Light-frame Wood Research Articles

Tsunami Forces and Effects on Structures

The 2004 Indian Ocean Tsunami claimed more than 220,000 lives. It was a low-probability high-consequence event. A similar disaster could strike elsewhere, particularly in the Pacific but also in Caribbean, Atlantic, and Mediterranean regions. Unlike in seismic ground shaking, there is usually a short lead-time precedes tsunami attack: from a few minutes for a local source to several hours for a distant source. Because mega-tsunamis are rare and because forewarning of these events is possible, the primary mitigation tactic to date has been evacuation. Hence, most efforts have focused on the development of effective warning systems, inundation maps, and tsunami awareness. This strategy makes sense from the standpoint of saving human lives. However, it does not address the devastating damage to buildings and critical coastal infrastructure, such as major coastal bridges, oil and LNG storage facilities, power plants, and ports and harbors. Failure in critical infrastructure creates enormous economic setbacks and collateral damage. The accelerating construction of critical infrastructure in the coastal zone demands a better understanding of design methodology in building tsunamiresistant structures. In some coastal areas such as low-elevation coastal spits or plains, evacuating people to higher ground may be impractical because they have no time to reach safety. In these situations, the only feasible way to minimize human casualties is to evacuate people to the upper floors of tsunami-resistant buildings. Such buildings must be designed and constructed to survive strong seismic ground shaking and subsequent tsunami impacts. The primary causes of structural failure subject to tsunami attack can be categorized into three groups: 1) hydrodynamic force, 2) impact force by water-born objects, and 3) scour and foundation failure. Tsunami behaviors are quite distinct, however, from other coastal hazards such as storm waves; hence the effects cannot be inferred from common knowledge or intuition. Recent research has addressed tsunami forces acting on coastal structures to develop appropriate design guidelines, and mechanisms leading to tsunamigenerated scour and foundation failures. This special issue is a compilation of 14 papers addressing tsunami effects on buildings and infrastructure. The four main groupings begin with two papers on tsunami force acting on vertical walls. Arikawa experimentally investigates the structural performance of wooden and concrete walls using a large-scale laboratory tank in Japan. Also using a similar large-scale tsunami flume but in the US, Oshnack et al. study force reduction by small onshore seawalls in front of a vertical wall. The second grouping focuses on tsunami force on 3-D structures. Arnason et al. present a basic laboratory study on the hydrodynamics of bore impingement on a vertical column. Fujima et al. examine the two types of formulae for tsunami force evaluation: the one calculated from flow depth alone and the other based on the Euler number. Lukkunaprasit et al. demonstrate the validity of force computation recommended in a recently published design guideline (FEMA P646) by the US Federal Emergency Management Agency. The other two papers look into the specific types of structures: one is for light-frame wood buildings by van de Lindt et al, and the other is for oil storage tanks by Sakakiyama et al. The topic of debris impact force is the focus of the third grouping. Matsutomi summarizes his previous research on impact force by driftwoods, followed by the collision force of shipping containers by Yeom et al. Yim and Zhang numerically simulate tsunami impact on a vertical cylinder; this paper is included in this grouping because their numerical approach is similar to that of Yeom et al. As for the fourth grouping, Shuto presents field observations on foundation failures and scours, and Fujii et al. discuss the erosion processes of soil embankments. There are two more papers: those are the application of fragility analysis to tsunami damage assessment by Koshimura et al. and evaluation of an offshore cabled observatory by Matsumoto and Kaneda. The topics presented here are undoubtedly in progress, and many revisions and improvements are still needed in order to achieve better predictability for tsunami effects on buildings and infrastructure. We hope you find the papers in this issue intriguing and the information useful, and become further interested in this important natural hazard. Lastly, we wish to express our appreciation to the authors for their timely contributions, and to the reviewers for their diligent and time-consuming efforts.

Wave Impact Study on a Residential Building

Recent natural disasters around the world including both tsunamis and hurricanes, have highlighted the inability of wood buildings to withstand wave and surge loading during these extreme events. Little is known about the interaction between coastal residential light-frame wood buildings and wave and surge loading because often little is left of the buildings. This leaves minimal opportunity for forensic investigations. This paper summarizes the results of a study whose objective was to begin to better understand the interaction between North American style residential structures and wave loading. To do this, one-sixth scale residential building models typical of North American coastal construction, were subjected to tsunami wave bores generated from waves of heights varying from 10 cm to 60 cm. The lateral force produced by the wave bores were, as expected, found to vary nonlinearly with parent wave height.

