Engineers urge limits on prescriptive IRC methods for three‑story multiplexes; suggest engineered design in higher seismic areas

Technical advisory group members debated whether three‑story multiplex buildings should be allowed to use the IRC prescriptive lateral systems (braced wall panels and other prescriptive methods) or whether projects should require an engineering design under the IBC for certain seismic zones or building configurations.

Engineers on the TAG said gravity (vertical) loading on stud walls is generally manageable, but seismic demands—particularly overturning and complex lateral load paths—raise concerns for stacked multiplex configurations that are more common in urban infill than single‑family or townhouse prototypes used to validate IRC prescriptive rules.

"My concern is that 3‑story multiplex units designed to the IRC do not give the same seismic performance as those designed to the International Building Code," said Michael Wright (TAG member/structural engineer). Several participants suggested limiting prescriptive IRC methods to lower seismic design categories (SDC A and B) and requiring engineered designs for SDC C, D, or sites with complex configurations.

John (TAG member/engineer) noted that national discussions are under way about whether braced‑wall panel tables and prescriptive methods—developed with townhouses and single‑family prototypes in mind—are appropriate for multiunit stacked buildings. Darren pointed out Table 602.10.3 (seismic bracing) already restricts some methods in D2 and higher.

The TAG did not set a final rule but suggested language options: allow IRC prescriptive methods in SDC A/B or where all braced walls are aligned and prescriptive criteria are met; require an engineer when the project is three stories in SDC C/D or when braced wall lines do not stack. Staff were asked to draft a proposed 108‑series section that clarifies thresholds for engineered design and prescriptive allowance.

Next steps: staff to circulate draft language that limits prescriptive methods by seismic design category and lists conditions under which an engineered lateral system is required.