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2 Technical Aspects of a Large-scale Wind Test Facility Introduction In the discussion that follows, the terms ''large-scale" and "full-scale" are used in specific ways. "Large-scale" refers to structural models, or components of structures, with length-scale factors greater than, or equal to, 1:4. "Full-scale" refers to components, subsystems, or entire structures with length-scale factors of 1:1. Thus, full-scale testing is a subset of large-scale testing. Although the committee was not asked to evaluate a specific design, the type of LSWTF under consideration is assumed to be of the wind-tunnel type (as opposed to an actuator or pressure-chamber system) suitable for experimentation on large-scale components of structures, as well as testing (to failure) of full-scale selected structures (e.g., manufactured homes, residential buildings, and light commercial buildings). Experiments on these scales would require sustained wind speeds of 150 to 200 mph (~ 65 to 90 m/s), with a reasonable representation of atmospheric turbulence, over an area large enough to engulf a residential, single-family dwelling or other structure of similar size. The flow structure and size of the facility's wind stream would have to be sufficient to create a realistic flow around the structure and thereby generate appropriate and representative spatial and temporal variations of wind-induced pressures. At the present time, there are significant gaps in the meteorological data for severe wind events that would have to be filled before the design parameters and capability requirements for an LSWTF could be stipulated. Previous Assessments Although there is general consensus in the wind engineering community about the need for large-scale data on the effects of extreme winds on structures, there is no consensus about the need for an LSWTF. The value of an LSWTF has been discussed in several assessments of research needs in wind engineering, including Assessment of Wind Engineering Issues in the United States (NRC, 1993); Severe Windstorm Testing Workshop (O'Brien, 1996); Workshop on Large-scale Testing Needs in Wind Engineering (AAWE, 1997b); and Workshop on Research Needs in Wind Engineering (Marshall, 1995), and was cited by several respondents to the committee's questionnaire. All of these assessments agreed that large-scale data are needed to improve structural performance and that an LSWTF could be a valuable tool for determining the effects of extreme winds on structures. These reports, however, also point out that other methods of data collection are available (e.g., full-scale field testing in natural wind) that may be able to
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answer many of the same questions. These reports concluded that the most effective research framework for wind-hazard reduction would be a combination of current methods of wind engineering research, such as full-scale field studies, wind tunnel and numerical modeling, component testing, and post-storm inspections. The reports emphasized that a coordinated national wind-hazard reduction program is necessary to mitigate wind-induced losses effectively, and they cautioned that an LSWTF alone would not provide answers to all outstanding questions in wind engineering (AAWE, 1997b; NRC, 1993). Some existing facilities in the United States and abroad might be modified for large-scale wind testing (AAWE, 1997a); another possibility is an international cooperative research program (NRC, 1993). Wind-Hazard Research Minimizing the loss of life, property damage, and disruptions of economic activities from windstorms are primary objectives of wind engineering research. Consequently, any proposed national program or facility must be evaluated in light of whether it contributes significantly toward meeting these objectives. The Federal Emergency Management Agency (FEMA) and the insurance industry have both determined that significant improvements in the wind resistance of buildings will only be achieved when there is a demand for wind-resistant or hazard-resistant construction at the local and individual level (Cermak, 1998; FEMA, 1992). As a result, both FEMA and the insurance industry have embarked on pilot demonstration projects to highlight the benefits of hazard-resistant construction and other wind-hazard mitigation measures. Called Project Impact (FEMA, 1998) and the Show Case Communities (IBHS, 1998), these new projects have not yet demonstrated tangible results. The research, engineering, and scientific communities have provided some of the technical underpinnings for reducing the vulnerability of buildings and other structures to wind damage. Significant work remains to be done in this area to ensure that key vulnerabilities of a particular structure are identified and that technically sound, cost-effective solutions are developed and implemented. Unfortunately, reducing vulnerability to wind-hazards is not just a question of developing appropriate technical solutions. First, wind-hazards are created by a variety of random events with large uncertainties in the magnitudes and characteristics of the winds. Second, the relevant government agencies and programs, as well as the construction industry, are fragmented. Third, implementation requires action by owners and the public, who may not consider hazard reduction a high priority. As a result, solving the wind-vulnerability problem will require coordinated work in scientific research, technology development, education, public policy, the behavioral sciences, and technology transfer. In the past decade, several proposals have been put forward identifying the need for a national program of wind research, technology development, and education to address the technical needs for reducing losses associated with severe windstorms (NRC, 1993; Jones et al., 1995; Marshall, 1995; O'Brien, 1996; AAWE, 1997a). Despite these efforts, no national effort has been made to integrate wind research, technology development, and education into broader programs for natural hazard preparedness and disaster recovery (Cermak, 1997). Ultimately, losses associated with severe windstorms can only be significantly reduced if existing buildings and structures are modified and new buildings are designed, constructed, inspected, and maintained with wind resistance in mind.
