Pouria Bahmani

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 prone to collapse during moderate to large earthquakes due to a characteristic deficiency in strength and stiffness in their first story. In order to propose and validate retrofit methods for these at-risk buildings, a full-scale four-story soft-story wood-frame building was constructed, retrofitted, and subjected to ground motions of various intensities. The tests were conducted to validate retrofit guidelines proposed in a “Federal Emergency Management Agency’s recent soft-story seismic retrofit guideline for wood buildings” and a performance-based seismic retrofit (PBSR) methodology developed as part of the NEES-Soft project. This paper is the second in a set of companion papers and presents the full-scale shake table test results using the two new approaches. The companion paper to this paper presents the desig...

Displacement-Based Design of Buildings with Torsion: Theory and Verification

AbstractDirect displacement design (DDD) is a procedure that allows one to distribute the forces induced by an earthquake to the levels of a multistory building to ensure that the desired level of interstory drift is not exceeded. To date, DDD has only been applied to buildings that do not exhibit significant torsional response. This paper presents a methodology to perform displacement-based design (DBD) on multistory buildings with in-plane torsional irregularities, thereby generalizing DBD for buildings with torsion. The procedure includes decoupling the contribution of the deformation that results from translation and torsion by using an existing approximation technique. The approach is validated by using detailed finite-element models of asymmetric buildings; it is found to accurately reproduce the desired dynamic structural properties. Both linear and nonlinear (elastic-perfectly plastic) systems are demonstrated and the accuracy is verified. The method is shown to be very accurate for linear systems...

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.

Direct Displacement Design of Vertically and Horizontally Irregular Woodframe Buildings

The seismic performance of multi-story buildings can be ascertained from structural deformations and accelerations. Direct displacement design (DDD), is a procedure that allows one to distribute the vertical stiffness to the levels of a multistory building such that all stories of the building meet the desired performance criteria. DDD, in its original form, provides a simple and effective procedure to ensure a multi-story building meets the desired story drift requirements when subjected to the specified seismic intensity by considering the stiffness and strength degradation at target story drift level believed or demonstrated to provide the desired building performance. However, this design methodology has not been able to be applied to buildings with severe in-plane torsional irregularities. This paper presents the details of a methodology to perform DDD on multi-story buildings with any level of translational and torsional response and with or without a soft story, thereby generalizing DDD for all buildings. The proposed method can be applied to any nonlinear system, and is illustrated here for woodframe buildings. The design method, which does not require time history analysis, was found to provide a design with the target performance and for building with torsion, was shown to provide significantly better accuracy than DDD in its original form.

Overview of the NEES-soft experimental program for seismic risk reduction of soft-story woodframe buildings

The existence of thousands of soft-story woodframe buildings in California has been recognized as a disaster preparedness problem resulting in mitigation efforts throughout the state. The considerable presence of these large multi-family buildings in San Francisco prompted the city to mandate their retrofitting over the next seven years. The NEES-Soft project, whose full title is “Seismic Risk Reduction for SoftStory Woodframe Buildings,” is a five-university multi-industry three-year project which has many facets including improved nonlinear numerical modeling, outreach, retrofit methodology development, and full-scale system-level experimental validation of soft-story retrofit techniques. In 2013, two full-scale buildings were tested within NEES-Soft. A hybrid test of a three-story building consisting of a onestory numerical substructure and a two-story physical structure above at the University at Buffalo, and a shake table test of a four-story building at the University of California – San Diego. A series of retrofits, based on methodologies ranging from FEMA P-807 to performance-based seismic retrofits developed as part of the project, were tested at both sites. Collapse testing for both building specimens was also conducted at the end of each test program. This paper presents a summary of selected test results for these full-scale building tests within the NEES-Soft project.

Full-scale testing of soft-story woodframe buildings with stiffness-based retrofits

The existence of thousands of soft-story woodframe buildings in California has been recognized as a disaster preparedness problem with concerted mitigation efforts underway in many cities throughout the state. The NEES-Soft project, whose full title is “Seismic Risk Reduction for Soft-Story Woodframe Buildings,” is a five-university multi-industry three-year project which has many facets including improved nonlinear numerical modeling, outreach, design method development, and full-scale system-level experimental validation of soft-story retrofit techniques. This paper summarizes the retrofit and test results for two full-scale buildings that were tested in 2013. The first is a three-story building at the University at Buffalo NEES facility using slow pseudo-dynamic testing. The bottom story, representing a soft story with garage openings, was the numerical substructure and reproduced by computer, while the damage to the two upper stories, representing the physical substructure, was observed in the lab. Then, testing of a full-scale four-story 4,000 sq-ft soft-story building at the UCSD NEES outdoor shake table was conducted. Retrofits ranged from crosslaminated timber rocking walls with rod hold downs to steel special moment frames combined with wood structural panels. 1 George T. Abell Distinguished Professor in Infrastructure, Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado, 80523-1372. 2 Ph.D. Candidate, Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado. 3 Ph.D. Candidate, Civil and Environmental Engineering, Colorado State University, Fort Collins, Colorado. 4 Assistant Professor, Glenn Department of Civil Engineering, Clemson University, Clemson, South Carolina. 5 Ph.D. Candidate, Glenn Department of Civil Engineering, Clemson University, Clemson, South Carolina. 6 Principal, Structural Solutions Inc, Walnut Creek, California. 7 Professional Practice Professor, Civil Engineering, Cal Poly, Pomona, California. 8 International Director of Building Systems, Simpson Strong-Tie, Pleasanton, California. 9 Assistant Professor, Civil and Environmental Engineering, Western Michigan University, Kalamazoo, Michigan. 10 Associate Professor, Civil and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, New York. 11 Ph.D. Candidate, Civil and Environmental Engineering, Rensselaer Polytechnic Institute, Troy, New York. 12 Research Engineer, Forest Products Laboratory, Madison, Wisconsin. van de Lindt, J.W., Bahmani, P., Jennings, E.N., Pang, W., Ziaei, E., Mochizuki, G., Gershfeld, M., Pryor, S., Shao, X., Symans, M., Tian, J., Rammer, D. Full-scale testing of a soft-story woodframe building with stiffness-based retrofits. Proceedings of the 10 National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. Full-Scale Testing of Soft-Story Woodframe Buildings with StiffnessBased Retrofits J.W. van de Lindt P. Bahmani, E.N. Jennings, W. Pang, E. Ziaei, G. Mochizuki, M. Gershfeld, S. Pryor, X. Shao, M. Symans, J. Tian, D. Rammer

Hybrid simulation of a wood shear wall frame

Hybrid simulations of a wood shear wall frame were conducted at the newly constructed Structural Engineering Laboratory at the University of Alabama. Preliminary cyclic testing and single degree of freedom pseudodynamic simulations were conducted to characterize the testing system and develop hybrid simulation control methods. Then full scale hybrid simulations were conducted of a two story wood frame with a numerical second story a physical first story of a single 4.88m wood shear wall with typical door and window openings. The lateral displacement response was determined using the Newmark integration algorithm and applied to the physical first story with the restoring force measured at the interface fed back to the numerical simulation of the second story. The experiments served two purposes: 1) to characterize the highly nonlinear seismic behavior of woodframe construction; and 2) to verify the developed pseudodynamic hybrid simulation controllers and their application to wood frame structures for eventual expansion to full buildings since this is the first time hybrid simulation of a wood frame structure has been achieved. A detailed overview of the experimental setup, procedure, and results are presented herein.