Temporary Structures Need Wind-Load Standards

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SOURCE: Engineering News Record · POSTED: 10/23/08 · AUTHOR: William B. Gorlin, PE

How strong is strong enough in resisting wind loads for temporary structures? Engineers have long wrestled with that question and its important corollary, how to erect a temporary structure that balances costs with safety benefits.

The fact is there are no definitive standards governing the wind-resisting strength of a spectrum of temporary structures, from concert stages and tents to temporary display structures, screens, roofs and shade structures, to lighting and speaker towers.

Engineers charged with designing a temporary display assembly, for instance, must 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 we sorely need authoritative guidelines on wind loads for all types of temporary structures. (The Entertainment Services and Technology Association, or ESTA, has promulgated an excellent standard for temporary concert stage roofs, but its scope is limited to just that kind of structure).

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 range 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.

While the ASCE standards offer a blueprint on wind loads from a purely engineering standpoint, they fail to incorporate the human element. 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 use our judgment to determine a wind-speed threshold above which action is required to eliminate risk, and gauge the proper level of manpower, equipment and time to dismantle the assembly safely and timely.

The fact remains, however, that codified guidelines are lacking to address these issues. Organizations like the ASCE or ESTA should take the lead in establishing broader-reaching standards for temporary structures that consider factors such as designing for a range of wind thresholds, maximum time for dismantling structures, and monitoring and operational procedures. An anemometer (wind gauge), for example, should be required on site and monitored continuously and weather forecasts should be reviewed routinely.

The lack of standards hurts a project’s bottom line, too. If a hurricane were approaching, 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, absent other guidelines, the building codes apparently require us to design for hurricanes, which needlessly drive up costs without increasing safety.

Event owners are often at the mercy of local building officials who may be unfamiliar with temporary wind loads and lacking the engineering expertise to distinguish 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 70 mph in increments of 10 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 achieve and properly documented. Furthermore, the threshold could be 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 must not be a haphazard, case-by-case exercise. Authoritative standards must 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.

See also: Wind-Load Concerns for Temporary Structures