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Approaches to Reducing Wind-Induced Damage in the United States
Copyright © IF AC Control in Natural IFAC Natural Disasters, Tokyo, Japan, 1998
APPROACHES TO REDUCING WIND-INDUCED DAMAGE IN THE UNITED STATES Timothy A. Reinhold 1
1
Associate Professor, Department of Civil Engineering, Clemson University, Clemson, South Carolina, USA
Abstract Abstract:: Dramatic losses from Hurricanes Hugo and Andrew demonstrated that low-rise buildings erected using conventional US construction techniques are extremely vulnerable wlnerable to damage in severe ,"ind "'ind events. events. These losses continue to spur the development of programs and efforts aimed at reducing wind-induced damage in the United States. States. The conclusions drawn regarding reasons for the damage and losses and the types of programs developed have varied depending on the groups assessing the damage, their perspectives and their biases. biases. Many of the programs initiated in recent years are focusing on community awareness and developing incentives for mitigation. mitigation. In most cases, the needs for developing the technological underpinnings of effective mitigation are being ignored. There is a clear need for a coordinated national effort to develop effective strategies for increasing public commitment to implementing mitigation, awareness of hazards, initiating and fostering a conunitment and the development of effective mitigation technologies. technologies. This paper provides a brief description of current initiatives and provides an overview of research progress and research needs for development of effective mitigation strategies. 1FA C strategies. Copyright @ 1998 19981FA Keywords Keywords:: Wind, damage, prevention, risk, structural properties
1.
INrRODUCTION INfRODUCTION
vulnerability have revealed numerous other areas of wlnerability poor as well as frequent building code violations and JX>Or construction. quality construction.
Six years after Hurricane Andrew ravaged South Florida, and nine years after Hurricane Hugo hit South Carolina, the US public and private sectors are still struggling to come to grips with the problems eX1X>sed by these storms. storms. For example, historical fastener schedules used for connecting roof sheathing to rafters or trusses have proven to be woefully inadequate for the uplift loads associated with severe ,"indstorms. windstorms. Toenailed connections between the roof rafters or trusses and walls have also been shown to be inadequate in all but the most mild wind climates. climates. Unprotected glass in windows and doors are easily broken by wind \"ind borne debris. debris. Thus, most existing low-rise structures contain at least three critical areas of \ulnerability, vulnerability, which are easily exploited during severe windstorms. Beyond weaknesses, damage investigations these omious weaknesses,
Often, after an event such as Hurricane Hugo or Hurricane Andrew, initial responses focus on increasing the design wind speeds, improving the local building codes and standards and improving building code enforcement. Within the past couple of years, a number of groups have focused on fostering community based activities which seek to -motivate communities to identify and address the local hazards. hazards. Two such initiatives are the Federal Emergency Management Agency's (FEMA) Project Impact and the Institute for Business and Home Safety's (IBHS) Showcase Communities. Communities. These activities and programs are intended to build public support, and perhaps demand, for reducing risks and wlnerabilities. vulnerabilities. 135
It is generally accepted that low-rise residential and commercial structures are the greatest area of windstorms. Approaches and vulnerability in severe windstorms. efforts to address these vulnerabilities and improve the wind resistance of low-rise buildings can be categorized as follows:
• • • • •
not have any immediate effect on the construction of the most vulnerable structures, the homes and light standards. commercial structures built to prescriptive standards. Wind speeds reported during a hurricane strike and repeated in the media after the storm are notoriously poor indications of the magnitude of the winds at location. Frequently, the effects of any partiCular location. anemometer heights, terrain roughness and averaging times are not reported and the public becomes convinced that they experienced wind speeds well above the values supported by careful analysis, and well above the values used in design. engineering design.
Increasing design wind speeds Improving building codes and enforcement Fostering community organization and action. Demonstration projects and technology transfer Developing engineering solutions
This paper provides an overview of these efforts and describes the pros and cons of the various Ultimately, a coherent, coordinated approaches. national program, which encompasses a number of these approaches, is needed if the US is going to in reducing its make measurable progress ill windstorms. Engineering vulnerability to severe ·windstorms. research and development must be an integral part of that program if it is going to be successful.
The engineering community has struggled for a long time with the debates generated by differences between forecast, forecast, reported, and measured wind speeds in hurricanes and other severe "'ind wind storms. storms. Some of the differences can be attributed to the different reference wind speeds used by the engineering and meteorological communities in the US. The engineering community in the US has used US. a fastest-mile wind speed, which has a variable averaging time as its reference standard, while the meteorological community has reported gust or one minute sustained wind speeds. In 1995, the ASCE 7 standard abandoned the fastest-mile wind speed in and adopted a peak gust wind speed at 10 meters in open terrain as the standard reference wind (ASCE 1995).
2. INCREASING DESIGN WIND SPEEDS
On several occasions, state legislatures have arbitrarily increased the design wind speeds for their states as an initial response to a large and costly windstorm disaster. While this is understandable, given the high wind speeds normally reported by the media and subsequently associated with a particular storm, it is a political response rather than an engineering response. In nearly every case, the wind speed increase is quietly reversed after the initial furor has died down.
Nevertheless, wind speeds have continued to be a point of contention in recent storms including Hurricanes Opal, Fran and Bonnie. Bonnie. Doppler radar and drop-sonde measurements of upper level wind speeds have indicated high wind speeds at heights of 1 to 3 km. km. Meteorologists have been concerned that strong convective cells could bring these wind speeds down to ground level and have tended to make conservative forecasts of ground level wind speeds. The engineering community also has to deal with the effects of changing technology and the possibilities of spurious wind speed gust measurements. The US Air Force has installed a new generation of hot wire anemometers at most of its bases. bases. These instruments have provided a consistent pattern of questionable readings in Hurricanes Opal and Fran and in Typhoon Paka. Preliminary indications are that if too much rain collects on the sensor it boils off causing the instrument to indicate a very high gust speed.
