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Sep. 9, 2013

Wind Speeds Change, But Little Difference for Design

By Jeffrey B. Stone, Ph.D., Florida Building Commissioner

A comparison of the wind maps for Florida between the new 2010 Florida Building Code (which became effective in March 2012) and the previous edition of the code shows an increase of 20 to 30 miles per hour for wind speeds along much of Florida’s coast. What used to be a design wind speed of 130 or 140 mph is now 160 or 170 mph, raising eyebrows among designers. Location of speed contours have also changed.

However, if developers think this will mean drastic changes for the design of buildings, they are being misled. That’s because the design methodology associated with the higher wind speeds has also changed in the new version of ASCE 7 Minimum Design Loads for Buildings and Other Structures (2010 edition). While designers must use the new higher winds speeds, they are factored in such a way that wind pressures—the design forces that exterior walls and roofs of building must be designed to resist—have not significantly changed for the majority of buildings.

“This could be causing some confusion among designers,” says Brad Douglas, PE, vice president of engineering for the American Wood Council, the major standards producer for the wood industry. “If designers are using the new wind speeds with their old design tables or software, they are making a costly mistake.”

The current wind speed maps are based on new uniform recurrence interval wind speed contours developed by wind researchers. But the ASCE 7 standard also carries new factors to use with the new maps, which results in wind pressure on buildings being about the same for most buildings. Near the shoreline, pressures may increase for some buildings; in other regions, design pressures have actually gone down.

This does not mean that construction requirements in hurricane prone areas have been relaxed. On the contrary, all buildings in Florida, even residential buildings, are now required to be designed by qualified designers in order to make sure building construction can resist hurricane-force winds. This has been a considerable transition for the residential construction industry, which used to rely on prescriptive framing techniques. Industry is meeting the demand for current design documents that are based on state-of-the-art structural engineering, but can be used by builders and code officials as well as designers. One example is the American Wood Council’s, Wood Frame Construction Manual 2012 (WFCM), which is based on ASCE 7– 2010.

One challenge that continues to hound builders is keeping the siding and roofing on their buildings when high winds strike. “The key to meeting these requirements is using products that have been tested for the specific application, and installing them correctly” says Douglas.”If you don’t protect the integrity of the structure with adequate attached siding and roofing, they could come off and expose the rest of the structure to much greater damage.” In addition, greater emphasis is being placed on the performance of windows and doors. Codes are evolving to ensure these components are designed to effectively mitigate damage. In addition, the Florida Building Commission provides a thorough product review and approval program that verifies building envelope products comply with the wind provisions of the Florida Building Code.

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