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An additional problem is the time it would take for the benefits of a coordinated plan to be observed. Only a small percentage of structures are replaced or added each year. Therefore, it would be many years before improvements in construction practices became prevalent. The adoption and implementation of remedial measures for existing structures is even more difficult to accomplish because the public often does not perceive a problem until a disastrous event occurs. The benefits and limitations of any single research facility must be carefully evaluated in light of the absence of coordinated action at the national level. Value of Large-scale Testing Testing of full-scale structures has been a part of wind engineering research for decades (Davenport, 1975), much of it associated with field measurements of wind characteristics, wind loads, and wind effects. These measurements have provided insight into the nature of various types of windstorms and benchmarks for evaluating analysis and design methods. Field studies continue to be an indispensable part of wind engineering research. Data on the structure and characteristics of winds in severe windstorms are meager, however. Frequently, instrumentation, primary and backup power sources, and recording devices fail in severe windstorms, and the resultant data gaps leave large uncertainties about the magnitude and structure of winds in extreme events. The problem is complicated by the random structure and very large spatial gradients of wind, which makes it extremely difficult to characterize. For example, substantial differences in wind speeds and characteristics can be caused by changes in elevation and by averaging time associated with a particular observation, as well as the topography and roughness of the upwind terrain. In an effort to reduce observational uncertainties in wind characteristics for extreme events, the National Oceanic and Atmospheric Administration (NOAA), the DOE, the National Institute of Standards and Technology (NIST), and several universities are attempting to measure wind magnitudes and wind characteristics in severe windstorms. New technologies are being employed, including new satellite imagery, airborne and ground-based Doppler radar (including two Doppler-on-wheels systems), wind profilers, Global Positioning System dropsondes, rapidly deployable trailers with anemometer masts, and new types of anemometers (Marks et al., 1998). All of these technologies were used during several recent hurricanes, which has led to considerable debate in the scientific, meteorological, and engineering communities regarding what is actually being observed and the implications of these observations. It will probably take several years of using these technologies before a coherent picture emerges. Field studies of wind loads and wind effects on buildings have been even more limited (Eaton and Mayne, 1975; Hoxey and Richards, 1993; Levitan and Mehta, 1992a, 1992b; Marshall, 1975; Marshall, 1977; Robertson, 1991). No data are available on wind loads on buildings in the eye wall of a hurricane or in a tornado. No data on buildings subjected to thunderstorms and tropical storms have been reported in the literature. Experience with wind-tunnel model studies has shown that the gust structure of the wind plays an important role in the development of wind loads on structures. However, most of the existing field data on wind loads are limited to simple building shapes in open exposures subjected to winds generated by the passage of frontal systems rather than severe windstorms. The lack of knowledge about wind loading and structural response in severe windstorms is a significant impediment to establishing meaningful standards for structural systems and for improving structural performance.
