Building Expansion Joint for Concrete Frame Structures: Design Principles

Expansion joints in concrete frame buildings divide the structure into independent units that can move freely relative to each other. Correct design of these joints prevents cracking and structural damage caused by thermal movement, creep, shrinkage, and differential settlement. Joint spacing in concrete frame buildings is determined by the thermal movement and the allowable crack width in the concrete. For a building with a temperature range of 40 degrees C and a coefficient of thermal expansion of 10 x 10^-6 per degree C, the thermal strain is 400 microstrains. If the allowable crack width is 0.2 mm, the maximum joint spacing is 0.2/0.0004 = 500 mm... but this is for a fully restrained structure. In practice, the concrete is not fully restrained, and joint spacings of 30-60 meters are typical for concrete frame buildings. Shrinkage of concrete contributes to the movement at expansion joints. Concrete shrinks by 200-400 microstrains during the first year after casting, with most of the shrinkage occurring in the first 90 days. For a 50-meter building section, this shrinkage corresponds to a movement of 10-20 mm. The joint must be designed to accommodate this shrinkage movement in addition to the thermal movement. Structural continuity at expansion joints requires careful detailing. The joint must allow free movement in the horizontal direction while maintaining vertical load transfer between the two building sections. This is typically achieved by using sliding bearings at the joint location, which allow horizontal movement while transferring vertical loads. Waterproofing of expansion joints in buildings is required at the roof level and at any below-grade locations. The joint must maintain waterproofing throughout the movement range of the joint. Flexible waterproofing membranes that bridge the joint gap are used for roof joints. Waterstop systems are used for below-grade joints. Fire resistance of expansion joints must comply with the building fire resistance requirements. The joint must maintain its fire resistance rating for the required period. Intumescent fire stops that expand when heated to seal the joint gap are commonly used for fire-resistant expansion joints.