Presented by: Civil Engineering Academy

Footings Presented by: Civil Engineering Academy

Due to high forces carried by columns, footings must be used where their ends bear against soil. The bearing stress is decreased by increasing the bearing area. Most material for footings has been covered in the Soil Mechanics Module (CERM Ch. 36, 38).

This lecture will cover much of Ch. 55 of the CERM

dealing with the structural portion of footings.

Review: A column can have a uniform bearing

pressure from a column load. It can also have a

moment placed on it making it an eccentrically

loaded with the result of a non-uniform bearing

pressure.

For the column loaded as shown, what is the

appropriate bearing stress distribution?

P=120 kips

M=30 k-ft

a)

b)

c)

d)

Footings are designed in two steps: ◦ 1) Selecting the footing area.

◦ 2) Selecting the footing thickness and

reinforcement.

Footing area is chosen so that the soil contact

pressure is within limits.

Thickness and reinforcement is chosen to

keep shear and bending stresses within limits.

For the general case of a footing with L and B,

carrying a service load, Ps, and a service

moment, Ms, the service soil pressure is:

For a wall footing, the factored load per unit

length is:

Basically 1.2D + 1.6L

The critical shear happens at a depth d (top

of footing to center of reinforcement).

Φ = 0.85 or 0.75 per ACI 318 App. C or

Ch. 9.

Vc = shear stress of concrete, Vc=2 𝑓′𝑐

qu = ultimate load

t = thickness of column

B = width of footing

The main steel (in x direction) resists flexure and is perpendicular to the wall face. Orthogonal steel (in and out of page, z) runs parallel to the wall and is placed in contact with the main steel. The total thickness of the wall footing is:

The depth and thickness is controlled by

shear.

You can have either one way or two way

shear but the majority of cases is two-way

shear.

Two-way failure happens with punching. The

area of punching is given by the following

equation:

Breadth questions dealing with structural

footing design will most likely be theory

based.

Depth questions will go into computations

and calculations of actually calculating the

size and reinforcement needed for the

foundations.

Footing problems!

Next topic: Retaining Walls!