Handbook of Civil Engineering Calculations

Working with the "Flexural Strength Parameters" table in the Appendix of the AISC

LRFD Specification, the compactness of the beam (for a doubly symmetric I shape bend-

ing about its major axis) should first be checked:

Flange

A_*_£_JiJn__ 9 .o

t 2tf 2 x 1 in

For the definition of b for a welded I shape, see the AISC LRFD Manual.

Flange

L-W-&"*

For the flange, A < Ap. Therefore, the flange is compact, and Mn* = Mpx for the limit state

of flange local buckling (FLB).

Web

Ap=p ^= *L»L = 128 .o

tw V

7

XiS in

Web

. 640 640 ....
A =
^TVsT

106

7

Web

A=970 970

v/TV36

= 161

7

For the web, (Ap = 106.7) < (A = 128.0) < (Ar = 161.7). The web is noncompact: Mn < M^
< Mpx; for the limit state of web local buckling (WLB); M^. is determined from AISC
LRFD Manual Eq. (A-F 1-3).

2. Analyze the lateral bracing relating to the limit state of lateral-
   torsional buckling (LTB)
   For this continuously braced member Lb = O; Mn^. = Mpx for LTB. Summarizing:

Limit State M^

LTB Mpx

FLB Mpx

WLB Mrx<Mnx<Mpx

The limit state of WLB (with minimum Mm) governs. To determine Mpx, Mn, and Mm for
a doubly symmetric I-shaped member bending about the major axis, refer again to the
AISC LRFD Manual table. There Mpx = FyZx, M^ = FySx for WLB and from Eq. (A-F 1-3)
(for WLB):