AISC LRFD 1.pdf

210 DESIGN FOR FLEXURE [Comm. F1.Lpd⎛ E ⎞= 0. 086⎜⎟Fr y(C-F1-4)⎝ y ⎠F2. DESIGN FOR SHEARFor unstiffened webs k v = 5.0, therefore1.10 Ek / F = 2.45 E / F , and1.37 Ek / F = 3.07 E / Fv yw yw v yw ywFor webs with h/ tw ≤110 . Ekv / Fyw, the nominal shear strength V n is based onshear yielding of the web, Equation F2-1 and Equation A-F2-1. This h/t w limitwas determined by setting the critical stress causing shear buckling F crequal to the yield stress of the web F yw in Equation 35 of Cooper, Galambos, andRavindra (1978) and Timoshenko and Gere (1961). When h/ tw >110 . Ekv / Fyw,the web shear strength is based on buckling. Basler (1961) suggested taking theproportional limit as 80 percent of the yield stress of the web. This corresponds toh / t w = (. 110 / 0.) 8 Ek v / Fyw. Thus, when h/ tw >137 . Ekv / Fyw, the webstrength is determined from the elastic buckling stress given by Equation 6 of Cooperet al. (1978) and Timoshenko and Gere (1961):Fcr2p Ekv=12 1 /22( - v )( h tw)(C-F2-1)The nominal shear strength, given by Equation F2-3 and A-F2-3, was obtained bymultiplying F cr by the web area and using E = 29,000 ksi (200 000 MPa) and v = 0.3.A straight line transition, Equation F2-2 and A-F2-2, is used between the limits110 . Ek v / Fyw and137 . Ek v / Fyw.The shear strength of flexural members follows the approach used in the AISCASD Specification, except for two simplifications. First, the expression for theplate buckling coefficient k v has been simplified; it corresponds to that given byAASHTO Standard Specification for Highway Bridges (1996). The earlier expressionfor k v was a curve fit to the exact expression; the new expression is just as accurate.Second, the alternate method (tension field action) for web shear strength isplaced in Appendix G because it was desired that only one method appear in themain body of the Specification with alternate methods given in the Appendix.When designing plate girders, thicker unstiffened webs will frequently be lesscostly than lighter stiffened web designs because of the additional fabrication. If astiffened girder design has economic advantages, the tension field method inAppendix G will require fewer stiffeners.The equations in this section were established assuming monotonically increasingloads. If a flexural member is subjected to load reversals causing cyclic yieldingover large portions of a web, such as may occur during a major earthquake, specialdesign considerations may apply (Popov, 1980).F4. BEAMS AND GIRDERS WITH WEB OPENINGSWeb openings in structural floor members may be necessary to accommodate variousmechanical, electrical, and other systems. Strength limit states, including localLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION