist most of the wind load, while the inner columns carry most of the weight. In modern
skyscrapers, computercontrolled masses of hundreds of tons, called tuned mass dampers, move to counter the building
sway. Today skyscrapers are also designed to move with earthquakes rather then stress the building frames.
The terrorist attack on World Trade Center (see page 525) has highlighted the need for better fireproofing of steel
beams, and technology is once more providing the solution. But terrorist acts do not deter the human spirit, which like the
skyscrapers themselves still soars. In the sands of United Arab Emirates, the next tallest skyscraper is rising. Called Burj
Dubai, or the Dubai tower, it will be twice the height of the Empire State Building. January, 2010 M. Vable 7.3* Mechanics of Materials: Deflection of Symmetric Beams 7 354 SUPERPOSITION The assumptions and limitations that were imposed in deriving the simplest theory for beam bending ensured that we have a
linear theory. As a consequence, the differential equations governing beam deflection, Equations (7.1) and (7.5), are linear
differential equations, and hence the principle of superposition can be applied to beam deflection.
The leftmost beam in Figure 7.26 is loaded with a uniformly distributed load w and a concentrated load P1. The superposition
principle says that the deflection of a beam with uniform load w and point force P1 is equal to the sum of the deflections calculated
by considering each load separately, as shown on the right two beams in Figure 7.26. Although the example in Figure 7.26 demonstrates the principle of superposition, there is no intrinsic gain in calculating the deflection of each load separately and adding to find
the final answer. But if the solutions to basic cases are tabulated, as in Table C.3, then the principle of superposition becomes a very
useful tool to obtain results quickly. Thus the maximum deflection of the beam on the left can be found using the results of cases 1
and 3 in Table C.3. Comparing the loading of the two beams in Figure 7.26 to those shown for cases 1 and 3, we note that P = −
P1 = −wL and p0 = −w, a = L, and b = 0. Substituting these values into vmax given in Table C.3 and adding, we obtain
3 4 4 ( wL ) L wL
11 wL
v max = −  –  = – 3 EI
8 EI
24 EI
y P1 w wL y x (7.12.a)
wL P1 y x w
x L (m) L (m) L (m) Figure 7.26 Example of superposition principle. Another very useful application of superposition is the deflection of statically indeterminate beams. Consider a beam built in at
one end and simply supported at the other end with a uniformly distributed load, as shown in Figure 7.27. The support at A can be
replaced by a reaction force, and once more the total loading can be shown as the sum of two individual loads, as shown at right in
Figure 7.27. Comparing the loading of the two beams in Figure 7.26 to those shown for cases 1 and 3 in Table C.3, we note that
P = RA, p0 = −w, a = L, and b = 0. vmax is at point A in both cases. Substituting these values into vmax given in Table C.3 and adding,
we obtain
3 R A L wL 4
v A =  – 3 EI 8 EI
y y w
x y
x A
L (m) Figure 7.27 (7.12.b) x A
L (m) w RA A
L (m) Example of use of superposition principle in solving statically indeterminate beam deflection. But the deflection at A must be zero in the original beam. Thus we can solve for the reaction force as RA = 3wL /8EI. Now the
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm solution of v(x) given in Table C.3 can be superposed to obtain
2 2
RAx
( –w ) x 2
2
v ( x ) =  ( 3 L – x ) +  ( x – 4 Lx + 6 L )
6 EI
24 EI (7.12.c) Substituting for RA and simplifying, the solution for the elastic curve is
2 2 2 wx ( – 2 x + 5 Lx – 3 L )
v ( x ) = 48 EI January, 2010 (7.12.d) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 355 EXAMPLE 7.8
For the beam shown in Figure 7.28, using the principle of superposition and Table C.3, determine (a) the reactions at A; (b) the maximum
deflection.
y Figure 7.28 w (kN/m)
x Beam in Example 7.8. A
L (m) PLAN
(a) The wall at A can be replaced by a force reaction RA and a moment reaction MA. Thus the beam would be a cantilever beam with a uniformly distributed load, a point force at the end, and a point moment at the end, corresponding to the first three cases in Table C.3. Superposing the slope and deflection values from Table C.3 and equating the result to zero would generate two equations in the two unknowns RA and
MA which give the reactions at A. (b) From the symmetry of the problem, we can conclude that the maximum deflection will occur at the
center. Substituting x = L/2 in the elastic curve equation of Table C.3 and adding the results, we can find the maximum deflection of the
beam. S O L U T IO N
(a) The right wall at A can be replaced by a reaction force and a reaction moment, as shown at left in Figure 7.29. The total loading on
the beam can be considered as the sum of the three loadings shown at right in Figure 7.29.
y y w (kN/m) y y MA
x A
L (m) w (kN/m) MA
x RA A x
RA L (m) A x A
L (m) L (m) Figure 7.29 Superposition of three loadings in Example 7.8.
Comparing the three beam loadings in Figure 7.29 to that shown for cases 1 through 3 in Table C.3, we obtain P = RA, M = –MA, and p
= –w, a = L, and b = 0. Noting that vmax and θmax shown in Table C.3 for the cantilever beam occur at point A, we can substitute the load
values and superpose to obtain the deflection vA and the slope at θA. Noting that at the wall at A the deflection vA and the slope at θA must
be zero, we obtain two simultaneous equations in RA and MA,
3 2 RA L ( –MA ) L ( –w ) L4
v A =  +  +  = 0
3 EI
2 EI
8 EI or 8 R A L – 12 M A = 3 wL 2 RA L ( –MA ) L ( –w ) L3
θ A =  +  +  = 0
2 EI
6 EI
EI
Equations (E1) and (E2) can be solved to obtain RA and MA. 3 R A L – 6 M A = wL or 2 (E1) 2 (E2) 2 wL
wL
(E3)
R A = M A = 12
2
(b) The maximum deflection would occur at the center of the beam. Substituting x = L/2, P = RA = wL/2, M = –MA = –wL/12, and p ANS. = –w in the equation of the elastic curve for cases 1 through 3 in Table C.3 and superposing the solution, we obtain
2 2 2 2 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm L
( wL /2 ) ( L /2 )
L
L – ( w L /12 ) ( L /2 )  ( – w ) ( L /2 )  L 2
2
v max = v ⎛  ⎞ =  ⎛ 3 L –  ⎞ +  +  ⎛  ⎞ – 4 L ⎛  ⎞ + 6 L
⎝ 2⎠
⎝ 2⎠
⎝
⎝ 2⎠
6 EI
2⎠
2 EI
24 EI
4 4 or (E4) 4 5 wL
wL
17 wL
v max =  –  – 96 EI 96 EI 384 EI (E5)
4 ANS. wL
v max = – 384 EI COMMENTS
1. All terms in Equations (E1) and (E2) have the same dimension, as they should. If this were not the case, then we would need to examine the equations obtained using superposition and the subsequent simplifications carefully to ensure dimensional homogeneity.
