Fastener Diameters

Minimum bolt diameters are 1⁄2 in for buildings and railroad bridges. In highway-bridge members carrying calculated stress, 3⁄4-in fasteners are the smallest permitted, in general, but 5⁄8-in fasteners may be used in 21⁄2-in stressed legs of angles and in flanges of sections requiring 5⁄8-in fasteners (controlled by required installation clearance to web and minimum edge distance). Structural shapes that do not permit use of 5⁄8-in fasteners may be used only in handrails.
In general, a connection with a few large-diameter fasteners costs less than one of the same capacity with many small-diameter fasteners. The fewer the fasteners, the fewer the number of holes to be formed and the less installation work. Larger-diameter fasteners are particularly favorable in connections where shear governs, because the load capacity of a fastener in shear varies with the square of the fastener diameter. For practical reasons, however, 3⁄4-and 7⁄8-in-diameter fasteners are preferred.
Maximum Fastener Diameters in Angles. In bridges, the diameter of fasteners in angles carrying calculated stress may not exceed one-fourth the width of the leg in which they are placed. In angles where the size is not determined by calculated stress, 1-in fasteners may be used in 31⁄2-in legs, 7⁄8-in fasteners in 3-in legs, and 3⁄4-in fasteners in 21⁄2-in legs. In addition, in highway bridges, 5â??„8-in fasteners may be used in 2-in legs.

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