Wood-frame Buildings Research Articles

Experimental Seismic Collapse Study of a Full-Scale, 4-Story, Soft-Story, Wood-Frame Building

AbstractIn the San Francisco Bay Area and throughout much of California, there are a large number of wood-frame buildings with garage space at ground level, resulting in open fronts on one or two sides. This type of geometry results in a soft and weak first story, and buildings of this archetype are generally referred to as soft-story buildings. During an earthquake, these buildings are susceptible to severe damage and collapse and have been recognized as a disaster-preparedness problem. The five-university Network for Earthquake Engineering Simulation (NEES)-Soft project culminated in a series of full-scale soft-story wood-frame building tests to validate two different retrofit philosophies and included a 2-month test program encompassing four different retrofits. The building had 370 m2 of living space and was designed to be generally representative of older San Francisco Marina and Mission District construction, circa 1950s. Following the retrofit testing, which only moderately damaged the test buildin...

Commentary on FEMA P-807 for Retrofit of Wood-Frame Soft-Story Buildings

The FEMA P-807 guideline titled Seismic Evaluation and Retrofit of Multi-Unit Wood-Frame Buildings with Weak First Stories is a new approach for building retrofit. It goes beyond current building codes and standards by quantifying performance in probabilistic terms. The authors performed an independent review of P-807, and the resulting commentary herein covers five topics: (1) building lateral force-drift (backbone) curves, (2) building interstory drift limits, (3) lateral strength requirements, (4) collapse safety margins, and (5) site class effects. It was concluded that P-807 may be an efficient way for relative ranking and selection of retrofit designs—but it has questionable accuracy for predicting actual building performance (i.e., the probability that a particular seismic intensity will result in a meaningful state of damage for a specific building).

Full-Scale Experimental Verification of Soft-Story-Only Retrofits of Wood-Frame Buildings using Hybrid Testing

The FEMA P-807 Guidelines were developed for retrofitting soft-story wood-frame buildings based on existing data, and the method had not been verified through full-scale experimental testing. This article presents two different retrofit designs based directly on the FEMA P-807 Guidelines that were examined at several different seismic intensity levels. The effects of the retrofits on damage to the upper stories were investigated. The results from the hybrid testing verify that designs following the FEMA P-807 Guidelines meet specified performance levels and appear to successfully prevent collapse at significantly higher seismic intensity levels well beyond for which they were designed. Based on the test results presented in this article, it is recommended that the soft-story-only retrofit procedure can be followed when financial or other constraints limit the retrofit from bringing the soft-story building up to current code or applying performance-based procedures.

Retrofit of a soft-story woodframe building using SMA devices with full-scale hybrid test verification

Soft-story woodframe buildings are prevalent in much of California and for decades have been recognized as a disaster preparedness problem for both the owners/tenants and community as a whole. Retrofit programs for these at-risk buildings have been proposed with the first such program in a major city mandated in San Francisco in 2013. In this paper, a seismic retrofit methodology for soft-story woodframe buildings is presented. The methodology utilizes seismic response modification devices that consist of a compact steel section for energy dissipation and shape memory alloy (SMA) wire for re-centering capability, should that be needed. The device can be installed within a scissor-jack brace which is known for its compact footprint and high displacement magnification factor. These characteristics make it suitable mainly because of the need for placement in or near short wall segments characteristic of the bottom floor of soft-story woodframe buildings. The retrofit methodology using the SMA-steel device in a scissor-jack brace was developed and exemplified on a three-story soft-story woodframe building. Nonlinear time history analysis was conducted to quantify the performance of the retrofitted building for a suite of earthquakes. Experimental validation consisted of a full-scale hybrid test of the three-story building with the seismic retrofit represented numerically in the first-story serving as the numerical substructure, and the upper two stories represented physically in the laboratory. The results of the full-scale hybrid test are presented thereby validating the proposed seismic retrofit using SMA-steel devices in scissor-jack braces.

Genetic Optimization for Seismic Retrofit of Soft-Story Woodframe Buildings Using FEMA P-807 Methodology

AbstractRecent earthquakes around the world are reminders of the destructiveness of earthquakes and the effect they exact on lives and property. In 1989, the Loma Prieta earthquake resulted in significant damage and fatalities in the San Francisco Bay Area. Multiunit woodframe buildings suffered severe damage as a result of moderate to strong ground motions. Just 5 years later in southern California, the Northridge earthquake caused even more extensive damage to soft-story woodframe buildings. These multifamily residential buildings have upper stories with dense walls but have a large amount of openings for automobile parking at ground level, making them soft (and weak) story buildings. These buildings pose very significant safety risks to tenants, financial risks to owners, and recovery risks to all levels of government. They have been recognized as a disaster preparedness problem, and mitigation efforts are underway in San Francisco with other California cities beginning to follow. In May 2009, FEMA and...

