Sheathing Nail Bending-Yield Stress: Effect on Cyclic Performance of Wood Shear Walls

This study investigated the effects of sheathing nail bending-yield stress (fyb) on connection properties and shear wall performance under cyclic loading. Four sets of nails were specially manufactured with average fyb of 87, 115, 145, and 241 ksi. Nail bending-yield stress and the hysteretic behavior of single-nail lateral connections were determined. The parameters of the lateral nail tests were used in a numerical model to predict shear wall performance and hysteretic parameters. The competency of the numerical model was assessed by full-scale cyclic tests of shear walls framed with Douglas-fir lumber and sheathed with oriented strandboard (OSB). The parameters of the shear wall model were used in another program to predict shear wall performance for a suite of seismic ground motions. The single-nail connection tests and wall model computations suggested that increased fyb of the sheathing nails should lead to improved wall stiffness and capacity. In both single-nail lateral connection and shear wall tests, the probability of nonductile failure modes increased as fyb increased. The peak capacity of the walls increased as fyb of the sheathing nails increased up to 145 ksi, but wall initial stiffness, displacement at peak capacity, and energy dissipation were not significantly affected by fyb. Sheathing nail fyb greater than 145 ksi did not enhance the overall cyclic behavior of wood shear walls

Hybridized framing to modify load paths and enhance wood shearwall performance

Heavy timber framing relies primarily on bracing to withstand lateral loads due to earthquakes and wind events. Bracing configurations in heavy timber framed buildings vary widely and include cross bracing, knee bracing, and other geometries. Many heavy timber frames constructed during colonial American times are still standing, exceeding the expected life of many structures being built today. Limited research has been conducted on the lateral resistance of heavy timber frames and their connections and design aids and procedures are not readily available for engineers to assist in the design of these structures. This method of wood construction has been largely replaced with the development of light-framed wood buildings, which utilize sheathing (typically plywood or OSB) attached to the frame to resist lateral loads. Today, the primary form of wood construction is light-frame. These structures rely on shearwalls to resist lateral loads. The shearwall consists of 2x4 or 2x6 studs regularly spaced with wood structural panel sheathing attached to the wall frame. This assembly is lightweight and ductile. Extensive research has been conducted on light-frame shearwalls since the 1950’s. The effects of construction variables (i.e., fastener schedule, sheathing thickness and grade, anchorage, and openings) on shearwall performance have been cataloged through numerous studies. Studies have found the sheathing-frame connection, particularly the perimeter connection, is critical to the performance of a shearwall. This connection is typically nailed, although sometimes staples or adhesives are used. The lateral load path in light-frame shearwalls relies on the sheathing-framing connection. If the load path can be modified then shearwall design can more fully utilize compressive and tensile properties of the wood materials and be less sensitive to the sheathing-framing connection properties. The idea of combining bracing typical of heavy timber framing with techniques used in light-frame construction has not been widely explored by research or analysis. This study investigates the use of bracing in conjunction with light-frame construction (a hybrid framing) to relieve the sheathing nails as the critical load path and enhance the shearwall performance under lateral loading. A 4 by 8-ft. shearwall was designed consisting of an internal cross brace without intermediate framing studs and a lapped connection at the cross intersection. A 4x4 top-plate was used to improve vertical capacity of the braced shearwall because no intermediate stud was included. Four different types of shearwalls were tested under cyclic loading following the CUREE protocol; a conventional light-framed shearwall, a cross-braced shearwall with no mechanical connection at the corners of the walls, a cross-braced shearwall with plywood gusset plates at the corners of the walls, and a cross-braced shearwall with metal truss plates at the corners of the walls. The conventional shearwall and the braced shearwall without mechanical connections at the corner of the wall performed similarly - the sheathing-frame connections controlled their performance. Withdrawal of the sheathing nails was the dominate failure mode. The braced shearwalls with the plywood gusset plate and the metal truss plates at the corners exhibited greater ultimate loads, greater initial stiffness and dissipated more energy compared to the conventional shearwall. The modes of failure for these walls were shear failures in the plywood gusset plates and buckling in the metal truss plates. Some failure was observed in the sheathing nails, however, to a lesser degree than observed in the conventional shearwall. The load path of vertical forces must be addressed in areas where intermediate studs are excluded due to the bracing configuration. Four additional walls were tested under vertical loading; two conventional shearwalls and two cross-braced shearwalls with metal truss plates at the corners. The braced shearwalls proved to adequately resist service level vertical loads similar to those resisted by the conventional shearwall. Overall, using a hybridized shearwall as a part of light-frame construction appears to be viable option to enhance the lateral performance

