Polaris was engaged to evaluate the partial collapse of the roof of a greenhouse structure in Texas.  The greenhouse consisted of four (4) approximately 45-ft wide by 300-ft long bays—one (1) office/administrative bay and three (3) growing bays.  Upon arrival, Polaris observed that the roof structure at the central portions of the three (3) growing bays had collapsed.  The growing bays each had exhaust fan cupolas within the collapse area, each of which contained six (6) exhaust fans.  The 18 exhaust fans had been replaced shortly prior to the collapse.  

Polaris’s scope of work included determining the cause of the collapse.  Polaris’s structural engineering experts did not identify any design or construction deficiencies which would have caused or contributed to the failure of the roof trusses.  Polaris reviewed historical weather data on and around the date of collapse.  No significant weather events occurred; and therefore, there was no wind or other loading conditions in excess of those considered during design.  Review of the construction drawings, specifications, and truss designs also revealed that the weight of the newly installed exhaust fans would not have resulted in loading conditions in excess of those considered during the design.     

During Polaris’s site visit, it was reported that the collapse occurred during testing of the newly installed exhaust fans.  All 18 of the exhaust fans were running during the testing. The building automation system was believed to have been manually bypassed; however, the manual bypass was unsuccessful, which resulted in the automatic closure of the exhaust vents on both ends of the growing bays.

Polaris’s mechanical engineering experts evaluated the conditions within the greenhouse caused by the 18 running exhaust fans and the closing of the vents—specifically, the negative pressure created due to the amount of air exhausted from the structure and the lack of make-up air caused by the closing of the vents.  Polaris determined that the negative pressure within the growing bays would have been several orders of magnitude higher than the code-required loading conditions (e.g., dead, live, and wind loads) utilized in the design of the roof structure.  Therefore, Polaris attributed the collapse of the structure to loads caused by the testing of the exhaust fans, in conjunction with the unexpected closing of the vents.