WIND DAMAGE TO ENVELOPES OF HOUSES AND CONSEQUENT INSURANCE LOSSES

Examination of insurance claim files from Hurricanes Hugo and Andrew has revealed that most wind damage to houses is restricted to the envelope of the building. Rain entering the building then causes the insurance loss to be magnified by a factor ranging from two, at lower wind speeds, to nine at higher speeds. In wooded and urban areas near the coast, 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 reaches about 70 m/s, at which point the average loss is approximately 12% of the insured value. Between 70 and 82 m/s (the upper limit observed in Hurricane Andrew) the average loss increases rapidly to 75%, although some small areas may experience losses over 90%. This rapid increase is associated with the loss of roof sheathing and damage to windows and doors. Probabilistic relationships are developed for expected insurance losses. These show that most hurricane-prone cities are more vulnerable to damage than inland cities, but South Florida represents an extreme risk. To reduce the vulnerability of future housing, it is recommended that envelopes be designed for the same probability of failure as the main structural system. A program to determine design loads and 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 programs are initiated.