Thang N. Dao

Dual-Objective-Based Tornado Design Philosophy

AbstractTornadoes represent a unique natural hazard because of the very low probability of occurrence, short warning times (on the order of only a few minutes), and the intense and destructive forces imposed on engineered and nonengineered buildings. The very low-probability/very high-consequence nature of a tornado strike makes designing for survival and reducing damage under typical financial constraints a substantial challenge. On April 27, 2011, an enhanced Fujita (EF) 4 (EF4) tornado devastated an almost 10-km (5.9-mi) long, 0.8-km-wide (1/2-mi-wide) path, through the city of Tuscaloosa, Alabama, and continued on the ground for 130 km (80 mi). This paper presents the design concept that resulted following a week-long data reconnaissance deployment throughout the city of Tuscaloosa by the authors. The dual-objective philosophy proposed herein is intended to focus on both building damage and loss reduction in low-to-moderate tornado wind speeds and building occupant life safety in more damaging wind-sp...

Methodology for Wind-Driven Rainwater Intrusion Fragilities for Light-Frame Wood Roof Systems

Light-frame wood buildings represent most of residential structures throughout the United States. Approximately half of the U.S. population lives within 50 miles of the coast with many of those dwellings in the Gulf Coast region or along the eastern seaboard, both of which can be negatively impacted by hurricanes. The majority of damage during a hurricane is the result of wind-driven rainwater entering a building through openings caused by strong wind. To date, wind fragility approaches to examine the probability of damage to a light-frame wood building have focused only on component or subassembly strength, thereby providing information up to the point of first failure of the building envelope. These previous analyses, while valuable, will not allow hurricane engineering research to progress to fully mechanistic loss modeling, which is needed to mitigate losses caused by these events. In this paper a methodology to develop fragility curves and fragility surfaces for the volume of rainwater intrusion is summarized and demonstrated on an example structure. To do this, nonlinear structural analysis, computational fluid dynamics, and reliability theory are combined with particle dynamics for rainwater trajectory modeling, essentially providing the first fragilities of their kind and going beyond first failure of the building envelope. It should be emphasized that it is the methodology that is the focus and some level of calibration is still necessary.

Loss Analysis for Wood Frame Buildings during Hurricanes. I: Structure and Hazard Modeling

AbstractHurricanes present a serious financial threat to the Gulf Coast and eastern seaboard of the United States. This paper is the first in a set of companion papers that detail a comprehensive mechanistic loss model for wind and rainwater intrusion damage to light-frame wood buildings. In this paper the structural and hazard modeling for a passing hurricane is explained and demonstrated. The structural model consists of a nonlinear nail model integrated into a finite-element model, and a hazard model is developed using a combination of existing wind tunnel data and a rainwater intrusion model. A methodology to compute the rainwater entry volume is presented. The approach differs from previous methods in that it allows any time period to be examined by dividing the hurricane passage period into smaller increments of time and allows both load and resistance to change from time period to time period. Thus, the time and location of damage and rainwater intrusion is known. The companion paper utilizes the v...

Seismic Performance of an Innovative Light-Frame Cold-Formed Steel Frame for Midrise Construction

This paper presents the method and results of a study whose objective was to investigate the seismic performance of a new light- frame cold-formed steel-frame system. The system includes floor trusses, open panels, V-braced panels, columns, and connections. To do this, aseriesofreversed-cyclictestswereconductedonsubassemblies.Theresultsofthisexperimentalprogramprovidedinsightintothebehaviorof the subassemblies under cyclic loading, which in turn provided key information for numerical modeling of the subassemblies and the whole systemaswellasfailuremechanismsoftheframe.Nonlineartimehistoryanalyseswereperformedusinganewformulation fora4-storyframe usingasuiteofearthquakegroundmotionstodemonstratethebehaviorofatypicalV-bracedsysteminamoderateearthquake.Bothtestresults andnumericalanalysisindicatethatthesystemhasgoodductilityasaresultofthescrewedplateconnectionsbetweenthelight-gaugemembers and between the light-gauge members and square columns. The frame system performed very well at four stories making it a viable midrise construction option in seismic regions of the United States and around the world. DOI: 10.1061/(ASCE)ST.1943-541X.0000683.