2. By symmetry we know that the reaction forces at each wall must be equal. Hence the value of the reaction forces should be wL/2, as
calculated in Equation (E3). January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 356 EXAMPLE 7.9
The end of one cantilever beam rests on the end of another cantilever beam, as shown in Figure 7.30. Both beams have length L and
bending rigidity EI. Determine the deflection at A and the wall reactions at B and C in terms of w, L, E, and I. w
B
wL L2 A
L2 L Figure 7.30 Two cantilever beams in Example 7.9. C PLAN
The two beams can be separated by putting an unknown force RA that is equal but opposite in direction on each beam at point A. From case
1 in Table C.3 for beam AB, the deflection at A can be found in terms of RA. From cases 1 and 3 for beam AC, the deflection at A can be
found by superposition. By equating the deflection at A for the two beams, we can find the force RA. (a) Once RA is known, the deflection
at A is found from the equation written for beam AB. (b) The reactions at B and C can be found using equilibrium equations on each
beam’s freebody diagram. S O L U T IO N
(a) The assembly of the beams shown in Figure 7.30 can be represented by two beams with a force RA that acts in equal but opposite
directions, as shown in Figure 7.31a and b. The loading on the beam in Figure 7.31b can be represented as the sum of the two loadings
shown in Figure 7.31c and d.
w B
RA
L C wL (a) C wL L2 A A RA Figure 7.31 w L2 A L2 RA (b) C
L2 A L2 L2 (c) (d) Analysis of beam assembly by superposition in Example 7.9. Comparing the beam of Figure 7.31a to that shown in case 1 in Table C.3, we obtain P = –RA, a = L, and b = 0. Noting that vmax in case 1
occurs at A, the deflection at A can be written as
2 3 ( –RA ) L
RA L
v A =  2 L = – (E1)
6 EI
3 EI
Comparing the beam in Figure 7.31c to that of case 1 in Table C.3, we obtain P = RA − wL, a = L, and b = 0. Comparing the beam in Fig ure 7.31d to that of case 3 in Table C.3, we obtain p0 = −w, a = L/2, and b = L/2. Since vmax for both cases occurs at A, by superposition
the deflection at A can be written as
2 3 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 3
4
( R A – wL ) L
( R A – wL ) L
( –w ) ( L ⁄ 2 ) ( 3 L ⁄ 2 + 4 L ⁄ 2 )
7 wL
v A =  2 L +  =  – 6 EI
3 EI
384 EI
24 EI
Equating Equations (E1) and (E2), give the reaction RA:
3 3 4
RA L
( R A – wL ) L
7 wL
–  =  – or
3 EI
3 EI
384 EI
Substituting Equation (E3) into Equation (E1), we obtain the deflection at A. (E2) 135 wL
R A = 256 (E3)
4 ANS. 45 wL
v A = – 256 EI (b) The reactions at the wall can be found from the freebody diagrams of each beam, as shown in Figure 7.32. By equilibrium of forces
in the y direction and the moments about B in Figure 7.32a, the reactions at B can be found,
RB = RA (E4) MB = RA L ANS. January, 2010 135 wL
R B = 256 2 135 wL
M B = 256 M. Vable Mechanics of Materials: Deflection of Symmetric Beams (a) 7 (b) MB B L4
wL 2 RA RB 357 C
MC L A wL
RC
L A Figure 7.32 (a) Freebody diagrams in Example 7.9. RA L4 By equilibrium of forces in the y direction and the moments about C in Figure 7.32b, the reactions at C can be found,
wL
249 wL
R C = wL +  – R A = 2
256
wL L
M C = wL ( L ) +  ⎛  ⎞ + R A L
2 ⎝ 4⎠ (E5)
2 153 wL
= 256 (E6)
2 249 wL
R C = 256 ANS. 153 wL M C = 256 COMMENT
1. This example demonstrates how the principle of superposition can significantly simplify the analysis and design of structures. Handbooks now document an extensive number of cases for which beam deflections are known. These apply to a wide variety of beam
assemblies. But to develop a list of formulas (as in Table C.3) requires a knowledge of the methods described in Sections 7.1 and 7.2. 7.4* DEFLECTION BY DISCONTINUITY FUNCTIONS Thus far, we have used different functions to represent the distributed load py, or moment Mz, for different parts of the beam.
We then had to determine the integration constants that satisfy the continuity conditions and equilibrium conditions at the
junctions xj. These tedious and algebraically intensive tasks, may be unavoidable for a complicated distributed loading function. But for many engineering problems, where the distributed loads either are constant or vary linearly, there is an alternative method that avoids the algebraic tedium. The method is based on the concept of discontinuity functions. 7 .4.1 Discontinuity Functions Consider a distributed load p and an equivalent load P = pε, as shown in Figure 7.33. Suppose we now let the intensity of the
distributed load increase continuously to infinity. At the same time, we decrease the length over which the distributed force is
applied to zero so that the area pε remains a finite quantity. We then obtain a concentrated force P applied at x = a. Mathematically,
P = lim lim ( p ε )
p→∞ ε→0 –1 Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Rather than write the limit operations, we can represent a concentrated force with, P 〈 x – a〉 .
x 2 2 a 1 x a
p a
P x a p Figure 7.33 Delta function.
2
January, 2010 2 M. Vable The function 〈 x – a〉 Mechanics of Materials: Deflection of Symmetric Beams
–1 7 358 is called the Dirac delta function, or delta function. The delta function is zero except in an infinites imal region near a. As x tends toward a, the delta function tends to infinity, but the area under the function is equal to 1. Mathematically, the delta function is defined as 〈 x – a〉 –1 ⎧ 0,
=⎨
⎩ ∞, x≠a ⎫
⎬
x → a⎭ a+ε ∫ a − ε 〈 x – a〉 –1 dx = 1 (7.13) Now consider the following integral of the delta function:
x ∫ –∞ 〈 x – a〉 –1 dx The lower limit of minus infinity emphasizes that the point is before a. If x < a , then in the interval of integration, the delta
function is zero at all points; hence the integral value is zero. If x > a , then the integral can be written as the sum of three integrals,
a−ε ∫ –∞ 〈 x – a〉 –1 dx + a+ε ∫a − ε 〈 x – a〉 –1 x ∫a + ε 〈 x – a〉 dx + Step function
x a –1 dx Ramp function 0 x Figure 7.34 Discontinuity functions. a 1 x a x x a a 2 x a The first and third integrals are zero because the delta function is zero at all points in the interval of integration, whereas the
second integral is equal to 1 as per Equation (7.13). Thus the integral · x ∫–∞ 〈 x – a〉 –1 dx is zero before a and one after a. It is
0 called the step function as shown in Figure 7.34 and is represented by the notation 〈 x – a〉 .
