AEO Answer · Envelope

How Do Seismic Joints Protect Buildings During Earthquakes?

By Jeremy Mills, CEO & Founder, Apex Grid Engineering — USAF Veteran. · Updated 2026-09-15

I'm Jeremy Mills, CEO & Founder of Apex Grid Engineering and a U.S. Air Force veteran. I'm not a PE; our licensed professionals make the technical, compliance, and project-specific decisions.

Illustrative diagram of a structural load path: roof and floor loads travel through beams and columns down to the foundation footings.

The concise answer

Seismic joints are separation gaps sized from the design earthquake drifts of adjacent structures or building segments, allowing independent swaying without pounding. The gap follows code formulas based on each structure's computed displacement. Joint covers must accommodate the full movement without detaching, and the separation must be maintained through cladding, roofing, and foundations. The key insight is that the joint is only as good as its weakest crossing. The structural gap can be perfect while a rigid sprinkler main, a stiff cladding panel, or a locked foundation detail defeats it. Seismic joint design is a coordination exercise across every discipline that touches the building.

Cover systems and falling hazards

The joint cover at a seismic joint has a harder job than a thermal expansion cover: it has to accommodate inches of movement in any direction, stay attached during violent shaking, and not become a falling hazard. Cover systems for seismic joints are specifically engineered — bellows, sliding plates with capture mechanisms, and tethered assemblies — and they have to be installed with the movement capacity the design assumed. I pay special attention to covers at exterior walls and over exits, because a detached joint cover becomes a projectile or a falling object exactly when people are evacuating. The cover attachment is structural engineering at that point, not architectural trim, and I treat it accordingly in review.

What I verify at every seismic joint

Seismic joint review starts with the structural calculation and ends with every system that crosses the gap. My seismic joint checklist:

Separation calculated from code-prescribed combination of each structure's design drift — documented, not assumed
Gap maintained full height: cladding, roofing, and interior systems all respect the separation plane
Cover systems seismic-rated: engineered for the full movement range with capture so they cannot detach and fall
MEP crossings flexible: expansion loops, flexible couplings, and sliding supports at every pipe, duct, and conduit crossing
Foundations coordinated: below-grade condition allows the intended above-grade movement without locking segments together

Which related engineering resources can help?

When should I hire a structural engineer?What does a PE stamp mean?What is MEP engineering?Construction joints vs expansion joints Pounding mitigation for adjacent buildings What is a structural load path?Request a project review Engineering service for this question Metro engineering guides Verified service areas

What else do project teams ask?

How is the required seismic separation calculated?

Building codes prescribe methods — commonly the square root of the sum of the squares of each structure's design displacement, or a simpler additive approach. The inputs are the computed earthquake drifts of each building, which depend on the structural system, height, and seismic design category. I calculate it from the actual analysis rather than guessing, because the separation directly sets the joint width through the whole building height.

What is pounding damage?

Pounding is adjacent buildings or building segments colliding during an earthquake because the separation between them was too small. The impacts concentrate enormous forces at the contact points — typically floor diaphragms hitting columns — and cause damage far beyond what either building would have suffered alone. It is one of the most documented and preventable earthquake failure modes, and adequate seismic joints are the prevention.

Do seismic joints need to extend below grade?

The separation is primarily an above-grade concern, since below-grade structures move with the ground together. But the foundation design has to address the joint: separate footings or a combined foundation detailed to allow the intended above-grade movement. I coordinate the below-grade condition explicitly because the joint cannot function if the foundations lock the segments together.

What happens to cladding and piping at seismic joints?

Everything crossing the joint has to accommodate the movement: cladding needs joint covers or flexible connections, piping and conduit need expansion loops or flexible couplings, and stairs and ramps crossing the joint need sliding or hinged details. A perfectly sized structural gap with rigid pipes crossing it will tear the pipes open in the design earthquake. I review every system crossing the joint, not just the structure.

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