By: Tony Barnes, P.E., Senior Structural Design Engineer
Recent collapses due to high wind events at outdoor performance facilities have highlighted the need for both design professionals and facility managers to consider plans to mitigate risk to life and property. The most notable of these collapses occurred at the Indiana State Fair in 2011. The collapse of a concert stage occurred as the result of a sudden wind gust. The stage was heavily damaged, seven people were killed, and 58 injured. Numerous other failures occur every year with property damage being the primary consequence.
While permanent structures are required to be designed to resist wind loads per building codes, the same may not be true for temporary structures. The International Building Code (IBC) directs engineers to the “American Society of Civil Engineers Minimum Design Loads for Buildings (ASCE 7),” to determine wind loads acting on permanent structures. These wind loads are based on a 50-year wind event and the associated wind speeds for typical Midwestern locations is 90 mph. Wind speeds are higher in coastal and some mountain regions. Wind speeds, and thus loads, on temporary structures may be reduced at the discretion of the engineer and/or jurisdictional authority per ASCE 37 – “Design Loads on Structures During Construction.”
Responsibility and requirements for designing temporary structures are not always clear. In the wake of the Indiana State Fair disaster, several states have responded by requiring even temporary structures to be designed as if they were permanent structures. Other states require “High Wind Action Plans” that involve a detailed evacuation plan and a team of on-site professionals using electronics and other systems to closely monitor the current weather conditions during an event. While a few states have proactively set some of these guidelines, the majority of states have no specific structural requirements for temporary structures or have requirements that are vague. Designing temporary structures for the wind loads used for permanent structures will likely result in significant cost increases and a structure that is over-designed while high wind action plans require costly labor and depend on the accuracy of weather predictions and efficacy of evacuation plans to ensure user safety.
In the absence of clear code requirements, it is recommended that design professionals, facility managers and facility owners utilize a risk management analysis. The risk analysis should consider the following:
• Intended occupancy and risk to human life– how many people will be present during use?
• Location – coastal region or mountainous region?
• Size of structure – is a single platform used or are multiple platforms and supporting structure used?
• Intended use of structure – display, stage, gathering, etc.
• Time of year used – certain months are more likely to yield strong winds.
• Length of time erected
• Acceptability of property damage – damage to elements may be acceptable if collapse is prevented; it may be less costly to replace some items rather than prevent all damage.
This simple risk analysis allows all parties involved to determine an appropriate course of action. In all cases, a detailed code review must be performed. Codes are ever-evolving and many states have taken forward steps in addressing the issue of wind loads on temporary structures but actual code changes lag behind. Staging Concepts has the in-house capability to perform a full wind analysis on any structural configuration and currently recommends, minimally, the use of ballast to prevent structural systems from overturning or becoming dangerous wind-borne debris. Permanent ballast and/or structural anchorages along with special details are used for permanent or higher risk structures. Contact a Staging Concepts representative to discuss your specific project.