Strength and stiffness of metal building rod brace anchor connections
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Rod or cable bracing is often provided in metal buildings to develop lateral resistance against wind or seismic loads in the ‘long’ direction of the building, i.e., in the direction perpendicular to the primary frames. This bracing is oriented in an X-configuration between frame bays, with bracing termination points occurring in the primary frame webs. The rods or cables connect to the primary frames with hillside washers that accommodate the X-bracing approach angle that vary based on the building height and bay spacing. The hillside washer mounts inside a punched hole in the web, and the performance of the bracing, both stiffness and ultimate capacity, is dependent upon the web out-of-plane plate bending stiffness and web-to-flange weld lengths and orientations, and the location of the anchorage point relative to the frame flanges. X-bracing stiffness also dictates lateral load sharing between frames and exterior cladding. The research study summarized herein revisits rod bracing design for metal buildings, focusing on local strength limit states including web bearing and tearing and documenting frame-anchor load-deformation response up to anchor failure. The work started with an industry survey of common rod anchor bracing details. The survey gleaned metal building frame geometry (flange, web dimensions) and anchor geometry (angle relative to web, location of anchor in the frame cross-section) that were used to design a test matrix for an experimental program. The results from the testing program are reviewed and discussed, including a comparison of the observed limit states to those laid out by Sinno et al. that are currently being used by MBMA members in design. Updated rod brace anchor design recommendations are provided based on this comparison.