David O. Prevatt

Making the Case for Improved Structural Design: Tornado Outbreaks of 2011

A total of 1,625 tornadoes occurred in the United States in 2011, resulting in economic losses that exceeded

Engineering Perspectives on Reducing Hurricane Damage to Housing in CARICOM Caribbean Islands

25 billion. Two tornado outbreaks stand out because they caused more than half of those losses. The tornadoes that cut through Tuscaloosa, Alabama, on April 27 and Joplin, Missouri, on May 22 were responsible for a combined 223 fatalities and more than 13,000 damaged buildings in the two cities. Although the economic losses associated with tornado damage are well documented, the writers argue that the overall impact should encompass longer term, broader considerations such as the social dis- ruption and psychological effects that impact communities. This paper examines observations by tornado damage assessment teams led by the first author in these two medium-sized cities and suggests that the evolution of building codes and past approaches to construction have led to conditions that made this extent of damage possible. The authors outline a multidisci- plinary path forward that incorporates engineering research and social and economic studies into a new design paradigm leading to building code changes and social practices that will improve resistance and mitigate future losses at a community level from tornadoes.

Failure Progression Analysis of Observed Residential Structural Damage within a Tornado Wind Field

The annual hurricane related losses to Caribbean residential buildings in the past two decades highlight the societal and socioeconomic vulnerability of these islands. Despite considerable hurricane disaster mitigation activity in the Caribbean Community and Common Market (CARICOM), the housing sector in particular remains at an elevated risk for damage. Improving the structural resiliency of homes has been largely ineffective, as was illustrated by damage to most of the Grenadian housing stock from Hurricane Ivan in 2004. While international and regional interventions have provided emergency postdisaster aid, sustained efforts to improve design and construction practices and foster sustainable mitigation activity are lacking, with the limited projects not gaining much traction in the construction sector. This paper provides an exploratory framework of the issues surrounding wind damage mitigation and the construction of residential structures to improve overall hurricane resilience in the CARICOM states. It synthesizes 35 years of research findings about Caribbean hazard mitigation initiatives, many of which are not available to researchers in other geographic settings. The uniqueness of the Caribbean housing stock, the importance of small-scale economies, the existing socioeconomic constraints, and the effectiveness of regional and international residential construction mitigation strategies are also evaluated. Finally, the paper identifies and prioritizes critical avenues of future research to improve the resiliency of the CARICOM residential building stock.

GIS for the Geo-Referenced Analysis and Rapid Dissemination of Forensic Evidence Collected in the Aftermath of the Tuscaloosa Tornado

Data collected from recent tornadoes in Tuscaloosa, Joplin, and Moore shows a consistent pattern of damage to residential structures. For an EF-4 or EF-5 tornado, damage levels increase from the outer edges toward to the center line of a tornado track. This is not just because of higher wind speeds at the center of a tornado vortex; the wind velocity fields around structures are also different at the tornado center. Analysis from the damage pattern from the tornado showed that the failure progression of residential structures within a tornado wind field depends on the relative location and direction of the house to the tornado track. With the same wind speed, different damage levels can be observed if structures located in different relative distances from the center-line of a tornado damage track. This should be considered when predicting tornado wind speed based on residential structural damage.

Empirical Approach to Evaluating the Tornado Fragility of Residential Structures

This study investigates the utilization of Geographic Information System (GIS) technology to rapidly disseminate damage and failure mode data related to wood-frame construction collected in the aftermath of the Tuscaloosa tornado on April 27, 2011. The City of Tuscaloosa was in the direct path of a major super cell tornado that bisected the city in a south-west to north-east direction with a half mile wide destruction path. Immediately after the tornado, forensic data collection activities were conducted. Forensic evidence was collected through both active and passive modes. Active data collection occurred at specific case study sites and along transects that ran approximately perpendicular to the direction of the tornado path. Passive data collection modes captured forensic evidence as researchers moved throughout the affected area. Tracking both the time and the location of field activities set the basis for the fusion of the collected data. Thus, by using the time of data collection, forensic data was correlated to a geo-location in full-day GPS track logs and uploaded in a GIS web portal. The web portal enabled 1) the geo-referenced analyses of the collected data, and 2) the rapid dissemination of the forensic evidence to the scientific community and to anyone with an internet connection.

An Engineering-Based Approach to Predict Tornado-Induced Damage

AbstractTornado-induced wind load modeling has advanced significantly in recent years, but comparison of the experimental or numerical models to observed tornado damage is limited. This paper descr...

