Soft-story Wood-frame Buildings Research Articles

Performance-based seismic retrofit procedure with shake table validation

Recent earthquakes such as Loma Prieta and Northridge in California have highlighted the poor performance of a specific class of existing buildings known as soft-story building. This is because many older buildings were designed prior to the implementation of modern seismic design codes. Although building codes have clearly evolved, the problem is still unresolved for older buildings that are code-deficient such as soft-story wood-frame buildings. Retrofitting these types of buildings came to the forefront after the 1989 Loma Prieta, 1992 Cape Mendocino, and 1994 Northridge earthquakes. This paper presents a new performance-based seismic retrofit (PBSR) methodology for existing wood-frame buildings with an extreme soft-story and torsional irregularity at their first story, i.e. the most common case for these types of buildings in the San Francisco Bay Area. The method was validated numerically using non-linear time history analysis and then was validated experimentally by conducting full-scale shake table tests on a four-story wood-frame building at NHERI @ UC San Diego (formerly NEES @ UCSD), the largest outdoor shake table in the United States. The method is shown to be applicable to these types of buildings and some advantages of the PBSR method over soft-story only retrofit methodologies are presented.

Mass Timber Rocking Panel Retrofit of a Four-Story Soft-Story Building with Full-Scale Shake Table Validation

Soft-story wood-frame buildings have been recognized as a disaster preparedness problem for decades. There are tens of thousands of these multi-family three- and four-story structures throughout California and other cities in the United States. The majority were constructed between 1920 and 1970, with many being prevalent in the San Francisco Bay Area in California. The NEES-Soft project was a five-university multi-industry effort that culminated in a series of full-scale soft-story wood-frame building tests to validate retrofit philosophies proposed by (1) the Federal Emergency Management Agency (FEMA) P-807 guidelines and (2) a performance-based seismic retrofit (PBSR) approach developed within the project. Four different retrofit designs were developed and validated at full-scale, each with specified performance objectives, which were typically not the same. This paper focuses on the retrofit design using cross laminated timber (CLT) rocking panels and presents the experimental results of the full-scale shake table test of a four-story 370 m2 (4000 ft2) soft-story test building with that FEMA P-807 focused retrofit in place. The building was subjected to the 1989 Loma Prieta and 1992 Cape Mendocino ground motions scaled to 5% damped spectral accelerations ranging from 0.2 to 0.9 g.

Closure to “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. II: Shake Table Test Results” by John W. van de Lindt, Pouria Bahmani, Gary Mochizuki, Steven E. Pryor, Mikhail Gershfeld, Jingjing Tian, Michael D. Symans, and Douglas Rammer

Closure to “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. II: Shake Table Test Results” by John W. van de Lindt, Pouria Bahmani, Gary Mochizuki, Steven E. Pryor, Mikhail Gershfeld, Jingjing Tian, Michael D. Symans, and Douglas Rammer

Discussion of “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. I: Building Design, Retrofit Methodology, and Numerical Validation” by Pouria Bahmani, John W. van de Lindt, Mikhail Gershfeld, Gary L. Mochizuki, Steven E. Pryor, and Douglas Rammer

Discussion of “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. I: Building Design, Retrofit Methodology, and Numerical Validation” by Pouria Bahmani, John W. van de Lindt, Mikhail Gershfeld, Gary L. Mochizuki, Steven E. Pryor, and Douglas Rammer

Closure to “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. I: Building Design, Retrofit Methodology, and Numerical Validation” by Pouria Bahmani, John W. van de Lindt, Mikhail Gershfeld, Gary L. Mochizuki, Steven E. Pryor, and Douglas Rammer

Closure to “Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. I: Building Design, Retrofit Methodology, and Numerical Validation” by Pouria Bahmani, John W. van de Lindt, Mikhail Gershfeld, Gary L. Mochizuki, Steven E. Pryor, and Douglas Rammer

