Dual Objective Design Philosophy for Tornado Engineering John W. van de Lindt 1 , M. ASCE, Shiling Pei 2 , M. ASCE, David O. Prevatt 3 , M. ASCE, Thang Dao 1 , A.M. ASCE, William Coulbourne 4 , M. ASCE, Andrew J. Graettinger 1 , M. ASCE, and Rakesh Gupta 5 , M. ASCE. 1 Department of Civil, Construction, and Environmental Engineering, University of Alabama, Tuscaloosa, AL. 2 Department of Civil and Environmental Engineering, South Dakota State University, Brookings, SD. 3 Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL. 4 Applied Technology Council, Rehoboth Beach, DE. 5 Wood Science and Engineering, Oregon State University, Corvallis, OR. ABSTRACT Tornadoes like all natural hazards posses a full range of intensities with the majority having winds below 130 mph. In this paper, a dual objective-based tornado engineering design philosophy is explained that has the simultaneous objectives of (1) reducing monetary losses due to damage; and (2) reducing loss of human life. While these objectives may seem an obvious goal for any design code related to natural hazards they have not yet been adequately addressed within the context of tornado hazard by engineers and scientists. Consider that at the center of a tornado swath for a large Enhanced Fujita (EF) 5 tornado there are slabs swept clean of the residential building that once stood there, corresponding to a degree of damage (DOD) of level 10. Moving out perpendicular to the direction of the tornado path the DOD reduces at some gradient to a DOD of 1, which is the threshold of visible damage. To date, it has been widely agreed upon that there is little that can be done against intense tornadoes in woodframe buildings except seeking shelter. There are two considerations or design objectives for the approach outlined in this paper: damage (D) and life safety (L). Damage can be controlled at lower levels of the Enhanced Fujita scale wind speeds, i.e. EF0 and EF1, through the use of engineered connectors, design ensuring continuous vertical uplift load paths, and horizontal load distribution and load path. This is handled typically at the component design level, i.e. connectors, single load paths. For wind speeds corresponding to EF2 and EF3 level, both component and system- level loading must be considered to enable better performance. System level performance is related to load sharing amongst wall lines and distribution of the lateral load path as a whole throughout the building as it is racked by wind and amplified further by windborne debris catching on the structure. In tornadoes with wind speeds corresponding to EF4 and EF5-level damage, the major issue becomes system effects and focuses on other alternatives to provide life safety to the building occupants. These alternatives are above or below ground safe rooms and neighborhood safe shelters. INTRODUCTION Tornadoes, like all natural hazards, possess a full range of intensities from Enhanced Fujita (EF) 0 that removes shingles from houses to EF5 that cause total destruction. Currently in structural engineering design, tornado forces are not considered because of their very low probability of 965 Structures Congress 2012 © ASCE 2012 Structures Congress 2012 Downloaded from ascelibrary.org by Oregon State University on 08/22/12. For personal use only. No other uses without permission. Copyright (c) 2012. American Society of Civil Engineers. All rights reserved.