Page 161 - Handbook of Civil Engineering Calculations, Second Edition

1.144 STRUCTURAL STEEL ENGINEERING AND DESIGN

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
b b f 18 in.
9.0
t 2t f 2 × 1 in.
For the definition of b for a welded I shape, see the AISC LRFD Manual.

Flange
65 65
p 10.833

F y 36
For the flange, < p . Therefore, the flange is compact, and M nx M px for the limit state
of flange local buckling (FLB).
Web
56 in.
h c
128.0

7
i n
t w / 16
Web
640 640
p 106.7
F y 36

Web
970 970
r 161.7
F y 36

For the web, ( p 106.7) < ( 128.0) < ( r 161.7). The web is noncompact: M rx <
M nx < M px ; for the limit state of web local buckling (WLB); M nx is determined from AISC
LRFD Manual Eq. (A-F1-3).
2. Analyze the lateral bracing relating to the limit state of lateral-torsional
buckling (LTB)
For this continuously braced member L b 0; M nx M px for LTB. Summarizing:
Limit State M nx
LTB M px
FLB M px
WLB M rx < M nx < M px

The limit state of WLB (with minimum M nx ) governs. To determine M px , M rx , and M nx for
a doubly symmetric I-shaped member bending about the major axis, refer again to the
AISC LRFD Manual table. There M px F y Z x , M rx F y S x for WLB and from Eq. (A-F1-3)
(for WLB):