On the morning following a spring windstorm in Chicago, workers in a 22-story office building noticed mortar and debris on the penthouse floor. A 16-foot high, 75-foot long concrete masonry interior wall was severely damaged, with many step and horizontal cracks. The top of the 6-inch-thick wall, which served as a backup to an aluminum curtain wall, leaned inward about 6 inches and was wedged tightly to the underside of the roof slab.Investigators observed that the aluminum curtain wall was designed with open gutter joints between the curtain wall panels. Originally, foam material had been installed in these joints to reduce air leakage. But at the time of the investigation, this material had deteriorated. In addition, the masonry wall was not mechanically supported at the top. Unanticipated lateral forces on the masonry wall from the air leakage -- plus lack of lateral support of the masonry wall -- caused the failure. Wind loads on the exterior curtain wall caused a pressure differential. The open curtain wall joints allowed the air pressure in the space between the masonry and the curtain wall to approximately equalize with the exterior pressure. This, in turn, caused a pressure difference between both sides of the interior masonry wall.Clearly, designers need to consider the proximity of interior masonry walls to the building's exterior and whether outside conditions may affect the interior walls. But excessive lateral pressures may be due to unanticipated interior conditions, as well. For example, the air pressure difference between an elevator shaft and an adjacent air shaft in one high-rise office building caused horizontal displacement of the masonry walls separating the two shafts.

Empirical Design
Wall failures of the type previously described can be avoided by recognizing that certain interior masonry walls can be exposed to lateral loads greater than the traditional interior-wall design load of 5 pounds per square foot and by structurally designing these walls to support the increased load.