Building Design and Construction Handbook - Merritt - Ventech!

7.50 SECTION SEVEN
TABLE 7.11 Tension on Gross Area
Allowable tensile
stress (ASD)
Fy, ksi Ft, ksi
Unit design tensile
strength (LRFD)
F y, ksi �P n/A g
36 21.6 36 32.4
42 25.2 42 37.8
45 27.0 45 30.5
50 30.0 50 45.0
55 33.0 55 49.5
60 36.0 60 54.0
A36 steel, the allowable tension stresses for gross and net areas are 22.0 and 29.0
ksi, respectively, and in the case of pin plates, 16.2 ksi.
7.18.2 LRFD of Tension Members
Design tensile strength �P n, kips, of the gross area A g,in 2 , should not exceed
0.90F y, where F y is the minimum yield stress of the steel (Table 7.9) and P n �
A gF y. Nor should the design tensile strength �P n, kips, exceed 0.75F u on the net
area A e,in 2 , of the member. Other criteria control the design tensile strength of pinconnected
members. (Refer to the AISC specification for LRFD.)
7.19 COLUMNS AND OTHER
COMPRESSION MEMBERS
The principal factors governing the proportioning of members carrying compressive
forces are overall column buckling, local buckling, and gross section area. The
effect of overall column buckling depends on the slenderness ratio Kl/r, where Kl
is the effective length, in, of the column, l is the unbraced length, and r is the least
radius of gyration, in, of the cross section. The effect of local buckling depends on
the width-thickness ratios of the individual elements of the member cross section.
W shapes with depths of 8, 10, 12, and 14 in are most commonly used for
building columns and other compression members. For unbraced compression
members, the most efficient shape is one where the value of r y with respect to the
minor axis approaches the value of r x with respect to the major axis.
When built-up sections are used as compression members, the element joining
the principal load-carrying elements, such as lacing bars, should have a shear capacity
of at least 2% of the axial load.
7.19.1 Effective Column Length
Proper application of the column capacity formulas for ASD or LRFD depends on
judicious selection of K. This term is defined as the ratio of effective column length
to actual unbraced length.