Systematic Seismic Design for Manageable Loss in Wood-Framed Buildings

Light frame wood structures make up the vast majority of the residential building stock in the United States. Because of this, earthquake-induced losses for this category of building from a significant earthquake would have a substantial financial impact on the regional economy, as well as on the building owner. Current wood-framed structural design philosophy focuses only on life safety and only limits damage through implicit assumptions. The concept of loss-based seismic design is introduced in this paper with typical loss-based design statements explicitly formulated with the intent of addressing the concerns, e.g., financial loss, of the building end-user. The loss-based design procedure was established based on a loss estimation framework that relied on the existing concept of assembly-based vulnerability (ABV). With the help of an automated dynamic and loss analysis package developed for wood-framed structures (SAPWood™) at Colorado State University, loss-based seismic design for a typical North American single family residential building was conducted for several different explicitly stated loss targets. The results from the numerical examples showed that loss-based seismic design for wood-framed structures is a viable concept that can serve as an important step in the evolution of end-user oriented, performance-based seismic design (PBSD).

Experimental Seismic Response of a Full-Scale Light-Frame Wood Building

A full-scale, two-story, light-frame wood townhouse building, designed according to modern U.S. engineered seismic design requirements, was tested on two triaxial shake tables operating in unison. The main objective of this experimental study was to determine the dynamic characteristics and the seismic performance of the test building under various base input intensities, representative of both ordinary and near-field ground motions in southern California. The building was tested with and without interior (gypsum wallboard) and exterior (stucco) wall finishes. The test results revealed that the installation of gypsum wallboard to the interior surfaces of structural wood sheathed walls substantially improved the seismic response of the test building. The application of exterior stucco further improved the seismic response of the test building, particularly in its longitudinal direction, where the shear response of low aspect ratio wall piers dominated. These shake table test results provide the evidence of...

Formulation of Seismic Fragilities for a Wood-Frame Building Based on Visually Determined Damage Indexes

The study of earthquake engineering has made significant strides over the last one-half century with scientists developing methods to better understand the basis and mechanisms of earthquakes and engineers working to mitigate economic loss and fatalities. A paradigm known as performance-based seismic design (PBSD) not only provides life safety to building occupants, but seeks to control structural and nonstructural damage in buildings and other structures. The development of fragility curves based on the well-known Park-Ang damage index is examined herein. This type of formulation can provide the information needed to assess the seismic vulnerability of a structure. Existing shake table test data from the NEESWood Project’s test of a 223 m2 (1,800 sq ft) two-story house was combined with a participant survey to calibrate a damage model. The result was the development of damage fragilities based exclusively on nonlinear time history analysis. Then, the proposed numerical damage model was applied and fragility curves were developed for a six-story light-frame wood condominium building. The results appear logical based on observations of system-level shake table tests over the last decade, and thus the method shows promise provided significant torsion is not present in the system.

Three-Dimensional Seismic Response of a Full-Scale Light-Frame Wood Building: Numerical Study

The experimental seismic responses of a full-scale two-story light-frame wood townhouse building, designed to modern U.S. engineered seismic design requirements, were compared against the predictions of a new software package entitled seismic analysis package for woodframe structures (SAPWood) developed recently within the NEESWood Project. The main objective of this paper was to verify the accuracy of the predictions from the SAPWood model, which incorporates shear deformations of shear walls as well as cumulative floor displacements caused by the out-of-plane rotations of the floor and ceiling diaphragms. A comparison was conducted on interstory drifts and shear wall deformations for various structural configurations (construction phases) of the test building and excitation levels. Good agreement was found between the numerical predictions and test results for the four different construction phases. The SAPWood model was shown to be a promising numerical tool for predicting the seismic response of light-frame wood structures.

