Hurricanes, tornadoes, and high wind events, expose tens of millions of people to devastating property losses year after year, particularly those along the eastern seaboard and Gulf Coast states. It is vital to address these extreme wind hazards by understanding the historical loss exposure and developing a methodical engineered approach to effectively manage/reduce wind losses.
Historically, extreme wind events—hurricanes, tornadoes, and high wind events—expose tens of millions of people to devastating property losses year after year, particularly those along the eastern seaboard and Gulf Coast states living in the path of hurricanes. As the population center shifts from the north to the south, these losses can only be expected to continue to rise. Corporate risk managers, insurers, and reinsurers that manage this increasing risk are looking for alternative vehicles to reduce the losses to wind storms. This article discusses extreme wind hazards, historical loss exposure, and a methodical engineered approach in effectively managing and reducing wind losses to the built environment.
Need for a Proactive Wind Risk-Reduction Approach History continues to demonstrate the vulnerability of the U.S. infrastructure to extreme wind storms. The last several years have served as examples where tornadoes continue to invade the Midwest, and Hurricanes Hugo in 1989, Bob in 1991, Andrew and Iniki in 1992, Erin and Opal in 1995, and Bertha and Fran in 1996 underscore the vulnerability of the United States to severe windstorms—a vulnerability that continues to increase as populations continue to expand and develop in hurricane-prone coastal areas even in the face of these hazards. It is important to recognize—particularly in the cases of the hurricanes mentioned—that these were not the worst-case events, either in intensity or path. However, the vulnerability of buildings in many regions of the United States to these types of extreme wind events has been made evident by these storms. While earthquakes are often considered the most terrifying of natural disasters, windstorms, including hurricanes and tornadoes, actually produce greater annual losses than those of earthquakes. Data from the Property Claim Service, Jersey City, New Jersey (Table 1) shows that wind storms in total contributed to slightly greater than 50 percent of the total insured property damage dollar loss, adjusted to 2002 dollars, for the costliest catastrophes affecting the United States. Wind storms, tornadoes, and hurricanes kill more Americans and destroy more property than any other natural disaster in the United States.
Table 1. Costliest Catastrophes and Estimated Insured Loss
A Adjusted to 2002 dollars. (ISO Property Claim Services, Jersey City, NJ)
The high percentage of damage caused by windstorms may also be attributed to the slow evolution and adoption of more stringent wind load provisions in model building codes, and the division of funding support for natural hazards research. Model building codes are just now beginning to be revised to address key parameters in building design and construction important to favorable performance during extreme wind storm events. Until recently (1992 Hurricane Andrew) there has not been a significant revision to model building code wind load provisions since the mid-1970s. Hurricane Andrew sparked a flurry of code revisions, particularly in the then adopted South Florida Building Code (SFBC) where new code provisions were adopted in the 1994 Edition to address the resistance of building external cladding and glazing systems to wind-borne debris impacts (see Figures 1 and 2). The Southern Building Code also instituted similar provisions; however, mandatory compliance is not required at this time.
Exterior Cladding Damage From Wind-Borne Debris, Hurricane Andrew
Same Building as in Figure 1 Showing Interior Damage Resulting From a Breach in the Exterior Cladding
Clearly, it is in the interest of all parties—insurers, reinsurers, risk managers, the engineering community, and ultimately property owners—to find ways to effectively mitigate the impact of extreme wind storm hazards.
Wind Mitigation Tools
With historical lessons and the advance of technology, the reduction of wind risk can be effectively achieved through a brute force engineered approach for new building construction going forward. Very recent adoption of new wind load design provisions in the building codes and advances in computer technology allow engineers to design more wind-resistant building structures. New building code wind design provisions and computational fluid dynamics programs are discussed as means to mitigate wind risk to the built environment.
Building Codes and Wind Design Provisions. The minimum requirements for structural design of buildings are typically set by locally adopted and enforced building codes. In the past, most building codes in use in the United States were based on one of three model-building codes: the National Building Code (NBC), Standard Building Code (SBC), and Uniform Building Code (UBC). However, with the development of the 2000 International Building Code by the International Code Council, (and the recently released 2003 Edition), the three model codes are being replaced by the single IBC code as state and municipal jurisdictions move to adopt the code. These codes are developed and published by industry associations representing building officials.