0 〈 x – a〉 = x ∫ – ∞ 〈 x – a〉 –1 x<a ⎧ 0,
dx = ⎨
⎩ 1, (7.14) x>a Now consider the integral of the step function,
x ∫–∞ 〈 x – a〉 0 dx If x < a, then in the interval of integration the step function is zero at all points. Hence the integral value is zero. If x > a, then
we can write the integral as the sum of two integrals,
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm a ∫–∞ 〈 x – a〉 0 dx + x ∫a 〈 x – a〉 0 dx The first integral is zero because the step function is zero at all points in the interval of integration, whereas the second integral value is x – a. The integral x · ∫ –∞ 〈 x – a〉 0 1 dx is called the ramp function. It is represented by the notation 〈 x – a〉 and is shown in Figure 7.34. Proceeding in this manner we can define an entire class of functions, which are represented mathematically as follows:
⎧ 0,
n
〈 x – a〉 = ⎨
n
⎩(x – a) , x≤a
x>a We can also generate the following integral formula from Equation (7.15):
January, 2010 (7.15) M. Vable Mechanics of Materials: Deflection of Symmetric Beams ∫ n+1 x 〈 x – a〉
n
〈 x – a〉 dx =  ,
n+1
–∞ 7 359 n≥0 (7.16) We define one more function, called the doublet function. It is represented by the notation 〈 x – a〉 –2 and is defined mathe matically as
〈 x – a〉
The delta function 〈 x – a〉 –1 –2 ⎧ 0,
=⎨
⎩ ∞, x≠a x ∫ – ∞ 〈 x – a〉 x→a and the doublet function 〈 x – a〉 –2 –2 dx = 〈 x – a〉 –1 (7.17) become infinite at x = a, that is, they are singular at x = a and
n are referred to as singularity functions. The entire class of functions 〈 x – a〉 for positive and negative n are called discontinuity functions.
The discontinuity functions are zero if the argument is negative. By differentiating Equations (7.14), (7.16), and (7.17) we
can obtain the following formulas:
–1 d 〈 x – a〉
 = 〈 x – a〉 –2
dx 0 d 〈 x – a〉
 = 〈 x – a〉 – 1
dx n d 〈 x – a〉
 = n 〈 x – a〉 n – 1 ,
dx 7.4.2 (7.18)
(7.19) n≥1 Use of Discontinuity Functions Before proceeding to develop a method for solving for the elastic curve using discontinuity functions, we discuss the process
by which the internal moment Mz and the distributed force py can be written using the discontinuity functions. We will
develop the procedure using a simple example of a cantilever beam subject to different types of loading, as shown in Figure
7.35. Then we will generalize the procedure to more general loading and types of support.
(a) P y y
M (b) x
a Mz Vy Vy a x
0
M x
x Mz Mx a p
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Mz a x x
a
w P Vy a Mz (c) x
a Mz M w y Mx a a
a
0 Mz x 0
P(x a) Mz
2 Px a p Px a 1 x
x a
a a)2 w(x
2 Mz
1 x 0
Mz p x a 2 a
2
w x a0
w x a Figure 7.35 Use of discontinuity functions. When we make an imaginary cut before x = a in the cantilever beams shown in Figure 7.35, the internal moment Mz will be
zero. If the imaginary cut is made after x = a, then the internal moment Mz will not be zero and can be determined using a freebody diagram. Once the moment expression is known, then it can be rewritten using the discontinuity functions. This moment
expression can be used to find displacement using the secondorder differential equation, Equation (7.1). However, if the
fourthorder differential equation, Equation (7.5), has to be solved, then the expression of the distributed force py is needed.
Now the distributed force can be obtained from the moment expression using the identity that is obtained by substituting
2 2 Equation (6.18) into Equation (6.17), or d M z ⁄ d x = p y . By using Equation (7.19) we can obtain the distributed force expression from the moment expression, as shown in Figure 7.35. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 360 If the distributed load is as shown in Figure 7.35c, then the expression for it can be obtained directly, without the freebody
diagram, and the moment expression can be obtained by integrating twice. For the concentrated force and moment also, it is not
difficult to recognize the type of discontinuity function that will be used in the representation. The difficulty lies in obtaining the
correct sign in the expression for the internal moment Mz. We shall overcome this problem by using a template to guide us.
A template is created by making an imaginary cut beyond the applied load. On the imaginary cut the internal moment is
drawn according to the sign convention discussed in Section 6.2.6. A moment equilibrium equation is written. If the applied load
is in the assumed direction on the template, then the sign used is the sign in the moment equilibrium equation. If the direction of
the applied load is opposite to that on the template, then the sign in the equilibrium equation is changed. The beams shown in
Figure 7.35 are like templates for the given coordinate systems. EXAMPLE 7.10
Write the moment and distributed force expressions using discontinuity functions for the three templates shown in Figure 7.36.
Case 1 y x=a
x=b x Figure 7.36 Case 3
y x x=a w P Case 2 M x=a x y Three cases of Example 7.10. PLAN
For cases 1 and 2 we can make an imaginary cut after x = a and draw the shear force and bending moment according to the sign convention in Section 6.2.6. By equilibrium we can obtain the moment expression and rewrite it using discontinuity functions. By differentiating twice, we can obtain the distributed force expression. For case 3 we can write the expression for the distributed force using
discontinuity functions and integrate twice to obtain the moment expression. S O L U T IO N
Case 1: We make an imaginary cut at x > a and draw the freebody diagram using the sign convention in Section 6.2.6 as shown in Figure 7.37a. By equilibrium we obtain
(E7) Mz = M is valid only after x > a. Using the step function we can write the moment expression, and by differentiating twice as per Equation (7.19)
we obtain our result. (a) Mz Vy P (b) M ANS. M z = M 〈 x – a〉
w x x=a Vy x=a x=b x=a Vy Mz –2 y Mz y
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm p y = M 〈 x – a〉 (c) y
x Figure 7.37 0 x w (a) Case 1, (b) Case 2, (c) Case 3 in Example 7.10. Case 2: We make an imaginary cut at x > a and draw the freebody diagram using the sign convention in Section 6.2.6 as shown in Figure 7.37b. By equilibrium we obtain
Mz = P ( x – a ) (E8) is valid only after x > a. Using the ramp function we can write the moment expression, and by differentiating twice as per Equation
(7.19) we obtain our result.
ANS. M z = P 〈 x – a〉 1 p y = P 〈 x – a〉 –1 Case 3: The distributed force is in the negative y direction. Its start can be represented by the step function at x = a. The end of the distributed force can also be represented by a step function using a sign opposite to that used at the start as shown in Figure 7.37b.
ANS. 0 p y = – w 〈 x – a 〉 + w 〈 x – b〉 0 Integrating Equation (E9) twice and using Equation (7.16), we obtain the moment expression
January, 2010 (E9) M. Vable Mechanics of Materials: Deflection of Symmetric Beams ANS. 7 361 w
w
2
2
M z = –  〈 x – a〉 +  〈 x – b〉
2
2 (E10) COMMENTS
1. The three cases shown could be part of a beam with more complex loading. But the contribution for each of the loads would be calculated as shown in the example.
2. In obtaining Equation (E10) we did not yet write integration constants. When we integrate for displacements, we will determine these
from boundary conditions.
3. In case 3 we did not have to draw the freebody diagram. This is an advantage when the distributed load changes character over the
length of the beam. Even for statically determinate beams, it may be advantageous to start with the fourthorder, rather than the secondorder differential equation. EXAMPLE 7.11
Using discontinuity functions, determine the equation of the elastic curve in terms of E, I, L, P, and x for the beam shown in Figure 7.38.
P y
2PL C A
x Figure 7.38 Beam and loading in Example 7.11. B L L PLAN
Two templates can be created, one for an applied moment and one for the applied force. With the templates as a guide, the moment
expression in terms of discontinuity functions can be written. The secondorder differential equation, Equation (7.1), can be written and
solved using the zero deflection boundary conditions at A and C to obtain the elastic curve. S O L U T IO N
Figure 7.39 shows two templates. By equilibrium, the moment expressions for the two templates can be written
M z = M 〈 x – a〉 0 M z = F 〈 x – a〉 1 (E1) y Mz
x Figure 7.39 Templates for Example 7.11. x a
M y Mz
x O x O a Vy Vy
F Figure 7.40 shows the freebody diagram of the beam. By equilibrium of moment at C, the reaction at A can be found as R A = 3 P ⁄ 2 .
We can write the moment expressions using the templates in Figure 7.39 to guide us. The reaction force is in the same direction as the
force in the template. Hence the term in Equation (E2) will have the same sign as shown in the template equation.
The applied moment at point A has an opposite direction to that shown in the template in Figure 7.39. Hence the term in the moment
expression in Equation (E2) will have a negative sign to that shown in the template equation. The force P at B has an opposite sign to that
shown on the template, and hence the term in the moment expression will have a negative sign, as shown in Equation (E2). Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 3P
1
0
1
M z =  〈 x〉 – 2 PL 〈 x〉 – P 〈 x – L〉
2 (E2)
y
A C
x 2PL Figure 7.40 P L Freebody diagram in Example 7.11. RA B L
RC Substituting Equation (E2) into Equation (7.1) and writing the zero deflection conditions at A and C, we obtain the boundaryvalue problem:
• Differential equation:
2 3P
dv
1
0
1
EI zz  =  〈 x〉 – 2 PL 〈 x〉 – P 〈 x – L〉 ,
2
2
dx January, 2010 0 ≤ x < 2L (E3) M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 362 • Boundary conditions:
v(0) = 0 (E4) v(2L) = 0
Integrating Equation (E3) twice using Equation (7.16), we obtain (E5) dv
3P
P
2
1
2
EI zz  =  〈 x〉 – 2 PL 〈 x〉 –  〈 x – L〉 + c 1
dx
4
2 (E6) P
3
2P
3
EI zz v =  〈 x〉 – PL 〈 x〉 –  〈 x – L〉 + c 1 x + c 2
4
6
Substituting Equation (E7) into Equation (E4), we obtain the constant c2:
P
 〈 0〉 3 – PL 〈 0〉 2 – P 〈 – L〉 3 + c 2 = 0
4
6
Substituting Equation (E7) into Equation (E5), we obtain the constant c1: or (E7) (E8) c2 = 0 P
P
13 2
 〈 2 L〉 3 – PL 〈 2 L〉 2 –  〈 L〉 3 + c 1 ( 2 L ) = 0
or
c 1 =  PL
6
4
12
Substituting Equations (E8) and (E9) into Equation (E7), we obtain the elastic curve. ANS. (E9) P
3
2
3
2
v ( x ) =  [ 3 〈 x〉 – 12 L 〈 x〉 – 2 〈 x – L〉 + 13 L x ]
12 EI zz (E10) Dimension check: All terms in brackets are dimensionally homogeneous as all have the dimensions of length cubed. But we can also
check whether the lefthand side and any one term of the righthand side have the same dimension,
P → O(F) x → O(L) F
E → O ⎛  ⎞
⎝ L 2⎠ 4 I zz → O ( L ) 3
⎛ FL 3 ⎞
Px
 → O ⎜  ⎟ → O ( L ) → checks
2
4
EI zz
⎝ (F ⁄ L )L ⎠ v → O(L) COMMENTS
1. Comparing the boundaryvalue problem in this example with that of Example 7.2, we note the following: (i) There is only one differential equation here representing the two differential equations of Example 7.2. (ii) There are no continuity equations at x = L as there
were in Example 7.2. The net impact of these two features is a significant reduction in the algebra in this example compared to the
algebra in Example 7.2.
3 2. Equation (E10) represents the two equations of the elastic curve in Example 7.2. We note that 〈 x – L〉 = 0 for 0 ≤ x < L. Hence Equation (E10) can be written v(x) = P(3x − 12Lx + 13L x)/12 EIzz, which is same as Equation (E18) in Example 7.2. For L ≤ x < 2L,
3 3 2 2 3 3 2 2 3 the term 〈 x – L〉 = ( x – L ) . Hence Equation (E10) can be written v ( x ) = P [ 3 x – 12 Lx + 13 L x – 2 ( x – L ) ] ⁄ 12 EI zz , which is
same as Equation (E19) in Example 7.2. EXAMPLE 7.12
A beam with a bending rigidity EI = 42,000 N · m2 is shown in Figure 7.41. Determine: (a) the deflection at point B; (b) the moment
and shear force just before and after B.
y
A Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Figure 7.41 Beam and loading in Example 7.12. 5 kN 12 kN m
x
2.0 m 4 kN/m
D C B
5 kN m
1.0 m 3.0 m PLAN
The coordinate system in this example is the same as in Example 7.11, and hence we can use the templates in Figure 7.39. Differentiating
the template equations twice, we obtain the template equation for the distributed forces, and we write the distributed force expression in
terms of discontinuity functions. Using Equation (7.5) and the boundary conditions at A and D, we can write the boundaryvalue problem
and solve it to obtain the elastic curve. (a) Substituting x = 2 m in the elastic curve, we can obtain the deflection at B. (b) Substituting
x = 2.5 in the shear force expression, we can obtain the shear force value. S O L U T IO N
(a) The templates of Example 7.11 are repeated in Figure 7.42. The moment expression is differentiated twice to obtain the template
equations for the distributed force,
January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams M z = M 〈 x – a〉
p = M 〈 x – a〉 0 M z = F 〈 x – a〉 –2 p = F 〈 x – a〉 7 1 (E1) –1 y (E2)
y Mz
x x a Mz
x x a Vy M Figure 7.42 Templates for Example 7.12. 363 Vy
F We note that the distributed force in segment AB is positive, starts at zero, and ends at x = 2. The distributed force in segment CD is negative, starts at x = 3, and is over the rest of the beam. Using the template equations and Figure 7.42, we can write the distributed force