Special Issue on Performance of Timber and Hybrid Structures

Special Issue on Performance of Timber and Hybrid Structures

Evaluation of the thermal and structural performance of potential energy efficient wall systems for mid-rise wood-frame buildings

Approximately 30% of energy use in Canada is consumed in buildings, specifically space heating, which can be reduced by constructing thermally-resistant building envelopes. This study evaluates potential innovative energy-efficient wall systems for mid-rise (four to six storeys) wood-frame buildings in terms of thermal and structural performances. Regarding the thermal resistance performance, four wall systems are developed, installed in a full-scale test house, and examined, along with a baseline wall system, using the field data collected. The selection of the wall systems is based on current practice, structural analysis, pre-fabricability, and energy-efficiency. Several sensors are installed on each wall system to measure temperature, heat flow, and relative humidity. In addition, structural tests are conducted to determine the compressive loading capacity of the tested wall systems for both concentric and eccentric loads, where full-scale panels are constructed and tested in laboratory. As a general finding, all the tested wall systems achieved ASHRAE's minimum assembly R-value recommendation of RSI 3.45, where the I-joist wall system had the highest R-value, while the conventional wall system had the highest load-bearing capacity. This paper recommends I-Joist wall systems for their higher energy efficiency suggesting more future research on efficient end connections to achieve consistent structural performance.

AHP based life cycle sustainability assessment (LCSA) framework: a case study of six storey wood frame and concrete frame buildings in Vancouver

Construction and building industry is in dire need for developing sustainability assessment frameworks that can evaluate and integrate related environmental and socioeconomic impacts. This paper discusses an analytic hierarchy process (AHP) based sustainability evaluation framework for mid-rise residential buildings based on a broad range of environmental and socioeconomic criteria. A cradle to grave life cycle assessment technique was applied to identify, classify, and assess triple bottom line (TBL) sustainability performance indicators of buildings. Then, the AHP was applied to aggregate the impacts into a unified sustainability index. The framework is demonstrated through a case study to investigate two six storey structural systems (i.e. concrete and wood) in Vancouver, Canada. The results of this paper show that the environmental performance of a building in Canada, even in regions with milder weather such as Vancouver, is highly dependent on service life energy, rather than structural materials.

What are the hygrothermal consequences of applying exterior air barriers in timber frame construction in Europe?

Moisture-induced damage is one of the major causes of degradations and reduced thermal performance in wood frame buildings. It is therefore crucial to incorporate the hygrothermal assessment of new timber frame building envelopes systems from the early development phase onwards. The article at hand presents the simulation results studying the hygrothermal performance of various timber frame wall configurations with exterior air barrier systems. A parameter analysis explores the impact of different European climates, insulation materials, exterior air barrier materials and verifies in addition to the impact of bad workmanship in the installation of the insulation layer. This study reveals that the application of mineral wool (MW) insulated timber frame walls in combination with exterior air barriers results in increased moisture loads. Moreover, small air gap channels between the MW and the adjacent exterior air barrier significantly increase natural convection and add up to harmful moisture levels. Yet the simulations indicate that the use of blown-in cellulose insulation can avoid these issues. The study further indicates that the technique of exterior air barrier is more suitable for continental climates rather than for moderate sea climates in Europe.

Numerical Retrofit Study of Light-Frame Wood Buildings Using Shape Memory Alloy Devices as Seismic Response Modification Devices

Woodframe buildings have historically performed well during earthquakes primarily because of their high strength/stiffness to weight ratio. Even still, these structures possess the potential for significant damage and even collapse when exposed to very large earthquakes. New design techniques and advanced technologies can provide an alternative for improved building performance. The use of dampers and other seismic response modification devices is one such method being explored in earthquake engineering. In this paper, shape memory alloy (SMA) devices are investigated for the response modification of light-frame wood buildings during strong earthquakes. A numerical model for a suite of SMA wood shearwalls is developed based on existing data. The numerical models of the shearwalls are examined using incremental dynamic analysis at the single wall level. Design charts are developed for the shearwall models for a range of seismic weights and targeted performance levels. The wall database is then used to demonstrate retrofit of a four-story apartment building. The interstory drift was reduced by over 55% when SMA devices are used for response modification of the four-story apartment building.

