design of spread foundation

Selection of materials.

Spread footings are usually designed to use 3

ksi

<

f'

c

< 4

ksi

, whereas modern structural members frequently use concrete between the range of 4

ksi

<

f'

c

< 8

ksi

. A higher concrete

strength helps reduction the member

’

s size. However, the footing

’

s design is govern by the

bearing capacity and settlement. That means that the strength of

the soil might be limiting

factor, and a higher concrete strength would not be relevant.

Where a footing must carry a load greater than 500 kips, an

f'

c= 5

ksi might be justified. Since flexural stresses are usually small, a grade 40 steel woul d usually be adequate, although it is currently unavailable in the US. The most common grade use d for construction is Grade 60 steel, which is almost universally used in the world today. The typical details of a spread footing, as sketched for draftin

g. The standard thicknesses T are given in English Units as multiples of 3": 12", 15", 18"..., etc

. A high precision in specifying

the depth of excavation

D

f

is unnecessary. ACI code specifies that at least 3 inches of concrete cover must be included from ground contact, which takes into con sideration irregularities in the excavation and corrosion factors

The same footing as built. Note that forming is required in this

site due to the presence of a clean gravelly sand, that would not stand vertically at the sides of the excavation