Page 283 - Civil Engineering Formulas

BUILDING AND STRUCTURES FORMULAS 217

TABLE 9.3 Allowable Bearing Stress, F , on Concrete and
p
Masonry*
Full area of concrete support 0.35f c

Less than full area of concrete 0.35f c A 1 0.70f c
support
BA 2
Sandstone and limestone 0.40
Brick in cement mortar 0.25
*Units in MPa 6.895 ksi.

With N established, usually rounded to full inches (millimeters), the mini-
mum width of plate B, in (mm), may be calculated by dividing A by N and
1
then rounded off to full inches (millimeters), so that BN A . Actual bearing
1
pressure f , ksi (MPa), under the plate then is
p
R
f p (9.36)
BN
The plate thickness usually is determined with the assumption of cantilever
bending of the plate:
1 3f p
t 2 B k B F b (9.37)

where t minimum plate thickness, in (mm)
k distance, in (mm), from beam bottom to top of web fillet
F allowable bending stress of plate, ksi (MPa)
b

COLUMN BASE PLATES

2
2
The area A , in (mm ), required for a base plate under a column supported by
1
concrete should be taken as the larger of the values calculated from the equa-
tion cited earlier, with R taken as the total column load, kip (kN), or
R
A 1 (9.38)
0.70 f c
Unless the projections of the plate beyond the column are small, the plate may
be designed as a cantilever assumed to be fixed at the edges of a rectangle with
sides equal to 0.80b and 0.95d, where b is the column flange width, in (mm), and
d is the column depth, in (mm).