47.5 MB - The Whole Building Design Guide

3.1.1 Nature of High WindsA variety of windstorm types occur in different areas of the UnitedStates. The characteristics of the types of storms that can affect thesite should be considered by the design team. The primary stormtypes are listed below.Straight-line wind: This type of wind generally blows in a straightline and is the most common. Straight-line wind speeds rangefrom very low to very high. High winds associated with intenselow pressure can last for approximately a day at a given location.Straight-line winds occur throughout the United States and itsterritories.Down-slope wind: Wind blowing down the slope of mountains is referredto as down-slope wind. Down-slope winds with very highspeeds frequently occur in Alaska and Colorado. In the continentalUnited States, mountainous areasare referred to as “special wind regions”ASCE 7, Minimum Design Loads for(see Figure 3-1). Neither ASCE 7 nor modelBuildings and Other Structures, providesbuilding codes provide specific wind speedsguidance for determining wind loads onin special wind regions. ASCE 7 does providebuildings. The IBC and NFPA 5000 referguidance on how to determine design windto ASCE 7 for wind load determination.speeds in these regions. If the local buildingdepartment has not established the basicspeed, use of regional climatic data and consultationwith a wind engineer or meteorologist is advised.Thunderstorm: This type of storm can form rapidly and producehigh wind speeds. Approximately 10,000 severe thunderstormsoccur in the United States each year, typically in the spring andsummer. They are most common in the Southeast and Midwest.Besides producing high winds, they often create heavy rain andsometimes spawn tornadoes and hail storms. Thunderstorms commonlymove through an area quite rapidly, causing high winds foronly a few minutes at a given location. However, thunderstormscan also stall and become virtually stationary.Downburst: Also known as a microburst, this is a powerful downdraftassociated with a thunderstorm. When the downdraftreaches the ground, it spreads out horizontally, and may form oneor more horizontal vortex rings around the downdraft. The out-3-2 MAKING CRITICAL FACILITIES SAFE FROM High Wind