© 2003 Door & Access Systems
Publish Date: Summer 2003
Author: Tom Wadsworth
Page 62

What Dealers Should Know
Part III

In this final installment of our three-part series on wind resistance, we suggest what dealers need to find out about the building site as it pertains to garage door wind resistance.

Rule of Thumb: If a “low-rise structure” is less shielded from the wind, it will likely experience more wind pressure. Various codes and standards (particularly ASCE 7) contain definitions of exposure conditions, but here are five basic conditions to keep in mind:

1. Structure Classified as Exposure B. This exposure is the most common. The definition from ASCE 7 is “urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single family dwellings or larger.” This is often referred to as a default exposure since the next four conditions usually need to be proven.
2. Structure Exposed to Open Land. Considered an Exposure C condition, ASCE 7 describes it as “open terrain with scattered obstructions having heights generally less than 30 feet.” This exposure can increase wind pressures by 20 to 35 percent over those in Exposure B areas.
3. Structure Near a Hurricane-Prone Coastline. Within one mile of this coastline area, a structure is considered to be in an Exposure C condition. Contrary to popular belief, these areas do not experience worst-case exposure conditions. Reason: Hurricane conditions (tides, storm surges) affect the wind differently than winds within storms in interior regions.
4. Structure Near Inland Water. This is the worst-case exposure condition, known as Exposure D. ASCE 7 describes this condition as “flat, unobstructed areas exposed to wind flowing over open water, excluding shorelines in hurricane-prone areas, for a distance of at least one mile.” ASCE 7 adds that structures within 1500 feet of such shoreline would be classified as Exposure D. This exposure can increase wind pressures by 45 to 60 percent over those in Exposure B areas.
5. Structure at the Base of a Large Hill or Mountain. The technical term is wind escarpment. Here the wind pressures can be significantly increased over Exposure B areas. You may need to consult a design professional or a local building official to determine pressures in these situations.

If you ever have any doubt about the effect of wind related exposure on a specific garage door installation, contact a local building official or a local design professional. If local exposure conditions affect the type of product you need, you should also work with your garage door supplier.