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_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:

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The AISC specification for allowable stress design for buildings includes three interaction formulas for combined axial compression and bending.

When the ratio of computed axial stress to allowable axial stress f /F _a exceeds 0.15, both of the following equations must be satisfied:

( f _a / F _a ) + ( C _{m x} f _{b x} ) / (1– f _a /F ^‘ _{e x} ) F _{b x} + C _{m y} f _{b y} / (1 – f _a / F ^‘ _{e y} ) F _{b y} ? 1

f _a / 0.60F _y + f _{b x} /F _{b x} + f _{b y} / F _{b y} ? 1

when f _a /F _a ? 0.15, the following equation may be used instead of the preceding two:

f _a / F _a + f _{b x} / F _{b x} + f _{b y} / F _{b y} ? 1

In the preceding equations, subscripts x and y indicate the axis of bending about which the stress occurs, and

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Horizontal deflections in the planes of bents and shear walls can be computed on the assumption that they act as cantilevers. Deflections of braced bents can be calculated
by the dummy-unit-load method or a matrix method. Deflections of rigid frames can be computed by adding the drifts of the stories, as determined by moment distribution
or a matrix method.

Figure showing Building frame resists lateral forces with (a) wind bents or (g) shear walls or a combination of the two. Bents may be braced in any of several ways, including (b) X bracing, (c) K bracing, (d) inverted V bracing, (e) knee bracing, and (f) rigid connections.

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Provision should be made for all structures to transmit lateral loads, such as those from wind, earthquakes, and traction and braking of vehicles, to foundations and their supports

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For greatest resistance to bending, as much of a plate girder cross section as practicable should be concentrated in the flanges, at the greatest distance from the neutral axis. This might require, however, a web so thin that the girder would fail by web buckling before it reached its bending capacity.

To preclude this, the AISC specification limits h/t.

For an unstiffened web, this ratio should not exceed.

where F_y = yield strength of compression flange,ksi (MPa).

Larger values of h/t may be used, however, if the web is stiffened at appropriate intervals.
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