Ongoing seismic testing and analysis program in the CUREe-Caltech woodframe project in California

Transcription

1 Ongoing seismic testing and analysis program in the CUREe-Caltech woodframe project in California Filiatrault, A. 1 Uang, C-M. 2 and Seible, F. 1 ABSTRACT The CUREe-Caltech Woodframe Project is currently underway in California as a combined research and implementation project to improve the seismic performance of woodframe buildings, a need which was brought to light by the January 14, 1994 Northridge, California Earthquake in the Los Angeles metropolitan region. The project, funded by FEMA, has five main elements, which together with a management element have the common objectives advancing the engineering of woodframe buildings and improving the efficiency of their construction technology for targeted seismic performance levels. The main research components of the project are included in the Testing and Analysis Element managed at the University of California, San Diego (UCSD). The objective of this paper is to briefly describe the projects currently underway in the Element 1 - Testing and Analysis of the CUREe-Caltech Woodframe Project. RESEARCH STRATEGY One clear issue that has emerged from a recent Invitational Workshop on Seismic Testing, Analysis and Design of Woodframe Construction (Seible et al., 1999), is the lack of understanding of the seismic behavior of woodframe structural systems. Very few numerical models capable of analyzing the seismic behavior of three-dimensional woodframe structures currently exist. Also, only limited experimental data have been generated at the system level. Recognizing these deficiencies, Element 1 of the CUREe-Caltech Woodframe Project is emphasizing the testing and analysis at both the component and system levels. The research strategy of Element 1 is illustrated in Fig. 1. The plan incorporates five main research tasks (1.1 to 1.5), centering on the shake table tests of large-scale woodframe systems. The other elements of the CUREe-Caltech Woodframe Project are Element 2 Field Investigations, Element 3 Codes and Standards, Element 4 Economic Applications, and Element 5 Education and Outreach. These five tasks are briefly discussed in the following sections. RESEARCH TASK 1.1 SHAKE TABLE TESTS The level of confidence associated with the seismic analysis and design of woodframe construction is much lower than for concrete or steel construction. There is a need for more experimental data on the seismic response of complete fullscale woodframe structures to improve the understanding of the state-of-the-art and the state-of-practice of analysis and design. Given the low weight-to-strength ratio of wood and the availability of high performance shake tables in California, shake table testing is viewed to be the most realistic procedure for system testing. Three different shake table projects are being conducted: tests of a simplified full-scale two-story single family house (Task 1.1.1), tests of a full-scale multi-story apartment building with tuck-under parking garages (Task 1.1.2), and tests of a simplified box-type woodframe building model (Task 1.1.3). This last project is being conducted at the University of British Columbia (UBC) in Vancouver, Canada. These tests are part of an already funded research project at UBC. The test structure for Task represents a simplified full-scale two-story single family house, with design details that are typical of current California residential construction practice (Filiatrault et al., 1999). Figure 2 shows a photograph of the building currently under construction at the time of writing. 1 Professor, Dept. Structural Eng., University of California, San Diego, Mail Code 0085, La Jolla, CA 92093, USA 2 Associate Professor, Dept. Structural Eng., University of California, San Diego, Mail Code 0085, La Jolla, CA 92093, USA

2 CUREe - CALTECH WOODFRAME PROJECT ELEMENT 1 TESTING AND ANALYSIS RESEARCH STRATEGY Shake Table Tests of a Single-Family House Shake Table Tests of a Multi-Story Apartment Building Shake Table Tests of a Simplified Model 1.2 International Benchmark Rate of Loading + Loading Protocol Effects Testing Protocols Dynamic Characteristics Anchorage Diaphragms Cripple Walls Shear Walls Soft/Weak Stories Wall Finish Materials Innovative Systems Connections Analysis Software Demand Aspects Reliability Analysis Figure 1. Research strategy of Element 1 Testing and Analysis of the CUREe-Caltech Woodframe Project. The footprint of the structure is 16 ft x 20 ft and is anchored to the shake table such that shaking will occur along the short dimension of the structure (north-south direction). The construction is at full scale, however the plan dimensions of the structure are smaller than would be of a typical residence due to the restrictions of the shake table. An attempt was made to maintain the character of typical shear walls within the smaller plan dimensions. Figure 2. Photograph of the two-story CUREe-Caltech house under construction on the uniaxial earthquake simulation system at the University of California, San Diego.