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A versatile, well conceived LSWTF could be used for a number of studies to identify and address data and knowledge gaps. Table 2-1 shows potential applications and technical capabilities that could be provided to the community of wind engineers and scientists. In addition, a number of other types of experiments might also be conducted, depending on costs and the availability of the facility. For the purpose of reducing wind-hazards, an LSWTF would be most useful for conducting destructive experiments of large-scale structural systems, for fostering the development and validation of computational models, and for improving test methods. During the course of discussions and the review of responses to the questionnaire, the committee identified three topical investigations of buildings and structures that could be accomplished in an LSWTF: the performance of the building envelope, new construction techniques, and retrofitting technology. Performance of the building envelope. Economic assessments of damage following windstorms have shown that once a building envelope is compromised, losses increase dramatically (Cermak, 1998). An LSWTF could offer a practical approach to determining the wind speed at which the building envelope is compromised in a full-scale building. Validation of construction techniques, practices, materials, and building code provisions. Numerous remedial measures have the potential for improving the wind resistance of a building, and it is a relatively straightforward matter to test these measures at the component level. It is far more difficult, however, to assess the effectiveness of these measures in a full-scale system where their attributes interact synergistically. An LSWTF could provide an opportunity for assessing these measures under a range of controlled conditions thereby reducing the uncertainties about their effectiveness in severe winds. Significant advancements could be made in construction practices if the properties of a total building system could be evaluated in a full-scale turbulent wind flow representative of a hurricane, thunderstorm, or other extreme wind event. Retrofitting techniques. A comprehensive wind-hazard reduction program must include improvements to existing buildings. Retrofitting techniques can be tested as components of a system, but their value to the behavior of the full-scale building system can be determined only by testing a full-scale, complete system. Destructive testing could include the following: Testing of sheathing systems by applying realistic spatial and temporal variations of wind loads. Current test methods apply loads uniformly over the surface of the specimen and have not included combined in-plane and out-of-plane loading. Testing of the performance of the building envelope with emphasis on system performance relative to window and roof performance. With current design criteria and construction practices, roof and wall systems may be more vulnerable to failure or water damage than protected windows and doors. Testing of variations in internal pressures in a building with multiple rooms. A breach of the building envelope, such as the failure of a window, can lead to
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pressurization of the building. Little is known about how pressurization is propagated throughout a building. Validating retrofitting techniques in the context of overall building performance. The benefits of a particular retrofitting measure or set of retrofitting measures could be ascertained, as well as whether the buildings will simply fail in another mode at a slightly higher wind speed. TABLE 2-1 Technical Capabilities of a Large-Scale Wind Testing Facility (LSWTF) Building Tests Code Development and Validation Other Applications Instrumentation/Testing High Reynolds Number testing of structural components Validation of full-scale computational resistance models Determining wind loads on floating offshore systems Testing and calibration of new wind sensors Water penetration experiments Validation of computational fluid dynamics (CFD) models Evaluating vehicle aerodynamics Development of instrumentation concepts Destructive testing of full-scale systems, including relationships to Saffir-Simpson Scale destruction categories Validation of construction techniques, practices, materials, and building code provisions Testing refinery systems (Reynolds Number) Evaluation of wind generators Sheathing system tests and evaluation that include spatial loads Improving load/resistance characteristics Tests of multiple steel stacks Simplification of test methods Strong room evaluations for residential structures Validation of systemic retrofitting techniques Fatigue of elements and connections in a full-scale system Development of damage fragility curves Window and roof system behavior relative to building envelope performance Development of wind flow and energy use relationships Internal pressure distributions on internal walls and ceilings Damage sensitivity to wind speed characterization (peak gust, sustained wind) Windborne debris injection and transport Windborne debris impact phenomena Behavior of roof top appurtenances Behavior of roof edge attachment details
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Testing of windborne debris injection and transport. It has been well established that severe windstorms generate and transport debris that can damage buildings and structures. Models of debris injection and subsequent transport are extremely sensitive to assumptions about wind speeds required to initiate the failure that produces the debris. Testing of the performance of rooftop appurtenances. Failure of mechanical system components and other rooftop appurtenances have caused significant damage to the interiors and contents of buildings. Testing of the performance of porch roofs and roof overhangs. Roof failures frequently originate at porches and roof overhang areas. Uses of an LSWTF related to improving analytical models and simplified test methods could include the following: Validation of full-scale computational resistance models. Intense loading generally produces nonlinear structural behavior in certain components, connections, and at the system level. More realistic load modeling would result in more realistic modeling of the behavior of structural systems. Validation of construction techniques, practices, materials, and building code provisions. Rather than waiting for a storm to provide validation, it would be possible to create representative wind loading conditions in a controlled environment. Realistic simulation of complex loading patterns and the response of the structural system to these loads. Idealized loads specified in building code provisions and simplified analytic procedures sometimes lead to design requirements