Page 156 - Handbook of Civil Engineering Calculations, Second Edition

STRUCTURAL STEEL DESIGN 1.139

where axial shortening, in. (cm); P unfactored axial force in member, kips (kg); I
length of member, in. (cm); E modulus of elasticity of steel 29,000 ksi (199.8
MPa); A g cross sectional area of member, sq.in. (sq cm).
2. Compute the column axial shortening
Substituting,
Pl 250 kips × (10.0 ft × 12 in./ft)
Shortening,
EA g 29,000 kips/sq.in. × 14.4 sq.in.
0.072 in. (0.183 cm).

Related Calculations: Use this equation to compute axial shortening of any steel col-
umn in LRFD work. This procedure is the work of Abraham J. Rokach, MSCE, American
Institute of Steel Construction.

DETERMINING THE COMPRESSIVE
STRENGTH OF A WELDED SECTION

The structural section in Fig. 36a is used as a 40-ft (12.2-m) column. Its effective length
factor K x K y 1.0. Determine the design compressive strength if the steel is A36.

Calculation Procedure:

1. Choose a design compressive strength
The design compressive strength is given by:

c P n c F cr A g
The values of c F cr can be obtained from the Table, “Design Stress for Compression
Members of 36 ksi Specified Yield-Stress Steel, 0.85” in the AISC Manual, if Kl/r is
known. With Kl 1.0 40.0 ft 12 in/ft 480 in. (1219 cm), then
r I

A
2
2
A (18 in.) – (17 in.) 35.0 sq.in.
2
(18 in.) – (17 in.) 4
4
I x I y I 1788 in (0.00074 m )
4
12
2. Find the Kl/r ratio for this section
With the data we have,
r 4 7.15 in.
1788 in

35.0 in 4
KI 480 in.
67.2
r 7.15 in.