National Wind Institute

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Now showing 1 - 20 of 37
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    Physical Modeling of Thunderstorm Downbursts
    (2001-08) Chay, Michael Thomas
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    The Performance of Low-Rise Open Span Heavy Steel Structures in Extreme Winds.
    (1997-06) Charlton, Joe R.
    This report is an engineering study of the field performance of open span lowrise steel frame structures that have been subjected to extreme wind events such as hurricanes and tornadoes. The wind velocities in these events either approached or slightly exceeded the normal design values specified in ASCE 7-95. This report focuses specifically on the performance of heavy steel structures and does not include pre-engineered metal buildings. All types of building failures are observed and analyzed in this report, including roofing and secondary cladding component failures as well as main structural failures. In each case study, the probable cause of failure is determined and through an analysis of the different case studies, patterns of failure are identified. Through an analysis of the patterns of failure, recommendations for general design improvements are made and areas requiring further study are identified. The study found that the main structural systems of heavy steel structures performed very well in these extreme winds. Virtually no damage was observed to any of the components of the main structural systems of the buildings, even when the wind velocities exceeded design values by as much as 30 percent. However, the components and cladding did not perform as well. In almost every instance of failure, at least some portion of the roof decking was removed. In most cases the damaged area was restricted to the windward edge of the roof/wall intersection. Another weak component was the overhead doors. In over half of the instances of damage, the overhead door was the first point of failure. The failure of the overhead door(s) then caused the failure of other building components.
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    Investigation of the Mechanics of Windborne Missile Impact on Window Glass
    (1999-12) Bole, Scott A.
    Engineers have recognized that failure of the building envelope is one mechanism that can lead to severe damage of structures during windstorms. The building envelope consists of the roof, doorways, windows, and cladding components that form the exterior wall system of a building. Failure of the building envelope results in internal pressurization of the structure which may lead to structural failure. For this reason, engineers have begun to focus on ways to make the building envelope resistant to the effects of severe windstorms. Window glass is one type of cladding material. Of the threats posed by a windstorm, the major threat to window glass consists of windborne debris. ASTM E1886, ASTM E1996, and SSTD 12-99 address the issue of resistance to windborne missile impacts. This thesis concludes that a simple statement of an object's kinetic energy upon impact by itself cannot serve to predict the outcome of the impact. Conservation of angular momentum occurs during a missile impact on window glass. Finally, energy is lost during a missile impact on window glass. v
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    Direct Measurement of Wind Loads on a Low-Rise Test Building
    (1998-05) Bird, Mark Wesley
    Wind loads on low-rise structures are studied in the field primarily so that the results obtained can be used to provide a base-line data set and to assist in wind tunnel studies. The objective of this work is to develop a means of directly measuring total wind loads in the field on a low-rise test building. This project is part of the Texas Tech University I Colorado State University Cooperative Program in Wind Engineering to study wind effects on low-rise buildings. The experimental facility utilized is the Wind Engineering Research Field Laboratory (WERFL) on the campus of Texas Tech University. This thesis contains development of an experimental procedure for directly measuring total wind-induced loads (total horizontal shear and uplift) on the 30 ft x 45 ft x 13 ft test building along with some preliminary results. These loads are measured by supporting the entire building on four load cells( one at each corner). The load cells were constructed in the laboratory to register force in the three orthogonal directions. Forces recorded by the load cells and pressures recorded at the 12 pressure taps (windward wall, roof and leeward wall) are used to measure wind-induced loads. Data utilized in this project is limited to records with winds normal to long axis of the building collected in February and March of 1995. A total of twelve 15- minute records are used in the analysis. Analysis of data indicates difficulty in obtaining a reference load (when wind is zero). A procedure to overcome this difficulty is suggested. Preliminary results show that overall loads measured using the load cells are smaller than the ones obtained from the integration of pressure tap values. However, there is significant uncertainty and scatter in the results obtained from the load cells.
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    Insurance Loss Analysis of Single Family Dwellings Damaged in Hurricane Andrew
    (Clemson University, 1995-05) Bhinderwala, Shiraj
    In recent years hurricanes have caused billions of dollars of damage to houses, putting the insurance industry under severe strain. In order to set appropriate premium rates in future and to guide damage mitigation policies, an understanding of the relationships between insurance losses and types of damage, and wind speed are required. Using loss data from Hurricane Andrew provided by two insurers, relationships have been established between average loss ratios and gradient wind speed for zip code areas. A more detailed analysis of data provided by one insurer enabled average loss ratios and damage severity for building components such as roofs, doors, windows, walls and external facilities to be determined. From this information it was possible to determine a loss magnifier relating the total damage to a building and its contents to damage to its envelope. At gradient wind speeds below 70 m/s and overall loss ratios less than 12% the total damage averaged twice the direct wind damage to the envelope. Above 70 mis the total damage increased to nine times the average envelope damage and resulted in average overall loss ratios greater than 60%. A comparison of the results of this study with the relationship between insurance losses and wind speed developed by Friedman, and used by some sectors of the insurance industry, indicated that such usage would seriously underestimate losses in typical zip code areas of 100 square kilometers. Friedman's relationship might prove satisfactory for a typical county of approximately 2500 square kilometers, the averaging area used by Friedman.