that are inconsistent with the observed performance of buildings in severe windstorms. Development of improved component tests. Many of the current tests for structural components and connections do not adequately reflect the actual physical processes at work in a severe windstorm. Although this discussion has indicated that an LSWTF would be useful for wind engineering research, the rationale for establishing such a facility involves more than its capability to provide needed information. Many of the items listed above can be accomplished by other means (e.g., computational resistance models can be validated through full-scale measurements in natural wind or through comprehensive post-storm investigations). The low level of funding available for wind engineering research has been a major impediment to the development of new instrumentation, testing, and analytical technologies. It has also been a major impediment to the full and effective use of existing technologies to capture the variability of loads and resistance through wind-tunnel tests and component tests. The committee noted that none of the major engineered structures in the world underwent full-scale testing to evaluate overall structural performance before it was built. With careful engineering, the wind resistance of low-rise residential and commercial structures could be dramatically improved. Given the current state of knowledge, a number of assumptions and considerable engineering judgments are necessary in the design of low-rise structures. In most cases, these assumptions and judgments lead to conservative designs. Thus, reducing the
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uncertainties could lead to economical designs more consistent with the actual level of risk. The real benefit of improved large-scale testing would be the savings and improved reliability of designs based on these investigations compared to engineered designs developed without the advantage of these experiments. Thus, the economic benefits of improved large-scale test methods, including an LSWTF, should be determined in terms of the savings expected compared to the cost of implementing better engineering design procedures, and not simply in terms of the potential savings over future construction using existing methods. INEEL has proposed a pilot LSWTF to test manufactured housing. The committee believes that a more economical solution would be to deploy instrumented manufactured homes in the paths of hurricanes, surrounded by sufficient instrumentation to quantify the winds in the storm. The committee also believes that a large-scale pilot project is not a practical first step toward an LSWTF because the facility would have limited capabilities, could not provide the required data, and might preempt the development of a more general LSWTF. Role of a Large-scale Wind Test Facility in Wind Engineering Research Even with the modest funding currently available for wind engineering research, advances are being made in a number of areas, such as the characterization of wind fields and the evaluation of the performance of the building envelope (AAWE, 1997a). Two critical questions regarding the need for an LSWTF (as opposed to the desirability of having one) are whether it is uniquely capable of providing needed data and whether it can provide this information at lower cost than other alternatives. It may be that if the general level of funding for wind engineering research were significantly increased, much more could be accomplished in other ways, at lower cost, than by means of an LSWTF. A variety of tools for research and development are available for determining the characteristics of wind-resistant structures, including analysis, numerical computation, wind-tunnel testing of small-scale models, wind-tunnel testing of large-scale or full-scale components, full-scale testing in the natural environment, and large-scale or full-scale testing of components and structures in simulated wind conditions under forces generated by actuators. Table 2-2 shows the scope and efficacy of a number of concepts for wind test structures. These tools have contributed to a growing understanding of how a wide range of structures, including tall buildings, low-rise commercial, industrial, and institutional buildings, residential buildings, and suspended-span bridges perform in high winds. Their potential for improving the economy and performance of structures of all types remains high. However, this knowledge alone has not been sufficient for the widespread implementation of improved designs and construction methods. There are social, economic, and institutional barriers to the deployment of technological improvements that engineering research alone cannot address (Cermak, 1998). Therefore, although an LSWTF would be an additional tool that could potentially help to improve design and construction technology, the effective transfer of the information produced by such a facility into practice would have to overcome similar barriers. Evaluating the efficacy of a wind engineering research method or facility requires first comparing its potential contributions with those of other experimental tools that could provide the same or equivalent information. To develop funding priorities, the relative costs of these
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TABLE 2-2 Concepts for Wind Testing of Structures. Source: adapted from Cermak, 1998. Concept Sufficiency good to fair —poor to not feasible Loading Mechanism Natural Wind Wind Speed (m/s) Artificial Wind Number of Fans Modeled Wind Boundary-Layer Wind Tunnel Synthetic Wind Actuators Scale of Model Structurea >45 <45 10–30b 1–2 mock-up testsc new large conventional pressure chambers dynamic wind load actuators FULL-SCALE (1:1) Full Structure — — — d Partial Structure — — d LARGE-SCALE (1:1–1:4) Full Structure — — — d Partial Structure — — — d MEDIUM-SCALE (1:4–1:25) Full Structure — — — — d Partial Structure — — — — — d SMALL-SCALE (1:25–1:250) Full Structure — — — — d d — — Partial Structure — — — — — — — — a one-story, one-family house b creating a test section large enough to test large-scale structures (e.g., "Wall of Wind" in O'Brien, 1996) c for example, a building glazing unit d determine influence functions and compute structural system loads
