Peter R. Sparks

Wind damage to envelopes of houses and consequent insurance losses

Abstract 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 iniated.

Long-term hurricane risk assessment and expected damage to residential structures

Abstract This paper presents results from a study to evaluate long-term hurricane risks in the Southeastern United States using event-based simulation procedures. These risks are defined by (1) the statistical extreme wind climate, and (2) the expected insured losses from damage to residential structures. A probabilistic hurricane event model developed by the authors is used to evaluate long-term risks. The event model parameters were derived from a statistical analysis of storms affecting the Southeastern United States and include radius of maximum winds, central pressure difference, landfall location, storm track, and decay rate. The 50-year mean recurrence interval (MRI) gradient-level and surface gust wind speeds are evaluated for the region investigated using results from the simulation analysis. When coupled with a damage model, also developed by the authors, the results from the event-based simulation analysis are used to provide estimates of the expected losses. The states of North Carolina, South Carolina, and Florida are used to demonstrate the applicability of this procedure for evaluating expected losses. Implications for setting design wind speeds as well as risk-consistent insurance rates are discussed.

Hurricane simulation techniques for the evaluation of wind-speeds and expected insurance losses

Abstract This paper describes the development of event-based hurricane simulation techniques for the evaluation of long-term risks in the Southeastern United States. Numerical simulation techniques were used to evaluate the 50-yr mean recurrence interval gradient wind-speeds for hurricane-prone regions in the study area. Using a damage model derived from actual insurance loss data and developed by the authors, the expected annual insurance losses were evaluated. The state of South Carolina was used as a case study to demonstrate the applicability of this system.

Gust factors and surface-to-gradient wind-speed ratios in tropical cyclones

Abstract Digital wind-speed data from tropical cyclones that have impacted the United States during the last decade have been analysed. Using data from automatic weather stations, dropsondes, radiosondes, and Doppler radar, relationships between surface and gradient wind speeds have been determined. In addition, gust factors have been computed for offshore, coastal, and inland sites. It has been found that the wind-speed characteristics of tropical cyclones are essentially the same as extra-tropical cyclones. Unusually high gust factors have been found, but they appear to be rare, and are usually associated with spiral rain-bands feeding into the tropical cyclone, rather than the main body of the storm. It has also been found that, as a result of unjustified assumptions regarding the relationship between wind speeds over the sea and over the land, the design wind speeds in some coastal areas of the United States are probably overstated by approximately 25%.

On the failure of single-story wood-framed houses in severe storms

Abstract Wind-tunnel pressure measurements are used to define the wind forces at the roof to wall interface of typical single-story, wood-framed houses. These forces are then combined with the weight of the roof, connector capacities and the effects of internal pressure to estimate the most likely wind speeds at which a roof would separate from the walls. This is then extended to predict the risk of serious structural damage in a typical design hurricane, taking into account the form of construction, amount of shelter and the degree to which the windows are protected. Conclusions are drawn regarding the suitability of damage observations to estimate wind speeds and the susceptibility of various forms of construction to damage in severe storms.

Wind conditions in Hurricane Hugo by measurement, inference, and experience

Abstract An analysis of wind conditions in Hurricane Hugo as it crossed the states of North and South Carolina on September 21 and 22, 1989, is described. The types of surface measurements available are discussed. It is shown that exposure, height, fetch, and gust factors developed for extratropical cyclones can be used to justify the observed wind characteristics within 250km of the point of landfall. As the storm began to break up gust factors became much larger than standard design values. A wind-field model is used to determine wind conditions in areas where no surface measurements were made. Results from this model are compared with storm surge height and building performance in an area which experienced the worst conditions during the storm. The general performance of buildings, the applicability of wind design criteria, and wind-speed recurrence-intervals are discussed.

A comparison of pedestrian comfort criteria applied to a city center

Abstract Use is made of readily available meteorological data in conjunction with the results of a wind-tunnel study to determine the pedestrian level wind environment in a city center. A comparison is then made of the acceptability of the conditions based on four different pedestrian confort criteria. Considerable differences in interpretation of the data are shown to result from the different criteria and comments aremmade on the suitability of each.

The response of low-rise non-engineered structures to extreme wind conditions

Abstract In 1984 over US

A micro-computer-based expert-system for the design of wood-framed houses to resist wind loads

250 million worth of damage was caused by two severe storms in the states of North and South Carolina. In March a series of tornadoes caused the majority of the damage and killed 57 people. In September, Hurricane Diana came ashore near the border between the two states but due to a fortuitous set of meteorological and legislative conditions the damage was relatively minor. The tornadoes passed through an area where strong winds are relatively rare and building codes are generally poorly enforced or non-existent. Hurricane Diana struck an area which had previously experienced a number of severe hurricanes. As a consequence of this an extremely stringent and specific residential building code had been developed and enforced. This paper uses a survey of the damage in these two storms to assess the extreme wind performance of non-engineered structures, i.e. those that derive their structural form from tradition or legislation. It considers the weaknesses in traditional forms of timber and masonry construction, examines the advantages to be gained by appropriate legislation and makes recommendations for improving the wind-resistance of such structures.

The influence of structural performance on the distribution of wind loads on buildings

Abstract Extensive damage to wood-framed houses in severe wind storms has indicated shortcomings in the normal forms of construction of these buildings. Many of the damaged structures had been built in jurisdictions that use the CABO One and Two Family Dwelling Code. The expert system described in this paper uses as its basis the prescriptive One and Two Family Dwelling Code and the wind loading provisions of ANSI A58.1, 1982. Using inputs of location, external dimensions and roof pitch of the house, the program can be provide (a) a listing of the design wind-loads suitable for use by a professional designer and (b) a list of modifications to the One and Two Family Dwelling Code for use when the form of construction described in the code fails to provide sufficient resistance to calculated wind-loads. An example of the use of the program is given and the suitability of the One and Two Dwelling Code is discussed for various exposures and design wind-speeds.