Florida Building Code 2001 Wind Load Design Provisions. The State of Florida has recently developed its own model building code—Florida Building Code 2001, mandated as the statewide design code. The Florida Building Code 2001 is modeled after the structural provisions of the Standard Building Code 1997 Edition, and the structural requirements of the South Florida Building Code for regions that fall within the "High Velocity Hurricane Zone."
The 2001 Florida Building Code invokes the provisions of ASCE 7 "Minimum Design Loads for Buildings and Other Structures" for the calculation of wind loads and pressures for the main wind force resisting system, and components and cladding systems. The current edition of ASCE 7 is the ASCE 7-02. The ASCE 7 Standard has only recently begun to reflect the state-of-the-art in wind design. Wind load provisions really began with the ANSI A58.1-1982 Standard. The conversion of the ANSI Standard to an ASCE Standard occurred in the late 1980s with the issuance of ASCE 7-88. There were essentially no technical changes in the wind load provisions from the ANSI Standard to the ASCE 7-88 version, just a conversion to the ASCE format. Major changes in wind design criteria occurred with the ASCE 7-95 and ASCE 7-98 Standards. Both reflect significant changes in the wind design provisions between the ANSI standard provisions essentially used up to the mid-1990s.
The current Edition of ASCE 7 is greatly expanded to reflect the latest in the field of wind engineering. The most notable change is in the basic wind speed map where wind velocities are reported as 3-second gust instead of fastest-mile. The new wind map was also developed using a greater body of recorded data than previously available. Additional refinement has also been incorporated into the determination of hurricane force winds at the immediate coastal sections and inland areas, as well as the calculation of wind pressures for components and cladding for buildings less than 60 feet in height, due to new wind data and recent wind tunnel research, respectively.
Wind-Borne Debris Impact. Although wind-borne debris has been identified as a principal initiator of building architectural and structural damage, building codes have only recently adopted wind-borne debris impact standards.
In the aftermath of Hurricane Andrew, the Dade County Commission adopted debris impact test requirements for roof and cladding systems. Up to this time there were no specific debris impact design or test requirements in the South Florida Building Code. These requirements were incorporated into the 1994 Edition of the South Florida Building Code effective September 1, 1994. The debris criteria is applicable to roofing materials, wall cladding, outside doors, skylights, glazing, glass block, shutters and other external protection devices. Test projectiles include a small missile (2 gm at 80 ft./sec.) impact for cladding and components and cladding used in the building above 30 ft. in height, and a large missile (9 lbs, 2x4 at 50 ft./sec.) for components and cladding less between grade and 30 feet. Cyclic wind pressure loading following either the small or large missile tests are contained in three Dade County Protocols—PA 201-94, PA 202-94, and PA 203-94. These same criteria have been adopted by the new Florida Building Code 2001 previously discussed.
New Wind Design Tool: Computational Fluid Dynamics to Model Wind Flow. With the advent of advanced computational methods, such as Computational Fluid Dynamics (CFD), wind flow patterns can be modeled around both proposed and existing structures. These simulations, which often replaced costly wind tunnel testing, are used to quantify the wind patterns flowing through or around a building or site. Figure 3 below shows an example of the flow patterns around a structure using CFD modeling. The CFD simulations can account for building configurations and spatial effects of shielding from nearby structures, similar to the results obtained from wind tunnel testing. Quantification of the wind flow patterns enable designers to determine the forces on the main structural system as well as the demands on the individual cladding systems. In addition, CFD models are often used to determine the downwind impacts on nearby buildings, particularly effects associated with odors, air intake exposures, and visual impairments.
Example of Wind Flow Around a Building Using CFD Modeling
As history has demonstrated, damage losses to wind storms greatly exceed the losses from all other catastrophes. Hurricanes and high wind storms are among the most destructive natural disasters that regularly affect the United States. As people migrate south and the built environment grows in the coastal areas subject to hurricanes, greater property losses and human suffering can only be expected. It therefore becomes incumbent upon the building owner, risk manager, and insurer to understand the wind risk in order to effectively manage that risk. The importance of properly designing, constructing, and enforcing wind-resistant building construction in order to reduce future losses due to extreme windstorm events cannot be overstated.
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