Roof Collapse Investigation

On May 2009, a severe weather event caused a roof collapse at a shopping center in the Dallas area. N-J was retained to determine the cause of the failure. N-J performed extensive field documentation of the as-built condition, weighed building materials to obtain an accurate estimate of the roof load, and obtained weather-related information to determine the cause of the collapse. It was determined that winds did not appear very high at the site, but there was heavy rain.

N-J personnel identified several design and maintenance flaws with the structure during site visits. Multiple layers of ceiling tiles and grids suspended from the roof structure were observed, which unnecessarily added to the suspended load on the structure. When the previous re-roofing was done, the existing roof was not removed, adding to the weight of an already heavy roof system. Roof deck concerns were observed including rusted-through areas (partially caused by a former dry cleaning tenant), poor lap slices, improper repairs, and inadequate attachment of the roof deck to walls.

N-J developed a computer model of the structure to analyze potential causes of the collapse. Results of the analysis showed an overstress with just the roof dead load alone. In addition to the structural analysis, a ponding-instability analysis was performed, which is a test of the flexibility of the structure to determine if the weight of accumulated water can cause sufficient sag in the structure to attract more water, and ultimately result in a collapse. It was determined that ponding was the triggering factor that caused the collapse.