Performance-Based Design of Wood Shearwalls Considering Performance of the Overall Structure

Transcription

1 Performance-Based Design of Wood Shearwalls Considering Performance of the Overall Structure David V. Rosowsky Ph.D. (99), Johns Hopkins University Richardson Chair in Wood Engineering Professor of Civil Engineering Oregon State University, Corvallis, OR, USA Jun Hee Kim Ph.D. (23), Oregon State University Post-Doctoral Researcher Department of Civil Engineering Oregon State University, Corvallis, OR, USA Summary This paper describes the extension of a risk-based methodology for seismic design of wood shearwalls with specific consideration given to the role of the individual shearwall in the performance of the overall structure. The work reported is part of a larger study by the authors having the following objectives: () to evaluate the contributions of various sources of uncertainty to shearwall performance under dynamic loading, (2) to statistically characterize the peak response obtained using a suite of ordinary ground motion records selected to characterize the seismic hazard in a particular region, and (3) to develop a risk-based procedure for performance-based design of wood shearwalls. Performance curves and design charts developed using this procedure could be used to determine limits on the seismic weight to ensure target non-exceedence probabilities for the different specified performance (drift) levels. The contributions of nonstructural finish materials also are examined. Finally, implications for specification of shearwalls are discussed in the context of emerging performance-based design methodologies for wood structures.. INTRODUCTION Woodframe structures in the U.S. often are permitted to be designed using prescriptive requirements specified in the applicable building codes or approved standards. Such prescriptive design generally involves selection of members, fasteners and amount of lateral-force bracing from tables. Fully engineered design becomes more common for other occupancies and for dwellings in the more populated areas of California and other western states, and can still involve some member selection from tables. Fastening and lateral force bracing are most often designed using methods based on engineering principles. There exists a need for preengineered prescriptive requirements in a form which is accessible by designers as well as builders. These might also be used by engineers as an approach to (or check) engineered designs, and by code developers as a check on engineered design criteria. Since wood shearwalls typically are constructed using only a few different member sizes, framing arrangements, sheathing materials, fastener types, and so forth, the process of shearwall design becomes one of selecting an appropriate combination of materials and fastener schedules. Thus, a prescriptive design aid might simply specify the combination of design parameters needed to meet a particular performance requirement. Shearwalls are integral components in the lateral force resisting system in woodframe structures, and an understanding of their behavior under dynamic loading is essential to predicting the seismic performance of woodframe buildings. Work at Oregon State University [Kim, 23; Rosowsky, 22; Rosowsky and Kim, 2a,b, 23, 24] is focusing on predicting shearwall performance considering different structural configurations, material and fastener properties, and regions of seismic hazard. The computational analysis is performed using a suite of programs that start at the fastener hysteretic property level and move up to the level of the complete structure [Rosowsky and Kim, 23]. By characterizing the seismic hazard with a suite of representative, scaled ordinary ground motion records and taking proper account of contributory sources of uncertainty (see: Rosowsky, 22), the numerical modeling procedure is able to probabilistically predict shearwall performance. Most recently, this work has been extended to develop fragility curves which describe predicted performance conditioned on different levels of spectral acceleration, the parameter used to scale the earthquake records to given seismic hazard level. These fragility curves provide information on expected performance at given hazard levels in a very concise manner and are easily interpreted by design engineers. Fragility curves are central components to a probabilistic safety (performance) analysis of

