WIND DAMAGE TO ENVELOPES OF HOUSES AND CONSEQUENT INSURANCE LOSSES

Examination of insurance claim files from Hurricanes Hugo and Andrew h as revealed that most wind damage to houses is restricted to the envel ope of the building. Rain entering the building then causes the insura nce loss to be magnified by a factor ranging from two, at lower wind s peeds, to nine at higher speeds. In wooded and urban areas near the co ast, damage to buildings and their contents generally begins when the gradient wind speed reaches 40 m/s. There is a linear increase in the average insurance loss with wind speed until the gradient speed reache s about 70 m/s, at which point the average loss is approximately 12% o f the insured value. Between 70 and 82 m/s (the upper limit observed i n Hurricane Andrew) the average loss increases rapidly to 75%, althoug h some small areas may experience losses over 90%. This rapid increase is associated with the loss of roof sheathing and damage to windows a nd doors. Probabilistic relationships are developed for expected insur ance losses. These show that most hurricane-prone cities are more vuln erable to damage than inland cities, but South Florida represents an e xtreme risk. To reduce the vulnerability of future housing, it is reco mmended that envelopes be designed for the same probability of failure as the main structural system. A program to determine design loads an d envelope component resistance is described. However, improvements in the wind resistance of the building stock will be slow and hurricane losses will remain high, unless large and aggressive retro-fitting pro grams are initiated.