Local Buckling

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For I-shaped beams and similar flexural members with thin webs, only
the web area between flange plates should be used in calculating Aw.
Table 9 gives equations for support shears, V, for beams for several
(5) Local Buckling.  To insure that a steel beam will attain fully
plastic behavior and possess the assumed ductility at plastic hinge
locations, the elements of the beam section must meet the minimum thickness
requirements sufficient to prevent a local buckling failure.  Adopting the
plastic design requirements of the AISC Specification, the width-thickness
ratio for flanges of rolled I- and W-shapes, and similar built-up single web
shapes that would be subjected to compression involving plastic hinge
rotation, shall not exceed the following values:
Fy(ksi)
bf/2tf
36
8.5
42
8.0
45
7.4
50
7.0
55
6.6
60
6.3
65
6.0
where,
Fy = specific minimum static yield stress for steel
bf = flange width
tf = flange thickness.
(a) The width-thickness ratio of similarly compressed flange
plates in box sections and cover plates shall not exceed 19O/(Fy)1/2.
For this purpose, the width of a cover plate shall be taken as the distance
between longitudinal lines of connecting rivets, high-strength bolts or
welds.
(b) The depth-thickness ratio of webs subjected to plastic
bending shall not exceed the value given by Equations (93a) or (93b)
depending on the value of P/Py.  When P/Py is less than 0.27:
EQUATION:
d/tw = [412/(Fy)1/2][l - l.4P/Py]
(93a)
When P/Py is greater than 0.27:
EQUATION:
d/tw = 257/(Fy)1/2
(93b)
where,
Py = plastic axial load equal to the cross-sectional area
times the specified minimum static yield stress, Fy.
2.08-134