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Unformatted text preview: ermined as follows:
k, =: Average contact pressure
Average settlement of the raft APPENDIX
C
( Clauses 5.1.1, 52.1 and B4.1 )
RIGIDiTY OF SUPERSTRUCTURE AND FOUNDATION
Cl. DETERMINATION OF THE RIGIDITY OF THE STRUCTURE Cl.1 The flexural rigidity EI of the structure of any section may be estimated according to the relation given below ( see also Fig. 2):
EI =
where
EL = modulus of elasticity
material ) in kg/cma, of the infilling material Ii = moment of inertia of the infilling in cm4, b = length or breadth
bending, ( wall H= of the structure in the direction of total height of the intiling in cm, Es = modulus of elasticity of frame material in kg/cmg,
IO = moment of inertia of the beam in cm”,
Z’t4 =
I’& = I*
K’
h h,
14 ..  .“. _ _“.. ___ __ IS : 2950 ( Part I )  1981
I’,, = Ib
9 I I = spacing of the columns in cm, h, = length of the upper column in cm, h, = length of the lower column in cm,
If
_(
I I’, = I, = moment of inertia of the upper column in cm’, II = moment of inertia of the lower column in cm4, and I/ = moment of inertia of the foundation beam or raft in cm’. NOTE The summation is to be done over all the storeys, including the foundation
beam of raft. In the case of the foundation, I’freplaces I’a and Ir becomes zero,
whereas for the topmost beam;’ I’” becomes zero. 1 FOUNDATION
RAFT
FIG. 2 ATION
DETERMIN OF RIGIDITY OF A &RUCTURE C2. RELATIVE STIFFNESS FACTOR K
C2.1 Whether a structure behaves as rigid or flexible depends on the relative
stiffness of the structure and the foundation soil. This relation is expressed
IS .__._l L ___.__
.._..
_ ._____II. IS : 2950( Part I )  1981
by the relative stiffness factor K given below:
a) For the whole structure K = EE&
0 b) For rectangular rafts or beams K = & 1 8
c) For circular rafts K = i2~~ 1 where
EZ = flexural rigidity of the structure over the length ( a ) in
Wm2,
Es = modulus of compressibility of the foundation soil in
kg/cm”,
I length of the section in the bending axis in cm,
b
= length perpendicular to the section under investigation in
cm,
= thickness of the raft or beam in cm, and a
d
R= radius of the raft in cm. C2.1.1 For K >
( see 5.2.1). 0.5, the foundation C3. DETERMINATION OF CRITICAL may be considered COLUMN as rigid SPACING C3.1 Evaluation of the characteristics h is made as follows:
h4
 kB
J 4E,Z where
k = modulus of subgrade reaction in kg/cm* for
width B in cm ( see Appendix B ).
B = width of raft in cm
EC = modulus of elasticity of concrete in kgf/cms
Z = moment of inertia of the raft in cm* 16 footing of IS : 2950 ( Part I )  1981 APPENDIX D ( Clause 5.1.2 )
CALCULATION OF PRESSURE DISTRIBUTION BY
CONVENTIONAL METHOD
Dl. DETERMINATION OF PRESSURE DISTRIBUTION Dl.1 The pressure distribution ( q ) under the raft shall be determined by the following formula: Qe; Qek I’YrtI,X
e 4= +i Y where
Q = total vertical load on the raft,
A’ = total area of the raft, ek, ei, ZL, Z; = eccentricities and moments of inertia about the principal
axes through the centroid of the section, and
x, y = coordinates of any given point on the raft with respect
to the x and y axes passing through the centroid of the
area of the raft.
Zi, $, ei, e; may be calculated from the following equations: z; =I,z; = Iv  1:”
T, V ‘Zv
7
i? Z
I, ei = ee  25 e;=ev  ev, and Z zy em
zv where
I,, Zv = moment of...
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This note was uploaded on 03/14/2014 for the course CE 684 taught by Professor Prof.deepankarchoudhury during the Spring '13 term at IIT Bombay.
 Spring '13
 PROF.DEEPANKARCHOUDHURY

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