In the US, the main problem is that low-rise residential construction is governed by prescriptive building code requirements, which specify how a building can be built so that it is "deemed to comply" with the more general engineering performance requirements. Thus, increasing the design wind requirements. speeds will not immediately affect house designs unless the prescriptive requirements are changed accordingly. Furthermore, there have historically been significant gaps between even the best prescriptive requirements and engineering design based on the latest codes and standards. Fastener schedules for roof sheathing attachment and roof to wall connections are two examples where the capacities developed by prescriptive requirements do not match the loads calculated from a standard such as the American Society of Civil Engineers ASCE 7-93 or ASCE 795(ASCE 1993 and 1995). Thus, arbitrarily increasing the design wind speeds will \\'ind \\'ill penalize structures which are designed by engineers but will
3. IMPROVING BUILDING CODES AND 3. ENFORCEMENT Wide spread reports of building code violations and poor code enforcement in South Florida produced several interesting debates and results. The 136
homebuilders used the code violations and poor enforcement as a basis for arguing that the damage was due to the deficient construction. Consequently, they suggested that if the construction had met the code, the damage would have been avoided and they have fought any attempts to strengthen the building codes. The insurance industry also focused on the building code enforcement issue as a means of ensuring that future construction would perform adequately. They have sought to institutionalize this by establishing a building department rating system with the ex-pectation e","pectation that future insurance rates for a community "ill will depend on that rating. rating.
requirements. In some cases, the overall forces of requirements. the wind on the roof system could be more than doubled by the failure of a large window. The experience of Hurricane Andrew did lead to changes in the South Florida Building Code. Code. However, a number of the changes were later overturned as the measures were debated more fully and greater political pressure was brought to bear on the process. One measure, which has remained., remained, is the requirement for window protection on all new construction. At present, Dade, Broward and Palm Beach Counties are the only counties in the US with requirements for window protection. protection. The SBCCI wind loads committee has consistently tried., tried, without \\ithout success, to introduce similar measures in the 1997b). The Standard Building Code (SBCCI 1997b). closest that the committee has been able to come to getting the SBCCI to address this issue is to publish appendix. However, the a test standard in an appendix. governing body has not yet allowed any reference to the appendix to be included in the body of the code. code.
This approach to reducing wind-induced losses is limited to new construction. construction. Consequently, it will likely affect only a small fraction of the building stock in a particular community for a number of years. Furthermore, it assumes that close adherence to the code will in fact produce buildings which are less vulnerable to \\'ind-induced wind-induced damage and losses. Historically, building codes in the US have focused on life safety issues. issues. Consequently, most provisions and requirements focus on structural components systems. Significantly less attention is paid to and systems. wall and roof coverings, coverings, items which are considered envelope. It is only after to be part of the building envelope. losses mount in a severe wind \\'ind or earthquake event, that calls are raised for provisions that also target the losses. reduction of economic losses.
The Building Department Rating System has produced positive results. results. Some local jurisdictions, which had not adopted one of the model codes, are . moving in the direction of adopting one and some states are finally establishing a statewide building code. Thus, the anticipation of higher insurance code. premiums for communities with poor ratings seems to be producing results. Others have taken advantage of subsidized training of inspectors as a means of trying to improve their inspection process and their ratings.
While the reduction of construction flaws will result in better construction, construction, there are still questions about the adequacy of US construction for events that meet or exceed design levels. levels. First, much of the damage and loss is associated with \\'ith breaching of the building envelope which includes the roof covering, roof sheathing, sheathing, v.indows, windows, doors, doors. and siding. siding. Most of these elements are selected from catalogs and typically are not chosen with the design wind v.ind event in mind. Second, Second., the prescriptive requirements are based on a certain set of assumptions which may not be fully realized for a particular site or building.
At present, the building code rating system does not differentiate between different model building codes. An implicit assumption is that any of the three major model building codes will provide comparable levels of protection. However, when wind provisions are considered, considered., there are significant differences between both the performance and prescriptive requirements.
For example, one of the best sets of prescriptive requirements available in the US for wind \\ind resistant construction of residential buildings is the Southern Building Code Congress International SS-TD-lO (SBCCI 1997a). However, the loads used in developing these prO\.isions prOvisions most closely match ASCE 7 loads for suburban areas and assume that the building remains closed (no \\indows or doors fail). fail) . At present. the primary jurisdictions using these provisions are on barrier islands where the suburban assumption is not valid. Also, the code does not include any direct requirement that window protection be installed. installed. Thus. Thus. \\indow \\indmv failure from \\ind borne debris could invalidate the load wind assumptions used to develop the prescriptive
COMMUNITY ORGANIZATION 4. FOSTERING COlv1MUNITY AND ACTION Two of the most recent national initiatives aimed at mitigating the effects of natural disasters have been the Federal Emergency Management Agency's Project Impact and the Institute for Business and Home Safety's Showcase Communities. These are model programs aimed at developing community awareness of the local risks associated with potential hazards. The initiatives seek to bring natural hazards. community leaders from the public and private sectors together to identify the local risks and devise plans for mitigation efforts.