3 The lateral load resisting system of the test structure parallel to the shaking direction (east and west elevations) consists of exterior shear walls along with two interior partition walls on the second floor. The openings of these walls have been designed in an attempt to reproduce the torsional eccentricity that would be induced by a large garage door opening on one side of a residence. These shear walls also support the gravity loads in combination with an interior bearing wall. In the north and south elevations, exterior shear walls provide a torsional restraint to the test structure. The window openings in these walls, which are perpendicular to the shaking direction, have been kept conservative and symmetrical to aid in simplifying the interpretation of the experimental results All wood structural panels will are sheathed with 3/8 -OSB and are fastened to the framing with 8d gun nails. The second floor structural walls are tied to the first floor walls by steel straps. The first floor walls are tied-down to the foundation by steel connector devices. To maximize the experimental data that can be extracted from the test structure, multiple quasi-static and shake table tests will be conducted at various stages of construction. The test structure will be repaired between test stages to return the lateral load resisting system to its initial strength and stiffness. RESEARCH TASK 1.2 INTERNATIONAL BENCHMARK There is a significant amount of research on the seismic behavior of woodframe construction being conducted outside California. It would be most beneficial to the Woodframe Project to tie together these research activities. To foster this collaboration, an International Benchmark is being organized in which US researchers and design professionals, inside and outside California, as well as the international community will be invited to blind-predict the inelastic seismic response of one of the woodframe buildings tested in Task 1.1. This will provide a unique opportunity to assess the capability of available numerical models incorporating widely different levels of sophistication and to foster cooperation between the CUREe-Caltech Woodframe Project and other related research activities. RESEARCH TASK 1.3 TESTING PROTOCOLS Common testing protocols must be developed for all testing programs of Element 1. These testing protocols must reflect the realistic seismic demand expected on woodframe buildings and must consider the dynamic characteristics of these buildings. Also, it is necessary to establish how various testing protocols used in the past influence the response of woodframe structures, and how these past results can be translated into a comparable format useful for the Woodframe Project. The effects of loading rate and near field ground motions also need to be investigated. The main objective of Task 1.3 is to develop the required testing protocols for the Woodframe Project. The task is composed of one main project on the development of testing protocols (Task 1.3.2) and two supporting projects (Tasks and 1.3.3). These three projects are briefly described below. Task Rate of loading and loading protocol effects The seismic behavior of woodframe structures can be influenced by the rate of loading and by the loading protocol used. In order to develop common testing protocols for the Woodframe Project, the influence of these two parameters need to be evaluated experimentally in the early stage of the project on a reduced number of specimens (e.g., shear walls). The main objectives of Task are: 1) Perform static and dynamic cyclic tests on shear wall specimens with and without nonstructural finish materials; 2) Relate the cyclic response obtained from other loading protocols (e.g., Sequential Phased Displacement Protocol) with the response obtained with the Woodframe Project Test Protocol; and 3) Evaluate the near-fault effect on the behavior of shear walls. Task Testing protocols Several testing protocols have been proposed for the cyclic testing of woodframe structural components. It is necessary to establish common testing protocols for all component tests and shake table tests for the Woodframe Project. For shake table testing, multiple hazards need to be established and multi-axis excitations need to be considered.

