David B. Roueche

Empirical Approach to Evaluating the Tornado Fragility of Residential Structures

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...

An Engineering-Based Approach to Predict Tornado-Induced Damage

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.

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

Performance-based engineering (PBE) is a methodology that requires specification on a range of performances or target reliabilities for structures of different importance. Information on these ‘performance levels’ require a probabilistic assessment of the potential factors that may influence a design, including information on the hazard, load, resistance, loss estimates, expert opinion and public perception. This paper describes one such probabilistic assessment in the development of empirically-based fragility functions for tornadoes using damage assessment data and a tornado wind field model for the 22 May 2011 Joplin, MO tornado. The damage assessment data was collected during field surveys of more than 1,240 structures in the aftermath of the tornado, using provisions of the Enhanced Fujita (EF) Scale to assess the damage. The wind field model was developed from the tree-fall patterns noted in the damage path of the tornado. Fragility functions were developed for the Degrees of Damage (DOD) associated with One- and Two-Family Residences in the EF Scale. The empiricallyderived fragility functions were progressive in nature, with median wind speeds varying from 33.6 m/s for initiation of visible damage to 85.2 m/s for complete destruction. These functions were compared to existing fragility functions for straightline winds to evaluate potential differences in failure mechanisms for structures exposed to tornadoes. Wind speeds associated with the median failure probability were used to estimate load factors, defined as the square of the ratio of the straightline wind speed to the tornado wind speed. Structures tended to fail at lower wind speeds in tornadoes than in straightline winds, with load factors between 1.32 and 1.51. A fragility assessment in the context of PBE naturally requires attribution and quantification of all uncertainties. Uncertainties in the both the damage and wind speed estimation in the development of fragilities are quantified and assessed using Monte Carlo methods. Preliminary results show variance in fragility parameters is higher for higher damage states but all damage states have relatively low coefficients of variation.

Wind Uplift Capacity of Foam-Retrofitted Roof Sheathing Panels Subjected to Rainwater Intrusion

AbstractWidespread damage to residential roofing systems during Hurricane Andrew in 1992 highlighted the structural inadequacy of wood roof sheathing attachment in existing Florida homes. Prior research found that retrofitting wood decks with closed-cell spray-applied polyurethane foam (ccSPF) could provide adequate wind resistance to pre-1994 roofs. However, it is unknown whether this retrofit method increases the potential for moisture to accumulate in the roof sheathing, and if so, what effect this has on the strength of the bond between the ccSPF and the wood substrate. Five full-scale roofs were subjected to natural weathering for 3 months, and 131 small-scale laboratory samples of ccSPF bonded to wood sheathing were intermittently sprayed with water and stored in a high-humidity environment for up to 16 weeks. These tests enabled systematic assessment of the effects of water leakage on ccSPF-retrofitted roof structures. The attic roofs were retrofitted with two ccSPF configurations and covered with ...

Using Tornado Damage Surveys to Improve Laboratory Tornado Simulations

Laboratory tornado simulators, intended for estimating tornado-induced wind loading on structures, have been in development for a little more than ten years with the first such facility being developed at Iowa State University. The ideal validation of such facilities would compare pressures on buildings in the lab to pressures on buildings in the field. Since this is nearly impossible from a practical standpoint, other validations must be sought. In the past, the Iowa State facilitys performance has been compared with Doppler radar velocity profiles, with surface pressure profiles and with damage patterns for individual structures. This paper summarizes how the simulators results compare with the newly acquired damage data from the Moore, Oklahoma tornado of May 21, 2013. The data from this tornado consists of geo-located, structural damage data. The structural damage was plotted as a function of radial distance from the center of the damage path. This function was then compared with lab simulator estimates of how tornado-induced forces vary with distance from the center of the vortex. The lab simulator over-predicted the damage for most radial positions, but the analysis highlights the promise of the lab simulation. The shape of the curves was reasonably consistent and could be improved with a more rational connection between predicted forces and resulting damage.

