What Is Seismic Lateral Force-Resisting System Selection?
Selecting the seismic lateral force-resisting system (LFRS) — moment frames, shear walls, braced frames, dual systems — is the structural design's defining decision: the system determines the building's seismic performance, the architectural freedom, and the cost. The selection balances the seismic hazard, the building's height and occupancy, the architectural constraints, and the construction economy. The system is the strategy; everything else is tactics.
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The system options
Moment frames (the beam-column rigid joints — the architectural openness; the expensive steel tonnage or the concrete congestion; the drift-controlled), shear walls (the concrete or masonry walls — the stiff, economical; the architectural walls consumed), braced frames (the steel diagonals — the efficient; the architectural diagonals negotiated), and dual systems (the moment frame plus the shear wall/braced frame — the redundant, high-performance; the tall-building standard). Each with the ordinary/intermediate/special detailing levels (the ductility hierarchy — the special detailing for the high-seismic).
Selection criteria
Seismic hazard (the SDC — the detailing level mandated; the near-fault effects), the building height (the code's height limits per system — the system permitted to the height), the occupancy (the Risk Category — the essential facility's performance demanding the better system), the architecture (the open floor plan — the moment frame; the core walls — the shear wall; the expressed structure — the braced frame), and the construction (the local economy — the concrete town vs. the steel town; the schedule — the system's erection speed).
The R-factor trade-off
The response modification coefficient (R — the code's reward for ductility: special moment frames R=8, ordinary R=3): the higher-R system designs for lower forces but requires the special detailing (the ductile connections, the capacity design — the labor and the inspection). The selection optimizes the total cost (the lower forces vs. the detailing premium) — the engineer's judgment, informed by the local construction economics. The peer review (the high-seismic selection validated — the second opinion on the strategy).
The bottom line
Select the seismic LFRS (moment frame, shear wall, braced frame, dual) by hazard, height, occupancy, architecture, and construction economy — balancing the R-factor's lower forces against the special detailing's premium.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.