A 17,000 sq ft portion of the 225,000 sq ft, low-sloped warehouse roof collapsed during a short, but intense, rainstorm. The structural system for the roof consisted of metal deck supported on open-web steel joists and joist girders supported on steel columns. SGH investigated the roof collapse and determined that the root cause of the failure was inadequate stiffness and strength of the roof structure. We determined the rain load during the rain event and performed an iterative structural analyses to determine the design rain load plus the additional ponding rain load. We collaborated with the structural engineer of record and provided repair recommendations to stiffen and strengthen the roof structure.
Design requirements for rain loads and ponding loads (ponding instability) have been a part of the structural code provisions throughout the U.S. for some time, but are not always well-coordinated amongst all the parties or adequately understood. SGH is applying lessons learned from this failure and other failures to help shape future modifications to the rain load provisions in ASCE 7-15 (Minimum Design Loads for Buildings and Other Structures).