The effects of nail bending-yield stress and biological deterioration on the cyclic performance of shearwalls

Key parts of the lateral force resisting system in wood-frame buildings are the shearwafls and the connections. The connections in wooden buildings are the primary source of ductility and energy dissipation; these are essential properties when buildings are exposed to lateral forces, such as wind and earthquakes. Shearwall design is based on new materials and a monotonic testing method, which departs from the actual situation because buildings age and are subjected to cyclic loads during wind and earthquake events. After the property losses experienced in the Loma Prieta and Northridge earthquakes, the engineering community realized there was a need to further investigate wooden shearwall performance especially with respect to condition and cyclic loading. Individual sheathing-framing connections can be designed with respect to capacity and yield mode by using the yield mode equations. However, the relationship between individual connection characteristics and the performance of a shearwall remains unclear. The objective of this study was to investigate the relationship between individual connections and shearwall performance where nail bending-yield stress (fyb) and biological deterioration of the wood were sources of variation in physical and simulation experiments

Wood materials and shearwalls of older light-frame residential structures

Light-frame construction practices and materials have changed greatly over the past 100 years. Contemporary research has focused on modern construction; thus, we know a great deal about the behavior of modern lightframe buildings under lateral forces. However, there are many light-frame buildings that were built prior to the introduction of modern building codes and material standards, and these buildings are still in service. The material and performance databases for these older structures are limited, so risk assessment and condition assessment are challenged for seismic or wind events. The project objective is to establish a basis for probabilistic assessment of the seismic performance of older construction by examining the performance of shearwalls, connections, and wood materials from older light-frame buildings Nineteen structures built between 1900 and 1970, scheduled for demolition, were sampled for material and connection tests as well as full-size shearwall tests. The scope of tests for each source structure was based on the availability of full-size shearwalls and the type of sheathing material used in the structure. Two exterior sheathing types were found in the source structures, horizontal plank sheathing and plywood. Wood lath-and-plaster was the characteristic interior wall covering in buildings of this era. Specific gravity was determined, and embedment tests were performed on the wood framing and sheathing materials. Bending-yield tests were performed on the sheathing nails (typically 0.113 by 2.5- in.), and lateral single-nail connection tests were performed on extracted connections. Full-scale shearwall racking tests were done both monotonically and cyclically using the basic CUREE loading protocol. The average specific gravity of the wood materials was 0.46. The material extracted from the source structures had an embedment strength that was statistically similar to the National Design Specifications (NDS) table value for a specific gravity of 0.46 (4.0 ksi). The results of the nail bending-yield test showed no significant change over time. Nails had average bending-yield strength of 97.3 ksi, which is similar to the NDS stated value of 100 ksi. In general, the connection tests showed agreement with the European Yield Model (EYM) equations for connection strength. The full-size shearwall capacities were in agreement with known values for walls with each type of sheathing. Based on the limited testing done in this study, no adverse effects due to age and service life were observed. The materials and assemblies performed according to modern standards for new construction. Insect damage and fungi deterioration were present in many of the structures, and because these conditions were avoided as much as possible, no inferences are made regarding the effects of insect and fungi damage on lateral shear strength. These tests show that a structure built in the early 1900’s will meet modern design expectations as long as the material has been kept dry and free of damage due to insects. The principal threats to hazard performance observed during this study were the construction practices in the early twentieth century. Most of the source structures had no anchorage to the foundation, shearwalls were connected to roof diaphragms with limited toe nail connections, most structures were sheathed with horizontal planks, and many of the source structures had few walls that met the modern prescribed aspect ratio for structural shearwalls of 2:1 for full table design capacity. The results of this research can be integrated with the Federal Emergency Management Administration (FEMA) document on seismic rehabilitation for buildings