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    Loads: The Basis of Repeated Loading and Impact Loading Criteria
    (James Cook University of North Queensland, 1978) Beck, V.R.
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    Robust Fractal Characterization of One-Dimensional and Two-Dimensional Signals
    (1993-08) Avadhanam, Niranjan
    Fractals have been shown to be useful in the analysis of time series data and in classification of natural shapes and textures. A Maximum Likelihood Estimator is used to measure the parameter H which is directly related to the fractal dimension. The robustness of the estimator is shown in the presence of noise. The performance of the method is demonstrated on datasets generated using a variety of techniques. Finally the performance of the estimator is shown by characterization of homogenous textures and by the segmentation of noisy composite images of natural textures
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    Regionalization of Tornado Hazard Probability
    (1981-12) Allen, Barry Scott
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    Study of Load Effects on a Dome Due to Fluctuating Wind Loads
    (2001-05-07) Agarwalla, Kamalesh
    The present study attempts to compare three contemporary techniques available to estimate peak load effects on structures from limited pressure histories, obtained from Wind Tunnel tests. A wind tunnel test was performed to study fluctuating wind loads on smooth and rough surface dome. The dome model was instrumented with 85 pressure taps. Pressure coefficient history at each tap location was obtained from the wind tunnel test. A Finite Element Model of the dome was created using structural analysis software RISA 3D at the Computer Laboratory of Wind Science and Engineering Research Center of Texas tech University. Structural analysis of the dome was performed for unit load at each of the 85 pressure tap locations and the load effects (displacement and moment) at the apex and stagnation point of the dome were calculated Thus the influence coefficients for load effects at apex and stagnation point were obtained for each of the 85 pressure taps. The influence coefficients were used to estimate the peak load effect coefficients at apex and stagnation point of the dome using three methods, viz., Actual Analysis, Quasi Steady theory and Covariance Integration method. The peak load effect coefficients obtained from the three methods were compared with each other. Load Response Correlation (LRC) technique was used to estimate the extreme pressure coefficient distributions on the surface of the dome, which are expected to cause peak load effects at the apex and stagnation point of the dome. The LRC extreme pressure coefficient distributions were calculated for two values of 'g~ one, same as that used for Covariance Integration method and the other, estimated from the peak load effect coefficients obtained from Actual Analysis. The LRC extreme pressure coefficients were compared with the actual pressure coefficients causing peak load effect coefficients in Actual Analysis. The pressure coefficient distributions obtained from LRC for two values of 'g ', were then applied on the surface of the dome and structural analysis of the dome was performed to calculate the peak load effect coefficients at the apex and stagnation point of the dome. This was done to confirm the peak load effect coefficients obtained from Actual Analysis and Covariance Integration method. All of the above calculations were performed for both smooth and rough surface dome. The peak load effect coefficients from the three methods, for smooth and rough surface dome were compared to understand the effect of surface finish of the dome on peak load effect coefficients.
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    A Comparative Study of Wind Load Standards
    (1985-08) Das, Nirmal K.
    The subject of this thesis is a comparative study of the wind load provisions specified in the major national standards. The basic premise is that two structures of identical shape and size and subjected to the same wind environment will experience the same wind effects and should be designed for the same wind loads even though they may be located on two different continents. In order to test this hypothesis, the wind load provisions of four national standards are compared: 1. American National Standard ANSI A58.1-1982 2. Australian Standard SAA 1170, Part 2-1983 3. British Standard CP3 wind loads, 1972 4. National Building Code of Canada, 1980 These standards are developed on the basis of research and prevalent professional practice in their respective countries. Since they are developed independently, a comparison of provisions in the standards provides a measure of the sophistication achieved in addressing wind loads.....This simple case study highlights significant differences in the provisions of the standards. The concluding chapter indicates similarities and differences in provisions of the four standards.