Statistical and analytical models for roof components in existing light-framed wood structures

Residential wood-framed construction failures account for the majority of economic losses following hurricanes. A common failure in these constructions during high wind events is loss of roof sheathing, especially in corner areas. Less common perhaps, but usually catastrophic, is the failure of the roof-to-wall connections in these structures. The main objective of the current research project is to evaluate the in-situ capacity of roof-to-wall connections and sheathing to rafter fasteners in light-framed wood constructions. The unique opportunity provided by Clemson University to access four residential structures located within a residential complex enabled the collection of perishable yet statistically significant data on the strengths of existing residential structures. The uplift capacities of 100 roof-to-wall toenail connections and 34 plank sheathing units were evaluated from field and laboratory tests. Realizing the key role of probability distributions in developing fragility estimates and loss prediction models, distribution fits and parameters for these structural components are postulated. One conclusion drawn is that the uplift capacities of two and three nail connections are best described by a lognormal distribution. The initial stiffness and the vertical displacement at peak load of both two nail and three nail connections follow a normal and Weibull distribution respectively. The uplift capacity of plank sheathing follows a lognormal distribution. An analytical model designed to approximate the uplift behavior of toenail connections is developed to facilitate modeling of roof systems. These probabilistic and analytical models developed by this study allow for the performance of detailed reliability based studies on light-framed wood roof structures.

Tsunami bore forces on a compliant residential building model

The forces exerted on light-frame wood buildings as a result of surge and waves are not fully understood. With a better understanding of these types of forces, it may eventually be possible to build coastal structures to better withstand the loads. In this paper, a recent two part experimental study that focused on determining the forces induced in a structurally compliant model of a typical Gulf Coast residential building is summarized. The one-sixth scale building was designed to approximately behave as the full scale building would under wave loading using rules of energy-based similitude. The compliant model was subjected to solitary wave loading in the Network for Earthquake Engineering Simulation (NEES) tsunami wave basin (TWB) at Oregon State University with wave heights ranging from 0.1 m to 0.6 m. Then, at Colorado State University, lateral force–deformation tests on a nominally identical model were performed in order to determine the force–deformation relationship for the building. The structural deformation produced by solitary waves in the wave basin was combined with the experimentally measured deformation in the structural laboratory to determine the force induced by the waves between 0.2m and 0.6 m. Finally, a simplified force equation constant similar to the existing design code formats was found to be 0.31.

Capacities of OSB-Sheathed Light-Frame Shear-Wall Panels with or without Perforations

Walls of low-rise wood light-frame buildings are commonly employed as the primary means for resisting damaging effects of horizontal forces caused by events like hurricanes and earthquakes. Here test data show how openings for doors and windows influence the in-plane shear stiffness and load capacity of light-frame wall panels sheathed with oriented strand board. Findings contribute to improved design data.

Performance-Based Wind Engineering for Wood-Frame Buildings

The majority of buildings and the vast majority of residential structures in North America are constructed using light-frame wood. Many of these structures are subjected to high winds along the eastern seaboard and gulf coast, and as a result routinely suffer damage resulting in financial losses. A relatively new paradigm in the field of earthquake engineering is performance-based design. This paper presents the concept of performance-based wind engineering for wood-frame buildings through the development and application of fragilities to form different owner/user performance expectations, namely, occupant comfort, continued occupancy, life safety, and structural integrity. The approach includes the development and application of fragility curves. An example analysis and design for a simple light-frame wood building for all four performance expectations is presented. The final design of the building needed to achieve the specified levels of structural performance are discussed for each performance expectation level.

Identification of multiple criteria for the evaluation of light-frame wood wall assemblies

Design of light-frame wood wall assemblies should consider several performance criteria. Possible alternatives must respect minimum code requirements for the studied context. Then, additional performance characteristics may be considered to achieve the most suitable configuration for a specific project. It is, however, not simple to define how several performance criteria can be evaluated simultaneously and which scale should be used to compare assemblies. Existing tools to evaluate performance criteria consider mostly one parameter at the time. An evaluation methodology taking into account at once different performance characteristics would be beneficial in the decision process. With the final aim of developing a multi-criteria framework to support the evaluation of improved factory-built wood-frame exterior walls, this article presents a methodology for quantitative evaluation of a set of performance characteristics. The identification of appropriate criteria is followed by the choice of quantification mechanisms. A case study comparing five wall assemblies exemplifies this methodology.