137
The goal is to develop local ownership of the assessment and programs in hopes that this will efforts. There are result in more effective mitigation efforts. a number of requirements that must be met for a local jurisdiction to become one of these model commuruues. communities. The requirements range from adoption and enforcement of one of the model building codes to local promotion of the program. In andlor IBHS have also committed most cases, FEMA and/or funds towards projects aimed at reducing the vulnerability of essential facilities or infrastructure. infrastructure. One part of the initiatives involves raising public hazards. awareness of the risks from natural hazards. Another is fostering development of incentives for individuals and businesses to take action to reduce vulnerability. Incentives under consideration their vulnerability. include reductions in insurance premiums, reductions in deductibles, reductions in mortgage interest rates or points, reductions or waiving of taxes for permits related to mitigation efforts, reductions in rea1-estate sales fees, fees , and moratoriums on increases in tax appraisals for the value of the mitigation efforts. As might be e>.:pected, little progress has been made in implementing these incentives, but at least the parties are talking. talking.
paper, a new rate structure which reflects the perceived risks, without regard to regulation, and the perceived benefits of various mitigation measures such as shutters or hurricane straps. While these models are based on engineering analysis and expert opinion, there is little hard data available for validation. In the end most of the modelers are forced to smear all of the elements together in order to compare loss predictions with accumulated data for selected storms. storms. Usually, the loss data is only available on a postal code basis and there is no data for evaluation of individual components within the models. models. A significant focus of the FEMA and IBHS initiatives lruuauves is on improving the ability of a community to prepare for and recover from an event. However, from an engineering perspective, both the Project Impact and Showcase Community initiatives are very weak when it comes to specific recommendations for the retrofit of existing structures. The exception is the engineering analysis and design associated with reducing the vulnerability of the essential facilities and infrastructure targeted for specific mitigation efforts. efforts. When it comes to dealing with homes or low-rise commercial and light industrial buildings, the literature typically contains statements like, "install window protection" or "make sure your roof sheathing is properly attached" and refers the owner to the building code or the local building department.
Raising public awareness and the development of incentives are designed to increase the likelihood that mitigation efforts will in fact be implemented. implemented. However, However, raising ralSlng public awareness without providing guidance on effective mitigation measures shell. One of the could turn out to be a hollow shell. problems with offering incentives is convincing the various parties that there is a benefit to them other than altruism for providing the incentives. incentives.
These initiatives are still in the early stages of implementation and it will be some time before their effectiveness can be properly assessed. assessed.
Within the insurance community, it has been difficult to develop effective incentives because premiums in areas at risk are too low according to incentives. their analysis to allow more than token incentives. Furthermore, insurance commissioners are requiring that the industry provide actuarially based rates and incentives. The industry has not been collecting incentives. sufficiently detailed information on their building stock and the losses to support analysis of the potential benefits of proposed mitigation efforts. efforts. Similarly, the construction and engineering communities do not posses the testing facilities to allow scientific evaluation of potential mitigation measures. Finally, while post disaster investigations measures. for wind events have been conducted for more than 25 years, the observations tend to be anecdotal rather than statistical and the winds associated with the debate . event are almost always subject to debate.
5. TECHNOLOGY TRANSFER AND 5.
DEMONSTRATION PROJECTS A frequent theme of presentations on reducing the vulnerability of the built environment in the US is that the solutions exist. The critical task, according to the presenters, is simply to implement what is already known. This argument implies that there is a consensus on how to strengthen buildings and that asSUTarlce that these consensus there is some level of assurance ideas will in fact reduce the vulnerability of buildings and structures to both structural damage and economic losses. All that one has to do is attend building code committee meetings or try to develop specific recommendations for reducing the vulnerability of a particular building to know that there is no such consensus within the building community. As noted in the preceding section, the community. lack of systematic statistical evidence on the performance of building features in windstorms is at the heart of the difficulty in developing incentives for mitigation. mitigation.
Since databases on performance and losses do not exist, the insurance industry is turning to modeling as an alternative. There are efforts currently under way in the state of Florida to develop, at least on 138
The state of Florida has recently initiated two projects, which are unique in their approach to tackling the issues involved in developing effective mitigation. One is the Residential Coastal mitigation. Mitigation Program (RCMP) and the other is a residential monitoring project.
will be selected at various locations around the coast of Florida, evaluated using RCMP procedures and retrofitted with an agreement to allow installation of wind pressure sensors and anemometers. anemometers. The plan is to retrofit and pre-wire about twenty homes in the initial phase of the project. These homes would be located every ten to fifteen miles along the coast in strikes. the areas with the greatest risk of hurricane strikes.
The RCMP uses a fraction of the funds in the Florida Wind Pool to initiate retrofit projects in the state. state. The long-term objective is to depopulate the Florida Wind Pool, which provides coverage for homeowners who have been unable to obtain carriers. In the first phase of insurance from private carriers. project, the RCMP is pro\iding assistance for the project, retrofitting 600 houses in Dade, Broward and Palm Beach Counties of South Florida. Letters were sent to selected homeowners in these counties offering them the opportunity to obtain up to $10,000 in a forgive-able loan for retrofitting their homes. If the homeowner elects to participate, an inspection is carried out of the home and the results are analyzed to determine weaknesses and to evaluate the cost effectiveness of various retrofits. retrofits. If the retrofits are found to be cost effective and less than $10,000, the program will pay for the retrofits and forgive the loan if the homeowner remains in the house for five years. years. If the cost is more than $10,000, the homeowner can still participate provided they pay the excess costs. costs.
When a storm is forecast for one of the coastal segments, where pre-wired homes are located, computers and sensors would be installed and activated The instrumentation system Vl-ill \\oill contain two low cost anemometers capable of surviving gust speeds of 70 mls m/s and twenty-eight pressure sensors. In order to avoid drilling holes in the homes and/or running reference tubing over the surface of the used. These house, absolute pressure sensors will be used. are solid state units that have been conditioned to +2.5 provide a resolution of 0.01 kPa over a range of +2.5 pressure. to -15 kPa relative to ambient atmospheric pressure. In addition to the inexpensive anemometers at the homes, mobile trailers equipped with instrumentation quality R M Young anemometers capable of surviving 70 to 90 mls m/s wind gusts will be deployed within the final 24 hours of landfall. This project offers the potential for collecting high events. fidelity wind and pressure data in hurricane events. The project builds on technology developed at Clemson University in projects sponsored by the South Carolina Sea Grant Consortium and the Idaho National Engineering and Environmental Laboratory. The contractors involved in the Laboratory. retrofitting the houses will carry out pre-wiring and installation brackets. of sensor mounting brackets. Deployment of the instrumentation will be accomplished by Clemson University and a consortium of Florida Universities. Universities. It is anticipated that deployment of the home monitoring systems will require two to three hours per house and that the trailers can be set up in about half an hour.
