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

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 than six weeks is 56% of that applied to a permanent structure, due to its reduced exposure to wind. Engineers have equated this probability with that of a temporary structure erected for a similar time.

While the ASCE standards offer direction on wind loads from a strictly engineering standpoint, they fail to take into account the human element. For instance, if a homeowner climbs an extension ladder—a temporary structure—to clean the gutters on his roof, he will not wait until a “56% of code wind” figure is reached before descending the ladder. A strong enough breeze will coax him down or discourage him from going up in the first place. People use forecasting and good judgment in everyday circumstances to deploy temporary structures—village placards, farmers’ market tents, shade structures, umbrellas—in wind speeds much lower than codes stipulate. In almost all these scenarios, the structures are dismantled in time.

Engineers attempt to apply this common sense approach to more significant engineered structures. We must use our judgment to arrive at a wind-speed threshold above which action is required to eliminate risk. We must also establish the appropriate level of manpower, equipment, and time to dismantle the assembly safely and in a timely manner.

Such practical approaches are not entirely absent on the regulatory landscape. The entertainment industry, for example, promulgates a standard for temporary concert stage roofs that considers the limited duration of exposure and human factors. The standard, ESTA E-1.21–2006, should form the framework for the development of a wider standard to address wind and other environmental loads on temporary structures.

On the whole, however, scant rules-based guidance governs the wind-resisting strength of temporary structures. Organizations such as ASCE or ESTA (Entertainment Services and Technology Association) should take a lead role in establishing more comprehensive standards for temporary structures that consider factors such as designing for a range of wind threshold levels, maximum time for dismantling structures, and monitoring and operational procedures. For instance, an anemometer (wind gauge) should be required on-site and monitored continuously, and weather forecasts should be reviewed routinely.

The following example illustrates the need for widely accepted standards from an authoritative industry group. Let’s say a tower is to be erected to raise a large video screen for an outdoor entertainment event. If windy conditions are forecast, at what wind speed should the video screen be taken down so it doesn’t collect wind like a sail? The equipment owner may recommend a minimum threshold—say, 40 miles per hour (mph). Without established guidelines, engineers would rely on that guidance combined with their own professional judgment in determining appropriate thresholds.

But this example prompts other questions: What is the safest way to dismantle the screen? What personnel are needed? If the wind doesn’t reach the $40\mathrm{mph}$ threshold, can the structure still be taken down safely? When weather forecasts have predicted the wind condition, is there enough time from the forecast notice to take it down? These questions emphasize the need for more definitive, broad-based standards addressing various potential wind thresholds, dismantlement times, and other concerns.

Minus clear-cut standards, some enlightened entities have filled the void by setting their own prerequisites for wind resistance. For example, one rigging equipment rental company allows event producers to choose required ballast (counterweight) for wind thresholds of 40, 50, or 60 mph for a video screen. The counterweight chosen depends on how much ballast the producer is willing to pay for to accommodate a higher wind threshold. The lower the wind threshold, the higher the probability of the wind occurring.

Instances such as these are the exception, however. Those charged with designing and building a temporary structure are often at the mercy of local building officials who may be unfamiliar with temporary wind loads and lack the engineering expertise to discern various wind-load scenarios. Event owners should have defined parameters for the levels of financial exposure they will face in dismantling operations compared with the cost of upgrading to a higher wind threshold.

For each possible wind threshold—such as 40, 50, 60, and 70 mph—the standard should include the maximum time required to dismantle the system, so that the structure can be taken down safely before the wind is forecast or likely to arrive. The dismantling approach must be realistic to accomplish and properly documented. The threshold could also be staggered in steps, such as lowering a video wall and speakers at 40 mph and dismantling the entire truss structure at 60 mph.

Decisions on wind loads for temporary structures should not be made on an ad hoc, case-by-case basis. Conclusive standards must be established to address different wind thresholds and time schedules for dismantling, to achieve optimum safety, and to permit cost-effective staging and operation.