Buildings Move - Expansion Joints Accommodate That Movement Without Damaging the Structure
All buildings experience movement due to thermal expansion and contraction, moisture changes, live load deflection, foundation settlement, and, in seismic zones, earthquake-induced displacement. When that movement is restrained - when two parts of a building fight against each other rather than moving independently - the result is cracking, spalling, buckled finishes, failed waterproofing, and in severe cases, structural distress. Expansion joints, contraction joints, and seismic joints allow different parts of a building (or a building and an adjacent building) to move independently while maintaining weathertightness and continuity of finishes across the joint. Understanding where joints are required, how they differ, and how they are detailed is tested on the ARE PDD exam as a construction documents coordination and detailing skill.
Types of Movement Joints
Expansion joints accommodate thermal expansion of building materials. As temperature rises, materials expand; as it falls, they contract. Outdoor or partially enclosed structures (parking garages, bridge decks, exposed concrete) experience the largest thermal range and require the most carefully spaced expansion joints. The required spacing and opening size depend on the material's coefficient of thermal expansion and the expected temperature range. Concrete structures require expansion joints approximately every 150–200 feet; steel framing can span somewhat further due to its lower coefficient of thermal expansion; masonry veneer requires control joints at closer intervals (12–20 feet for brick, panel widths for CMU).
Control joints are planned cracks in materials (concrete, masonry, plaster) that concentrate shrinkage and thermal movement at defined locations rather than allowing random cracking throughout the material. A control joint weakens the material at a specific line (by reducing the cross section or by embedding a bond breaker) so that when the material cracks due to shrinkage, it cracks at the control joint rather than randomly. Control joints do not separate the building into independent structural sections - they are within a continuous structural element.
Seismic joints (also called seismic separation joints) physically separate adjacent buildings or building sections so that they can move independently during an earthquake without colliding. Under seismic ground motion, adjacent buildings sway at different frequencies and amplitudes; if they are not separated by an adequate gap, they can impact each other (a phenomenon called "pounding") and cause severe structural damage. IBC and ASCE 7 specify minimum seismic separation widths based on the calculated maximum displacements of each structure.
Building Expansion Joint Locations
Expansion joints are typically required: where building wings meet at re-entrant corners (the junction of two wings is a high-stress location where differential movement concentrates); where a new addition meets an existing building; where the building transitions between significantly different structural systems (concrete to steel, for example); in very long buildings where thermal movement would accumulate to unacceptable levels without interruption; and where the structure steps in section (different heights that cause differential movement).
Expansion Joint Details
The joint must be continuous from the roof through the walls to the foundation - any connection across the joint will fight against the movement the joint is intended to accommodate. At the roof, an expansion joint cover flashing must be waterproof while accommodating movement in all directions. At walls, joint covers (aluminum or rubber extrusions, or formed metal covers) span the gap while accommodating movement. At floors, joint covers provide a smooth transition at grade level and must accommodate pedestrian traffic loads while still allowing the joint to open and close. Coordination of expansion joints across all building systems - structure, envelope, interior finishes, plumbing, electrical - is one of the most complex detailing tasks in construction documents.
Key Exam Points
- Expansion joint: accommodates thermal movement; continuous from roof to foundation; required in long buildings, at wing junctions, at structural transitions.
- Control joint: planned crack location within a continuous element; accommodates shrinkage and minor thermal movement.
- Seismic joint: separates buildings or building sections to prevent earthquake pounding.
- Joint must be continuous through all building layers - any tie across joint restrains the movement the joint is meant to allow.
- Concrete buildings: expansion joints approximately every 150–200 feet; masonry veneer: control joints 12–20 feet.
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