The effect of biological deterioration on the performance of nailed oriented strand board sheathing to Douglas-fir framing member connections

Service life prediction models for light-framed wood structures require an extensive quantity of empirical data on deterioration pathologies for the numerous structural components, as well as mechanistic approaches to determine their capacity at various levels of deterioration. The data and models presented in this study satisfy a portion of the information intensive requirements of service life prediction models and will be used for continued development of these models. This study investigated single-shear mechanical properties of three nailed connection geometries of biodeteriorated aspen oriented strand board (OSB) sheathing and Douglas-fir framing members, typical in light-framed lateral force resisting systems. Mechanical properties of the nailed connections including ultimate and yield strength, stiffness, and energy dissipation were evaluated at increasing levels of deterioration caused by the brown rot fungus, Postia placenta, through monotonic and quasi-static fully-reversed cyclic testing. The OSB sheathing specific gravity was the strongest explanatory variable for the mechanical properties examined and controlled the behavior of the connections at increasing levels of fungal damage. The data suggested that nailed connections in light-framed lateral force resisting systems can tolerate a moderate amount of fungal attack prior to significant loss of connection capacity. The connection yield mode transitioned from nail bending (mode IIIs) to side member crushing (mode Is) as the weight loss of the OSB sheathing approached 30 percent. Nominal design capacity and yield mode of nailed sheathing to framing member connections with fungal deterioration can be estimated using existing yield models for dowel-type connections through evaluation of the dowel bearing strength of the decay-damaged wood materials. Various physical, mechanical, and chemical properties of the OSB sheathing were monitored in a parallel study at increasing levels of fungal deterioration including dowel bearing strength, shear strength, weight loss, and solubility in an aqueous solution of sodium hydroxide (NaOH). These properties were strongly correlated with the OSB sheathing specific gravity. Near infrared (NIR) spectroscopy, in combination with multivariate statistical methods, was used to develop predictive models for weight loss, shear strength, dowel bearing strength, and solubility. The NIR methods showed considerable promise as a field inspection tool based on the accuracy of models developed in this study

Risk analysis of light-frame wood construction due to multiple hazards

Light-frame wood buildings are widely built in the United States (U.S.). Natural hazards cause huge losses to light-frame wood construction. This study proposes methodologies and a framework to evaluate the performance and risk of light-frame wood construction. Performance-based engineering (PBE) aims to ensure that a building achieves the desired performance objectives when subjected to hazard loads. In this study, the collapse risk of a typical one-story light-frame wood building is determined using the Incremental Dynamic Analysis method. The collapse risks of buildings at four sites in the Eastern, Western, and Central regions of U.S. are evaluated. Various sources of uncertainties are considered in the collapse risk assessment so that the influence of uncertainties on the collapse risk of lightframe wood construction is evaluated. The collapse risks of the same building subjected to maximum considered earthquakes at different seismic zones are found to be non-uniform. In certain areas in the U.S., the snow accumulation is significant and causes huge economic losses and threatens life safety. Limited study has been performed to investigate the snow hazard when combined with a seismic hazard. A Filtered Poisson Process (FPP) model is developed in this study, overcoming the shortcomings of the typically used Bernoulli model. The FPP model is validated by comparing the simulation results to weather records obtained from the National Climatic Data Center. The FPP model is applied in the proposed framework to assess the risk of a light-frame wood building subjected to combined snow and earthquake loads. The snow accumulation has a significant influence on the seismic losses of the building. The Bernoulli snow model underestimates the seismic loss of buildings in areas with snow accumulation. An object-oriented framework is proposed in this study to performrisk assessment for lightframe wood construction. For home owners and stake holders, risks in terms of economic losses is much easier to understand than engineering parameters (e.g., inter story drift). The proposed framework is used in two applications. One is to assess the loss of the building subjected to mainshock-aftershock sequences. Aftershock and downtime costs are found to be important factors in the assessment of seismic losses. The framework is also applied to a wood building in the state of Washington to assess the loss of the building subjected to combined earthquake and snow loads. The proposed framework is proven to be an appropriate tool for risk assessment of buildings subjected to multiple hazards. Limitations and future works are also identified