To date, county building inspectors using predefined inspection forms has carried out the inspections in counties. The results of the inspections these three counties. are being analyzed by Applied Research Associates Raleigh, North Carolina. in Raleigh, Carolina. Potential retrofits being considered are limited to options accepted by the local code authority. authority. This has limited some of the possible retrofit considerations. considerations. The program is providing systematic information on buildings in these counties, which may prove useful for subsequent analyses of typical building stock. The limitation is that all of the properties belong to the Florida Wind Pool and as such may not be an accurate statistical sample of the total population of buildings. buildings. The retrofits will prOvide accurate estimates of costs for the various measures implemented. implemented. Here the limitation is on the types of measures allowed due to code restrictions. restrictions. Once the retrofits are completed there will be a reasonably large number of properties, which will provide valuable data on performance of the retrofit ne:-.1 storm strikes the area. area. measures when the ne:o.1 ...ill ex1end e:o.1end the program to other areas Latter phases ,\...ill \\oill be broader in future of the state so the coverage \\ill years. years.
The measurement program offers the potential for obtaining reliable high fidelity information on the rnagnitudes and characteristics ground level winds magnitudes and wind pressures on low-rise buildings in hurricanes. The trailers will provide prOvide high frequency hurricanes. wind data at five and ten meter heights. The pressure data from the buildings will serve a number of purposes. "\\ind purposes. First, it is needed for validation of "ind tunnel simulations of hurricane wind events. events. Second, Second., it can be used to improve or validate loads facilities. produced in component and system test facilities. Third, to a lesser e:o.1ent, e:-.1ent, the data will also serve as a Third., basis for evaluating code provisions and for investigating the effects of building features on wind loads. loads.
The monitoring project was conceived as a supplement to the RCMP. RCMP. In this program, houses 139
6. DEVELOPING ENGINEERING SOLUTIONS
load/deformation characteristics of the connections between components and systems are poorly understood. understood
Ultimately, the reduction in losses and damage from severe windstorms requires retrofitting of existing structures and modification of conventional US. Changes in construction techniques used in the US. the ways that individuals, businesses, communities and governments prepare for and respond to natural hazards can certainly reduce the human suffering and collateral losses associated with business interruption and disruption of essential services. However, if homes and buildings continue to suffer substantial damage in wind events that are generally at or below the design level, significant losses and disruption of lives and community function will be unavoidable. unavoidable.
Despite the large losses sustained in severe windstorms, US spending on wind engineering small. A frequent reaction research has been quite small. of casual observers is that simply installing a few problems. In some more fasteners can solve the problems. cases, certain failure modes could indeed be connections. prevented by simple changes in critical connections. However, this may result in a modest increase in overall performance, as another mode of failure becomes the trigger. trigger. In other cases, attempts to use limited data on loads and capacities can result in unreasonable construction requirements for some connections while ignoring other critical elements.
Attempts to change the way homes and businesses are constructed in the US, US, so that they are more wind resistant, face numerous social, political, technical problems. At the core of the problem and scientific problems. is the fact that the wind engineering community does not have facilities for full-scale wind testing which are equivalent to the shake tables used in earthquake engineering. Most typical building connections and engineering. systems composed of common building materials can not be scaled down for study in existing wind tunnel facilities. Furthermore, problems associated with v<.ith facilities. water penetration must be studied at full-scale.
The lack of technical and scientific certainty in the understanding of wind loads and the resistance of structures to wind have clearly affected the development of wind provisions in US building codes. When an adequate technical basis is not codes. available, the debate can quickly degenerate into the interests. Similarly, when analysis politics of special interests. and observations suggest that changes are warranted, it is easy to discount the recommendations because they are rarely based on adequate statistical evidence costlbenefit analyses. or unassailable cost/benefit
Existing methods and requirements for evaluating components, connections, connections, the performance of components, v<.ind loads structural systems and buildings under \\ind effects, including wind driven rain, are and wind effects, very elementary or are non-existent in the US. Unlike Japan, there are no formal requirements and test procedures for water penetration. penetration. Furthermore, many of the structural testing methods and requirements are limited to unidirectional quasiconnections. static loading of components and connections. These methods are not representative of the complex load combinations and load effects that typically ""inds. occur when a building is subjected to high ""inds.
7. 7. CONCLUSIONS Record losses in recent hurricanes have produced a variety of responses aimed at reducing losses from future storms. There is a clear need to integrate these efforts into a national program that addresses issues. It must promote local involvement three key issues. in mitigation efforts, develop meaningful incentives to motivate retrofitting of existing structures and support the development of comprehensive, technically sound, and cost effective solutions for new and existing buildings. buildings.
