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Building Design and Construction Handbook - Merritt - Ventech!

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STRUCTURAL STEEL CONSTRUCTION 7.57<br />

TABLE 7.13 Limiting Width-Thickness Ratios for LRFD of Columns<br />

Compression elements<br />

Flanges of W <strong>and</strong> other I shapes<br />

<strong>and</strong> channels; outst<strong>and</strong>ing legs of<br />

pairs of angles in continuous<br />

contact<br />

Flanges of square <strong>and</strong> rectangular<br />

box sections; flange cover plates<br />

<strong>and</strong> diaphragm plates between<br />

lines of fasteners or welds<br />

Legs of single angle struts <strong>and</strong><br />

double angle struts with<br />

separators; unstiffened elements<br />

(i.e., supported along one edge)<br />

Width<br />

thickness<br />

ratio<br />

Limiting width-thickness ratio � r<br />

General A36 steel A50 steel<br />

b/t 95/�F y 15.8 13.4<br />

b/t 238/�F � F*<br />

y r 47.7 (rolled)<br />

53.9<br />

(welded)<br />

b/t 76/�F y 12.7 10.7<br />

Stems of tees d/t 127/�F y 21.2 18.0<br />

All other stiffened elements<br />

(elements supported along two<br />

edges)<br />

b/t<br />

h c/t w<br />

253/�F y 42.2 35.8<br />

37.6<br />

(rolled)<br />

41.1<br />

(welded)<br />

* F y � compressive residual stress in flange: 10 ksi for rolled shapes, 16.5 ksi for welded sections.<br />

2<br />

Fcr � (0.877/� c)F y<br />

(7.11)<br />

Computations can be simplified by use of column load tables in the AISC LRFD<br />

‘‘Steel <strong>Construction</strong> Manual.’’<br />

For design of columns with elements having width-thickness ratios exceeding<br />

the limits in Table 7.13, refer to the AISC LRFD specification.<br />

(T. V. Galambos, ‘‘Guide to <strong>Design</strong> Criteria for Metal Compression Members,’’<br />

4th ed., John Wiley & Sons, Inc., New York.)<br />

7.20 BEAMS AND OTHER FLEXURAL MEMBERS<br />

The capacity of members subject to bending depends on the cross-section geometry,<br />

AISC ASD <strong>and</strong> LRFD procedures incorporate the concept of compact <strong>and</strong> noncompact<br />

sections.<br />

7.20.1 ASD of Flexural Members<br />

Beams classified as compact are allowed a bending stress, ksi, F b � 0.66F y for the<br />

extreme surfaces in both tension <strong>and</strong> compression, where F y, is the specified yield<br />

stress, ksi. Such members have an axis of symmetry in the plane of loading, their<br />

compression flange is adequately braced to prevent lateral displacement, <strong>and</strong> they<br />

develop their full plastic moment (section modulus times yield stress) before buckling.

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