Collapse behavior and performance improvement methods of multi-story composite frames with different span ratios

To investigate the disparity in collapse performance between multi-story composite frames with equal or unequal spans, experimental tests were statically conducted on two 1/3 scaled two-bay three-story composite planar frames, featuring span ratios of 1:1 and 4/3:1, respectively, subjected to an internal-column removal scenario. Analysis encompasses collapse resistance response, failure patterns, deformation characteristics, axial force development, and load-carrying mechanisms. The findings indicate that an increase in beam span correlates with a continuous decrease in both the maximum load at the bearing capacity stage and the first fractured load. Specifically, the unequal-span composite frame specimen exhibited its maximum collapse resistance at the flexural mechanism stage, while the equal-span frame reached its maximum collapse resistance at the ultimate failure point due to cooperative action. Subsequently, modeling methods were validated, and extensive investigations into resistance enhancement for unequal-span composite frames using steel braces were conducted. Detailed design methodologies featuring various brace types were proposed based on collaborative stress analysis of unequal-span double-span beams. The accuracy of these methodologies was validated through numerous numerical examples, underscoring their significant research and practical implications in effectively augmenting bearing capacities and mitigating collapse risks in unequal-span frame structures.