Analysis of the resistance of low-rise buildings to wind loads and wind effects is also fraught ""ith problems. problems. First, adequate quantification of wind loads and the variation of these loads in time and space are only now becoming available. available. Second, since the building's skin or cladding frequently plays an important role in the stiffness of the building, models need to be created which adequately account for the contribution of the skin to the structural resistance. Third, low-rise buildings are normally resistance. composed of a variety of materials and structural forms. Their load/deformation characteristics are understood; so frequently, frequently, they are not well understood; combined in ways that do not adequately account for their relative flexabilities. Furthermore, flexabilities. Furthermore, the
REFERENCES American Society of Civil Engineers (1993). Minimum Design Loads for Buildings and Structures, 7-93 , ASCE, New York, NY. NY. Structures, ASCE 7-93, American Society of Civil Engineers (1995). Minimum Design Loads for Buildings and Structures, ASCE, New York, NY. NY. Structures, ASCE 7-95, ASCE, Southern Building Code Congress International (1 997a). Guidelines for Wind Resistant (1997a). Construct., SS ID 10, SBCCI, Birmingham, AL. AL. Construct., Southern Building Code Congress International (1997b). Code , SBCCI, (1997b). Standard Building Code, Birmingham, AL. AL. 140
APPROACHES TO REDUCING WIND-INDUCED DAMAGE IN THE UNITED STATES Timothy A. Reinhold 1
1
Associate Professor, Department of Civil Engineering, Clemson University, Clemson, South Carolina, USA
Abstract Abstract:: Dramatic losses from Hurricanes Hugo and Andrew demonstrated that low-rise buildings erected using conventional US construction techniques are extremely vulnerable wlnerable to damage in severe ,"ind "'ind events. events. These losses continue to spur the development of programs and efforts aimed at reducing wind-induced damage in the United States. States. The conclusions drawn regarding reasons for the damage and losses and the types of programs developed have varied depending on the groups assessing the damage, their perspectives and their biases. biases. Many of the programs initiated in recent years are focusing on community awareness and developing incentives for mitigation. mitigation. In most cases, the needs for developing the technological underpinnings of effective mitigation are being ignored. There is a clear need for a coordinated national effort to develop effective strategies for increasing public commitment to implementing mitigation, awareness of hazards, initiating and fostering a conunitment and the development of effective mitigation technologies. technologies. This paper provides a brief description of current initiatives and provides an overview of research progress and research needs for development of effective mitigation strategies. 1FA C strategies. Copyright @ 1998 19981FA Keywords Keywords:: Wind, damage, prevention, risk, structural properties
1.
INrRODUCTION INfRODUCTION
vulnerability have revealed numerous other areas of wlnerability poor as well as frequent building code violations and JX>Or construction. quality construction.
Six years after Hurricane Andrew ravaged South Florida, and nine years after Hurricane Hugo hit South Carolina, the US public and private sectors are still struggling to come to grips with the problems eX1X>sed by these storms. storms. For example, historical fastener schedules used for connecting roof sheathing to rafters or trusses have proven to be woefully inadequate for the uplift loads associated with severe ,"indstorms. windstorms. Toenailed connections between the roof rafters or trusses and walls have also been shown to be inadequate in all but the most mild wind climates. climates. Unprotected glass in windows and doors are easily broken by wind \"ind borne debris. debris. Thus, most existing low-rise structures contain at least three critical areas of \ulnerability, vulnerability, which are easily exploited during severe windstorms. Beyond weaknesses, damage investigations these omious weaknesses,
Often, after an event such as Hurricane Hugo or Hurricane Andrew, initial responses focus on increasing the design wind speeds, improving the local building codes and standards and improving building code enforcement. Within the past couple of years, a number of groups have focused on fostering community based activities which seek to -motivate communities to identify and address the local hazards. hazards. Two such initiatives are the Federal Emergency Management Agency's (FEMA) Project Impact and the Institute for Business and Home Safety's (IBHS) Showcase Communities. Communities. These activities and programs are intended to build public support, and perhaps demand, for reducing risks and wlnerabilities. vulnerabilities. 135
It is generally accepted that low-rise residential and commercial structures are the greatest area of windstorms. Approaches and vulnerability in severe windstorms. efforts to address these vulnerabilities and improve the wind resistance of low-rise buildings can be categorized as follows:
• • • • •
not have any immediate effect on the construction of the most vulnerable structures, the homes and light standards. commercial structures built to prescriptive standards. Wind speeds reported during a hurricane strike and repeated in the media after the storm are notoriously poor indications of the magnitude of the winds at location. Frequently, the effects of any partiCular location. anemometer heights, terrain roughness and averaging times are not reported and the public becomes convinced that they experienced wind speeds well above the values supported by careful analysis, and well above the values used in design. engineering design.
Increasing design wind speeds Improving building codes and enforcement Fostering community organization and action. Demonstration projects and technology transfer Developing engineering solutions
This paper provides an overview of these efforts and describes the pros and cons of the various Ultimately, a coherent, coordinated approaches. national program, which encompasses a number of these approaches, is needed if the US is going to in reducing its make measurable progress ill windstorms. Engineering vulnerability to severe ·windstorms. research and development must be an integral part of that program if it is going to be successful.
The engineering community has struggled for a long time with the debates generated by differences between forecast, forecast, reported, and measured wind speeds in hurricanes and other severe "'ind wind storms. storms. Some of the differences can be attributed to the different reference wind speeds used by the engineering and meteorological communities in the US. The engineering community in the US has used US. a fastest-mile wind speed, which has a variable averaging time as its reference standard, while the meteorological community has reported gust or one minute sustained wind speeds. In 1995, the ASCE 7 standard abandoned the fastest-mile wind speed in and adopted a peak gust wind speed at 10 meters in open terrain as the standard reference wind (ASCE 1995).
2. INCREASING DESIGN WIND SPEEDS
On several occasions, state legislatures have arbitrarily increased the design wind speeds for their states as an initial response to a large and costly windstorm disaster. While this is understandable, given the high wind speeds normally reported by the media and subsequently associated with a particular storm, it is a political response rather than an engineering response. In nearly every case, the wind speed increase is quietly reversed after the initial furor has died down.
Nevertheless, wind speeds have continued to be a point of contention in recent storms including Hurricanes Opal, Fran and Bonnie. Bonnie. Doppler radar and drop-sonde measurements of upper level wind speeds have indicated high wind speeds at heights of 1 to 3 km. km. Meteorologists have been concerned that strong convective cells could bring these wind speeds down to ground level and have tended to make conservative forecasts of ground level wind speeds. The engineering community also has to deal with the effects of changing technology and the possibilities of spurious wind speed gust measurements. The US Air Force has installed a new generation of hot wire anemometers at most of its bases. bases. These instruments have provided a consistent pattern of questionable readings in Hurricanes Opal and Fran and in Typhoon Paka. Preliminary indications are that if too much rain collects on the sensor it boils off causing the instrument to indicate a very high gust speed.
