CIVIL ENGINEERING FORMULAS

COLUMN FORMULAS 91

The thickness of the base plate tp, in (mm), is the largest of the values given
by the equations that follow

(3.17)

(3.18)

(3.19)

wheremprojection of base plate beyond the flange and parallel to the web,
in (mm)
(N 0.95d)/2
nprojection of base plate beyond the edges of the flange and perpendi-
cular to the web, in (mm)
 (B0.80bf)/2





X

AMERICAN INSTITUTE OF STEEL CONSTRUCTION
ALLOWABLE-STRESS DESIGN APPROACH

The lowest columns of a structure usually are supported on a concrete founda-
tion. The area, in square inches (square millimeters), required is found from

(3.20)

wherePis the load, kip (N) and Fpis the allowable bearing pressure on sup-
port, ksi (MPa).
The allowable pressure depends on strength of concrete in the foundation
and relative sizes of base plate and concrete support area. If the base plate occu-
pies the full area of the support, , where is the 28-day compres-
sive strength of the concrete. If the base plate covers less than the full area,
, where A 1 is the base-plate area (BN), and A 2
is the full area of the concrete support.
Eccentricity of loading or presence of bending moment at the column base
increases the pressure on some parts of the base plate and decreases it on
other parts. To compute these effects, the base plate may be assumed com-
pletely rigid so that the pressure variation on the concrete is linear.
Plate thickness may be determined by treating projections mandnof the base
plate beyond the column as cantilevers.The cantilever dimensions mandnare

FP0.35fcA 2 /A 1 0.70fc

Fp0.35fc fc

A

P

FP

(4dbf)/(dbf)^2 ][Pu/(0.85fc 1 )

(2 2 X)/[1 2 (1X)]1.0

n (dbf)/4

tpn

B

2 Pu

0.9FyBN

tpn

B

2 Pu

0.9FyBN

tpm

B

2 Pu

0.9FyBN