Wind Uplift Resistance of Artificially and Naturally Aged Asphalt Shingles

The tornado risk assessment methodology currently used by both private and public agencies utilizes empirically derived loss models that rely on historical claims data for predicting future effects of tornadoes. The accuracy of these empirical models is dependent on many factors, including the quality and quantity of available historical data, accuracy of the tornado intensity models, and the universality of applying those empirical models from one region to another. A more rigorous approach may be the development of engineering-based damage assessment models, made applicable to construction in any region and to any tornado that varies in size and strength. This chapter presents a framework for an engineering-based tornado damage assessment (ETDA) for low-rise buildings. The model predicts damage on the most vulnerable sector of the built environment, nonengineered residential buildings. The model components include a translating tornado vortex model, a tornado-induced wind load calculation approach, a probabilistic wind-borne debris impact model, and a time-variant model for internal pressure changes within the structure. The time evolution of structural damage to a building is determined using successive time steps of component level wind loading vs. structural resistance as the tornado translates past the building. The output of this model is a percentage damage index for each component and the overall building damage ratio. The ETDA model is illustrated using four houses damaged in the 2011 Joplin, MO, tornado. Predicted damage using the ETDA model is in good agreement (within 15 %) of post-tornado damage observations reported by the third author.

What Do We Learn from Wind Uplift Tests of Roof Systems

AbstractDamage assessment surveys performed following the 2004 and 2008 hurricane seasons indicate that the vulnerability of shingle roofs to wind-induced damage increases with age. This paper presents two interrelated experiments on the wind uplift resistance of asphalt shingle seals. The objective is to identify whether and to what extent aging reduces the wind uplift capacity. In the first experiment, asphalt shingles were artificially aged, and the mechanical uplift resistance was measured at discrete intervals. In the second experiment, mechanical uplift tests were performed in situ on ≥9-year-old shingle roofs installed on four central Florida homes. Results from the first experiment demonstrate that heat exposure can reduce uplift resistance along its sealant strip in some products; however, the excess capacity in the bond may be sufficiently large to prevent the mechanical resistance from dropping below the design requirement. Inclusion of ultraviolet and water in the heat-exposure tests did not a...

Wind Loads on Single-Family Dwellings in Suburban Terrain — Comparing Field Data and Wind Tunnel Simulation

There are several industry-accepted test protocols to evaluate the wind uplift resistance of commercial roofing systems (ASTM E1592, UL1897, UL580, FM 4470 etc.). These tests apply pneumatic pressure to full-scale roof specimens installed on a pressure chamber, and increasing the pressure until failure. The wind uplift design pressure is then determined by reducing the peak pressure by an accepted factor of safety (e.g. 1.5 or 2.0). A critical question to be asked on the results is the meaning or validity of these static test results when hurricane winds produce dynamic roof pressures that are constantly fluctuating in time and space? The presenter reviews similarities and limitations of common wind uplift test methods for standing seam metal panels, and mechanically attached single ply roofing systems that have been the industry-standard for many years and reports on research results for these systems. Most of the test protocols provide comparative results, absent any verifiable relationship to roof performance in actual wind storms. Of particular interest now is the determination of the wind design pressure for lightframed wood roof systems, for which there exists no industry-accepted test protocol. The fastener schedules and minimum fasteners included in current ICC building code today appear to be based on research conducted immediately after Hurricane Andrew. Recent experimental studies at the University of Florida revisited those studies and developed a new dynamic test protocol for wind uplift testing of roof sheathing panels. The results have shown inconsistency in previous tests and that static pressure testing of wood roofs may over-estimate their failure capacity when compared with dynamic pressure test methods. The presentation concludes with a proposal for standardizing the wind uplift testing of wood roof systems based on dynamic pressure fluctuations. The conclusion has far-reaching implications as similar modifications may be required to calibrate current static pressure test methodologies for commercial roof systems so that they for hurricane-prone locations.

Development of Empirically-Based Fragilities of Residential Damage in the 2011 Joplin, Missouri, Tornado

Components and cladding on residential structures continue to be damaged in high winds despite improvements to building codes. Modern wind design codes that are helping to prevent structural damage to buildings are not as effective in preventing building envelope failures. Since 1998, a unique collaborative effort between Clemson University, the University of Florida, and Florida International University called the Florida Coastal Monitoring Program (FCMP) has been collecting full-scale pressure and wind speed data on residential buildings in suburban areas. The program, sponsored by the Florida Department of Community Affairs (FDCA), was developed to increase our limited full-scale data available on wind loadings of low rise buildings – none of which was obtained during sustained hurricane force winds until this project. Another objective was to determine the validity of current design wind load values that are based on wind tunnel tests using open-country exposure. This serves as the primary motivation to study pressure variation on roofs of suburban houses. This paper presents results of the field data collection and analysis of pressure coefficient data from field experiments and wind tunnel results obtained at Clemson University’s atmospheric boundary layer wind tunnel.