Foundation Design

Foundation is the base of any structure. Without a solid foundation, the structure would not hold for long. We have to be very cautious with the design of foundations because our entire structure rests on the foundation. The job of a foundation is to transfer the loads of the building safely to the ground.

The strength of the foundation determines the life of the structure. As we discussed in the earlier article, design of foundation depends on the type of soil, type of structure and its load. Higher the load bearing capacity of the soil, the larger the load it could safely carry.

Foundations are basically divided into Shallow Foundations and Deep Foundations.

In this article, we are going discuss the step by step guide to Column Footing Design for a shallow foundation.

Reinforced Concrete Footings

Footing comprises of the lower end of a column, pillar or wall which i enlarged with projecting courses so as to distribute load.

Footings shall be designed to sustain the applied loads, moments and forces and the induced reactions and to ensure that any settlement which may occur shall be as uniform as possible and the safe bearing capacity of soil is not exceeded.

In sloped or stepped footings, the effective cross-section in compression shall be limited by the area above the neutral plane, and the angle of slope or depth and location of steps should be such that the design requirements are satisfied at every section.

Design Procedure of Column Footings | Foundation Design

Here is a step-by-step guide to Column Footing Design:

Step 1

Area required for footing

Square = B = (w+w1)/P0

Where, Po = safe bearing capacity of soil

w1 = self weight of footing

w = self weight of footing

For Rectangle = b/d = B/D

A = b x d

Net upward pressure on the footing

q/p = W/A

Step 2

Bending Moment

Critical section for maximum bending moment is taken at the face of the column

For a square footing,

M xx = q x B/8 (L – a)2

M xx = q x L/8 (B – b)2

M yy = q x B/8 (L – a)2

Step 3

To fix the depth of the footing shall be greater of the following:

Depth from bending moment consideration

d = √(M/Qb)

where, Q = moment of required factor

Depth from shear consideration

Check for one way shear

Check for two way shear or punching shear

Critical shear for one way shear is considered at a distance ‘d’ from face of the column.

Shear force, V = qB [ ½(B – b) d]

Nominal shear stress, T v = k . T c

Tc = 0.16√fck

Step 4

Check for two way shear

Critical section for two way shear is considered at a distance at a distance d/2 from all the faces of the column.

SF, V = q [ B2 – (b + d)2]

SF, V = q [L x B – (a + d)(b + d)]

Nominal shear stress, T v = V/2((a+d)(b+d)d) ——- {for a rectangle

Tv = V/4((b+d)d) ——- {for a square

Tv = k . T c

k = 0.5 + β > 1 ; [Beta β = ratio of sides of the column

Tc = 0.16√fck

Area of steel, Ast = M/((σ)stjd)

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