Experimental Investigation of Foam-Filled CHS Braces under Cyclic Loading

The performance of seismic braced frames can become compromised due to premature local buckling–induced fracture of the hollow structural section (HSS) steel braces. A strategy to delay the fracture of the braces is to postpone local buckling by filling HSS braces. Two experimental campaigns were undertaken to assess the performance of circular hollow section (CHS) braces under representative seismic loading when filled with a lightweight, pourable, expanding polyurethane foam. To determine the influence of the foam infill, 12 brace members were tested under reverse cyclic loading. In general, the foam infill was able to delay the initiation of local buckling in the plastic hinge region, consequently leading to improvements in cyclic ductility. Reduced strain values within the plastic hinge region of the filled braces compared with those of the empty braces further highlighted the beneficial influence of the foam infill. Overall, the test results suggested that the diameter-to-thickness ductility limits for CHS members in the current seismic design provisions can be relaxed with the inclusion of the polyurethane foam as infill.