Question

a. How wide must a wall footing be if the load is 9,500 pounds per foot of wall length, and the footing rests on a sandy gravel? Show calculations and make a sketch. Assume that it is 18" thick. b. How large must a square column footing be to support a load of 65,000 pounds if it stands on silty gravel? Show calculations. Make a sketch of the footing, assuming that it is 18" thick. © Diane Reid

Answer 1

a) 38"

b) L = 5.7'

Explanation:

a) Given

P = 9,500 lb/ft

Thick = 18"

We assume an allowable bearing pressure of σ = 3000 lb/sf

If Stress = Force / Area

σ = P/A

Solving for area

A = P/σ ⇒ A = 9,500 lb/ft/3000 lb/sf

⇒ A = 3.167 sf, or 3.167 ft wide, per foot of length.

⇒ A = 38" wide

We can see the pic 1 in order to understand the answer.

b) Given

P = 65,000 pounds = 65,000 lb

Thick = 18" = 1.5'

We assume an allowable bearing pressure of σ = 2000 lb/sf

If Stress = Force / Area

σ = P/A

Solving for area

A = P/σ ⇒ A = 65,000 lb/2000 lb/sf

⇒ A = 32.5 sf

then A = L² ⇒ L = √A = √(32.5 sf) = 5.7 ft

Finally L = 5.7'

We can see the pic 2 in order to understand the answer.