Seismic Performance of a Hybrid Building System Consisting of a Light Wood Frame Structure and a Reinforced Masonry Core

AbstractStory limit of residential light wood-frame buildings in the province of British Columbia was raised from four to six in 2009. The increase in height leads to more flexible buildings, potentially requiring the stiffer elevator shaft and stairwell core made from reinforced masonry to reduce building deflection under lateral loads. Guidelines for designing such hybrid buildings connected with ductile connections are currently lacking. As a first step towards achieving this goal a six-story light wood-frame building was designed assuming different scenarios of attachment to a stiffer core. Seismic responses to 10 ground motions of the designed six-story light wood-frame building with and without connection to a reinforced masonry core were obtained through the use of a finite-element modeling procedure through commercial software. A user-developed subroutine to model the hysteretic performance of wood shear walls, masonry core and connections was used in the analyses. Results show that the maximum la...

Variability in Wood-Frame Building Damage using Broad-Band Synthetic Ground Motions: A Comparative Numerical Study with Recorded Motions

Earthquake damage to light-frame wood buildings is a major concern for North America because of the volume of this construction type. In order to estimate wood building damage using synthetic ground motions, we need to verify the ability of synthetically generated ground motions to simulate realistic damage for this structure type. Through a calibrated damage potential indicator, four different synthetic ground motion models are compared with the historically recorded ground motions at corresponding sites. We conclude that damage for sites farther from the fault (>20 km) is under-predicted on average and damage at closer sites is sometimes over-predicted.

Seismic Loss Estimation of Residential Wood-Frame Buildings in Southwestern British Columbia Considering Mainshock-Aftershock Sequences

AbstractEarthquakes are rare natural catastrophes, causing severe impacts on society due to loss of lives, damage to built facilities, and business interruptions. After a large earthquake (mainshock), many induced events (aftershocks) may be triggered and cause additional damage or even collapse to mainshock-damaged buildings. To reduce seismic hazard and risk of aftershocks and extend the current performance-based earthquake engineering framework, the effects of aftershocks on the seismic performance of structures should be incorporated. Considering an extensive set of real and artificial mainshock-aftershocks sequences, the vulnerability of residential wood-frame houses in southwestern British Columbia, Canada, is examined analytically using incremental dynamic analysis. Subsequently, seismic loss estimation is conducted for four wood-frame house types with different seismic capacities. The analysis results indicate: (1) moderate effects of aftershocks (5–20%) on maximum structural response and damage e...

Empirical Selection Equation for Friction Pendulum Seismic Isolation Bearings Applied to Multistory Woodframe Buildings

AbstractBase-isolation systems have been increasingly used in concrete and steel structures for seismic design, but their application to woodframe structures remains largely unexplored with only several hundred buildings isolated, primarily post-and-beam buildings in Japan. This paper examines the incorporation of friction pendulum (FP) bearings into the performance-based seismic design (PBSD) for woodframe structures using nonlinear time-history analysis. Specifically, a regressive equation for the selection of the bearing radius (R) based on the fundamental period and the seismic weight present on each FP bearing is developed. Six generalized models representative of 1- to 6-story woodframe buildings were analyzed and used to develop a regressive design equation. Based on rigorous multi-incremental dynamic analysis (M-IDA), a regressive linear relationship between the spectral acceleration and bearing radius of the FP bearing was developed. An illustrative application to a 4-story residential woodframe ...

Effect of Mainshock-Aftershock Sequences on Woodframe Building Damage Fragilities

AbstractAlthough aftershocks have the potential to cause severe damage to buildings and threaten life safety, their effect in seismic risk analysis is not explicitly accounted for in modern building design codes, nor in emerging methodologies such as performance-based seismic design. The ultimate objective of this study is to systematically integrate aftershock hazard into performance-based earthquake engineering (PBEE) through analytical studies with structural degradation models derived from publicly available experimental data. In this paper, the first step is made by introducing a procedure to compute the probability of a mainshock-damaged woodframe building entering different damage states as a result of aftershock. Aftershock fragilities are developed by performing incremental dynamic analysis (IDA) using a sequence of mainshock-aftershock ground motions. To compute the seismic response of the damaged building, IDA is performed using a sequence of mainshocks of different intensity levels combined wi...

