Page 153 - Civil Engineering Formulas

COLUMN FORMULAS 91

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

2P u
t p m (3.17)
B 0.9F y BN
2P u
t p n (3.18)
B 0.9F y BN
2P u
t p n (3.19)
B 0.9F y BN
where m projection of base plate beyond the flange and parallel to the web,
in (mm)
(N 0.95d)/2
n projection of base plate beyond the edges of the flange and perpendi-
cular to the web, in (mm)
(B 0.80b f )/2
n (db f )/4
(2 2 X)/[1 2 (1 X)] 1.0
X (4 db f )/(d b f ) ][Pu/( 0.85f c 1 )
2

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
P
A (3.20)
F P
where P is the load, kip (N) and F p is 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-
is the 28-day compres-
pies the full area of the support, F p 0.35f c , where f c
sive strength of the concrete. If the base plate covers less than the full area,
F P 0.35f c A 2 /A 1 0.70f c , where A 1 is the base-plate area (B N), 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 m and n of the base
plate beyond the column as cantilevers. The cantilever dimensions m and n are