2 structural systems subject to hazard loadings. A discussion on the role of fragility analysis in performancebased engineering is provided by Ellingwood et al. (24). Most experimental tests of wood shearwalls are performed on isolated shearwall assemblies, with or without nonstructural finish (NSF) materials, with solid walls (no openings) being the most common assembly tested. Although some shake table tests of full-scale structures have been performed recently, isolated shearwall assemblies remain the most common test configuration used to evaluate the performance of wood shearwalls under seismic loading. 2. ANALYSIS OF WALLS ACTING AS PART OF COMPLETE STRUCTURES This paper presents selected results from studies of shearwalls acting as part of complete woodframe structures. Comparisons are made to results from studies (testing/analysis) of isolated shearwall. A numerical model, SAWS [Folz and Filiatrault, 22], was used in this study to investigate the performance of shearwalls in complete structures. Nonlinear dynamic time-history analyses were performed using the SAWS program and a suite of 2 ordinary ground motion records, scaled appropriately to specified hazard levels. Seismic zone IV (LA) and soil profile type D (S D ) were assumed. Two structural configurations were considered, a one-story and a two-story structure. Both are described in detail elsewhere [Kim, 23]. Due to space limitations, only selected results from the analysis of the one-story structure are presented here. The model of the one-story structure was developed to be representative of typical southern California residential construction. The plan of this structure was 32 ft. 2 ft. (9.75 m 6. m) and the structure had openings for pedestrian doors and windows. The shearwalls in the structure were built using 3 / 8 -in. (9.5 mm) OSB, attached to the framing using a gun-driven nail. In most cases, a 6 /2 (5 mm / 3 mm) nailing schedules was used. The top-plate and end studs were double members, while the sole-plate and the interior studs were single members. The framing members were nominal 2 in. 4 in.(5 mm mm) spaced (in most cases) at 24 in. (6 mm) on-center. Properly installed hold-downs were assumed to be present. Nonstructural finish materials ( / 2 -in. (2.7 mm) gypsum wallboard and 7 / 8 -in. (22 mm) stucco) were assumed to be properly attached. Based on assumed weight tributary to the roof diaphragm and wall dead load, a calculated total seismic weight of 54 lb. (67 kn) was assigned at the roof level and an equivalent viscous damping of % of critical was assumed. The information in the figures shown in [Kim, 23] was used to develop the global hysteretic parameters for each shearwall using the program CASHEW [Folz and Filiatrault, 2, 2] and the procedure described in [Rosowsky and Kim, 22]. Distributions of peak wall displacement were constructed for each shearwall for three different hazard levels. The performance of all shearwalls was well below the suggested drift limit [FEMA 2a,b] at the low hazard level (IO, 5/5), however two of the walls, the South wall and North wall, performed less well in the high hazard level (CP, 2/5) because they had a number of openings (see: Kim, 23). The other two walls, the East wall and West wall, performed well at all hazard levels. 2. Performance of shearwalls with NSF materials Nonstructural finish materials are known to contribute (in some cases significantly) to the seismic resistance of wood frame structures. In many parts of the west coast of the U.S., stucco is commonly used as an exterior finish material. Typical construction throughout North America utilizes gypsum wallboard as an interior finish material. In most cases, the contributions of these nonstructural finish materials are ignored in the design process. However, recent experimental tests are shown that the influence of stucco and gypsum wallboard increase peak strength and initial stiffness and decrease deformation capacity of shearwalls compared to bare wall [Gatto and Uang, 2; Pardoen et al., 22]. This effect was investigated (for the case of isolated shearwalls) by Rosowsky and Kim (24) using a visual best-fit program to capture the shearwall hysteretic parameters from the results of actual cyclic shearwall tests. The performance of shearwalls with nonstructural finish materials (stucco and gypsum wallboard) in the complete one-story structure also was investigated. Two cases were considered: () walls with OSB and gypsum wallboard, and (2) walls with OSB, gypsum wallboard and stucco. In all cases reported here,

