What Is a Seismic Joint vs. an Expansion Joint?
Seismic joints separate building portions for earthquake movement (the wide — the 4-12+ inch gaps; the pounding prevention); expansion joints accommodate temperature movement (the narrower — the 1-2 inch; the thermal). The confusion: both are gaps in the building — but the seismic joint's width (the inelastic drifts) and the detailing (the fire, the waterproofing across the moving gap) differ fundamentally. The engineer sizes each for its movement.
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Seismic joints
The purpose (the pounding — the adjacent structures' or the wings' independent movement; the separation — the SRSS of the drifts), the width (the code — the inelastic displacements; the 4-12+ inches), and the detailing (the fire — the rated joint systems; the waterproofing — the bellows; the flooring — the cover plates accommodating).
Expansion joints
The purpose (the thermal — the concrete's expansion; the 200-foot rule of thumb — the joint spacing), the width (the temperature range — the 1-2 inches), and the detailing (the sealants — the watertight; the simpler — the non-seismic).
Combined and confused
The dual-purpose (the seismic joint serving the thermal — the wider governing), the retrofit (the pounding's mitigation — the joint created; the tying — the alternative), and the maintenance (the joint covers — the damage; the seals — the renewal). The drawings' joint schedule — the each joint's type and width explicit.
The bottom line
Seismic joints (4-12+ in, pounding prevention, SRSS of drifts) differ from expansion joints (1-2 in, thermal) — size each for its movement and detail fire/waterproofing across the gap.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.