Wind and Seismic Forces Wind and seismic forces cause a cyclical shearing action on the building framework, which distorts the rectangular shape to an angled parallelogram. This distortion, called racking , can result in cracking and crushing of partitions adjacent to columns, floors and structural ceilings. Light steel-frame buildings are diagonally braced with steel strapping. On larger buildings, racking is resisted by shear walls and wind-bracing without considering the strength added by finishing materials. Moreover, the partitions must be isolated from the structure to prevent cracking caused by racking movement and distortion.
All materials expand with an increase in temperature and contract with a decrease.
In tall concrete or steel-frame buildings, thermal expansion and contraction may cause cracking problems resulting from racking when exterior columns and beams are exposed or partially exposed to exterior temperatures. Since interior columns remain at a uniform temperature, they do not change in length. The amount of expansion or contraction of the exposed columns depends on the temperature difference and several other factors. However, the stiffness of the structure resists the movement and usually full unrestrained expansion is not reached. Racking, resulting from thermal movement, is greatest in the outside bays of upper floors in winter when temperature differentials are largest.