3 superior construction quality was assumed. Based on calculation, and assuming weight tributary to the roof diaphragm and wall dead load, a total seismic weight of 6793 lb. (75 kn) was estimated for the structure with OSB and gypsum walls, while a total seismic weight of 2952 lb. (93 kn) was estimated for the structure with OSB, gypsum wallboard, and stucco. Stucco was assumed to have a weight of psf (479 Pa). Equivalent viscous damping of % of critical was assumed in both cases. The hysteretic parameters for the stucco and gypsum wallboard were based on available experimental test data and were adjusted for the length of the wall. In the case of the partition walls, gypsum wallboard was attached on both sides, and it was assumed that the stiffness and strength was twice that of a single side of gypsum wallboard [Folz and Filiatrault, 22]. The resulting hysteretic parameters for each shearwall (with NSF materials) in the one-story structure may be found elsewhere [Kim, 23]. The SAWS model for the one-story structure (with NSF materials) is composed of 7 zero-height nonlinear shear spring elements, one each for: four OSB only layers, four stucco layers, and nine gypsum wallboard layers. If only the gypsum NSF materials are considered, the four stucco layers are removed. Peak displacement distributions were developed for each shearwall (OSB + different combinations of nonstructural finish materials) for the three different hazard levels (IO, LS, and CO). As expected, the performance of the shearwalls with NSF materials is better than OSB-only walls at all hazard levels. The addition of stucco dramatically improves the shearwall performance. This was especially evident at the highest seismic hazard level. 2.2 Effect of partition walls Typical partition walls are constructed with gypsum wallboard attached to both sides of the wall framing using mechanical fasteners (drywall screws). The partition walls usually are treated as nonstructural elements in a building (i.e., they are excluded in a structural analysis or in the design of the primary shearwalls), however they may contribute to the overall structural performance. This was investigated using the SAWS model of the one-story structure described previously. Of particular interest was the contribution of the partition walls to peak displacement. Partition walls were found to significantly influence the shearwall performance at all hazard levels. This can be seen in Figure. As expected, the OSB-only shearwalls (without partition walls) exhibited the worst performance, while the shearwalls with NSF materials (stucco and gypsum wallboard, and with partition walls) performed considerably better. All shearwalls (with or without NSF materials) analyzed with consideration of partition walls perform well below the drift limit at IO (5/5) and LS (/5) hazard levels. Also, the variability in peak wall displacement is reduced when the effect of partition walls is considered in the analysis. This was observed at all hazard levels. FX(x) Stucco Hazard Level: LS (%/5yrs) Wall: North Wall (NW) max (in.) Figure. Comparison of peak displacement distributions for the effect of partition walls and NSF materials, (LS, 5/5 hazard level) [ in. = 25.4 mm, ft. =.35 m] 2% Drift =.92 in. OSB only

4 2.3 Performance comparison for isolated wall and wall in one-story structure Using the north wall (which had the worst performance) in the model of the one-story structure, the difference between performance of an isolated shearwall and the same shearwall acting as part of a one-story structure was investigated. The seismic weight acting on the isolated shearwall was assumed to be one-half of that acting on the complete structure. A set of ten hysteretic parameters for the north wall in a one-story structure was obtained using the CASHEW program and assumed nail parameters (see: Rosowsky and Kim, 22). Figure 2 presents the peak displacement distributions for the isolated wall and the wall in the complete one-story structure for LS (/5) hazard level. The difference in peak displacement distributions was found to increase with increasing hazard level. This suggests that consideration of the performance of the complete structure system should be included in the design of wood shearwall assemblies, particularly at high hazard level events. This might be able to be accomplished using a simple modification factor (applied to peak drift), however this factor may be very structure-dependent. Obviously much more work is needed to quantify this effect for a range of structures, but the implication of these types of results (for example, in setting redundancy factors) is evident Hazard Level: LS (%/5yrs) Wall in system Isolated wall FX(x) max (in.) Figure 2. Comparison of peak displacement distributions for isolated shearwall and shearwall in complete one-story structure (LS, /5 hazard level) [ in. = 25.4 mm, ft. =.35 m] 2% Drift =.92 in. 2.4 Fragility curves Fragility curves were developed for both isolated shearwalls and walls acting as part of complete structures by Kim (23). Examples of fragility curves for shearwalls in the simple one-story structure described previously are presented here. The seismic demand (interface) variable is the spectral acceleration, S a. Fragility curves of this type can be used either as design aids or to assess risk consistency in current design provision. Fragility curves were developed for the North wall of the one-story structure, which has a pedestrian door and windows. The North wall exhibited the worst displacement performance in the one-story structure. To construct the fragilities, the suite of ground motion records was scaled to six different hazard levels ranging from 5% in 5 years (72-year MRI) to % in 5 years (4795-year MRI). The fragility curves for the North wall sheathed with OSB only, are shown in Figure 3. Drift limits of %, 2% and 3% of the total wall height were considered. Figure 4 presents the fragility curves for the North wall with different combinations of finish materials, and with and without consideration of the partition walls. Only the IO (5/5, % drift limit)