Loss Analysis for Wood Frame Buildings during Hurricanes. II: Loss Estimation

AbstractThis is the second paper in a set of companion papers. The first paper focused on modeling the hurricane hazard and the methodology to compute the volume of rainwater entering a roof system during a hurricane. This second paper explains a methodology to probabilistically compute losses beginning with the type and location of damage and volume and location of rainwater intrusion into the roof system. The ability to identify each contribution to damage and monetary loss is a necessary condition for development of performance-based wind engineering, thus providing the impetus for this second paper. The methodology summarized in this paper is capable of tracking damage and the individual damageable component contributions to the total loss probability. An illustrative example that compares losses for the same light-frame wood residential structure introduced in the first paper is presented. In addition, both damage and loss probability distributions are presented for each room in the structure, as wel...

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

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.

Displacement-Based Design of Buildings with Torsion: Theory and Verification

AbstractDirect displacement design (DDD) is a procedure that allows one to distribute the forces induced by an earthquake to the levels of a multistory building to ensure that the desired level of interstory drift is not exceeded. To date, DDD has only been applied to buildings that do not exhibit significant torsional response. This paper presents a methodology to perform displacement-based design (DBD) on multistory buildings with in-plane torsional irregularities, thereby generalizing DBD for buildings with torsion. The procedure includes decoupling the contribution of the deformation that results from translation and torsion by using an existing approximation technique. The approach is validated by using detailed finite-element models of asymmetric buildings; it is found to accurately reproduce the desired dynamic structural properties. Both linear and nonlinear (elastic-perfectly plastic) systems are demonstrated and the accuracy is verified. The method is shown to be very accurate for linear systems...

Performance of Residential Shelters during the May 20, 2013, Tornado in Moore, Oklahoma

AbstractResidential shelter performance, in the aftermath of the devastating May 20, 2013, EF5 tornado that struck Moore, Oklahoma, was investigated by a reconnaissance team comprised of researchers from several universities. The team observed and documented 75 residential shelters along the tornado damage path and noted that the concept of shared shelter use was responsible for saving many lives. All belowground shelters performed well with no evidence of structural compromise or shelter breach from debris missiles; however, debris was occasionally observed blocking the opening of the garage slab residential shelters and flooding was also noted in a few cases. Although less common, the majority of above ground residential shelters observed were found in areas that experienced EF3–EF4 estimated wind speeds. In all cases but one, the above ground safe rooms performed well. A debris missile breach of an insulated concrete form (ICF) safe room was documented. While poor construction quality, specifically poo...

Hybrid System of Unbonded Post-Tensioned CLT Panels and Light-Frame Wood Shear Walls

AbstractCross-laminated timber (CLT) is a relatively new type of massive timber system that has shown to possess excellent mechanical properties and structural behavior in building construction. When post-tensioned with high-strength tendons, CLT panels perform well under cyclic loadings because of two key characteristics: their rocking behavior and self-centering capacity. Although post-tensioned rocking CLT panels can carry heavy gravity loads, resist lateral loads, and self-center after a seismic event, they are heavy and form a pinched hysteresis, thereby limiting energy dissipation. Conversely, conventional light-frame wood shear walls (LiFS) provide a large amount of energy dissipation from fastener slip and, as their name implies, are lightweight, thereby reducing inertial forces during earthquakes. The combination of these different lateral behaviors can help improve the performance of buildings during strong ground shaking, but issues of deformation compatibility exist. This study presents the re...

Lidar-Based Methodology to Evaluate Fragility Models for Tornado-Induced Roof Damage

AbstractStructural fragility models have been developed to provide probabilistic estimates of building component damage states caused by exposure to differing wind speeds. Limitations in taking accurate measurements and gathering quantitative loss information in the aftermath of tornadoes has not allowed for comparing fragility-based information with actual building-loss data. This paper presents a methodology for extracting damaged building geometry and roof sheathing loss information from three-dimensional (3D) point cloud data collected with laser scanning technology. The observed building component damage states are then compared to fragility model predictions to validate the models. The methodology was tested on 3D data collected after the 2013 Moore, Oklahoma tornado. From this initial study, which was performed on 5 houses having 27 damaged roof surfaces, it was shown that fragility models underestimated the loss in the center of roof planes (Zone 1) and overestimated the loss in the corners of roo...