4 The main objectives of Task are: 1) Develop testing protocols for component tests; 2) Establish input ground motions for shake table tests; and 3) Develop testing protocols for force-controlled elements (e.g., hold-downs). Task Dynamic characteristics of woodframe buildings The representative dynamic characteristics (natural frequencies and damping) of woodframe construction are needed to develop the testing protocol in Task A code-type period formula is also necessary for design purposes. The main objectives of Task are: 1) Collect existing strong-motion data from instrumented buildings to perform system identification; 2) Perform field vibration tests on woodframe buildings to enhance the existing database; 3) Develop period formulae for different types of woodframe buildings. Through analysis of recorded earthquake response and by forced and ambient vibration testing, Task is developing a database of periods, damping ratios and mode shapes of woodframe buildings. This database will enable the development of a more accurate period formula for design purposes. Calibration tests of the full-scale two-story house at UCSD (Task 1.1.1) are also being used to evaluate the relationship between the modal parameters identified through field vibration tests and the equivalent linear modal parameters that can be expected during strong motion shaking where lateral stiffness may be lost because of damage to some components. A regression analysis is being performed on the period database. Various structural parameters that are expected to control the building periods are being examined to see if they make statistically significant contributions to the regression formula for periods. The goal is to produce a simple but reasonably accurate formula for predicting a building's fundamental period in the direction of each building axis. RESEARCH TASK 1.4 COMPONENT TESTING This research task is composed of eight different sub-tasks related to the testing of woodframe sub-assemblages. These projects are briefly described below. Task Anchorage If not properly anchored, woodframe buildings can move off their foundations during an earthquake. Such movements can cause fires from broken gas lines, and damage to foundations, floors, walls, windows, and other utility connections as well as building contents. It is very expensive to lift a woodframe building up, put it back on its foundation, and repair the damage. Note that shear wall hold-down connections are excluded from this Task. The first objective of Task (Task ) is to improve the understanding of the cyclic behavior of sill plate-tofoundation anchorage connections for a wide range of anchorage configurations. It is expected that a better understanding of this behavior for the full range of performance level will lead to an improved optimization of the capacity, reliability and performance of sill plate-to-foundation anchorage connections. Results of previous tests seem to indicate that failure occur mainly in the wood sill plate. Therefore, the principal failure mode to be investigated is the longitudinal splitting of sill plates at the anchors. Sill plate-to-foundation tests are being reviewed to determine the performance characteristics of sill plates. Shear wall tests are also being reviewed to extract pertinent information including, test setup, loading, and results. The performance of shear wall anchors in the tests is being considered in the testing component of this task. In general sill plate anchorage that appear to have performed well are being considered in the testing component of this task. The testing component of this task involves tests of sill plate anchorage in direct shear, but more importantly tests of sill plates considering direct shear coupled with the overturning component of the shear wall. The test setup uses a four-foot square section of shear wall, which is bolted to a simulated concrete foundation. The test setup is intended to model the inherent eccentricity between the sheathing and the anchor bolts, which creates cross grain bending in the sill plate as the sill plate lifts off the foundation. Cyclic loads, developed under Task are being applied to the test specimens to replicate direct shear forces and forces associated with overturning of the wall. Test variables have been established and include wood species, bolt diameter, washer size and shape, power-driven pins, sill plate thickness and width, dead loads, bolt location with respect to edges of sill plate. Information, obtained through the literature research has been incorporated into a database that will be used in the evaluation of sill plate anchorage.

5 The second objective of Task (Task ) is to improve the understanding of the behavior of diaphragm-tofoundation connections that are typical of hillside constructions. These types of anchorage connections can be subjected to simultaneous shear and normal forces. It is expected that a better understanding of this behavior over the full range of performance levels will lead to an improved optimization of the capacity, reliability and performance of diaphragm-tofoundation connections. Task Diaphragms Seismic lateral loads are transmitted to shear walls via floor and roof diaphragms. In most design applications, it is assumed that diaphragms are rigid. In reality, the flexibility of diaphragms can change the dynamic characteristics of the structure and the way the lateral loads are transmitted to shear walls. Also, the behavior of diaphragm-shear wall connections is not well understood. The main objective of Task is to evaluate the in-plane stiffness, strength and internal force distribution of floor and roof diaphragms of various constructions. Task Cripple walls In California, woodframe buildings frequently are built without a basement. A short wooden stud wall, called a cripple wall, on a concrete foundation often supports the first floor in such buildings. If the cripple walls are not braced laterally during an