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 (a) For the case of simple shear stresses, as encountered at
end supports, it is important to distinguish three ranges of behavior
depending on the magnitude of h/t. For large values of h/t, the maximum
shear stress is dictated by elastic buckling in shear, and for intermediate
h/t values the inelastic buckling of the web governs; whereas for very small
values of h/t, local buckling will not occur and failure will be caused by
yielding produced by shear stresses. This point is illustrated in Figure 42
for Fy = 40 ksi. The specific equations for use in design for Fy = 40,
60, and 80 ksi are summarized in Tables 12, 13, and 14, respectively.
(b) At the interior supports of continuous panels, high shear
bending moments combined with large shear forces are present and webs must
be checked for buckling due to these forces. The interaction formula
presented in the AISI Specification is given in terms of the allowable
stresses rather than critical stresses which produce buckling. In order to
adapt this interaction formula to ultimate load conditions, the problem of
inelastic buckling under combined stresses has been considered in the
development of the recommended design data.
(c) To minimize the amount and complexity of design
calculations, the allowable design shear stresses at the interior support of
a continuous member have been computed for different depth-to-thickness
ratios for Fy = 40, 60, and 80 ksi, and tabulated in Tables 12, 13, and
14.
(3) Web Crippling. In addition to shear problems, concentrated
loads or reactions at panel supports, applied over relatively short lengths,
can produce load intensities that can cripple unstiffened thin webs. For
blast resistant design, the values recommended by AISI are multiplied by a
safety factor of 1.50 to relate the crippling loads to the ultimate
conditions.
(a) For those sections that provide a high degree of
restraint against rotation of their webs such as I-beams made by welding two
angles to a channel, the ultimate crippling loads are given as follows:
Acceptable ultimate end support reaction
EQUATION:
(Qu)e = 1.5Fyt2[4.44 + 0.558(N/t)1/2]
(103)
Acceptable ultimate interior support reaction
EQUATION:
(Qu)i = 1.5Fyt2[6.66 + 1.446(N/t)1/2]
(104)
where,
Qu
=
ultimate support reaction
Fy
=
yield stress
N
=
bearing length
t
=
web thickness
2.08-140
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