A study in impermanence

When we talk about wind loads on building and structures, we’re almost always concerned with permanent installations: hospitals, skyscrapers, arenas, and so on. Such buildings are designed to withstand the severest of wind events, storms that have very high return intervals. The specific return interval chosen for a given structure depends on that structure’s importance (A power station is considered more important than an office complex.) and may be anywhere from 300 to 1,700 years.

The temporary building used by the Cardiff Library while its main building was under construction. It was cleverly designed to look like a shelf of books.
Temporary buildings, like this one used by the Cardiff Library (cleverly designed to look like a shelf of books), must still be resilient under many wind and weather conditions.

But what about structures that will be taken down in a year? Six months? Six weeks? A temporary building might house an organization whose main building is undergoing renovations. A concert venue might only need a simple shelter for the duration of a music festival. How are wind loads calculated for these kinds of structures? Certainly such limited-edition edifices don’t need to stand up to a wind event that’s only expected to occur every millennium and a half.

Dr. Jon Peterka, one of CPP’s founders, and Dr. Daryl Boggs, one of our longest tenured principals, investigated this very issue in the early 1990s and developed a theory around the idea of equivalent risk. Equivalent risk means that a temporary structure’s reliability should be consistent with that of a permanent structure.

In simple terms, every structure offers a certain resistance to the wind. If the structure is designed according to a relevant wind load standard (e.g., ASCE 7-10), then that resistance doesn’t just match the worst-case expected wind load, but exceeds it by a certain margin of safety. This design wind load has a certain probability of being exceeded during the structure’s lifetime.

In Boggs and Peterka’s analysis, the probability of failure is the working variable, not the wind load itself. But working backward from that probability, a design wind load, and therefore a design wind speed (itself having a specific return interval), is calculated. This ensures that when a temporary structure is built, it is as safe as its permanent counterparts. It is designed to withstand wind events that are as likely to occur as the wind events that dictate the design of a permanent structure.

Understanding special cases like temporary structures requires a specialized and nuanced understanding of wind’s inherently nondeterministic nature. A custom analysis is usually necessary for very short return intervals, but the building codes now provide guidance on the 10-to-1oo year range. Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) addresses building serviceability with special wind speed maps that represent 10, 25, 50, and 100-year return periods. Simply select the wind speed appropriate for your region, and perform the calculations as usual.