WEBS UNDER CONCENTRATED LOADS

Criteria for Buildings

The AISC specification for ASD for buildings places a limit on compressive stress in webs to prevent local web yielding. For a rolled beam, bearing stiffeners are required at a concentrated load if the stress f a , ksi (MPa), at the toe of the web fillet exceeds F a = 0.66F y w , where F y w is the minimum specified yield stress of the web steel, ksi (MPa). In the calculation of the stressed area, the load may be assumed distributed over the distance indicated in Fig. 9.4.

For a concentrated load applied at a distance larger than the depth of the beam from the end of the beam:

F a = R / f w ( N + 5K )

where R = concentrated load of reaction, kip (kN)

t w = web thickness, in (mm)

N = length of bearing, in (mm), (for end reaction, not less than k)

K = distance, in (mm), from outer face of flange to web toe of fillet

For a concentrated load applied close to the beam end:

f a = R / t w ( N + 2.5 k )

To prevent web crippling, the AISC specification requires that bearing stiffeners be provided on webs where concentrated loads occur when the compressive force
exceeds R, kip (kN), computed from the following:

For a concentrated load applied at a distance from the beam end of at least d/2, where d is the depth of beam:

R = 67.5 t 2 w [ 1 + 3 ( N / d ) ( t w / t f ) 1.5 ] (F y w t f / t w ) ½

where t f = flange thickness, in (mm)

For a concentrated load applied closer than d/2 from the beam end:

R = 34r 2 w [ 1 + 3 ( N / d ) ( t w / t f ) 1.5 ] ( F y w t f / t w ) ½

If stiffeners are provided and extend at least one-half of the web, R need not be computed.

Another consideration is prevention of sidesway web buckling. The AISC specification requires bearing stiffeners when the compressive force from a concentrated load
exceeds limits that depend on the relative slenderness of web and flange r w f and whether or not the loaded flange is restrained against rotation:

r w f = ( d c / t w ) / ( l / b f )

where l = largest unbraced length, in (mm), along either top or bottom flange at point of application of load

b = flange width, in (mm)

d c = web depth clear of fillets = d – 2k

Stiffeners are required if the concentrated load exceeds R, kip (kN), computed from

R = 6800 t 3 w / h ( 1 + 0.4 r 3 w f )

where h = clear distance, in (mm), between flanges, and r w f
is less than 2.3 when the loaded flange is restrained against rotation. If the loaded flange is not restrained and r w f is less than 1.7,

R = 0.4 r 3 w f ( 6800 t 3 w / h )

R need not be computed for larger values of r w f ..

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