expression,
0 0 0 –1 –2 p = 5 〈 x〉 – 5 〈 x – 2〉 – 4 〈 x – 3〉 – 5 〈 x – 2〉 – 12 〈 x – 2〉
Substituting Equation (E3) into Equation (7.5) and writing the boundary conditions, we obtain the boundaryvalue problem: (E3) • Differential equation:
2
2⎞
d ⎛
 ⎜ EI zz d v⎟ = 5 〈 x〉 0 – 5 〈 x – 2〉 0 – 4 〈 x – 3〉 0 – 5 〈 x – 2〉 –1 – 12 〈 x – 2〉 –2 ,
2
2
dx ⎠
dx ⎝ 0≤x<6 (E4) • Boundary conditions:
v(0) = 0 (E5) 2 dv
EI zz  ( 0 ) = 0
2
dx (E6) v(6) = 0 (E7) 2 dv
EI zz  ( 6 ) = 0
2
dx (E8) 2
d ⎛ EI d v⎞ = 5 〈 x〉 1 – 5 〈 x – 2〉 1 – 4 〈 x – 3〉 1 – 5 〈 x – 2〉 0 – 12 〈 x – 2〉 –1 + c
⎜ zz  ⎟
1
2
dx⎝
dx ⎠ (E9) Integrating Equation (E4) twice, we obtain 2 dv
5
5
2
2
2
1
0
EI zz  =  〈 x〉 –  〈 x – 2〉 – 2 〈 x – 3〉 – 5 〈 x – 2〉 – 12 〈 x – 2〉 + c 1 x + c 2
2
2
2
dx
Substituting Equation (E10) into Equation (E5), we obtain (E10) (E11) c2 = 0 Substituting Equation (E10) into Equation (E8), we obtain
5
5
 〈 6〉 2 –  〈 4〉 2 – 2 〈 3〉 2 – 5 〈 4〉 1 – 12 〈 4〉 0 + c 1 ( 6 ) = 0
or
c1 = 0
2
2
Substituting Equations (E11) and (E12) into Equation (E10) and integrating twice, we obtain (E12) dv
5
35
32
35
2
1
EI zz  =  〈 x〉 –  〈 x – 2〉 –  〈 x – 3〉 –  〈 x – 2〉 – 12 〈 x – 2〉 + c 3
dx
6
2
3
6 (E13) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 5524
4
45
3
2
EI zz v =  〈 x〉 –  〈 x – 2〉 –  〈 x – 3〉 –  〈 x – 2〉 – 6 〈 x – 2〉 + c 3 x + c 4
24
6
24
12
Substituting Equation (E14) into Equation (E5), we obtain (E14)
(E15) c4 = 0 Substituting Equation (E14) into Equation (E6), we obtain
25
55323
 〈 6〉 4 –  〈 4〉 4 –  〈 3〉 4 –  〈 4〉 3 – 6 〈 4〉 2 + c 3 ( 6 ) = 0
or
c 3 = –  = – 8.97
12
6
24
24
36
Substituting Equations (E15) and (E16) into Equation (E14) and simplifying, we obtain the elastic curve,
1
4
4
4
3
2
v =  [ 15 〈 x〉 – 15 〈 x – 2〉 – 12 〈 x – 3〉 – 60 〈 x – 2〉 – 432 〈 x – 2〉 – 646 x ]
72 EI zz (E16) (E17) Substituting x = 2 into Equation (E17), we obtain the deflection at point B,
1
4
4
4
3
2
v ( 2 ) =  [ 15 〈 2〉 – 15 〈 0〉 – 12 〈 – 1〉 – 60 〈 0〉 – 432 〈 0〉 – 646 ( 6 ) ]
3
72 [ ( 42 ) ( 10 ) ] (E18)
ANS. January, 2010 v(2) = −1.2 mm M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 364 (b) As stated in Equation (7.1), the moment Mz can be found from Equation (E10). And as seen in Equation (7.4), the shear force Vy is
the negative of the expression given in Equation (E11). Noting that the constants c1 and c2 are zero, we obtain the expressions for Mz and Vy:
Mz ( x ) = 5
5
 〈 x〉 2 –  〈 x – 2〉 2 – 2 〈 x – 3〉 2 – 5 〈 x – 2〉 1 – 12 〈 x – 2〉 0 kN · m
2
2
1 1 1 0 (E19) –1 V y ( x ) = [ – 5 〈 x〉 + 5 〈 x – 2〉 + 4 〈 x – 3〉 + 5 〈 x – 2〉 + 12 〈 x – 2〉 ] kN (E20) Point B is at x = 2. Just after point B, that is, at x = 2–, all terms except the first term in Equations (E19) and (E20) are zero.
M z ( 2– ) = 10 kN· m ANS. V y ( 2– ) = – 10 kN 0 Just after point B, that is, at x = 2+, the step function 〈 x – 2〉 is equal to 1. Hence this term along with the first term are the nonzero
terms in Equations (E19) and (E20).
M z ( 2+ ) = ( 10 – 12 ) = – 2 kN· m V y ( 2+ ) = ( – 10 + 5 ) = – 5 kN ANS. M z ( 2+ ) = – 2 kN· m (E21)
V y ( 2+ ) = – 5 kN COMMENT
1. We note that Mz(2+) − Mz(2−) = −12 kN · m and Vy(2+) − Vy(2−) = 5 kN · m, which are the values of the applied moment and
applied shear force. Thus the jump in the internal shear force and internal moment difference is captured by the step function. 7 .5* AREAMOMENT METHOD One last method is especially useful in finding the deflection or the slope of the beam is to be found at a specific point. Called
the areamoment method, it is based on graphical interpretation of the integrals that are generated by integration of Equation
(7.1).
Equation (7.1) can be written
Mz
d v′(x )
= EI zz
dx (7.20.a) where v ′ ( x ) = d v ( x ) / dx represents the slope of the elastic curve. Integrating the equation from any point A to any other
point x, we obtain
x v′(x) ∫v ′ ( x ) d v ′ ( x ) = ∫x
A v ′ ( x ) = v ′ ( xA ) + Mz
 dx 1 or
EI zz
A x ∫x Mz
 dx 1
EI zz
A (7.20.b) Integrating Equation (7.20.b) between point A and any point x, we obtain
x x Mz
 dx 1⎞ dx
⎠
EI zz
A (7.20.c) Mz
( x – x 1 )  dx 1
EI zz
A (7.20.d) ∫x ⎛ ∫x
⎝ v ( x ) = v ( xA ) + v ′ ( xA ) ( x – xA ) +
Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm A The last integral2 can be written as
x ∫x v ( x ) = v ( xA ) + v ′ ( xA ) ( x – xA ) + Assume EIzz is a constant for the beam. From Equations 7.20.b and 7.20.d, the slope and the deflection at point B can be written
2 By integrating by parts, it can be shown that
x x A A ∫x ∫x f ( x 1 ) dx 1 dx = x ∫x ( x – x 1 ) f ( x 1 ) dx 1 A Letting f(x1) = Mz /EIzz, we can obtain Equation (7.20.d) from Equation (7.20.c).
January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 1
v ′ ( x B ) = v ′ ( x A ) + EI zz xB ∫x Mz M z dx (7.21) xB ∫x ( x B – x ) M z dx xB (b) x (7.22) A Mz dx xA 365 A 1
v ( x B ) = v ( x A ) + v ′ ( x A ) ( x B – x A ) + EI zz
(a) 7 Mz xB AM x xB xA Mz dx Centroid of area AM x xA
x xB Figure 7.43 Graphical interpretation of integrals in areamoment method.
(a) x xB (b) The integral in Equation (7.21) can be interpreted as the area under the bending moment curve, as shown in Figure 7.43. The
moment diagram can be constructed as discussed in Section 6.4. Thus, if the slope v ′ ( x A ) at point A is known, then by adding
the area under the moment curve, we can obtain the slope at point B. The area AM will be considered positive if the moment
curve is in the upper plane and negative if it is in the lower plane.
