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Building Design and Construction Handbook - Merritt - Ventech!

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STRUCTURAL STEEL CONSTRUCTION 7.73<br />

tioned such that the maximum web depth-thickness ratio h/t does not exceed h/t<br />

given by (7.32) or (7.33), whichever is applicable.<br />

If a/h � 1.5,<br />

If a/h � 1.5,<br />

h 2000<br />

� (7.32)<br />

t �F yƒ<br />

h 14,000<br />

� (7.33)<br />

t �F ( F � F )<br />

yƒ yƒ r<br />

where a � clear distance between transverse stiffeners, in<br />

t � web thickness, in<br />

F yƒ � specified minimum yield stress of steel, ksi<br />

F r � compressive residual stress in flange � 16.5 ksi for plate girders<br />

Web stiffeners are frequently required to achieve an economical design. However,<br />

web stiffeners are not required if h/t � 260 <strong>and</strong> adequate shear strength is provided<br />

by the web. The criteria for the design of plate girders are given in the AISC LRFD<br />

Specification.<br />

<strong>Design</strong> Flexural Strength. The design flexural strength is �bMn, where �b � 0.90.<br />

If hc/t � 970 �F y,<br />

determine the nominal flexural strength as indicated in Art.<br />

7.15, for either compact or noncompact shapes. If hc/t � 970 �F , Mn is governed<br />

y<br />

by the limit states of tension-flange yielding or compression-flange buckling.<br />

The design strength is the smaller of the values of �bMn for yielding of the<br />

tension flange, which is<br />

<strong>and</strong> for buckling of the compression flange, which is<br />

� M � 0.90S R RF (7.34)<br />

b n xt PG e yt<br />

� M � 0.90S R R F (7.35)<br />

b n xc PG e cr<br />

where RPG � plate-girder bending-strength reduction factor<br />

� 1 � 0.0005ar(hc/t � 970/ �Fcr) � 1.0<br />

R e � hybrid girder factor<br />

� 1 � 0.1(1.3 � a r)(0.81 � m) � 1.0<br />

� 1 for nonhybrid girders<br />

a r � ratio of web area to compression-flange area<br />

m � ratio of web yield stress to flange yield stress or to F cr<br />

F cr � critical compression-flange stress, ksi<br />

F yt � yield stress of tension flange, ksi<br />

S xt � section modulus, in 3 , with respect to the tension flange<br />

S xc � section modulus, in 3 , with respect to the compression flange<br />

The critical stress F cr is different for different limit states. Its value is computed<br />

from the values of parameters that depend on the type of limit state: plate girder<br />

coefficient C PG, slenderness parameter �, limiting slenderness parameter � p for a<br />

compact element, <strong>and</strong> limiting slenderness parameter �, for a noncompact element.<br />

Thus, F cr may be computed from one of Eqs. (7.34) to (7.36) for the limit states

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