In the US, the main problem is that low-rise residential construction is governed by prescriptive building code requirements, which specify how a building can be built so that it is "deemed to comply" with the more general engineering performance requirements. Thus, increasing the design wind requirements. speeds will not immediately affect house designs unless the prescriptive requirements are changed accordingly. Furthermore, there have historically been significant gaps between even the best prescriptive requirements and engineering design based on the latest codes and standards. Fastener schedules for roof sheathing attachment and roof to wall connections are two examples where the capacities developed by prescriptive requirements do not match the loads calculated from a standard such as the American Society of Civil Engineers ASCE 7-93 or ASCE 795(ASCE 1993 and 1995). Thus, arbitrarily increasing the design wind speeds will \\'ind \\'ill penalize structures which are designed by engineers but will
3. IMPROVING BUILDING CODES AND 3. ENFORCEMENT Wide spread reports of building code violations and poor code enforcement in South Florida produced several interesting debates and results. The 136
homebuilders used the code violations and poor enforcement as a basis for arguing that the damage was due to the deficient construction. Consequently, they suggested that if the construction had met the code, the damage would have been avoided and they have fought any attempts to strengthen the building codes. The insurance industry also focused on the building code enforcement issue as a means of ensuring that future construction would perform adequately. They have sought to institutionalize this by establishing a building department rating system with the ex-pectation e","pectation that future insurance rates for a community "ill will depend on that rating. rating.
requirements. In some cases, the overall forces of requirements. the wind on the roof system could be more than doubled by the failure of a large window. The experience of Hurricane Andrew did lead to changes in the South Florida Building Code. Code. However, a number of the changes were later overturned as the measures were debated more fully and greater political pressure was brought to bear on the process. One measure, which has remained., remained, is the requirement for window protection on all new construction. At present, Dade, Broward and Palm Beach Counties are the only counties in the US with requirements for window protection. protection. The SBCCI wind loads committee has consistently tried., tried, without \\ithout success, to introduce similar measures in the 1997b). The Standard Building Code (SBCCI 1997b). closest that the committee has been able to come to getting the SBCCI to address this issue is to publish appendix. However, the a test standard in an appendix. governing body has not yet allowed any reference to the appendix to be included in the body of the code. code.
This approach to reducing wind-induced losses is limited to new construction. construction. Consequently, it will likely affect only a small fraction of the building stock in a particular community for a number of years. Furthermore, it assumes that close adherence to the code will in fact produce buildings which are less vulnerable to \\'ind-induced wind-induced damage and losses. Historically, building codes in the US have focused on life safety issues. issues. Consequently, most provisions and requirements focus on structural components systems. Significantly less attention is paid to and systems. wall and roof coverings, coverings, items which are considered envelope. It is only after to be part of the building envelope. losses mount in a severe wind \\'ind or earthquake event, that calls are raised for provisions that also target the losses. reduction of economic losses.
The Building Department Rating System has produced positive results. results. Some local jurisdictions, which had not adopted one of the model codes, are . moving in the direction of adopting one and some states are finally establishing a statewide building code. Thus, the anticipation of higher insurance code. premiums for communities with poor ratings seems to be producing results. Others have taken advantage of subsidized training of inspectors as a means of trying to improve their inspection process and their ratings.
While the reduction of construction flaws will result in better construction, construction, there are still questions about the adequacy of US construction for events that meet or exceed design levels. levels. First, much of the damage and loss is associated with \\'ith breaching of the building envelope which includes the roof covering, roof sheathing, sheathing, v.indows, windows, doors, doors. and siding. siding. Most of these elements are selected from catalogs and typically are not chosen with the design wind v.ind event in mind. Second, Second., the prescriptive requirements are based on a certain set of assumptions which may not be fully realized for a particular site or building.
At present, the building code rating system does not differentiate between different model building codes. An implicit assumption is that any of the three major model building codes will provide comparable levels of protection. However, when wind provisions are considered, considered., there are significant differences between both the performance and prescriptive requirements.
For example, one of the best sets of prescriptive requirements available in the US for wind \\ind resistant construction of residential buildings is the Southern Building Code Congress International SS-TD-lO (SBCCI 1997a). However, the loads used in developing these prO\.isions prOvisions most closely match ASCE 7 loads for suburban areas and assume that the building remains closed (no \\indows or doors fail). fail) . At present. the primary jurisdictions using these provisions are on barrier islands where the suburban assumption is not valid. Also, the code does not include any direct requirement that window protection be installed. installed. Thus. Thus. \\indow \\indmv failure from \\ind borne debris could invalidate the load wind assumptions used to develop the prescriptive
COMMUNITY ORGANIZATION 4. FOSTERING COlv1MUNITY AND ACTION Two of the most recent national initiatives aimed at mitigating the effects of natural disasters have been the Federal Emergency Management Agency's Project Impact and the Institute for Business and Home Safety's Showcase Communities. These are model programs aimed at developing community awareness of the local risks associated with potential hazards. The initiatives seek to bring natural hazards. community leaders from the public and private sectors together to identify the local risks and devise plans for mitigation efforts.
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The goal is to develop local ownership of the assessment and programs in hopes that this will efforts. There are result in more effective mitigation efforts. a number of requirements that must be met for a local jurisdiction to become one of these model commuruues. communities. The requirements range from adoption and enforcement of one of the model building codes to local promotion of the program. In andlor IBHS have also committed most cases, FEMA and/or funds towards projects aimed at reducing the vulnerability of essential facilities or infrastructure. infrastructure. One part of the initiatives involves raising public hazards. awareness of the risks from natural hazards. Another is fostering development of incentives for individuals and businesses to take action to reduce vulnerability. Incentives under consideration their vulnerability. include reductions in insurance premiums, reductions in deductibles, reductions in mortgage interest rates or points, reductions or waiving of taxes for permits related to mitigation efforts, reductions in rea1-estate sales fees, fees , and moratoriums on increases in tax appraisals for the value of the mitigation efforts. As might be e>.:pected, little progress has been made in implementing these incentives, but at least the parties are talking. talking.
