WEBS UNDER CONCENTRATED LOADS

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_yw , where F_yw 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.

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=67.5t²_w[1+3(N/d)(t_w/t_f)^1.5](F_ywt_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=34r²_w[1+3(N/d)(t_w/t_f)^1.5](F_ywt_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_wf and whether or not the loaded flange is restrained against rotation:

r_wf=(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_f=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=6800t³_w/h(1+0.4r³_wf)
where
h=clear distance, in (mm), between flanges, and r_wf
is less than 2.3 when the loaded flange is restrained against rotation. If the loaded flange is not restrained and r_wf is less than 1.7,

R=0.4r³_wf(6800t³_w/h)
R need not be computed for larger values of r_wf