Equation 7.28 is intended to assure yielding of the gross section rather than premature net area fracture. Although Eq. 7.28 does not include explicitly the force increase due to strain-hardening of the gross section, it should not be considered unconservative. This is because a portion of the web can aid the flange net area by providing additional fracture resistance, and a portion of the flange tensile stresses would be transmitted to the bracket by virtue of the friction at the beam or column interface. Note that in the beam flanges of the test specimens violating Eq. 7.28, fracture did not occur until severe flange buckling propagated into the net area. These observations appear to agree with recent experimental findings (Masuda et al. 1998) Reduction of column moment and axial force capacities due to column bolt holes need not be considered when checking column-beam moment ratio (Section 4.3.3), as long as Eq. 7.28 is satisfied. The recent Japanese study
indicated that 30% to 40% loss of flange area due to bolt
holes showed only about a 10% reduction in the yield moment capacity (Masuda et al. 1998), which could support this provision. Based on the experimental results, the minimum spacing of column bolts may be as small as times the bolt diameter. Weld Sizes. A weld metal with a specified Charpy VNotch toughness of 20 ft-lb at -20°F should be used for the haunch bracket fabrication. A sample weld detail is shown in Figure 7.11. A double-V bevel groove weld should be used to connect the haunch stiffener to the vertical leg. A single-V bevel groove weld may be used to connect the horizontal leg to the vertical leg. The end of the vertical leg may be offset up to 3/8 in. away from the outer surface of the horizontal leg to accommodate the steel backing, provided that at least 60% of the horizontal leg cross section is groove-welded. Although the current
Figure 7.11 Haunch Bracket Detail 68
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