DISSERTATION DAMAGE ANALYSIS AND MITIGATION FOR WOOD-FRAME STRUCTURES SUBJECTED TO TORNADO LOADING Submitted by Christine Diane Standohar-Alfano Department of Civil and Environmental Engineering In partial fulfillment of the requirements For the Degree of Doctor of Philosophy Colorado State University Fort Collins, Colorado Spring 2016 Doctoral Committee: Advisor: John W. van de Lindt Bruce R. Ellingwood Paul R. Heyliger Russ S. Schumacher Copyright by Christine D. Standohar-Alfano 2016 All Rights Reserved ABSTRACT DAMAGE ANALYSIS AND MITIGATION FOR WOOD-FRAME STRUCTURES SUBJECTED TO TORNADO LOADING Tornadoes are one of the most devastating natural hazards that occur in the United States. While there is an average of approximately 1200 tornadoes per year across the country, the annual likelihood of experiencing a tornado at a particular location is quite small due to their relatively small size. However, the high consequence of a tornado strike necessitates the determination of geographic tornado hazard. A methodology to estimate the annualized probabilistic tornado hazard over the contiguous U.S. was developed and used the most recent 38 years of climatological tornado data. Furthermore, with the use of detailed damage surveys after the April 3-4, 1974 and April-May, 2011 tornado outbreaks, an empirical method was developed and applied to account for the gradient of wind speed along a tornado’s path length and path width. From this, a probabilistic tornado hazard index was developed across the United States which quantified the annual probability of experiencing a tornado of any strength on the Enhanced Fujita scale. Tornado hazard curves were developed from the tornado hazard analysis at six illustrative locations which varied as a function of location-specific occurrence rates. Five different residential wood-frame building archetypes were designed at each of the locations based on current residential building code and/or practice. Fragilities for the roof sheathing, truss to wall top-plate, and wall-to-foundation connections were developed for each archetype. At each of the six locations, the fragility curves for the locally adopted residential building code were ii convolved with the tornado hazard curve at that specific location in order to compute annual failure probabilities for select components along the vertical load path. This was one of the first times unconditional risk of component failure due to tornadoes has been computed since the tornado hazard curve was convolved with the fragility curves. These probabilities quantify failure probabilities of residential wood-frame construction components to tornado winds. In addition, the more wind-resistant Florida residential building code is applied to other locations in the U.S., fragilities are developed and convolved, and failure probabilities for these modified buildings are computed. This resulted in a quantitative measure of risk reduction from tornadoes by using strengthened construction at various locations across the country. The convolved failure probabilities were first developed for individual components. The system level behavior of the entire structure was also assessed and included the correlated dependencies between individual components. Results indicate that stricter building codes may be beneficial in areas with a high annual tornado risk, such as Tornado Alley. The final portion of this work used a simplified property loss model applied to the April 25-28, 2011 tornado outbreak. This was one of the largest tornado outbreaks in U.S. history and resulted in over $5B in property loss. In order to determine property loss over a broad area, census data regarding household income and home market value was utilized. The performance of manufactured homes had to be considered in conjunction with wood-frame residential construction since the tornado outbreak impacted the southern U.S. which has a high number of manufactured homes. Using the system level fragility analysis, property loss was estimated based on both locally adopted residential codes and the stricter guidelines described in the Florida Residential Building Code. Results indicate that using strengthened construction methodologies would reduce property loss up to 40% as compared to current design guidelines. iii ACKNOWLEDGEMENTS First I would like to express my deepest gratitude to my advisor, Dr. John van de Lindt for seeing the potential of multidisciplinary work. His patience and understanding throughout my time as his student are greatly appreciated. He provided substantial guidance and motivation that pushed me outside my comfort zone so that I could successfully transition into the field of structural engineering. I would also like to thank and recognize my committee members Dr. Bruce Ellingwood, Dr. Paul Heyliger, and Dr. Russ Schumacher for providing me with additional support, ideas, and valuable critique. Additionally, I’d like to thank the Civil Engineering faculty members at Colorado State University and the University of Alabama. I would also like to thank the University of Alabama Graduate Council Fellowship Office and acknowledge the financial support of Colorado State University and the George T. Abell Professorship funds. Finally, I’d like to thank the many people who supported me during my graduate care. I am especially grateful to my colleagues Omar Amini, Eric Holt, Hassan Masoomi, Elaina Sutley, Negar Nazari, and countless others for their invaluable assistance and patience. In addition, I’d like to thank my dad and siblings for providing the encouragement necessary for completing my graduate career. Lastly, I’d like to thank my husband Joel for his patience and support. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................... ii ACKNOWLEDGEMENTS ........................................................................................................ iv LIST OF TABLES ..................................................................................................................... viii LIST OF FIGURES ..................................................................................................................... xi INTRODUCTION AND MOTIVATION ................................................................................... 1 1.1 Statement of Problem ............................................................................................................ 1 1.2 Impact of Tornadoes on Woodframe Residential Structures ................................................ 3 1.3 Framework of the Solution ................................................................................................... 9 1.4 Limitations of the Study ..................................................................................................... 11 TORNADO HAZARD ESTIMATION ..................................................................................... 16 2.1 Historical Evaluations of Tornado Risk .............................................................................. 16 2.2 Description of Data ............................................................................................................. 17 2.3 Methodology ....................................................................................................................... 23 2.3.1 Obtaining Tornado Area .............................................................................................. 23 2.3.2 Obtaining Wind Speed Probabilities from a Weibull Distribution .............................. 24 2.3.3 The Gradient Technique for Reduced Area ................................................................. 27 2.4 Annual Tornado Probability Estimation ............................................................................. 35 2.5 Determining the Annual Tornado Hazard at a Location ..................................................... 40 2.6 Results of Probabilistic Tornado Hazard Analysis ............................................................. 43 2.7 Development of Tornado Hazard Curves ........................................................................... 48 2.8 Uncertainty in the Tornado Hazard Analysis ..................................................................... 49 FRAGILITY ANALYSIS ........................................................................................................... 58 3.1 Introduction to Fragility Analysis ....................................................................................... 58 3.2 Sample Locations ................................................................................................................ 65 3.3 Uncertainty in Tornado Wind Loads .................................................................................. 67 3.4 Wind Load Statistics ........................................................................................................... 70 3.5 Dead Load and Resistance Statistics .................................................................................. 77 3.5.1 Roof Sheathing Connection ......................................................................................... 77 3.5.2 Roof-to-Wall Connection ............................................................................................. 80 v 3.5.3 Wall-to-Foundation Connection .................................................................................. 82 3.6 Results of the Fragility Analysis ........................................................................................