Page 154 - Civil Engineering Formulas

92 CHAPTER THREE

(76.2 mm) 1/4 n

3"
B b f 0.80b f
3"
n
(76.2 mm) N 1/4 2 2 For columns
10" or larger
1/4
d

m 0.95d m
FIGURE 3.6 Column welded to a base plate.

usually defined as shown in Fig. 3.6. (If the base plate is small, the area of the
base plate inside the column profile should be treated as a beam.) Yield-line
analysis shows that an equivalent cantilever dimension n can be defined as
1
n 42 dbf , and the required base plate thickness t p can be calculated from

f p
t p 2l (3.21)
B F y
where l max (m, n, ), in (mm)
n
f p P/(BN) F p , ksi (MPa)
F y yield strength of base plate, ksi (MPa)
P column axial load, kip (N)
For columns subjected only to direct load, the welds of column to base
plate, as shown in Fig. 3.6, are required principally for withstanding erection
stresses. For columns subjected to uplift, the welds must be proportioned to
resist the forces.

COMPOSITE COLUMNS

The AISC load-and-resistance factor design (LRFD) specification for structural
steel buildings contains provisions for design of concrete-encased compression
members. It sets the following requirements for qualification as a composite col-
umn: The cross-sectional area of the steel core—shapes, pipe, or tubing—should