Managing Extreme Wind (Hurricane) Losses
March 2004
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.
by Nathan
C. Gould, D.Sc., P.E., S.E. and Michael J. Griffin,
P.E.
ABS Consulting
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
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.
Figure 1
Figure 2
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.
Figure 3
Summary
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.
Mr. Griffin is a Technical Manager with ABS Consulting EQE Structural Engineers Division
located in St. Louis. He has more than 20 years professional experience in structural
engineering related to risk mitigation of natural hazards, with special expertise
in applying risk-based engineering methods to assess hazard vulnerability to
structures and nonstructural systems, equipment, and components subjected to
earthquake and extreme wind events. He has developed and designed cost-effective
solutions for mitigating earthquake and extreme wind risk for numerous public
and private clients throughout North America, the Caribbean, and Europe. He
has a BS in Civil Engineering and MS in Structural Mechanics, both from the
University of California, Irvine, and is licensed as a Professional Engineer
in California, South Carolina, Missouri, Ohio, and Tennessee.
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