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    Wind Parameters of Texas Tech University Field Site
    (1988-08) Chee, Vui Chok
    Wind parameters obtained from field data are generally simulated in wind tunnel for studying wind effects on structures. The result of the wind tunnel study depends on the reliability of field wind parameters and the simulation technique. The objective of this study is to assess wind parameters from field data. The National Science Foundation has sponsored a project at the Texas Tech University Wind Engineering Research Field Laboratory to study wind effects on low-rise building. Wind pressure and meteorological data are collected on the test building and meteorological tower respectively. Meteorological data which include wind speed, wind direction, temperature, barometric pressure and relative humidity data, are measured at four levels of the tower. Wind speed, wind direction and temperature data are used for assessment of wind parameters and characterization of terrain. A total of 63, 15-minute duration each, records are collected. Of these, 31 records are found to be suitable for analysis. These 31 records are analyzed to determine wind profile parameters for both power and logarithmic laws, turbulence intensity and longitudinal integral scale of turbulence. The wind profile parameters, mean wind directions and terrain features are used to characterize the field site terrain. Results of the analysis are presented in this report.
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    Probabilities of Wind-Induced Peak Pressures on a Low Building
    (1993-12) Atilli, Sai Sailaja Kameswari
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    Finite Element Modeling of Tornado Missile Impact on Reinforced Concrete Wall Panels
    (1993-12) Zhang, Yuming
    This study describes a finite element model for the impact of large tornado missiles on reinforced concrete wall panels. The analysis predicts the dynamic response of wall panels when impacted by a missile with a large contact area such as an automobile. The development and current status of tornado-generated missiles and their impact effects on concrete wall panels are summarized along with the failure criteria for concrete material in the theory of plasticity. Quadratic finite elements are used to discretize the domain of the wall panel. Fundamental assumptions are based on the Mindlin and Reinsser's plate theories. An "embedded" model is employed to account for the reinforcing bars. The nonlinear behaviors of concrete and of steel bars are analyzed by means of rate-dependent constitutive relationships. A new strain rate-stress relationship is proposed, and the relationship between the fluid parameter 7 and the coefficient a is set up. A model is proposed to describe the initial and subsequent yield surfaces of concrete material, which avoids underestimation of the effect of high hydrostatic stresses on the yielding behavior of concrete. Ottosen's four-parameter failure criterion is used to define the failure surface of concrete. A crack monitoring algorithm accounts for post-cracking and post-crushing behavior of concrete. An explicit time integration scheme is used to solve the nonlinear dynamic equations carried out using the finite element discretization of a concrete wall panel. A computer program is developed in accordance with the above conditions. Several impact tests were conducted in the Civil Engineering Testing Laboratory. A steel pipe 3-in diameter by 10-ft long was fired by Texas Tech missile cannon. The missile struck at the center of a .5 in thick by 12-in square steel plate which is attached to the center of the reinforced concrete panel so that the impact contact pressure is spread over the whole steel plate. Dynamic displacements of the concrete panel were measured with a linear variable displacement transducer (LVDT). The calculated results from the computer program were compared with the test results. The computer program for dynamic analysis of reinforced concrete wall panels is verified by the impact test results. The new relationship between stress and strain rate for viscoplastic analysis that is proposed is proven to be correct, and the formula for the critical time step is successfully derived. As a practical application of the analysis technique, the contact failure pressure for a particular panel geometry is calculated. The contact failure pressure and the elapsed time to failure after missile contact define an impulse loading to produce failure of the panel. Since automobile crashes tend to produce triangular impulse loads, the two pulses (failure and impact) can be compared to determine if a particular impact will fail a panel. Thus, a particular concrete panel can be analyzed to determine if it will fail under a postulated automobile impact.
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    Wind Erosion:Mechanics of Saltation and Dust Generation
    (1994-12) SIngh, Udai Bhan
    Soil erosion by wind and associated dust generation cause major social and economic problems in many parts of the world. There is a high cost associated with on-site and off-site damages due to wind erosion. Wind erosion of top soil causes loss of essential plant nutrients and soil productivity, resulting in loss of agricultural production. Visibility reduction on highways during dust storms can cause severe accidents. Fine dust particles suspended in air deteriorate the environmental air quality. Particulate matter smaller than 10 jim in size (PM,o) are also recognized as health hazards to human and animals. Off-site costs of wind erosion are much higher than on-site costs. The reference saltation height and reference concentration in the saltation zone must be determined to accurately predict dust concentrations and visibility at different heights in the atmosphere. The rate of dust generation depends on kinetic energy from saltating particles during the wind erosion process and dust potential in natural soil. The mechanics of saltation and dust generation and the relationship between kinetic energy of saltating particles and dust generation are not very well understood. An experiment was carried out in a wind tunnel to collect mean saltation height and maximum transport data for three different uniform particle sizes and a physically based model was developed to predict mean saltation height as a function of particle size, wind velocity, and material properties. Data on soil particle concentration with height in the saltation zone were analyzed to develop a general equation to predict soil particle concentration with height as a function of friction velocity and particle size. A methodology was also developed to connect dust generation to the wind erosion process. A controlled energy dust generator (CE/DG) was designed and developed to relate dust generation to the wind erosion process. The device was tested with seven different soil types to investigate the relationship between dust emission and kinetic energy from abrasion during wind erosion.