Building Design and Construction

Load Capacity of Bolts

Under service conditions, bolts may be loaded in tension, shear, or a combination of tension and shear. The load capacities specified in AISC ASD and LRFD specifications are closely related and are based on the ‘‘Specification for Structural Joints Using ASTM A325 or A490 Bolts,’’ Research Council on Structural Connections of the Engineering Foundation. Both bearing-type and slip-critical bolted connections are proportional for the shear forces on the gross area of bolts.

Allowable tension and shear stresses for bolts are listed in Table 7.24. The allowable bearing load at a bolt hole is 1.5Fudt, where Fu is the specified tensile strength, d is the nominal bolt diameter, and t = thickness of connected part.
Table 7.25 tabulates maximum sizes for standard, oversize, and slotted bolt holes. Oversize holes are permitted only in slip-critical connections. In slip-critical connections, slots may be formed without regard to the direction of loading; but in bearing-type connections, slot length should be placed normal to the direction of loading. Washers, hardened when used with high-strength bolts, should be placed over oversize and short-slot holes.

Long-slot holes may be used in only one ply of the connected parts at an individual faying surface. When the slot is in an outer ply, plate washers or a continuous bar with standard holes should be installed to cover the entire slot. Washers or bars for A325 or A490 bolts should be 5⁄16 in or more thick but need not be hardened. If hardened washers are required, they should be placed over the outer surface of a plate washer or bar.

LRFD for Bolts

The design strength of bolts or threaded parts is Rn (tabulated in Table 7.26) applied to the nominal body area of bolts and threaded parts except upset rods (see footnote h for Table 7.26). The applied load is the sum of the factored external loads plus the tension, if any resulting from prying action caused by deformation of connected parts. If high-strength bolts are required to support the applied loads by direct tension, they should be proportioned so that the average required strength (not including initial bolt tightening force) applied to the nominal bolt area will not exceed the design strength.
The design strength in tension for a bolt or threaded part subject to combined tension and shear stresses is also listed in Table 7.26. The value of ƒv, the shear caused by the factored loads producing tensile stress, should not exceed the values for shear alone given in Table 7.26.


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