Module 11 - NPTel

pedestal) by compression in concrete and steel and tension in steel.
Compression forces are transferred through direct bearing while tension forces
are transferred through developed reinforcement. The permissible bearing
stresses on full area of concrete shall be taken as given below from cl.34.4 of IS
456:
(11.5)
(11.6)
σ br = 0.25fck, in working stress method, and
σ br = 0.45fck, in limit state method
It has been mentioned in sec. 10.26.5 of Lesson 26 that the stress of concrete is
taken as 0.45fck while designing the column. Since the area of footing is much
larger, this bearing stress of concrete in column may be increased considering
the dispersion of the concentrated load of column to footing. Accordingly, the
permissible bearing stress of concrete in footing is given by (cl.34.4 of IS 456):
(11.7)
σ br = 0.45fck (A1/A2) 1/2
with a condition that
(A1/A2) 1/2 ≤ 2.0
(11.8)
where A1 = maximum supporting area of footing for bearing which is
geometrically similar to and concentric with the loaded area A2, as
shown in Fig.11.28.15
A2 = loaded area at the base of the column.
The above clause further stipulates that in sloped or stepped footings, A1 may be
taken as the area of the lower base of the largest frustum of a pyramid or cone
contained wholly within the footing and having for its upper base, the area
actually loaded and having side slope of one vertical to two horizontal, as shown
in Fig.11.28.15.
If the permissible bearing stress on concrete in column or in footing is
exceeded, reinforcement shall be provided for developing the excess force
(cl.34.4.1 of IS 456), either by extending the longitudinal bars of columns into the
footing (cl.34.4.2 of IS 456) or by providing dowels as stipulated in cl.34.4.3 of IS
456 and given below:
(i) Sufficient development length of the reinforcement shall be provided to
transfer the compression or tension to the supporting member in accordance with
Version 2 CE IIT, Kharagpur