From Figure 7.43a, we see that the integral in Equation (7.22) is the first moment of the area under the moment curve about
point B. This first moment of the area can be found by taking the distance of the centroid from B and multiplying by the area,
xB ∫x ( x B – x ) M z dx = ( x B – x ) A M (7.23.a) A With this interpretation the deflection of B can be found from Equation (7.22). Table C.2 in the Appendix lists the areas and
the centroids of the areas under various curves. These values can be used in calculating the integrals in Equations 7.21 and
7.22.
Consider the cantilever beam in Figure 7.44a and the associated bending moment diagram. At point A the slope and the
deflection at A are zero. Hence v ′ ( x A ) = 0 and v ( x A ) = 0 in Equations 7.21 and 7.22. The area AM, representing the integral in
2 Equation (7.21), is −PL(L)/2. Thus the slope at B is v ′ ( x B ) = – PL /2 EI . Since distance of the centroid from B is xB – x = 2L/3,
2 ( x B – x ) A M = ( 2 L /3 ) ( – PL /2 ) is the value of the integral equation, Equation (7.22). Thus the deflection at B is
3 v ( x B ) = – PL /3 EI .
y
y (a) P Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm x (b) A A
x P B L3 PL
4 Mz 2L 3
x AM 1 C
L2 L2 L
Mz B PL2 PL2
16 AM 16 2 x
PL Figure 7.44 AM PL2
2EI Application of areamoment method. L3
x1
¯ L3 L3
x2
¯ () L6 L6
L3
(b) Now consider the simply supported beam and the associated bending moment in Figure 7.44b. The value of the slope is not
known at any point on the beam. Thus before the deflection and slope at B can be determined, the slope at A must be found.
The deflection at A is zero. Treating the slope at A as an unknown constant, we equate the deflection at C from Equation (7.22)
to zero and obtain the slope at A,
January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 366 1
v ( x C ) = v ( x A ) + v ′ ( x A ) ( x C – x A ) +  [ A M1 ( x C – x 1 ) + A M 2 ( x C – x 2 ) ] = 0 or
EI
2 2 1 PL 2 L
PL L
v ′ ( x A ) ( L ) +   ⎛  ⎞ +  ⎛  ⎞
EI 16 ⎝ 3 ⎠ 16 ⎝ 3⎠ (7.23.b) 2 =0 PL
v ′ ( x A ) = – 16 EI or (7.23.c) Using Equation (7.22) once more, we find the deflection at point B: 1
v ( x B ) = v ( x A ) + v ′ ( x A ) ( x B – x A ) +  [ A M1 ( x B – x 1 ) ]
EI
2 2 3 1 PL L
PL
PL L
= –  ⎛  ⎞ +   ⎛  ⎞ = – 16 EI ⎝ 2⎠ EI 16 ⎝ 6⎠
48 (7.23.d) EXAMPLE 7.13
In terms of E, I, w, and L, determine the deflection and slope at point B for the beam and loading shown in Figure 7.45.
w (kips in) y
x A Figure 7.45 Beam and loading in Example 7.13. B L (in) C
L (in) PLAN
The reaction forces at A and C can be found and then the shear–moment diagram can be drawn as discussed in Section 6.4. The area
under the moment curve and the location of the centroids can then be determined. Because the deflection at A is zero, the deflection at C
can be written in terms of the unknown slope at A using Equation (7.22). Equating the deflection at C to zero, then gives the slope at A.
Slope and deflection at B can now be found using Equations 7.21 and 7.22, respectively. S O L U T IO N
From the freebody diagram of the entire beam, the reaction forces at the supports can be found and the shear–moment diagram drawn,
as shown in Figure 7.46. The moment curve in region BC is a quadratic, and the areas under the curves is the sum of three area. Table C.2
in the Appendix lists the formulas for the areas and centroids.
2 3 wL
L wL
A 1 =  ⎛ ⎞ = 8
2⎝ 4 ⎠
2 3 wL
L wL
A 2 =  ⎛ ⎞ = 4⎝ 4 ⎠
16
2 3 wL
2 L wL
A 3 =  ⎛  ⎞ ⎛ ⎞ = 192
3 ⎝ 4⎠ ⎝ 32 ⎠
2 3 9 wL
2 3 L 9 wL
A 4 =  ⎛ ⎞ ⎛  ⎞ = 64
3 ⎝ 4 ⎠ ⎝ 32 ⎠ 2
x 1 =  ( L )
3 (E1) L
9L
x 2 = L +  = 8
8 (E2) 5L
37 L
x 3 = L +  ⎛  ⎞ = 8 ⎝ 4⎠
32 (E3) 5 3L
49 L
x 4 = 2 L –  ⎛ ⎞ = 8⎝ 4 ⎠
32 (E4) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm w (kips in)
x A
RA
V B C L (in)
wL 4 L (in)
RC 3wL 4 Vy wL 4
(kips) 3L 4
L4 Figure 7.46 Shear–moment diagram in Example 7.13.
The deflection at C can be written as
January, 2010 Mz
(in kips) wL2 4
A1 A3 9wL2 32
A2 A4 3wL 4 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 367 1
v ( x C ) = v ( x A ) + v ′ ( x A ) ( x C – x A ) +  [ A 1 ( x C – x 1 ) + A 2 ( x C – x 2 ) + A 3 ( x C – x 3 ) + A 4 ( x C – x 4 ) ]
(E5)
EI
The deflections at the support are zero. Substituting v(xC) = 0 and v(xA) = 0 and the values of the areas and centroids in Equation (E5), we can find the slope at A.
3 3 3 3 wL
wL
9 wL
1 wL
2L
9L
37 L
49 L
v ′ ( x A ) ( 2 L ) +   ⎛ 2 L – ⎞ +  ⎛ 2 L –  ⎞ +  ⎛ 2 L – ⎞ +  ⎛ 2 L –  ⎞ = 0
EI 8 ⎝
3⎠
16 ⎝
8 ⎠ 192 ⎝
32 ⎠
64 ⎝
32 ⎠ 3 or 7 wL
v ′ ( x A ) = – 48 EI (E6) The deflection at B can be written as
1v ( x B ) = v ( x A ) + v ′ ( x A ) ( x B – x A ) +  [ A 1 ( x B – x 1 ) ]
EI
Substituting the calculated values we obtain the deflection at B,
3 (E7) 3 7 wL
1 wL
2L
v ( x B ) = –  ( L ) +  ⎛  ⎞ ⎛ L –  ⎞
48 EI
EI ⎝ 8 ⎠ ⎝
3⎠ (E8)
4 ANS. 5 wL
v ( x B ) = – 48 EI COMMENTS
1. The example demonstrates uses of the area moment method for finding slopes and deflection at a point in the beam.
2. If the elastic curve needs to be determined for an indeterminate beam, we can use the area moment method to determine the reactions
and then the secondorder differential equation to solve the problem. But if this approach is to have any computational advantage over
using the fourthorder differential equations, then it must be possible to draw the moment diagram quickly by inspection. PROBLEM SET 7.3
Superposition
7.57 Determine the deflection at the free end of the beam shown in Figure P7.20. 7.58 Determine the reaction force at support A in Figure P7.34. 7.59 Determine the deflection at point A on the beam shown in Figure P7.59 in terms of w, L, E, and I.
y Figure P7.59 7.60 w wL wL2 x A
L L Determine the reaction force and the slope at A for the beam shown in Figure P7.60, using superposition.
y w (kips in) Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm x Figure P7.60 A
L (in) L (in) 7.61 Two beams of length L and bending rigidity EI, shown in Figure P7.61, are simply supported at the ends and are in contact at the
center. Determine the deflection at the center in terms of P, L, E, and I.
