Expansion Joints in Buildings: Accommodating Thermal and Seismic Movement
Steel and concrete both expand when they heat up and contract when they cool, a small movement per unit length that adds up to something structurally significant once a building stretches long enough. A steel frame two hundred meters long can grow and shrink by a noticeable fraction of an inch between winter and summer temperature extremes, and if that entire frame were built as one continuous rigid structure with no relief anywhere along its length, the restrained thermal movement would build up internal stress with nowhere to go, stress that shows up eventually as cracked finishes, jammed doors, or, in the worst case, distress in the structural frame itself.
Thermal Joints Split the Building So It Can Move Freely
An expansion joint physically separates a building into two or more independent structural segments, each free to expand and contract on its own without dragging its neighbor along with it, connected only through a flexible joint cover that accommodates the relative movement at floors, walls, and roofs where the segments meet. Placement and spacing of thermal expansion joints in long buildings has traditionally followed rule-of-thumb maximum length guidance, though more detailed analysis accounts for actual anticipated temperature range, the building's construction type, and whether the structure is heated and cooled continuously or exposed to full seasonal swings, since an unconditioned parking structure exposed to direct sun sees a far larger effective temperature range than a climate-controlled office floor of the same length.
Every expansion joint in the structural frame has a corresponding joint in every finish and system crossing it, roofing membrane, flooring, curtain wall, and building enclosure elements like the ones discussed in curtain wall structural design, since a structural joint that isn't matched by a corresponding gap in the finishes simply relocates the restraint problem from the frame into whatever rigid finish tries to span across the joint uninterrupted.
Expansion joints are sometimes confused with control joints in concrete slabs and pavements, but the two solve different problems: a control joint in a slab is a deliberately weakened line meant to control where shrinkage cracking occurs, while a full structural expansion joint physically separates independent structural systems and has to accommodate real, ongoing relative movement over the building's entire service life, not just one-time shrinkage during curing.
Seismic Separation Joints Solve a Different Problem With Similar Hardware
Where two adjacent buildings, or two structurally distinct portions of the same building with different heights, stiffnesses, or dynamic periods, are close enough together that they could collide during an earthquake, a seismic separation joint provides a gap sized not for thermal movement but for the combined lateral displacement each structure could independently develop during the design earthquake. Pounding, the collision of adjacent structures swaying out of phase with each other during shaking, has caused documented structural damage at inadequately separated buildings, which is why seismic joint width is calculated from each structure's expected peak displacement added together, a fundamentally larger and more safety-critical gap than a thermal expansion joint typically requires.
Where a single building has an irregular plan or a significant difference in height or stiffness between wings, designers sometimes choose to seismically separate the wings into independent structures entirely, each with its own lateral system, rather than trying to force the whole irregular footprint to respond as one unified structure, a strategy that sidesteps some of the torsional and load-path irregularities that would otherwise complicate the design covered in load paths and structural redundancy.
Joint Cover Assemblies Have to Survive the Movement They're Designed For
The visible joint cover, the metal or elastomeric assembly bridging the gap at floors, walls, and roofs, has to physically accommodate the full design movement range repeatedly over the building's service life without losing weathertightness or becoming a trip hazard at floor level, and selecting the right joint cover system is as much a durability and maintenance question as a structural one, since a joint that structurally performs exactly as designed but leaks or jams after a few years of cyclic movement still represents a design that failed in practice.
ASCE 7 and guidance published through the National Institute of Building Sciences cover both thermal expansion joint spacing practice and seismic separation requirements, maintained through the National Institute of Building Sciences.