Performance of Light-Frame Wood Residential Construction Subjected to Earthquakes in Regions of Moderate Seismicity

The majority of single-family dwellings in the United States are light-frame wood construction. Residential construction practices have evolved gradually over the years, most light-frame wood structures have not been structurally engineered, and many residential buildings suffered significant damage in recent hurricane and earthquake disasters. As a result, new concepts and methodologies are evolving to better predict and evaluate the performance of wood frame structures exposed to natural hazards and to support improved residential building practices. This paper examines the behavior of typical light-frame wood structural systems in the central and eastern United States under earthquake hazards. Nonlinear structural analysis tools and stochastic methods are used to model the uncertainties in ground motion intensity and structural response. Fragility curves defining damage state probabilities as a function of ground motion intensity are developed for typical lateral force-resisting shear wall systems subjected to increasing levels of ground motion. A comparison of these fragilities with those embedded in HAZUS provides additional perspective on damage potential for residential construction in regions of low-to-moderate seismicity.

Experimental Investigation of the Effect of Multiple Earthquakes on Woodframe Structural Integrity

Woodframe (light-frame wood) structures make up the vast majority of the United States building stock including those in areas of high seismicity. Their life cycle is unique in that when they are damaged, repair is often nonstructural in nature consisting of drywall repair, and possibly wall replacement. The main reason for this is that most woodframe structures are residential and life safety is typically implied through prescriptive means. Thus, one important question that has yet to be answered is: What is the effect of previous earthquakes on the performance or integrity of a woodframe structure during the next earthquake? Because the primary lateral force resisting system in western style woodframe construction is the wood shearwall, an attempt is made herein to answer this question, albeit in part, based on a series of assembly-level tests conducted at Colorado State University on (1) isolated wood shearwall assemblies; and (2) corner assemblies consisting of a shearwall and short transverse wall. Both types of assemblies were tested with and without drywall and subjected to a series of six earthquakes of increasing intensity. As expected, the effect of the drywall was quite significant, reducing the peak drift of the isolated shearwalls by 30% and the corner assemblies by over 50%. Interestingly, the corner assembly without drywall was most severely damaged by the repetitive simulated earthquakes. In contrast, the corner assembly with drywall suffered very little damage simply because of the added stiffness and strength, so peak displacements were below 1% wall height. Woodframe assemblies appear to perform quite well, only suffering moderate degradation, when subjected to subsequent earthquakes provided peak drifts are kept below 1.5%.

Application and feasibility of coal fly ash and scrap tire fiber as wood wall insulation supplements in residential buildings

Each year, nearly 55% of the fly ash (FA) produced by coal burning power plants in the United States is disposed of in landfills and ash ponds, while the amount of recycled fiber from scrap tires that is beneficially used in end-user markets is virtually negligible. This paper presents the results of a study carried out to investigate whether it might be possible to increase the thermal efficiency of a light-frame residential structure through addition of a fly ash-scrap tire fiber composite to traditional fiberglass insulation in light-frame wood residential construction. This type of construction represents more than 80% of the building stock in North America. The results of this study suggest that the fly ash-scrap tire fiber composite provides a sustainable supplement to traditional insulation that not only increases the efficiency of traditional insulation but can also help significantly reduce the environmental issues associated with disposal of these waste products.

Performance-Based Seismic Design of Six-Story Woodframe Structure

This paper presents a performance-based seismic design of a six-story light-frame wood building using a new direct displacement design (DDD) procedure specifically developed for mid-rise wood buildings. The proposed displacement-based design procedure uses normalized modal analysis and equivalent linearization techniques, along with segmented shearwall concepts, and allows engineers to select shearwalls from a database of backbone curves. The multistory direct displacement-based procedure is a promising design tool for performance-based seismic design of mid-rise wood buildings because it allows consideration of multiple performance objectives and does not require nonlinear time-history analysis of the complete structure. The proposed procedure further does not require the engineer to provide an estimate of equivalent damping. The proposed procedure is illustrated on a six-story building and is validated using nonlinear time-history analysis results.