paper, a new rate structure which reflects the perceived risks, without regard to regulation, and the perceived benefits of various mitigation measures such as shutters or hurricane straps. While these models are based on engineering analysis and expert opinion, there is little hard data available for validation. In the end most of the modelers are forced to smear all of the elements together in order to compare loss predictions with accumulated data for selected storms. storms. Usually, the loss data is only available on a postal code basis and there is no data for evaluation of individual components within the models. models. A significant focus of the FEMA and IBHS initiatives lruuauves is on improving the ability of a community to prepare for and recover from an event. However, from an engineering perspective, both the Project Impact and Showcase Community initiatives are very weak when it comes to specific recommendations for the retrofit of existing structures. The exception is the engineering analysis and design associated with reducing the vulnerability of the essential facilities and infrastructure targeted for specific mitigation efforts. efforts. When it comes to dealing with homes or low-rise commercial and light industrial buildings, the literature typically contains statements like, "install window protection" or "make sure your roof sheathing is properly attached" and refers the owner to the building code or the local building department.
Raising public awareness and the development of incentives are designed to increase the likelihood that mitigation efforts will in fact be implemented. implemented. However, However, raising ralSlng public awareness without providing guidance on effective mitigation measures shell. One of the could turn out to be a hollow shell. problems with offering incentives is convincing the various parties that there is a benefit to them other than altruism for providing the incentives. incentives.
These initiatives are still in the early stages of implementation and it will be some time before their effectiveness can be properly assessed. assessed.
Within the insurance community, it has been difficult to develop effective incentives because premiums in areas at risk are too low according to incentives. their analysis to allow more than token incentives. Furthermore, insurance commissioners are requiring that the industry provide actuarially based rates and incentives. The industry has not been collecting incentives. sufficiently detailed information on their building stock and the losses to support analysis of the potential benefits of proposed mitigation efforts. efforts. Similarly, the construction and engineering communities do not posses the testing facilities to allow scientific evaluation of potential mitigation measures. Finally, while post disaster investigations measures. for wind events have been conducted for more than 25 years, the observations tend to be anecdotal rather than statistical and the winds associated with the debate . event are almost always subject to debate.
5. TECHNOLOGY TRANSFER AND 5.
DEMONSTRATION PROJECTS A frequent theme of presentations on reducing the vulnerability of the built environment in the US is that the solutions exist. The critical task, according to the presenters, is simply to implement what is already known. This argument implies that there is a consensus on how to strengthen buildings and that asSUTarlce that these consensus there is some level of assurance ideas will in fact reduce the vulnerability of buildings and structures to both structural damage and economic losses. All that one has to do is attend building code committee meetings or try to develop specific recommendations for reducing the vulnerability of a particular building to know that there is no such consensus within the building community. As noted in the preceding section, the community. lack of systematic statistical evidence on the performance of building features in windstorms is at the heart of the difficulty in developing incentives for mitigation. mitigation.
Since databases on performance and losses do not exist, the insurance industry is turning to modeling as an alternative. There are efforts currently under way in the state of Florida to develop, at least on 138
The state of Florida has recently initiated two projects, which are unique in their approach to tackling the issues involved in developing effective mitigation. One is the Residential Coastal mitigation. Mitigation Program (RCMP) and the other is a residential monitoring project.
will be selected at various locations around the coast of Florida, evaluated using RCMP procedures and retrofitted with an agreement to allow installation of wind pressure sensors and anemometers. anemometers. The plan is to retrofit and pre-wire about twenty homes in the initial phase of the project. These homes would be located every ten to fifteen miles along the coast in strikes. the areas with the greatest risk of hurricane strikes.
The RCMP uses a fraction of the funds in the Florida Wind Pool to initiate retrofit projects in the state. state. The long-term objective is to depopulate the Florida Wind Pool, which provides coverage for homeowners who have been unable to obtain carriers. In the first phase of insurance from private carriers. project, the RCMP is pro\iding assistance for the project, retrofitting 600 houses in Dade, Broward and Palm Beach Counties of South Florida. Letters were sent to selected homeowners in these counties offering them the opportunity to obtain up to $10,000 in a forgive-able loan for retrofitting their homes. If the homeowner elects to participate, an inspection is carried out of the home and the results are analyzed to determine weaknesses and to evaluate the cost effectiveness of various retrofits. retrofits. If the retrofits are found to be cost effective and less than $10,000, the program will pay for the retrofits and forgive the loan if the homeowner remains in the house for five years. years. If the cost is more than $10,000, the homeowner can still participate provided they pay the excess costs. costs.
When a storm is forecast for one of the coastal segments, where pre-wired homes are located, computers and sensors would be installed and activated The instrumentation system Vl-ill \\oill contain two low cost anemometers capable of surviving gust speeds of 70 mls m/s and twenty-eight pressure sensors. In order to avoid drilling holes in the homes and/or running reference tubing over the surface of the used. These house, absolute pressure sensors will be used. are solid state units that have been conditioned to +2.5 provide a resolution of 0.01 kPa over a range of +2.5 pressure. to -15 kPa relative to ambient atmospheric pressure. In addition to the inexpensive anemometers at the homes, mobile trailers equipped with instrumentation quality R M Young anemometers capable of surviving 70 to 90 mls m/s wind gusts will be deployed within the final 24 hours of landfall. This project offers the potential for collecting high events. fidelity wind and pressure data in hurricane events. The project builds on technology developed at Clemson University in projects sponsored by the South Carolina Sea Grant Consortium and the Idaho National Engineering and Environmental Laboratory. The contractors involved in the Laboratory. retrofitting the houses will carry out pre-wiring and installation brackets. of sensor mounting brackets. Deployment of the instrumentation will be accomplished by Clemson University and a consortium of Florida Universities. Universities. It is anticipated that deployment of the home monitoring systems will require two to three hours per house and that the trailers can be set up in about half an hour.
