Question

Select the most efficient member for the following LRFD required moments and unsupported lengths. Assume Cb = 1.0.

Mr	Lb
630 kip-ft	30 ft
160 kip-ft	10 ft
350 kip-ft	16 ft
400 kip-ft	8 ft

fy=50

Answer 1

Using AISC Manual Table 3-10 fy=50 Cb=1

• find unbraced length on horizontal axis.
• Obtain the section, from the appropriate intersection of the Mr corresponding to LRFD column on vertical axis such that Mr is greater than or equal to required moment in the question with the unbraced length,

The selected moment should be the next greater value,so that the section would be the most efficient.

Selected sections are

Mr Lb Selected Members

630 kip-ft 30 ft W24*104

160 kip-ft 10 ft W12*35

350 kip-ft 16 ft W21*62

400 kip-ft 8 ft W21*57