Simplified Design for Footings - Simplified Design of Reinforced Concrete Buildings - page 213

may be consulted to determine the bearing capacities to be used in design. In the absence of such information or
for conditions where the nature of the soils is unknown, borings or load tests should be made. For larger buildings,
borings or load tests should always be made.

In general, the base area of the footing is determined using unfactored loads and allowable soil pressures
(ACI 15.2.2), while the footing thickness and reinforcement are obtained using factored loads (ACI 15.2.1).

7.4 SURCHARGE

In cases where the top of the footing is appreciably below grade (for example, below the frost line) allowances
need to be made for the weight of soil on top of the footing. In general, an allowance of 100 pcf is adequate
for soil surcharge; unless wet packed conditions exist that warrant a higher value (say 130 pcf). Total surcharge
(or overburden) above base of footing can include the loads from a slab on grade, the soil surcharge, and the
footing weight.

7.5 ONE-STEP THICKNESS DESIGN FOR REINFORCED CONCRETE FOOTINGS

A simplified footing thickness equation can be derived for individual footings with minimum reinforcement
using the strength design data developed in Reference 7.1. The following derivation is valid for › = 4000 psi,
f y = 60,000 psi, and a minimum reinforcement ratio of 0.0018 (ACI 10.5.4).

Set ρ= 0.0018 × 1.11 = 0.002 *

⎛
⎜

0.5pf y
0.85

⎞
⎟

R n =ρ f y 1 −

f c ʹ

0.5 × 0.002 × 60,000

⎛
⎜

⎞
⎟

= 0.002 × 60,000 1 −

0.85 × 4000

= 117.9 psi

For a 1ft wide design strip:

M u
φ

M u × 1000

d reqd

2

=

R n = 117.9 =

9.43M u

0.9 ×

where M u is in ft-kips.
Referring to Fig. 7-1, the factored moment M u at the face of the column (or wall) is (ACI 15.4.2):

⎛
⎜

c 2
2

⎞
⎟ =

P u
A f

⎛
⎜

c 2
2

⎞
⎟

M u = q u

where c is the largest footing projection from face of column (or wall). Substituting M u into the equation for
d 2 reqd results in the following:

* The minimum value of ρ is multiplied by 1.11 to account for the ratio of effective depth d to overall thickness h, assumed as d/h 0.9.