Sears, Texas, Roof Structure Engineering Report Completed

TR2, The Roof Reinforcer, completed and submitted its final evaluation report for this eastern Texas major retailer today. Tim McCarthy P.E. was consulted to investigate a deflected 20T RTU mounted on the roof. "We performed two site visits altogether," said Mr. McCarthy, principal with The Roof Reinforcer. The first visit yielded measurements of deflections at the top of the RTU and the surrounding roof membrane. The second visit was necessary to measure structural element elevations inaccessible without proper equipment."

The project required two weeks to investigate the structure, analyze the acquired data and develop conclusions.

"The deflection in the roof mounted equipment was obvious," noted Paul, Southern Region Project Manager for TR2. "And there were elevation divergences in the roofing measurements and the structural supporting elements too. But what was unusual was that the RTU measurements and the roof top and structural measurements were telling two different stories. The RTU measurements indicated an unacceptably large deflection to one side, unacceptable from the viewpoint of HVAC performance. The structural measurements corresponded in direction but were much less in magnitude, a magnitude that was well within design standards." The expected solution, an inadequate structural roof system for the support of the HVAC, was not indicated by these measurements. Further investigation was indicated.

Tim McCarthy P.E.: "We performed calculations of the center of gravity of the 20 ton RTU in relation to the supporting structure and found it to be significantly off center, about 50%. We further discovered that the mounting system was significantly smaller than the HVAC base. The combination of these two geometries renders this particular RTU significantly vulnerable to laterally imposed loading conditions." Further investigation revealed that this store was within the current code prescribed hurricane region and likely subject to significant wind gusting.

"This appears to be a classic case study for the significant acceleration in recent codes of wind loads on roof mounted equipment," noted Tim McCarthy. "It's only been since 2000 that industry research and the building codes have directly addressed wind gust loads on roof mounted equipment. And the results have been surprising. Current wind codes mandate design loads frequently double or even triple what many current buildings have been designed for."

The report recommends a contemporary wind analysis be performed for this deflected RTU, that a proper load distributing curb be designed, and anchorages installed in accordance with the wind design.