earthquake, they can collapse and the building will fall, causing damage to the foundation, floor, walls, windows, and utility connections as well as the contents of the buildings. It may also cause fires from broken gas lines. Again, it becomes very expensive to repair the damage. The main objective of Task is to improve the understanding of the cyclic behavior of full-scale cripple walls of different constructions. It is anticipated that a better understanding of this behavior will lead to validations or recommendations for revisions of current retrofit techniques and of current code conventional construction provisions for new structures. One important issue to be addressed is the development of appropriate lateral stiffness and strength limits for cripple walls to insure compatibility of cripple wall retrofits with existing construction at story levels above. A total of 26 cripple walls are being tested in this project, with 12 level cripple walls and 14 stepped cripple walls. Two heights (2' and 4') are being investigated for the level cripple walls, and two slopes (3 horizontal to 1 vertical, and 2 horizontal to 1 vertical) are being investigated for stepped cripple walls. All cripple walls are being anchored to a concrete foundation beam founded on well-compacted soil. Testing of the cripple walls are being carried out using the recently constructed soil-pile test facility at the University of California, Davis. A test setup is being designed to simulate a reversed curvature expected in cripple walls under seismic loading. Task Shear walls Woodframe construction using shear wall and diaphragm systems has been shown to be very cost-effective lateral load resisting systems. Although a substantial amount of experimental work has been done over the past few decades on the structural behavior of wood-based shear wall systems, several issues for seismic analysis and design still required consideration. The main objective of this Task is to extend the ongoing testing program at UC-Irvine for the City of Los Angeles. Twenty-five different 16-ft x 8-ft wall configurations are being tested with two walls of each configuration built to provide some indication of the variation of results among identical samples. While almost all the configurations are using the CUREe test protocol (Task 1.3.2), several quasi-static tests use the displacement test protocol defined for the UCSD shake table tests (Task 1.1.1). A series of tables and graphs are being developed displaying the envelope curves, 'backbone' curves, hysteresis loops and other relevant data for each wall group. In addition, nonlinear analytical models are being developed in order to understand the shear walls' behavior. Task Soft/weak stories Woodframe apartment buildings with a row of garages on one side of the ground floor are vulnerable to soft or weak story collapse. Experience from the Northridge Meadows Apartments during the Northridge earthquake has shown again the vulnerability of this type of buildings.

6 The main objective of Task is to evaluate various seismic design and retrofit strategies (steel frames, shear walls, bracing, etc.) for woodframe buildings with first-floor parking garages. Task Wall finish materials A large portion of the cost associated with the repair of a woodframe building after an earthquake is related to damage to exterior and interior non-structural wall finish materials (plaster, dry walls, stucco, etc.). The relation between damage to these non-structural components and structural response needs to be established for performance-based design. The main objectives of Task is to establish repair cost versus damage relationships for finish materials and to investigate new materials and connections to reduce the damage level to these non-structural elements. Task is interacting with Task (Shake table tests of a two-story single family house), Task (Shake table tests of a threestory apartment building) of Element 1 of the Woodframe Project and with Element 2 (Field Investigations). The project primarily involves cyclic testing and analysis of full-scale gypsum wall subassemblies designed to elucidate the common types of damage observed in post-earthquake inspections. The wall panel specimens measure 8 feet high by 16 feet and include openings and boundary conditions to replicate real conditions. Finite element analysis techniques are being used to predict and extrapolate the test results to other configurations. Testing is being conducted at San Jose State University and the analyses are being run at Stanford University. Task Innovative systems Innovative seismic design and retrofit strategies have been developed for steel and concrete buildings. So far, limited research has been conducted to develop similar systems for woodframe buildings. The main objective of this research project is to investigate numerically the suitability of fluid dampers for seismic protection of light-framed wood buildings. The objective is being met by performing an analytical/numerical study of wood building components (shear walls) and systems (3-D houses) with and without fluid viscous dampers. In addition, practical issues associated with implementation of fluid dampers within light-framed wood buildings are being investigated (e.g., number of dampers required, suitable location of dampers, and cost-effectiveness of dampers). Task Connections Although a substantial amount of experimental work has been performed on the monotonic and cyclic behavior of sheathing-to-framing connections, information is still needed for a thorough understanding of the behavior of various subelement inter-connections. Additional testing is needed to develop an appropriate database of connector hysteretic models of various sub-elements. The main objective of Task is to investigate the cyclic behavior of wood element connections of various types of wood elements and to develop a database over the full range of performance levels. The task is divided