Application of Energy Dissipation Devices for Seismic Protection of Soft-Story Wood-Frame Buildings in Accordance with FEMA Guidelines

AbstractRecent earthquakes have demonstrated the vulnerability of soft-story wood buildings. A performance-based retrofit option focusing on stiffening/strengthening only the soft ground story, as regulated by FEMA guidelines, was proposed in an attempt to achieve collapse prevention of the ground story while at the same time ensuring damage control of the upper stories. This paper focuses on an alternative retrofit approach that focuses on energy dissipation, in order to achieve a performance that is essentially comparable to the inherent performance target defined in FEMA guidelines. A viscous damper retrofit is designed based on conclusions drawn from parametric studies. Then, the proposed retrofit is validated via slow pseudodynamic hybrid testing of a full-scale three-story building. The test data are presented, interpreted, and compared against both pretest numerical predictions and posttest simulation results after model calibration. It is demonstrated that a damping retrofit, as compared to a stif...

Full-Scale Experimental Investigation of Second-Story Collapse Behavior in a Woodframe Building with an Over-Retrofitted First Story

AbstractSoft-story woodframe buildings have been identified as a disaster preparedness problem throughout California and are present in many other states of the United States. These buildings can be readily identified by their large openings at the ground floor, often for parking, which results in a soft and weak first story that is prone to collapse in moderate to severe earthquakes. This paper presents the hybrid test results of a full-scale collapse test program that was carried out on a 3-story soft-story woodframe building with an overretrofitted first story. The overretrofitted design was constrained to the soft story only, essentially representing a retrofit that would likely drive the soft-story failure mechanism into the upper stories. The objectives of the collapse testing were to (1) quantify the collapse shift into the upper stories when the first story is overstrengthened, (2) investigate the collapse mechanisms of a woodframe building constructed with archaic building materials and style in ...

Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. I: Building Design, Retrofit Methodology, and Numerical Validation

AbstractSoft-story wood-frame buildings have been recognized as a disaster preparedness problem for decades. There are tens of thousands of these multifamily three- and four-story structures throughout California and other parts of the United States. The majority were constructed between 1920 and 1970 and are prevalent in regions such as the San Francisco Bay Area in California. The NEES-Soft project was a five-university multiindustry effort that culminated in a series of full-scale soft-story wood-frame building tests to validate retrofit philosophies proposed by (1) Federal Emergency Management Agency’s recent soft-story seismic retrofit guideline for wood buildings and (2) a performance-based seismic retrofit (PBSR) approach developed as part of the NEES-Soft project. This paper is the first in a set of companion papers that presents the building design, retrofit objectives and designs, and full-scale shake table test results of a four-story 370-m2 (4,000-ft2) soft-story test building. Four different ...

Experimental Seismic Behavior of a Full-Scale Four-Story Soft-Story Wood-Frame Building with Retrofits. II: Shake Table Test Results

AbstractSoft-story wood-frame buildings have been recognized as a disaster preparedness problem for decades. The majority of these buildings were constructed from the 1920s to the 1960s and are pro...

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...

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...

Corotational Model for Cyclic Analysis of Light-Frame Wood Shear Walls and Diaphragms

AbstractThis paper presents a new two-dimensional shear-wall and diaphragm model developed as part of a Network for Earthquake Engineering Simulation (NEES) Project entitled NEES-Soft: Seismic Risk Reduction for Soft-Story Woodframe Buildings. A large portion of the older multistory buildings in the California region were constructed with a deficiency that makes them vulnerable to collapse in the first story during earthquakes. This deficiency is referred to as soft-story. The new model presented in this paper was developed using a corotational formulation, which makes it suitable for modeling the side-sway collapse of wood shear walls under large displacement as well as estimating the in-plane stiffness of floor diaphragms. To achieve high computational efficiency, a nodal condensation technique is used to eliminate the degrees of freedom (DOFs) associated with the nail connections from the global DOFs of the model. To verify the validity of the new model, the model was coded into a computer program and ...