P A Figure P7.61 January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 368 7.62 Two beams of length L and bending rigidity EI, shown in Figure P7.62, are simply supported at the ends and are in contact at the
center. Determine the deflection at the center in terms of w, L, E, and I.
w A Figure P7.62 7.63 A cantilever beam’s end rests on the middle of a simply supported beam, as shown in Figure P7.63. Both beams have length L and
bending rigidity EI. Determine the deflection at A and the reactions at the wall at C in terms of P, L, E, and I. C P B A
D Figure P7.63 7.64 A cantilever beam’s end rests on the middle of a simply supported beam, as shown in Figure P7.64. Both beams have length L and
bending rigidity EI. Determine the deflection at A and the reactions at the wall at C in terms of w, L, E, and I. w
C
B A
D Figure P7.64 7.65 The end of one cantilever beam rests on the end of another cantilever beam, as shown in Figure P7.65. Both beams have length L and
bending rigidity EI. Determine the deflection at A and the reactions at the wall at C in terms of w, L, E, and I.
w
C
B Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm Figure P7.65 A Discontinuity functions
7.66 A gymnast with a mass of 60 kg stands in the middle on a wooden balance beam as shown in Figure P7.66. The modulus of elasticity of the wood is 12.6 GPa. To bracket the elasticity of the support, two models are to be considered: (a) the supports are simply supported;
(b) the supports are built in ends. Determine the maximum deflection of the beam for both the cases.
120 mm B A Figure P7.66 January, 2010 3m 3m 150 mm 80 mm
Cross section M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 369 7.67 Solve Problem 7.17 using discontinuity functions. 7.68 Solve Problem 7.18 using discontinuity functions. 7.69 Solve Problem 7.19 using discontinuity functions. 7.70 Solve Problem 7.20 using discontinuity functions. 7.71 (a) Solve for the elastic curve for the beam and loading shown in Figure P7.23. (b) Determine the slope and deflection at point C. 7.72 Solve Problem 7.34 using discontinuity functions. 7.73 A beam is supported and loaded as shown in Figure P7.73. The spring constant in terms of beam stiffness is written as
3 k = α EI ⁄ L , where α is a proportionality factor. Determine the extension of the spring in terms of α, w, E, I, and L.
w
A B L/2 C k L/2 Figure P7.73 Areamoment method
7.74 Using the areamoment method, determine the deflection in the middle for the beam shown in Figure P7.2. 7.75 Using the areamoment method, determine the deflection in the middle of the beam shown in Figure P7.3. 7.76 Using the areamoment method, determine the deflection and slope at the free end of the beam shown in Figure P7.4. 7.77 Using the areamoment method, determine the slope at x = 0 and deflection at x = L of the beam shown in Figure P7.6. 7.78 Using the areamoment method, determine the slope at x = 0 and deflection at x = L of the beam shown in Figure P7.17. 7.79 Using the areamoment method, determine slope at x = 0 and deflection at x = L of the beam shown in Figure P7.18. 7.80 Using the areamoment method, determine the slope at the free end of the beam shown in Figure P7.20. Stretch Yourself
7.81 To improve the load carrying capacity of a wooden beam (EW = 2000 ksi) a steel strip (Es = 30,000 ksi) is securely fastened to it as
shown in Figure P7.81. Determine the deflection at x =L.
n 2i Steel Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm 0.25 in
20 lb/in.
w y
x Figure P7.81 * 7.6 Wood
4 in 6 ft 6 ft CONCEPT CONNECTOR Compared to a theory for the deflection of beams, our understanding of the strength of beams developed more intuitively, as
described in Section 6.7. The very term elastic curve for the deflection of a beam reflects the early impact of mathematics. January, 2010 M. Vable 7.6.1 Mechanics of Materials: Deflection of Symmetric Beams 7 370 History: Beam Deflection In the seventeenth century, procedures were in use to draw tangents (similar to differentiation) and areas swept by curves
(similar to integration). Issac Newton (16421727) in 1666 realized that the two procedures were inverse of each other and
developed a method called fluxional method, which he circulated among some of his friends but did not publish. Newton’s
method did not receive much attention. Nine years later in Germany, Gottfried Wilhelm Leibniz (16461716) developed nearly
the same method independently. Leibniz’s notation caught on, especially in Europe, and so did his name for the method, differential calculus. Members of the Bernoulli family were in the forefront of finding applications for this new mathematical tool.
One of the applications they considered was the determination of the elastic curve. Figure 7.47 Pioneers of beam deflection theories. Daniel Bernoulli Leonard Euler Jacob Bernoulli (1646–1716) and John Bernoulli (1667–1748) won acclaim for their mathematical work, which the French
Academy of Science recognized by making the brothers members in 1699. Daniel Bernoulli (1700–1782), John’s son, made
important contributions to hydrodynamics, while Leonard Euler (1707–1782), John’s pupil, introduced analytic methods used
today in practically every area of mathematics. His name is also associated with buckling theory, as we shall see in Chapter 11.
Both Daniel Bernoulli and Euler were pioneers in the theory of the elastic curve.
Jacob Bernoulli had started with Mariotte’s assumption that the neutral axis is tangent to the bottom (the concave side) of
the curve in a cantilever beam. From this he obtained a relationship between the curvature of the beam at any point and the
applied load, as described in Problem 7.46. Although Mariotte’s assumption proved incorrect, Bernoulli’s result was correct—
except for the value of the bending rigidity. Euler, on the suggestion of Daniel Bernoulli, approached the same problem by minimizing the strain energy in a beam, which yielded the correct relationship. Euler called the constant relating moment and curvature the moment of stiffness (rather than bending rigidity), but he recognized that it had to be determined experimentally. As we
saw in Section 3.12.1, much later Thomas Young made a similar observation concerning axial rigidity, and the modulus of elasticity is named after him. Such are the quirks of history.
ClaudeLouis Navier (1785–1836), whose work on the concept of stress we met in Section 1.5, was the first to solve for
the deflection of statically indeterminate beams. Navier carried the extra unknown reactions in the secondorder differential Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm equation and determined these reactions from conditions on the deflection and slopes at the support (see Problem 7.45).
Jean Claude SaintVenant, whose work we have seen in several chapters, analyzed the deflection of a cantilever beam
due to a force at the free end. He was the first to realize that it can be found without formally integrating the differential equations. This was the beginning of the areamoment method that we studied in Section 7.5*. Alfred Clebsch (1833–1872), in his
1862 book on elasticity, considered the deflection of a beam under concentrated forces (see Problem 7.47). His approach
later evolved into the discontinuity method, discussed in Section 7.4*. The English mathematician W. H. Macaulay formally
introduced the discontinuity functions in 1919.