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tools must also be considered while recognizing that certain vital information may only be available from one form of experimentation, perhaps at considerable cost. Finally, the role of experimental investigations relative to other areas of needed wind engineering research must be considered, as well as how the greatest benefits can be achieved from the prudent investment of resources. Many different technical approaches have been brought to bear to improve the performance of the nation's building stock and infrastructure relative to wind loads. Continuing human and economic losses suggest that there is more work to be done in both the development and implementation of research results. There is a general consensus, however, that many of the results of current research have not been implemented effectively (Cermak, 1998). The manufacturing sector needs to be involved in implementing research results because it supplies the large variety of materials and components that make up a constructed building. Engineers and contractors can only implement improvements if they have information on the performance of new products and materials. Because of the small market and difficulty of carrying out qualification tests on a limited budget, this information is not often developed. Therefore, a testing and certification mechanism should be established to assist manufacturers in qualifying proposed new items or concepts for improving the wind resistance of structures. To date, the experimental focus in wind engineering has been in the use of wind tunnels, mostly boundary-layer wind tunnels (Cermak, 1995). Wind-tunnel facilities have provided a wealth of data and understanding about the nature of wind loads on a wide range of structures, but wind tunnels can only test models and cannot test causes of failure of structural elements. Although more needs to be done in this area, calibrations with (albeit limited) full-scale data suggest that the results are consistent with expected loads and pressures on real structures (Cermak, 1995). The results of wind-tunnel investigations, and supporting analytical and numerical computations, have led to significant improvements in building codes in the past two decades (Cermak, 1995). Related investigations have focused on evaluating the response of structural and nonstructural components (e.g., shear walls, roofing systems) to wind-induced loads, with testing performed frequently at large-scale, or even full-scale. Commercial testing—often proprietary—is also quite common. A number of complementary full-scale field investigations involving the use of natural environmental winds have also been performed. To date, these investigations have not included testing to failure. The design of engineered structures has effectively incorporated aerodynamic characterizations obtained from wind-tunnel experiments, in some cases complemented by full-scale observations from the natural environment. For obvious reasons, no full-scale multistory building has ever been tested to failure under controlled conditions in an LSWTF. It is conceivable that at wind speed that would cause failure, experiments conducted on non-engineered structures in an LSWTF could provide information to improve current design practices. However, much can also be learned from analyses based on the results of component studies augmented by observations of failures in real events. Structures designed to resist actual fluctuating wind loads would perform more predictably than structures designed according to current wind-load criteria and could possibly be less costly to build. The savings could be used to upgrade components of the building to further improve its overall performance. Analyses to failure of wood-frame homes, manufactured housing, and low-rise commercial structures, in conjunction with component testing, could help to determine their behavior leading to failure and improve their design. Experiments in an LSWTF could be used to validate computational results based on component and other tests for
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both steel buildings and wood-frame houses. However, validation is also possible from full-scale measurements (generally nondestructive) or, in a statistical sense, from detailed analyses of post-disaster damages. It has been suggested that existing facilities in the United States or abroad could be modified for large-scale wind testing. The capabilities of at least one facility, the NASA Ames large-scale test facility, are described in the AAWE Report Workshop on Large-scale Testing Needs in Wind Engineering (AAWE, 1997b). Although this facility would have the capability to develop aerodynamic loading on structures as large as a manufactured house or a small residence, there would still be some significant difficulties in using it for wind-engineering investigations. The problems include the development of acceptably scaled turbulence and a significant concern that destructive testing would produce debris that could damage the wind tunnel or fans. Additional study would be required to determine if facilities of this type could be used for large-scale structural research. Priority of a Large-scale Wind Test Facility Although this review was initiated at the request of DOE in response to a proposal by the INEEL, this committee was not asked to evaluate a specific proposal for an LSWTF. However, some important issues should be considered before any proposal is considered. First, funding for wind engineering research, technology transfer, and education in the United States has historically been about $4 million per year (AAWE, 1997a). Because a large-scale test facility would be only one of many tools available to the wind engineering community, and one with specific capabilities and limitations, it would be prudent not to spend a disproportionate amount of the available funds in any given year on the construction, maintenance, and operating expenses of an LSWTF. Figure 2-1 illustrates the committee's view of the relative importance of an LSWTF for wind-hazard reduction. FIGURE 2-1 The Importance of an LSWTF in wind-hazard reduction. Given that a large-scale test facility has the potential to be used in the ways already discussed, it is conceivable that such a facility could be a part of a well organized, well funded national wind-hazard reduction program at a later date. However, given the current state of wind
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engineering research, from the standpoint of overall funding and the capacity for technology deployment, the construction of an LSWTF at this time would be premature. Before such a facility should be considered, a clear and objective plan for its use would have to be developed, describing exactly what capabilities the facility would include, the level of participation of the wind engineering research community in the research program, the specific questions that would be answered during the first few years of operation and at what cost, and the reasons these questions could not be answered more effectively, from both a technical and economic standpoint, by other means. Finally, there would have to be a clear understanding of how this facility and its research program and results would fit into a national wind-hazard reduction program.
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