Effects of Moisture Intrusion on the Performance of a Cross-Laminated Timber (CLT) Angle Bracket Connection

Cross-laminated timber (CLT) is revolutionizing the use of wood in the construction sector of North America as a solution for walls and diaphragms in mid-rise or even high-rise timber structures on account of its environmental advantages, high strength-to-weight ratio, fire-safety performance, and propensity for prefabrication. However, considering the hygroscopic nature of wood, moisture intrusion can affect material properties and, moreover, moisture increases the possibility of biological degradation, which can directly affect the durability of CLT structural members and their connections. The favorable seismic performance of connections in the CLT structural systems has been well researched in numerous studies. In addition, even though several research efforts have been conducted to understand the hygrothermal performance of CLT panels, knowledge of the CLT connections when subjected to moisture cycling is minimal. In this study, a CLT shear wall-to-diaphragm L-bracket connection is exposed to two high moisture exposure conditions - flood and simulated rain with increased humidity as well as different exposure durations to investigate the connection performance under the effects of moisture intrusion. Currently, there are four major species that are used for CLT, namely, Douglas-fir, Southern yellow pine, Norway spruce, and Spruce Pine Fir. All four species were incorporated into the study. A total of 264 cyclic tests were performed on wall-to-diaphragm L-bracket connection specimens to evaluate the connection performance in terms of strength, stiffness, and energy dissipation along with the development of two force-displacement engineering models. Results from both exposure studies suggest no significant degradation in connection performance after a moisture cycle of wetting and drying apart from a significant decrease in energy dissipation in flood exposure. However, the effects of multiple moisture cycling merit further study

