Wind Loads for Temporary Structures: Making the Case for Industrywide Standards

Abstract

Engineers are well versed in designing and building structures to comply with applicable codes. But one crucial area where we lack definitive regulatory guidance is for temporary structures. Engineers constantly ask themselves the question, Is this structure as safe, as wind-resistant, and as cost-effective as we can make it? Local building codes, preferences of the equipment owner, and the engineer’s own professional judgment all factor into the equation, but we need more than an ad hoc system for determining wind-load thresholds for all types of temporary structures. We need industrywide standards that not only govern the safety of these structures, but also balance costs with safety benefits. Temporary structures encompass a wide range of items such as concert and theatrical stages, tents, public art projects, temporary roofs and shade structures, lighting and speaker towers, temporary grandstands and bleachers, and many other facilities. Some building codes require temporary structures to comply with the wind-load guidelines applied to permanent buildings, and others leave this to the discretion of the building official. Accordingly, engineers are designing temporary structures to be strong enough to withstand once-a-century hurricanes when in fact these structures will be used for only a short period—sometimes just a day or two. If a hurricane were approaching, though, you would not erect a temporary structure, nor would you have it sheltering people during a heavy storm. In fact, strong winds would likely keep people away from the event altogether and may well prompt event owners to cancel or postpone the event. In any case, the most these structures are likely to face is a thunderstorm, but absent other guidelines, the building codes apparently require us to design for hurricanes, which needlessly drives up costs without increasing safety. Building codes define the design loads that structures are subject to in a gamut of environmental conditions—wind, rain, snow, varying temperatures, or earthquake. For wind loads, nearly all states and municipalities have adopted codes that incorporate ASCE 7, which sets parameters for minimum design loads on buildings. Most building codes, however, do not specify requirements for temporary structures, whose lifespan ranges from one day to several months. IBC 2006 states that structures erected for less than 180 days as temporary, but does not provide further guidance. Engineers may then look to another standard, ASCE 37, which addresses design loads on permanent structures under construction—short term, similar to temporary structures. ASCE 37 incorporates provisions for adjusting wind loads to lower them for short-term exposure during construction for up to five years, which is relevant here because temporary structures typically are erected for six weeks or less. Based on this standard, the wind load applied to a structure under construction for less