Reference: [JCM 7]
A
ld
Hook may be
used to get
more anchorage
Critical embedment must be sufficient to
prevent pullout or splitting prior to
yielding of the bar
ld
Load
Critical section
Highest stress
Possible crack paths
A
Not enough spacing
Section A-A
Reference: [JCM 6.10]
To limit cracks to acceptable sizes
Instead of using fewer large bars at large spacing, use
more smaller bars with moderate spacing
Use spacing about 9 inches or smaller
Members Subjected to
Max. Crack Width
(in)
Dry air
0.016
Moi
Ex: Flexural Crack Control
[Modified JCM Ex. 6.3, 9edt.]
Calculate the estimated width of flexural cracks that will occur in the beam.
The beam is exposed to moist air.
Determine if the crack width is satisfactory compared to the values specified in Table
Reference: [JCM 6.1]
Safety Related
Load carrying capacity
At ultimate level (i.e., capacity near ultimate failure)
Factored loads are used
Example: design of beam for bending
Mn Mu
Factored Moment
determined using factored loads
1.4 DL
U =
1.2 DL +
Design for Shear [JCM 8] [ACI 318-08 Chapter 11]
At neutral axis, high shear region
45o
If there is moment present, the crack may start at nearly vertical
flexural crack and then bends toward 45-degree as it
approaches the neutral axis.
[ACI 318-08, 11.4.
JCM Chapter 12
[JCM]
[JCM]
Design of Wall Footings (Continuous Footings)
[JCM Figure 12.5]
Design considerations:
1. one-way shear
2. bending moment
3. development length
[JCM Figure 12.3]
[JCM Figure 12.3]
L
ultimate soil pressure, qu
face of support
She
Slender Column [JCM 11]
secondary moment
or P effect
2nd order
Definition of Slender/long columns
1st order
Axial load + end moments + P moment
Capacity of col. Significantly reduced due to P moment
How much P moment is considered significant?
If slender
Short Column Subject to Axial Load & Bending Moment [JCM 10]
steel bar
P
C1
C2
e < 0.10 h (tied)
e < 0.05 h (spiral)
e P
C1
C2
e
(b) Axial load w/ small eccentricity (small moment)
Entire section in compression but unbalanced strain profile
Failure contro
Short Column [JCM 9]
Short compression block or Pedestal
A concrete block is classified as pedestal [ACI-318 2.2] if:
Pu
L 13 min(b or h)
<- steel reinforcement is not needed
Design bearing strength [ACI-318 10.14]
L
b
h
P Pn (0.85 f `c Ag )
u
0.65 for
Negative Moments
Pu
Example: cantilever beam
Flexural Steel on top
Top in Tension
Bottom in Comp.
L
Mu
Moment Diagram
-ve moment
-PuL
Reinforcement is placed at
top to carry tensile force
a (compression block)
Example: continuous beam
Wu
clear span
Ln
Ln