Multi-Scale Approach to Evaluating Moisture Durability of Wood-Based Composites

The majority of low-rise residential structures in the U.S. are constructed with wood. Wood-based composites are primary building materials in these structures, used as structural sheathing, joists, and beam components. Wood composites are susceptible to degradation upon exposure to high levels of moisture. Moisture durability is routinely assessed with accelerated weathering (AW) procedures. The goal of AW is to expose wood composites to the degrading influences of moisture in a suitable assessment time-frame, which can require severe conditions. Small specimens are evaluated to reduce the time necessary to observe degradation and increase replications within constraints of quality control and research laboratory equipment. Applying AW results to wood composites exposed to adverse conditions in service is hindered by severity of AW conditions and small specimen sizes evaluated. Moisture uptake can differ between small and large specimens. Edge effects, or the propensity for lower resistance to moisture uptake at the edges, are assumed to be less detrimental as specimen size increases. The influence of edge effects, and in particular, the ability to apply AW results of small specimens to sizes more representative of those in service are largely unknown. Moisture durability of larger wood composites and assemblies produced with wood composites have been investigated, but indicate that a knowledge gap remains. The influence of edge effects on moisture durability along with performance of structural-size specimens and assemblies were studied in this work with a multi-scale approach. At the small scale, the influence of specimen size on AW results was evaluated in three wood composite products commonly used in residential construction: laminated veneer lumber (LVL), oriented strand board (OSB), and plywood. Moisture durability of wood composite I-joists was evaluated using laboratory AW and outdoor weathering. The impact of moisture degradation in engineered wood structural panels on full-size shear walls was investigated by subjecting OSB and plywood to AW prior to shear wall assembly. Finally, a numerical model predicting moisture transport in OSB and plywood was developed to better understand moisture transport in these materials during cyclic changes in relative humidity. Specimen size was most influential on water absorption, where AW method-dependent relationships between specimen size and water absorption were observed. However, the edge effects observed for water absorption did not directly translate into mechanical property loss. Lack of distinct trends regarding edge effects indicates that a wider range of specimen sizes may be necessary to distinguish these effects. Short-span bending strength of I-joists was reduced by both forms of weathering. One month of outdoor exposure in a rainy, cool climate reduced bending strength twice as much as the full laboratory AW procedure. In addition, moisture exposure resulted in a shift in failure mode, indicating degradation of the OSB web and web-flange joint. Weathering OSB prior to shear wall construction resulted in statistically significant reductions in yield load, maximum load, and energy dissipation. Weathering plywood sheathing prior to shear wall construction had no influence on wall properties. The numerical model developed for moisture transport in OSB and plywood was validated with experimental measurements, where the average deviation between measured and predicted values was 9.0% and 10.2% for OSB and plywood, respectively. Information gained on the influence of specimen size will help guide future experiments to better understand these effects. Results from I-joist and shear wall tests could provide engineers with residual properties after exposure to adverse conditions to determine remedial actions. The moisture transport model can aide in developing and refining AW procedures and provide moisture field input into damage evolution models

Analyse multicritère des compositions de mur à ossature légère en bois

Le concepteur d'une composition de mur à ossature légère doit considérer simultanément plusieurs critères de performance, en plus des exigences définies par la réglementation. Ce projet de recherche développe un cadre d'analyse multicritère permettant l'évaluation de compositions améliorées de mur extérieur préfabriqué à ossature légère en bois. Un exemple d'application compare cinq compositions de mur selon six contextes décisionnels situés à Québec. Le cadre d'analyse identifie des contraintes de conception et des critères de performance. Les contraintes de conception proposées pour assurer l'acceptabilité de compositions de murs dans le contexte d'utilisation étudié sont: la résistance structurale, la performance au feu, la résistance thermique, la permeance à l'air et les mesures obligatoires de gestion de l'humidité. Les critères de performance, définis ensuite pour évaluer les avantages d'un mur, sont la gestion de l'humidité, l'atténuation des bruits aériens, les coûts de construction, d'entretien et d'énergie liée au chauffage, et les impacts environnementaux. Des méthodes d'évaluation et des échelles de mesures sont étudiées pour chaque critère de performance. Pour l'évaluation des impacts environnementaux, un indice de performance environnementale est défini en comparant des indicateurs actuels. Suivant une approche d'analyse du cycle de vie, l'inventaire fourni par le logiciel ATHENA est agrégé selon trois modèles d'analyse d'impact: IMPACT 2002+, Eco-indicator 99 et TRACI. Les impacts relatifs des différents'composants du mur et de l'énergie d'opération sont comparés. En conclusion, l'indicateur de changement climatique est accepté comme un indice approprié de performance environnementale dans le contexte de l'étude. Le cadre d'analyse permet l'agrégation des évaluations obtenues selon chaque critère de performance, en intégrant les préférences d'un décideur. Quatre procédures d'agrégation multicritères (somme pondérée, MACBETH, ELECTRE II et PROMETHEE) sont utilisées pour ranger les alternatives selon de l'information quantitative et qualitative. L'étude montre que la sélection de la procédure d'agrégation la plus appropriée dépend de la nature des échelles de mesure utilisées. De plus, la comparaison de rangements résultants de différentes approches permet un choix plus éclairé. L'ensemble du projet confirme la pertinence d'appliquer une approche d'aide à la décision multicritère pour considérer simultanément différents aspects de la performance d'un mur à ossature légère en bois