in to three different sub-tasks: Task : Effect of fastener head penetration on sheathing-to-wood connections; Task : Inter-story shear transfer in woodframe buildings; Task : Diaphragm-to-wall connections in woodframe construction. The main objective of Task is to quantify the effects of fastener head penetration on sheathing-to-wood connections. This objective is being met by conducting a series of wood joint tests with different fastener head depth penetrations. Many combinations of panel thickness, fastener type and size, and fastener head penetration are being investigated. The other objective of Task is to establish a database over the full range of performance of doweltype connections. The database includes studies pertaining to sheathing-to-wood, light-gauge sheet metal-to-wood, sheathing-to-light-gauge metal studs, and wood-to-wood connections. Studies involving a variety of fasteners (nails, bolts, wood screws, lag screws, and staples) are being included in the database. The database is being compiled on a CD- ROM with a user-friendly search engine. The common objective of Tasks and is to evaluate experimentally the inter-story shear transfer mechanism in woodframe buildings. At the time of writing, the scope these two tasks are being finalized. RESEARCH TASK 1.5 ANALYSIS

7 The three different projects of Task 1.5 are related to the seismic analysis of woodframe construction. These three projects are briefly described below. Task Analysis Software Commercial structural analysis packages are not very efficient in modeling woodframe buildings. Several inherent properties of woodframe constructions (e.g., material properties, lateral load-resisting systems, etc.) make analyses of woodframe construction less precise than analyses of concrete or steel structures. For example, hysteretic rules for woodframe members are usually not available in commercial nonlinear dynamic analysis packages. The main objective of Task is to develop a specialized computer platform for the nonlinear seismic analysis of woodframe buildings with user-friendly pre- and post-processors. This software package is being developed not just as a research tool, but with the view that the structural engineering community may use it at large. An existing computer program developed earlier (Filiatrault, 1990) for the static and dynamic analysis of timber shear walls is being revisited and modified. This Shear Wall Analysis Program (SWAP) is capable of predicting the nonlinear response (static and dynamic) of timber shear wall subassemblies when subjected to static or dynamic (earthquake) lateral loads. Since SWAP was written, numerous experimental studies have been performed on the cyclic behavior of shear walls. No specific numerical model, however, has been developed to cover this cyclic loading case (only ultimate load and dynamic models exist). SWAP is being extended to include a cyclic analysis option for any type of displacementcontrolled loading-protocol. The hysteric nail-behavior is being modeled using a variation of the simple Wayne Stewart model (Stewart, 1987). This model contains path following rules for general cyclic loading (not just monotonically increasing cyclic loading protocols). Also, gap elements between panels and hold-down elements are being considered for the shear wall model. In addition, the model is accounting for the presence of gravity loads on the wall and the resulting P-Delta effects. The completed, stand-alone, computer program is performing ultimate load and cyclic analysis of general shear wall configurations with openings. The modified SWAP analysis model is being evaluated extensively against available experimental data (e.g. testing program at UC-Irvine for the City of Los Angeles). Incorporation of strength degradation and cyclic fatigue rules for the hysteric elements in the shear wall model are being calibrated using this test data. The modified and validated SWAP model is also being used in conjunction with a three-dimensional nonlinear seismic analysis model. The first model that currently being considered is the pancake model developed in Task Shake table test of a two-story single-family house of the CUREe-Caltech Woodframe Project. The pancake model simulates the three-dimensional seismic response of a woodframe construction through a degenerated two-dimensional planar analysis. The general-purpose computer program RUAUMOKO (Carr, 1998) is used to construct the pancake model in Task The SWAP shear wall model can provide the required input parameters for each shear wall in a three-dimensional woodframe structure (using the Wayne Stewart hysteretic model, available within RUAUMOKO). Using this approach each shear wall element adds at most two degrees of freedom to the problem (in-plane and out-of-plane wall behavior). Consequently, the resulting pancake model has a manageable number of degrees of freedom for a complete threedimensional analysis of the structure. In turn, this will result in reasonable computing times for a full seismic analysis. Task Demand aspects In order to develop a performance-based seismic design procedure for woodframe construction, deformation and force demands must be established for various earthquake intensities. Furthermore, design engineers have questioned the applicability for woodframe construction of the redundancy factor, which was developed for other materials. The main objectives of Task are: 1) Evaluate the seismic demands on various components of both conventional and engineered woodframe structures; 2) Evaluate the deformation demands in deformation-controlled elements of woodframe structures (e.g., shear walls); and 3) Evaluate the force demands in force-controlled elements of woodframe structures (e.g., hold-downs). Task Reliability analysis Considering the large uncertainties inherent to wood properties and seismic loading, a performance-based seismic design procedure needs to be established in a reliability-based framework.

