A Mighty Wind (Load)


How strong should a temporary structure be made to withstand winds that may be encountered during its brief lifespan?

The answer is less definitive than you might think. The engineering community has long grappled with the question, “How strong is strong enough?” in resisting wind loads for temporary structures such as concert stages, tents, temporary display structures, temporary screens, temporary roofs and shade structures, and lighting and speaker towers. The corollary — and equally important question — is how to erect a temporary structure that balances costs with safety benefits.

For temporary concert stage roofs, there is a standard promulgated by the Entertainment Services and Technology Association (ESTA): American National Standard E-1.21-2006, Entertainment Technology, “Temporary Ground-Supported Overhead Structures Used to Cover the Stage Areas and Support Equipment in the Production of Outdoor Entertainment Events.” This is an excellent standard that considers the practical situations associated with such events.

Its effectiveness notwithstanding, this standard only addresses an important but narrow range of temporary structures. The fact is that there are no absolute standards governing the wind-resisting strength of a wide range of temporary structures. Engineers tasked with designing a temporary display assembly, for example, have to rely on their own judgment and that of the municipal or state jurisdiction in which the structure is built. Without appropriate standards, safety may be compromised or costs needlessly increased.

That is why our industry needs strong advocacy for the issuance of authoritative guidelines on wind loads for all types of temporary structures. To determine appropriate wind pressures, structural engineers start by consulting building codes. These codes define the design loads that structures are subject to in a gamut of environmental conditions, whether it be wind, rain, snow, varying temperatures, or earthquake. For wind loads, nearly all states and municipalities have adopted into their codes ASCE 7, a standard developed by the American Society of Civil Engineers that defines minimum design loads on buildings.

Most building codes, however, don't specify requirements for temporary structures, whose lifespan ranges from one day to two years. Accordingly, engineers may consult another standard, called ASCE 37, which addresses design loads on permanent structures under construction — short term, similar to temporary structures.

ASCE 37 includes provisions for modifying wind loads to reduce them for short-term exposure, which is relevant here because temporary structures such as stages and tents typically are erected for six weeks or less. Based on this standard, the wind load applied to a structure under construction is 56% of that applied to a permanent structure, because of its reduced exposure to wind. Engineers have equated this probability with that of a temporary structure.

While the ASCE standards offer a blueprint on wind loads from a purely engineering standpoint, they fail to incorporate the human element. For example, 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 sufficiently strong breeze will coax him down or discourage him from going up in the first place.

In everyday life, people use forecasting and good judgment to deploy temporary structures — village placards, farmers' market tents, shade structures, umbrellas — in wind speeds much lower than codes stipulate. In virtually all of these scenarios, the structures are dismantled in time.

Engineers attempt to apply this common-sense approach to more significant engineered structures, such as temporary stages, band shells, lighting and speaker towers, and display walls. We must use our judgment to determine a wind-speed threshold above which action is required to eliminate risk and must ascertain the appropriate level of manpower, equipment, and time to dismantle the assembly safely and in a timely fashion.

Such practical approaches that consider the limited duration of exposure and human factors have been utilized by ESTA E-1.21. This standard should form the framework for the development of a broader standard to address wind and other environmental loads on temporary structures.

The fact remains, though, that codified guidelines are lacking to address some of these issues. A broader standard should consider factors such as designing for a range of wind threshold levels, maximum time for dismantling structures, and monitoring and operational procedures. An anemometer (wind gauge), for example, should be required onsite and monitored continuously, and weather forecasts should be reviewed routinely.

The lack of standards hurts a project's bottom line, too. We have designed dozens of concert stages, ranging from large touring and live event sets for the Rolling Stones, Bon Jovi, and Madonna, to smaller ones for the Reverend Billy Graham and the New York Philharmonic. In all cases, we used ESTA E-1.21 combined with good judgment to determine appropriate wind thresholds and established proper operational guidelines.

If a hurricane were approaching, however, you would not erect the temporary structure, nor would you have the structure sheltering people during the storm. The most these structures are likely to face is a tropical storm, but in the absence of other guidelines, the building codes apparently require us to design for hurricanes, which needlessly drive up costs without increasing safety.

Cost-versus-safety considerations will only magnify as live outdoor events and major traveling productions — such as Cirque du Soleil and concert tours — grow more popular. Encouraging signs have emerged in recent years, including the introduction of ESTA E-1.21 as well as ESTA's Entertainment Technician Certification Program (ETCP), that strengthen certification standards for theatre and arena riggers and entertainment electricians. This is a great beginning but will hopefully expand to include more types of temporary assemblies.

Without clear-cut standards, some enlightened entities have filled the void by setting their own prerequisites for wind conditions. For instance, one rigging equipment rental company allows event producers to choose required ballast (counterweight) for wind thresholds of 40, 50, or 60mph for a video screen, depending on the producer's appetite for paying for extra ballast to accommodate a higher wind threshold. The lower the wind threshold, the higher the probability of the wind occurring.

Those instances are the exception, however. Event owners 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. 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 to 70mph in increments of 10mph, 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 achieve and properly documented. Furthermore, the threshold could be in steps, such as lowering a video wall and speakers at 40mph and dismantling the entire truss structure at 60mph.

Decisions on wind loads for temporary structures must not be a haphazard, case-by-case exercise. Authoritative standards can be established to account for different wind thresholds and timetables for dismantlement, to ensure optimum safety and to allow for cost-effective staging and operation.

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