Tips & Tricks | Seismic (4)

October 13, 2026

Guide To: Flexible Diaphragm Load Distribution for L-Shaped Buildings in RISA

L-shaped buildings can present unique design challenges because of their asymmetric geometry. One such challenge is the distribution of lateral loads especially when utilizing a flexible diaphragm. For background on flexible diaphragm load distribution, refer to the related article below for some details on the load distribution based on beam layout. L-shaped buildings require additional assumptions and clarifications from the general beam layout guidance described there. The program uses different distribution methods for wind and seismic loads based on the application of the load type. Wind load is generated using the full building face dimensions, while seismic load is generated using mass tributary area. In both cases, the generated loads are assigned to the lateral framing members that define each diaphragm region. The following sections compare three conditions: the theoretical distribution based on engineering fundamentals, the distribution generated by an initial integrated RISAFloor and RISA-3D model, and the distribution after modifying the member designation assumptions. Each condition uses the same floor plan, with equal-sized bays and lateral frame lines shown in red. Theoretical Results To understand and verify how RISA distributes lateral loads, first establish the theoretical distribution for an L-shaped building with a flexible diaphragm. These theoretical values provide…