Each aspect of beam theory thus had its own development. The normal stress in bending was relatively intuitive; shear
stress in bending was guided by experiment; and beam deflection was guided by mathematics. Together, they highlight the
importance of intuition, experimental evidence, and mathematical formalization. Engineers need them all to understand
nature. January, 2010 M. Vable 7.7 Mechanics of Materials: Deflection of Symmetric Beams 7 371 CHAPTER CONNECTOR Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm In this chapter we saw several methods for determining the deflection of beams. The preferred approach treats beam deflection as a secondorder boundaryvalue problem. However, other approaches may be needed if the distributed loads on the
beam are complicated functions or if we have only experimentally measured values for the distributed load. With the discontinuity function, a single differential equation can represent the loading on the entire beam. This method should be used if the
beam loading changes in a discrete manner across the beam. The areamoment method, a graphical technique, can yield quick
solutions for the beam deflection and slope at a point if the moment diagram can be constructed easily. The superposition
method is another versatile design tool. It can be used for determinate or indeterminate beams, provided we know the beam
deflection and slope. Handbooks supply these values for many basic cases.
Chapter 7 concludes the second major part of this book. Table 7.1 offers a synopsis of onedimensional structural elements,
described in Chapters 4 through 7. The table highlights the essential elements common to these theories. They allow us to obtain
deformation, strains, and stresses at any point in a onedimensional structural element. In the next three chapters we will use this
information in many ways.
In order to determine whether a structure will break under a given load, we need failure theories, which we study in
Chapter 10. To apply failure theories, we first need to determine the maximum normal and shear stresses at a point. Chapter 8,
on stress transformation, describes how to obtain these stresses from our onedimensional theories. Only experiment, however, can render the final verdict on designs based on the onedimensional theory. One popular experimental technique is to
measure strains using strain gages. And this technique requires a relationship between the strains obtained from onedimensional theory and the strains in any given direction. That relationship, known as strain transformation, is the topic of Chapter
9. Chapter 10 is the culmination of the first nine chapters. Here we study stresses and strains in structural elements subject to
combined axial, torsional, and bending loads. We also address the design and failure of structures and machine elements. January, 2010 M. Vable Mechanics of Materials: Deflection of Symmetric Beams Table 7.1 7 372 Synopsis of onedimensional structural theories.
y (v)
x (u) z(w) Axial (Rods) Displacements/
deformation Torsion (Shafts) u ( x, y, z ) = u ( x )
v=0 Strains u=0 y w=0 y
2 dφ
γ x θ = ρ  max ∫A σxx dA T= dv ε xx = – y 2 xx dx dx dφ
τ x θ = G γ x θ = ρ dx du
dx σ xx = E ε xx = E  N= v = v(x) x xx Internal forces and
moments dv
u ( x, y, z ) = – y dx w=0 φ ( x, y, z ) = φ ( x ) w=0 du
ε xx = dx Stresses v=0 Symmetric Bending (Beams) 2 dv σ xx = E ε xx = – Ey 2 τ xy ≠ 0 dx ∫A ρτx θ dA ∫A σxx dA = 0 ...Locates neutral axis
Mz = – ∫A y σxx dA Vy = ∫A τxy dA Sign convention N Stress formulas Vy T y Mz y x N
σ xx = A Tρ
τ x θ = xx max J Mz y
σ xx = – I zz xx Vy Qz τ sx = τ xs = – I zz t Deformation formulas du
N
 = dx
EA
u2 N(x – x ) 2
1
– u1 =  Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm EA
EA = axial rigidity January, 2010 dφ
T = dx
GJ 2
Mz
dv
 = 2
EI zz
dx T(x – x ) 2
1
φ 2 – φ 1 =  GJ
GJ = torsional rigidity v= Mz ∫ ⎛ ∫  dx⎞
⎝ EI zz ⎠ d x + C1 x + C2 EIzz = bending rigidity M. Vable Mechanics of Materials: Deflection of Symmetric Beams 7 373 POINTS AND FORMULAS TO REMEMBER
• The deflected curve of a beam represented by v(x) is called elastic curve. • dv
M z = EI zz 2
dx 2 • 2 dv
d⎛
EI zz  ⎞
2
d x⎝
dx ⎠ 2 •
• 2 2 dv
d⎛
EI zz  ⎞ = p (7.5)
2
2⎝
dx ⎠
dx
where v is the deflection of the beam at any x and is positive in the positive y direction; Mz is the internal bending
moment; Vy is the internal shear force; p is the distributed force on the beam and is positive in the positive y direction; EIzz
Vy = – (7.1) (7.4) 2 is the bending rigidity of the beam; and d v ⁄ dx is the curvature of the beam.
The mathematical statement listing all the differential equations and all the conditions necessary for solving for v(x) is
called boundaryvalue problem for beam deflection.
Boundary conditions for secondorder differential equations:
dv
(x ) = 0
dx A Builtin end at xA v ( x A ) = 0
Simple support at xA v ( x A ) = 0
Smooth slot at xA
• dv
(x ) = 0
dx A Continuity conditions at xj:
v1 ( xj ) = v2 ( xj ) •
•
•
•
• • d v1
d v2
 ( x j ) =  ( x j )
dx
dx Boundary conditions for fourthorder differential equations are determined at each boundary point by specifying:
( v or V y ) and ( d v ⁄ dx or M z ) (7.6)
In fourthorder boundaryvalue problems, at each point xj where the differential equation changes, the continuity conditions and equilibrium conditions must be specified.
The superposition method is a versatile design tool that can be used for solving problems of determinate and indeterminate beams provided the beam deflection and slope values are available for many basic cases, such as in a handbook.
In the discontinuity function method a single differential equation and conditions on deflection and slopes at support
describe the complete boundaryvalue problem.
Discontinuity functions:
x≤a
⎧0
–n
n
0
x≠a
〈 x – a〉 = ⎧
〈 x – a〉 = ⎨
⎨
n
⎩(x – a) x > a
⎩∞ x → a
Differentiation formulas:
–1 0 d 〈 x – a〉
 = 〈 x – a〉 –2
dx
• x –2 dx = 〈 x – a〉 x ∫–∞ 〈 x – a〉 –1 –1 dx = 〈 x – a〉 0 n≥1 x 〈 x – a〉 n +1 ∫–∞ 〈 x – a〉 dx = n+1
n n≥0 The areamoment method is a graphical technique that can yield quick solutions of beam deflection and slope at a point,
if the moment diagram can be constructed easily.
xB
1
v ′ ( x B ) = v ′ ( x A ) + M dx
EI zz x A z
(7.21) ∫ ⎧
⎪
⎨
⎪
⎩
AM 1
v ( x B ) = v ( x A ) + v ′ ( x A ) ( x B – x A ) + EI zz xB ∫x ( x B – x ) M z dx A ⎧
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎩ Printed from: http://www.me.mtu.edu/~mavable/MoM2nd.htm n d 〈 x – a〉
 = n 〈 x – a〉 n –1
dx Integration formulas: ∫ –∞ 〈 x – a〉
• d 〈 x – a〉
 = 〈 x – a〉 – 1
dx ( xB – x ) AM January, 2010 (7.22) ...
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 Spring '10
 SMITH

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