A New Method to Assess the Seismic Vulnerability of Existing Wood Frame Buildings

This paper aims at applying a direct displacement-based method to assess the seismic vulnerability of existing multi-storey wooden buildings. This procedure is consistent with Priestley’s direct methodology firstly developed for reinforced concrete structures. The distinctive characteristic of the proposed method is that the system response is quantified through the use of displacements instead of equivalent elastic strengths, according to the traditional force-based approaches. Consequently, in comparison to common force-based procedures, this method cannot only be considered as a rational alternative but also as a new seismic philosophy to design or assess structures. A representative timber construction system commonly used in Italy was selected as case study. The construction system illustrated in this work was analysed in detail, with special attention given to the mechanical connections typically used. The typical failure mechanisms and the energy dissipation capacity of the structure or of its members were identified on the basis of the mechanical properties of structural parts and connections, as well as of their geometry. In the proposed direct displacement-based assessment approach, the seismic intensity that would cause the limit state to be exceeded can be calculated by means of simple formulas. Therefore, the capacity to demand ratio can be simply derived. The procedure could be used to gauge the likelihood of losses, by combining it with simple loss models to account for probabilistic aspects.

Pounding of San Francisco–Type Soft-Story Midblock Buildings

Presented is a study into the effects of pounding on the collapse performance of midblock wood-frame soft-story buildings. This study analyzed various pounding situations and found that it can change the collapse risk when compared to the risk of the same building having no adjacent buildings (no-pounding). Key factors include relative building strengths, weights, and separation (gap) distances. When the buildings had similar strengths, it was found that the risk was about the same as that for no-pounding, independent of building relative weights and/or gap size. When the strengths varied, it was found that pounding could change the risk of certain buildings. The risk increased in the stronger and decreased in the weaker buildings, and the risk was biased toward the no-pounding risk of the heavier buildings. The risk generally increased with larger building separation distances, but there were exceptions.

Monitoring of Vertical Movement in a Four-Story Wood-Frame Building in Coastal British Columbia

Abstract Vertical movement and lumber moisture content (MC) were monitored during the construction of a four-story wood frame residential building in the winter of 2010–2011 in coastal British Columbia, Canada. The work was part of a long-term study to assemble field performance information and validate movement prediction methods to assist in the design of five- and six-story wood frame buildings. The MC readings of dimensional lumber generally remained around 20 % on average before the building was completely protected from rain with its roof and wall sheathing membrane under rainy construction conditions. With the data collection started when the roof sheathing was installed and continued into occupancy of the building, the vertical movement was found to occur during the process of wood drying and the installation of non-structural elements such as drywall and cladding etc., which increased the local loads. The total movement amount, contributed by wood shrinkage, gap closure (settlement), and other factors, reached about 34 mm at an exterior wall, 43 mm at an interior hallway shear wall, and 45 mm at an interior partition wall, after a total monitoring period of 17 months. These values were fairly comparable to the values predicted from wood shrinkage alone for this building.

Seismic Response of Midrise Light Wood-Frame Buildings with Portal Frames

The seismic response of midrise light wood-frame buildings incorporating portal frames and shear walls were investigated via numerical simulation. Because portal frames and shear walls exhibit different ductility characteristics, of specific interest in this paper is the seismic force-modification factor for the combined building system. An approach to estimate the ductility-related force-modification factor, Rd, for the hybrid light wood-frame building containing two types of lateral load-resisting systems with different Rd was proposed. One-, four-, and six-story buildings with layouts that rely on shear walls only or a combination of shear walls and portal frames were designed using the Rd estimated from the proposed approach. Frequency and nonlinear time history analyses were performed on these multistory hybrid buildings using a modified macro element model to represent the lateral-resistant element in finite-element program, ABAQUS. The results show that the use of a Rd value higher than the lowest Rd factor of the two lateral load-resisting systems is justified.

Analysis of Damage in Laminated Architectural Glazing Subjected to Wind Loading and Windborne Debris Impact

Wind loading and windborne debris (missile) impact are the two primary mechanisms that result in window glazing damage during hurricanes. Wind-borne debris is categorized into two types: small hard missiles; such as roof gravel; and large soft missiles representing lumber from wood-framed buildings. Laminated architectural glazing (LAG) may be used in buildings where impact resistance is needed. The glass plies in LAG undergo internal damage before total failure. The bulk of the published work on this topic either deals with the stress and dynamic analyses of undamaged LAG or the total failure of LAG. The pre-failure damage response of LAG due to the combination of wind loading and windborne debris impact is studied. A continuum damage mechanics (CDM) based constitutive model is developed and implemented via an axisymmetric finite element code to study the failure and damage behavior of laminated architectural glazing subjected to combined loading of wind and windborne debris impact. The effect of geometric and material properties on the damage pattern is studied parametrically.