Observations of Structural Damage Caused by Hurricane Katrina on the Mississippi Gulf Coast

The loads associated with Hurricane Katrina led to the destruction or severe damage of approximately 130,000 homes and over 200 deaths in the state of Mississippi. This paper discusses the results of a field inspection of structural damage along the state's Gulf Coast area caused by this hurricane. It was found that reinforced concrete, steel frame, and heavy timber structures generally performed well, with minimal structural damage. Precast concrete, light frame wood, and bridge structures generally performed poorly. Nonstructural components of all building types, in particular facades and interior partitions subjected to storm surge, were typically destroyed. For various structures, the primary cause of failure was found to be insufficient connection strength. A comparison of Katrina's storm surge and wind loads is made to those specified in current design standards. It was found that Katrina's forces exceeded those specified in design standards in many parts of the state.

Reliability of woodframe residential construction subjected to earthquakes

Most housing in the United States is light-frame wood construction (90% nationally, and 99% in California). Residential construction in the United States typically has received little or no structural engineering. The performance of wood residential construction to earthquake effects was apparent in the 1994 Northridge Earthquake, and underlines the need for new approaches to enhance building performance through improved prediction, evaluation and design methods. In this paper, methods of stochastic nonlinear dynamic analysis are used to simulate the behavior of lateral force-resisting shear wall systems typically found in residential construction subjected to earthquakes. The probability that shear wall drift limits are exceeded for uniform hazard earthquake ground motions with various return periods and intensities can be estimated from this analysis and can be related to performance levels for residential occupancy that have been suggested in concurrent research on performance-based engineering. The probability of failure under a spectrum of possible earthquakes is determined by convolving the structural fragility derived from the above analysis with the seismic hazard specified by the US Geological Survey.

The Effect of Biological Deterioration on the Seismic Performance of Woodframe Shearwalls

: This article describes a combined experimental and analytical investigation of the effect of biological deterioration on the performance of light-frame wood shearwalls subject to seismic loading. Deterioration due to fungal attack is believed to have a significant effect on the displacement and capacity performance of light-frame shearwalls. Some studies have shown that fungi tend to degrade properties of nailed connections common in light-frame structures, however, the effect on shearwall assemblies has not yet been quantified. The effect of biological deterioration (due to fungal attack) on cyclic performance of light-frame shearwalls was evaluated in this study by, first, fitting sets of hysteretic model parameters to single fastener sheathing-to-framing connections exposed to different levels of decay from brown rot fungi. Next, these model parameters were used as an input to a numerical model used to evaluate the cyclic behavior of complete shearwall assemblies. Finally, the output from this model was used in a nonlinear dynamic time-history analysis to evaluate displacement response of the shearwalls subject to a suite of ground motion records. The results were presented in the form of peak displacement distributions and fragility curves.

Hurricane damage to residential construction in the US: Importance of uncertainty modeling in risk assessment

The severe damage to residential construction and the social disruption caused by hurricanes in the past two decades in the United States demonstrate the need for a better understanding of residential building performance and improved performance prediction tools if the goal of mitigating hurricane hazards is to be achieved. In this paper, probabilistic risk assessment methods are developed to assess performance and reliability of low-rise light-frame wood residential construction in the United States subjected to hurricane hazards. In a fully coupled reliability analysis, structural system fragilities are convolved with hurricane hazard models expressed in terms of 3-s gust wind speed. Alternate models for structural fragility used in such analyses are tested statistically. Sources of inherent variability and epistemic (knowledge-based) uncertainties due to competing plausible hurricane wind speed models and databases on structural resistance are explicitly included in the reliability analysis. The impact of epistemic uncertainties on structural reliability and their implications for risk-informed decision-making are examined. Comparison of reliabilities of building components that are essential to maintain the integrity of the building envelope highlights the need to design and detail those components to reduce the overall vulnerability of a building to hurricane effects.

Fragility Analysis for Performance-Based Seismic Design of Engineered Wood Shearwalls

Performance-based engineering concepts applied to both structural design and assessment have gained interest among structural engineers and researchers in recent years. Performance-based design applied to building systems includes selection of appropriate structural systems and configurations to ensure that the structure has adequate strength, stiffness, and energy dissipation capacity to respond to loadings, including those arising from natural hazards, without exceeding permissible damage states. Although performance-based design has advanced for some materials and structural types, such as steel and reinforced concrete buildings and bridges built in high seismic regions, the application to light-frame wood structures has only recently been explored. This paper investigates the application of fragility techniques to the seismic design of engineered woodframe shearwalls. The methodology developed herein provides a technical basis for the development of future performance-based design provisions for shear...