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Building Design and Construction Handbook - Merritt - Ventech!

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7.66 SECTION SEVEN<br />

2<br />

Mcr � C b (�/L b) �EIyGJ � IC y w(�E/L b)<br />

(7.20)<br />

For solid rectangular bars <strong>and</strong> symmetric box sections,<br />

M � 57,000C �JA/(L /r ) (7.21)<br />

cr b b y<br />

Noncompact Beams. The nominal flexural strength M n for noncompact beams is<br />

the least value determined from the limit states of<br />

1. Lateral-torsional buckling (LTB)<br />

2. Flange local buckling (FLB)<br />

3. Web local buckling (WLB)<br />

The AISC LRFD specification for structural steel buildings presents formulas for<br />

determining limit-state moments. In most cases, LRFD computations for flexural<br />

members can be simplified by use of tables in the AISC ‘‘Manual of Steel <strong>Construction</strong>—LRFD.’’<br />

See also Art. 7.21.<br />

LRFD for Shear in Flexural Members. The design shear strength is �VVn, where<br />

�V � 0.90, <strong>and</strong> for rolled shapes <strong>and</strong> built-up members without stiffeners is governed<br />

by the web depth-thickness ratio. The design shear strength may be computed<br />

from<br />

h 418<br />

�Vn � 0.90 � 0.6FyAw � 0.54FyAw � (7.22)<br />

t �Fy 418/�Fy 418/�Fy<br />

418 h 523<br />

�Vn � 0.90 � 0.6 � 0.54FyAw � � (7.23)<br />

b/t h/t �F t �F<br />

y y<br />

132,000 119,000 h 523<br />

�Vn � 0.90 � Aw � � (7.24)<br />

2 2<br />

(h/t) (h/t) t �Fy where Vn � nominal shear strength, kips<br />

Aw � area of the web, in2 � dt<br />

d � overall depth, in<br />

t � thickness of web, in<br />

h � the following web dimensions, in: clear distance between fillets for<br />

rolled shapes; clear distance between flanges for welded sections<br />

Fy � specified minimum yield stress, ksi, of web steel<br />

See also Art. 7.21.2.<br />

7.20.4 Beam Penetrations<br />

Certain designs, especially buildings with minimal floor-to-floor heights, require<br />

penetrations, or openings, in the webs of beams to permit the routing of ductwork<br />

or piping. In general, such penetrations can safely be made at locations where the<br />

beam shear loading is low if the penetration height is limited to half the beam<br />

depth. The central span region of a beam carrying a uniform load is an example<br />

of a typical situation. The penetration should be centered on the mutual axis of the<br />

member <strong>and</strong> all re-entrant corners should have a generous radius.

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