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BEHA VIOR OF INDIVIDUAL FASTENERS

Connections are generally classified according to the manner of stressing the fastener Section, that is, tension, shear or combined tension and shear. Typical examples of connections subjecting fasteners to shear are splices and gusset plates in trusses. Bolts in tension are common in hanger connections and in beam-to-column connections. Some beam-to-column connections may also subject the bolts to combined tension and shear. It is apparent that, before a connection can be analyzed, the behavior of the component parts of the connection must be known. Therefore, the behavior of a single bolt subjected to the typical loading conditions of tension, shear, or combined tension and shear is discussed in this section.

Bolts Subjected to Tension

Since the behavior of a bolt subjected to an axial load is governed by the perfor-mance of its threaded part, load versus elongation characteristics of a bolt are more significant than the stress versus strain relationship of the fastener metal itself.

In the 1985 ASTM specifications for high-strength bolts, both the minimum tensile strength and proof load are specified. The proof load is about equivalent to the yield strength of the bolt or the load causing 0.2% offset. To determine the actual mechanical properties of a bolt, ASTM requires a direct tension test of most sizes and lengths of full-size bolts. In practice, the bolt preload force is usually introduced by tightening the nut against the resistance of the connected material. As this torque is applied to the nut, the portion not resisted by friction between the nut and the gripped material is transmitted to the bolt and, due to friction between bolt and nut threading, induces torsional stresses into the shank. This tightening procedure results in a combined tension-torsional stress condition in the bolt. Therefore, the load versus elongation relationship observed in a torqued tension test differs from the relationship obtained from a direct tension test. Specifically, torquing a bolt until failure results in a reduction in both ultimate load and ultimate deformation as compared with the corresponding values determined from a direct tension test. Typical load versus elongation curves for direct tension as well as torqued tension tests are shown in Fig. 4.4 for A325 bolts and A490 bolts. In torquing a bolt to failure, a reduction in ultimate strength of between 5 and 25% was experienced in tests on both A325 and A490 bolts. The average reduction is equal to 15%. Frequency distributions of the ratio T/Tu for both A325 and A490 bolts are also shown in Fig. 4.4.

As well as having a higher load, a bolt loaded to failure in direct tension also has more deformation capacity than one that is failed in torque tension. This is visible in the two specimens shown in Fig. 4.5. The differences in thread de-formation and necking of the critical section in the threaded part of the bolts are readily apparent.

To determine whether specified minimum tensile requirements are met, specifications require direct tension tests on full-size bolts if the bolts are longer than three diameters or if the bolt diameter is less than 1¼ in. for A325 bolts or 1 in. for A490 bolts. Bolts larger in diameter or shorter in length shall preferably be tested in full size:however, on long bolts tension tests on specimens machined from such bolts are allowed. Bolts shorter than three diameters need only meet minimum and maximum hardness requirements. Tests have illustrated that the actual tensile