8 The main objective of Task is to integrate the information from Tasks 1.4 and to develop a reliability-based framework for the seismic design of woodframe construction. CONCLUSION The research plan of Element 1 of the CUREe-Caltech Woodframe Project is being re-evaluated as the project progresses in collaboration with Element 3 (Building Codes and Standards) and with the Advisory Committee of the project. It is believed that this project will advance the engineering of woodframe buildings and improve the efficiency of their construction technology for targeted seismic performance levels. Comments and suggestions related to the research program of the CUREe-Caltech Woodframe Project are welcomed and can be sent via the web page of the project at Comments and suggestions can also be sent to the principal investigators of the various research tasks, as listed in Table 1. Table 1. Investigators for Element 1 Testing and Analysis of the CUREe-Caltech Woodframe Project. Task Investigator Shake table tests of a two-story single-family house Andre Filiatrault, UC-San Diego, afiliatrault@ucsd.edu Shake table tests of a three-story apartment building Stephen Mahin, UC-Berkeley, mahin@ce.berkeley.edu Shake table tests of a simplified woodframe building Frank Lam, University of British Columbia franklam@interchange.ubc.ca 1.2 International benchmark Chia-Ming Uang, UC-San Diego, cuang@ucsd.edu Rate of loading and loading protocol effects Testing protocols Helmut Krawinkler, Stanford University krawinkler@cive.stanford.edu Dynamic characteristics James Beck, California Institute of Technology jimbeck@cco.caltech.edu Anchorage James Mahaney, Wiss, Janney, Elstner Associates, Inc. JMahaney@wje.com Yan Xiao, USC, yanxiao@usc.edu Diaphragms James D. Dolan, Virginia Tech, jddolan@vt.edu Cripple walls Rob Chai, UC-Davis, yhchai@ucdavis.edu Shear walls Gerald Pardoen, UC-Irvine, gpardoen@uci.edu Soft/weak stories Stephen Mahin, UC-Berkeley, mahin@ce.berkeley.edu Wall finish materials Greg Deierlein, Stanford University, ggd@stanford.edu Kurt McMullin, San Jose State, mcmullin@ .sjsu.edu Innovative systems Michael Symans, Washington State University Connections Fernando Fonseca, Bringham Young University ffonseca@et.byu.edu Gerald Pardoen, UC-Irvine, gpardoen@uci.edu Ken Fridley, Washington State University fridley@wsu.edu Analysis software Bryan Folz, UC-San Diego, bfolz@ucsd.edu Demand aspects To be determined Reliability analysis To be determined REFERENCES Carr, A.J., RUAUMOKO User s Manual, Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand. Filiatrault, A., "Static and Dynamic Analysis of Timber Shear Walls", Canadian Journal of Civil Engineering, 17(4),

9 Filiatrault, A., Uang, C-M, and Seible, F Task Shake Table Tests of a Simplified Two-Story Single-Family House: Proposed Test Structure and Testing Protocol, CUREe-Caltech Woodframe Project, University of California, San Diego, CA. Seible, F., Filiatrault, A., and Uang, C-M. Editors, Proceedings of the Invitational Workshop on Seismic Testing, Analysis and Design of Woodframe Construction, CUREe Publication No. W-01, CUREe-Caltech Woodframe Project, Division of Structural Engineering, University of California, San Diego. Stewart, W.G., The Seismic Design of Plywood Sheathed Shear walls, Ph.D. Thesis, Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand. ACKNOWLEGEMENTS The authors gratefully acknowledge the Federal Emergency Management Agency (FEMA) as the primary source of funding for the CUREe-Caltech Woodframe Project as well as the Office of Emergency Services (OES) of California. The following organizations are also acknowledged for providing financial and in-kind support to Element 1 Testing and Analysis of the CUREe-Caltech Woodframe Project: Ainsworth Lumber Co., American Plywood Association (APA), Dixieline Lumber Co., International Staples, Nails and Tools Association (ISANTA), Johns-Manville Roofing Materials, Maruhachi Ceramics of America Inc., Structural Engineering Association of Northern California (SEAONC), Simpson Strong Tie Inc., Stimson Lumber Co., Valentine Construction Inc., Western Forest Product Association (WFPA), Willamette Industry.