Foundation Design

Foundation Design
Foundation is the base of any structure. Without a firm foundation, the structure cannot stand. That is the reason why we have to be very cautious with the design of foundations because our entire structure rests on the foundation.
Laying of Column Footing Reinforcement
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.
On that basis, the 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.

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

Here is a step-by-step guide to Column Footing Design:
Column Footing Plan and Section
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,
Mxx = q x B/8 (L – a)2
Mxx = q x L/8 (B – b)2
Myy = 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 =square root(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, Tv = k . Tc
Tc = 0.16square rootfck
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, Tv =  V/2{(a+d) (b+d)d} ——- {for a rectangle
Tv = V/4{(b+d)d}         ——- {for a square
Tv = k . Tc
k = 0.5 + Beta > 1                       ; [Beta = ratio of sides of the column
Tc = 0.16square rootfck
Area of steel, Ast = M/((sigma)stjd)


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