Development of empirically-based fragilities of residential damage in the 2011 Joplin, Missouri tornado

Performance-based engineering (PBE) is a methodology that requires specification on a range of performances or target reliabilities for structures of different importance. Information on these ‘performance levels’ require a probabilistic assessment of the potential factors that may influence a design, including information on the hazard, load, resistance, loss estimates, expert opinion and public perception. This paper describes one such probabilistic assessment in the development of empirically-based fragility functions for tornadoes using damage assessment data and a tornado wind field model for the 22 May 2011 Joplin, MO tornado. The damage assessment data was collected during field surveys of more than 1,240 structures in the aftermath of the tornado, using provisions of the Enhanced Fujita (EF) Scale to assess the damage. The wind field model was developed from the tree-fall patterns noted in the damage path of the tornado. Fragility functions were developed for the Degrees of Damage (DOD) associated with One- and Two-Family Residences in the EF Scale. The empiricallyderived fragility functions were progressive in nature, with median wind speeds varying from 33.6 m/s for initiation of visible damage to 85.2 m/s for complete destruction. These functions were compared to existing fragility functions for straightline winds to evaluate potential differences in failure mechanisms for structures exposed to tornadoes. Wind speeds associated with the median failure probability were used to estimate load factors, defined as the square of the ratio of the straightline wind speed to the tornado wind speed. Structures tended to fail at lower wind speeds in tornadoes than in straightline winds, with load factors between 1.32 and 1.51. A fragility assessment in the context of PBE naturally requires attribution and quantification of all uncertainties. Uncertainties in the both the damage and wind speed estimation in the development of fragilities are quantified and assessed using Monte Carlo methods. Preliminary results show variance in fragility parameters is higher for higher damage states but all damage states have relatively low coefficients of variation.

Wind Uplift Capacity of Foam-Retrofitted Roof Sheathing Subjected to Water Leaks

A well-known source of damage to houses in hurricanes occurs when water bypasses failed roof coverings that allow water to enter the interior through joints in the wood roof decks. Closed-cell spray-applied polyurethane foam (ccSPF) sprayed to the underside of the roof functions as a secondary water barrier to mitigate this damage, in addition to its primary function as a thermal barrier. Recent studies at the University of Florida revealed that ccSPF also significantly increases the wind uplift resistance of a wood roof deck due to its strong bond to wood substrates. This presentation describes a research project that investigated the effects of incidental water leakage on the strength of the ccSPF-to-wood bond and on moisture retention characteristics in a wood roof. The two-phased study consisted of the construction and long-term testing of full-scale roof attics exposed to outdoor environmental conditions in Gainesville, FL, and bench-type studies using small-scale roof deck samples. Each roof attic was retrofitted using ccSPF, selfadhered membrane underlayment and/or air gaps between the sheathing and ccSPF. Numerous 1⁄2 in. diameter holes (leak gaps) cut into the roofing created sources of water leaks, and we continuously monitored moisture content in the wood in real-time through a web-based application. The wind uplift capacity of roof panels (ultimate failure pressure), were determined at the end of each exposure period. Concurrently, small-scale testing was conducted to measure the tensile strength of the wood-to-ccSPF bond for samples exposed to up to 16 weeks of intermittent water sprays. The moisture distribution in 6 in. x 6 in. wood (OSB and plywood) roof deck samples was also determined, representing common construction patterns such as vertical or horizontal sheathing joints, and the configurations of full-scale retrofit systems. While ccSPF remains highly effective as a structural retrofit despite significant wetting, elevated moisture content occurs within the wood substrate. Successful techniques were demonstrated to mitigate moisture retention, such as use of self-adhered waterproofing membrane or including an underside-deck air gap within the ccSPF retrofit layer that resulted in substantial reduction (90% and 80%, respectively) in moisture contents within the sheathing. The study has led to recommendations for the installation and maintenance of ccSPF-retrofitted residential roofs, and the use of similar wood-foam composite systems in wood-framed buildings. BACKGROUND Failure of roof sheathing during extreme wind events is a common failure mode in residential roofs. The majority of hurricane-related losses are sustained by residential homes and 95% of these are from failures within roof-systems (Baskaran and Dutt, 1997). Inadequate fastening of wood sheathing to roof framing members is the most common failure mode. Roof 1st Residential Building Design & Construction Conference – February 20-21, 2013 at Sands Casino Resort, Bethlehem, PA PHRC.psu.edu