To date, county building inspectors using predefined inspection forms has carried out the inspections in counties. The results of the inspections these three counties. are being analyzed by Applied Research Associates Raleigh, North Carolina. in Raleigh, Carolina. Potential retrofits being considered are limited to options accepted by the local code authority. authority. This has limited some of the possible retrofit considerations. considerations. The program is providing systematic information on buildings in these counties, which may prove useful for subsequent analyses of typical building stock. The limitation is that all of the properties belong to the Florida Wind Pool and as such may not be an accurate statistical sample of the total population of buildings. buildings. The retrofits will prOvide accurate estimates of costs for the various measures implemented. implemented. Here the limitation is on the types of measures allowed due to code restrictions. restrictions. Once the retrofits are completed there will be a reasonably large number of properties, which will provide valuable data on performance of the retrofit ne:-.1 storm strikes the area. area. measures when the ne:o.1 ...ill ex1end e:o.1end the program to other areas Latter phases ,\...ill \\oill be broader in future of the state so the coverage \\ill years. years.
The measurement program offers the potential for obtaining reliable high fidelity information on the rnagnitudes and characteristics ground level winds magnitudes and wind pressures on low-rise buildings in hurricanes. The trailers will provide prOvide high frequency hurricanes. wind data at five and ten meter heights. The pressure data from the buildings will serve a number of purposes. "\\ind purposes. First, it is needed for validation of "ind tunnel simulations of hurricane wind events. events. Second, Second., it can be used to improve or validate loads facilities. produced in component and system test facilities. Third, to a lesser e:o.1ent, e:-.1ent, the data will also serve as a Third., basis for evaluating code provisions and for investigating the effects of building features on wind loads. loads.
The monitoring project was conceived as a supplement to the RCMP. RCMP. In this program, houses 139
6. DEVELOPING ENGINEERING SOLUTIONS
load/deformation characteristics of the connections between components and systems are poorly understood. understood
Ultimately, the reduction in losses and damage from severe windstorms requires retrofitting of existing structures and modification of conventional US. Changes in construction techniques used in the US. the ways that individuals, businesses, communities and governments prepare for and respond to natural hazards can certainly reduce the human suffering and collateral losses associated with business interruption and disruption of essential services. However, if homes and buildings continue to suffer substantial damage in wind events that are generally at or below the design level, significant losses and disruption of lives and community function will be unavoidable. unavoidable.
Despite the large losses sustained in severe windstorms, US spending on wind engineering small. A frequent reaction research has been quite small. of casual observers is that simply installing a few problems. In some more fasteners can solve the problems. cases, certain failure modes could indeed be connections. prevented by simple changes in critical connections. However, this may result in a modest increase in overall performance, as another mode of failure becomes the trigger. trigger. In other cases, attempts to use limited data on loads and capacities can result in unreasonable construction requirements for some connections while ignoring other critical elements.
Attempts to change the way homes and businesses are constructed in the US, US, so that they are more wind resistant, face numerous social, political, technical problems. At the core of the problem and scientific problems. is the fact that the wind engineering community does not have facilities for full-scale wind testing which are equivalent to the shake tables used in earthquake engineering. Most typical building connections and engineering. systems composed of common building materials can not be scaled down for study in existing wind tunnel facilities. Furthermore, problems associated with v<.ith facilities. water penetration must be studied at full-scale.
The lack of technical and scientific certainty in the understanding of wind loads and the resistance of structures to wind have clearly affected the development of wind provisions in US building codes. When an adequate technical basis is not codes. available, the debate can quickly degenerate into the interests. Similarly, when analysis politics of special interests. and observations suggest that changes are warranted, it is easy to discount the recommendations because they are rarely based on adequate statistical evidence costlbenefit analyses. or unassailable cost/benefit
Existing methods and requirements for evaluating components, connections, connections, the performance of components, v<.ind loads structural systems and buildings under \\ind effects, including wind driven rain, are and wind effects, very elementary or are non-existent in the US. Unlike Japan, there are no formal requirements and test procedures for water penetration. penetration. Furthermore, many of the structural testing methods and requirements are limited to unidirectional quasiconnections. static loading of components and connections. These methods are not representative of the complex load combinations and load effects that typically ""inds. occur when a building is subjected to high ""inds.
7. 7. CONCLUSIONS Record losses in recent hurricanes have produced a variety of responses aimed at reducing losses from future storms. There is a clear need to integrate these efforts into a national program that addresses issues. It must promote local involvement three key issues. in mitigation efforts, develop meaningful incentives to motivate retrofitting of existing structures and support the development of comprehensive, technically sound, and cost effective solutions for new and existing buildings. buildings.
Analysis of the resistance of low-rise buildings to wind loads and wind effects is also fraught ""ith problems. problems. First, adequate quantification of wind loads and the variation of these loads in time and space are only now becoming available. available. Second, since the building's skin or cladding frequently plays an important role in the stiffness of the building, models need to be created which adequately account for the contribution of the skin to the structural resistance. Third, low-rise buildings are normally resistance. composed of a variety of materials and structural forms. Their load/deformation characteristics are understood; so frequently, frequently, they are not well understood; combined in ways that do not adequately account for their relative flexabilities. Furthermore, flexabilities. Furthermore, the
REFERENCES American Society of Civil Engineers (1993). Minimum Design Loads for Buildings and Structures, 7-93 , ASCE, New York, NY. NY. Structures, ASCE 7-93, American Society of Civil Engineers (1995). Minimum Design Loads for Buildings and Structures, ASCE, New York, NY. NY. Structures, ASCE 7-95, ASCE, Southern Building Code Congress International (1 997a). Guidelines for Wind Resistant (1997a). Construct., SS ID 10, SBCCI, Birmingham, AL. AL. Construct., Southern Building Code Congress International (1997b). Code , SBCCI, (1997b). Standard Building Code, Birmingham, AL. AL. 140