AISC LRFD 1.pdf

74 FLANGES AND WEBS WITH CONCENTRATED FORCES [Sect. K1.33Ctr wt⎡ ⎛f h/t ⎞ ⎤wRn= ⎢1+ 0.42⎥(K1-6)h ⎢ ⎜l/b ⎟f⎥⎣ ⎝ ⎠ ⎦for (h / t w ) / (l / b f ) > 2.3, the limit state of sidesway web buckling does not apply.When the required strength of the web exceeds R n , local lateral bracing shall beprovided at the tension flange or either a pair of transverse stiffeners or a doublerplate, extending at least one-half the depth of the web, shall be provided adjacent tothe concentrated compressive force.When transverse stiffeners are required, they shall either bear on or be welded to theloaded flange to develop the full-applied force. The weld connecting transversestiffeners to the web shall be sized to transmit the force in the stiffener to the web.Also, see Section K1.9.Alternatively, when doubler plates are required, they shall be sized to develop thefull-applied force. Also, see Section K1.10.(b) If the compression flange is not restrained against rotation:For (h / t w ) / (l / b f ) 1.7,33Ctr wt⎡ ⎛f h/t ⎞ ⎤wRn= ⎢0.42⎥(K1-7)h ⎢ ⎜l/b ⎟f⎥⎣ ⎝ ⎠ ⎦for (h / t w ) / (l / b f ) > 1.7, the limit state of sidesway web buckling does not apply.When the required strength of the web exceeds R n , local lateral bracing shall beprovided at both flanges at the point of application of the concentrated forces.In Equations K1-6 and K1-7, the following definitions apply:l = largest laterally unbraced length along either flange at the point of load, in.(mm)b f = flange width, in. (mm)t f = flange thickness, in. (mm)t w = web thickness, in. (mm)h = clear distance between flanges less the fillet or corner radius for rolledshapes; distance between adjacent lines of fasteners or the clear distancebetween flanges when welds are used for built-up shapes, in. (mm)C r = 960,000 ksi (6.62x10 6 MPa) when M u < M y at the location of the force= 480,000 ksi (3.31x10 6 MPa) when M u M y at the location of the force6. Web Compression BucklingThis Section applies to a pair of compressive single-concentrated forces or the compressivecomponents in a pair of double-concentrated forces, applied at bothflanges of a member at the same location.Either a single transverse stiffener, or pair of transverse stiffeners, or a doublerplate, extending the full depth of the web, shall be provided adjacent to concentratedcompressive forces at both flanges when the required strength of the webexceeds R n ,LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION