2005-070 - Laserfiche WebLink

Home Browse Search

1%W Hurricane winds are unique in several ways : 1 ) They are more turbulent than winds in most other types of storms ; 2 ) They are sustained for a longer period of time (several hours ) than any other type of atmospheric disturbance ; 3 ) They change slowly in direction ; thus , they are able to seek out the most critical angle of attack on a given structure ; and 4 ) They generate large quantities of flying debris as the built environment is progressively damaged ; thus , amplifying their destructive power . In hurricanes , gusts of wind can be expected to exceed the sustained wind velocity by 25 % to 50 % . This means a hurricane with sustained winds of 150 mph will have wind gusts exceeding 200 mph . The wind ' s pressure against a fixed structure increases with the square of the velocity . For example a 100- mph wind will exert a pressure of approximately 40 pounds per square foot on a flat surface , while a 190- mph wind will exert a force of 122 pounds per square foot on that same structure . In terms of a 4- by 8-foot sheet of plywood nailed over a window , there would be 1 , 280 pounds of pressure against this sheet in a 100- mph wind , and 3 , 904 pounds or 1 . 95 tons of pressure against this sheet in a 190- mph wind . The external and internal pressures generated against a structure vary greatly with increases in elevation , shapes of buildings , openings in the structures , and the surrounding buildings and terrain . Buildings at ground level experience some reductions in wind forces simply because of the drag exerted by the ground against the lowest levels of the air column . High- rise buildings , particularly those located along the beach ront will receive the full strength of hurricane winds on their upper stories . Recent stuc ies estimate that wind speed increases by approximately 37 % just 15 feet above ground level . The wind stream generates uplift as it divides and flows around a structure . The stream following the longest path around a building , generally the path over the roof, speeds up to rejoin the wind streams following shorter paths , generally around the wa Is . This is the same phenomenon that generates uplift on an aircraft' s wing . The roof in effect becomes an airfoil that is attempting to "take off' from the rest of the building . Roof vortexes generally concentrate the wind ' s uplift force at the corners of a roof. These key points can experience uplift forces two to five times greater than those exerted on other parts of the roof. Once the envelope of the building has been breached through the loss of a window or door, or because of roof damage , wind pressure on internal surfaces becomes a factor . Openings may cause pressurizing or depressurizing of a building . Pressurizing pushes the walls out , while depressurizing will pull the walls in . Internal press re coupled with external suction adds to the withdrawal force on sheathing fasteners . Damages from internal pressure fluctuations may range from blowouts of windows and doors to total building collapse due to structural failure . During Hurricane Andrew, catastrophic failure of one- and two-sto wood -frame buildings in residential areas was observed more than catastrophic failures in other types of buildings . Single-family residential construction is particularly vulnerable because less engineering oversight is applied to its design and construction . As opposed to hospitals and public buildings , which are considered "fully engineered , " and office and industrial buildings , ` ftw which are considered " marginally engineered , " residential construction is considered " non -engineered . " Historically , the bulk of wind damage experienced nationwide has 4-21