5 performance level is shown since the other performance levels (LS, CP) result in very low failure probabilities for the walls built with NSF materials. Figure 4 confirms that NSF materials (stucco and gypsum wallboard) contribute significantly to the performance of shearwalls acting as part of complete structures under earthquake loading. It also shows that partition walls significantly influence the performance of shearwalls acting as part of a complete structure and subject to earthquake loading % 2% 3% Pf Sheathing: OSB ( 3 / 8 ") NSF: None W = 54 lb. total S a (g) Figure 3. Fragility curves for the North wall (OSB only) in the one-story structure (no partition walls) [ in. = 25.4 mm, ft. =.35 m, lb. = N].9 Pf OSB only + Stucco.3 Sheathing: OSB ( 3 / 8 ") NSF: GWB + Stucco Hazard Level: IO (5%/5yrs) S a (g) Figure 4. Comparison of fragility curves for the North wall in the one-story structure (IO, 5/5, % drift limit) [ in. = 25.4 mm, ft. =.35 m, lb. = N]

6 3. CONCLUSIONS This paper described selected results from a study to investigate the performance of a shearwall acting as part of a complete structure, and to compare it to the performance of an isolated shearwall. The contributions of nonstructural finish materials and partition walls also were investigated. Implications for seismic design of wood shearwalls were discussed in the context of emerging performance-based design methodologies for wood structures. The work reported in this paper is on-going and further results are expected in the coming year. Acknowledgments Support for this project was provided by the CUREE-Caltech Woodframe Project (Task.5.3) and the National Science Foundation (Grant No. CMS-4938). References Ellingwood, B.R., Rosowsky, D.V., Li, Y. and Kim, J.H. (24), Fragility Assessment of Light-Frame Wood Construction Subjected to Wind and Earthquake Hazards, to appear in ASCE Journal of Structural Engineering. FEMA (2a), Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, Washington, DC. FEMA (2b), Global Topics Reports on the Prestandard and Commentary for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, Washington, DC. Folz, B. and Filiatrault, A. (2), CASHEW Version.: A Computer Program for Cyclic Analysis of Wood Shear Walls, Report No. SSRP-2/, Struc. Sys. Res. Proj., Dept. of Structural Engineering, University of California San Diego, La Jolla, CA. Folz, B. and Filiatrault, A. (2), Cyclic Analysis of Wood Shear Walls, Journal of Structural Engineering, ASCE, 27(4): Folz, B. and Filiatrault, A. (22), SAWS Seismic Analysis of Woodframe Structures, Version., CUREE Publications No. W-2, Consortium of Universities for Research in Earthquake Engineering, Richmond, CA. Gatto, K. and Uang, C.M. (2), Cyclic Response of Woodframe Shearwalls: Loading Protocol and Rate of Loading Effects, Report No. SSRP-2/6, Struc. Sys. Res. Proj., Department of Structural Engineering, UC-San Diego, La Jolla, CA. ICBO (997), Uniform Building Code, International Conference of Building Officials, Whittier, CA. Isoda, H., Folz, B. and Filiatrault, A. (22), Seismic Modeling of Index Woodframe Buildings, CUREE Publication No. W-2, Consortium of Universities for Research In Earthquake Engineering, Richmond, CA. Kim, J.H. (23), Performance-Based Seismic Design of Light-Frame Shearwalls, Ph.D. Dissertation, Department of Civil, Construction, and Environmental Engineering, Oregon State University, Corvallis, OR. Pardoen, G.C., Kazanjy, R.P., Hamilton, C.H., Waltman, A. and Freund, E. (22), Testing and Analysis of One-Story and Two-Story Shearwalls under Cyclic Loading, Task.4.4. Draft Report, CUREE-Caltech Woodframe Project, UC-Irvine, Irvine, CA. Rosowsky, D.V. (22), Reliability-Based Seismic Design of Wood Shearwalls, ASCE Journal of Structural Engineering, 28(): Rosowsky, D.V. and Kim, J.H. (22a), Reliability Studies, CUREE Publication No. W-, CUREE-Caltech Woodframe Project, Richmond, CA. Rosowsky, D.V. and Kim, J.H. (22b), Performance-Based Seismic Design of Wood Shearwalls, Proceedings: World Conference on Timber Engineering (WCTE 22), Selangor, Malaysia, August 22. Rosowsky, D.V. and Kim, J.H. (23), A Probabilistic Framework for Performance-Based Design of Wood Shearwalls, Proceedings: 9 th International Conference on Applications of Statistics and Probability in Civil Engineering (ICASP9), San Francisco, CA, July 23. Rosowsky, D.V. and Kim, J.H. (24), Incorporating Nonstructural Finish Effects and Construction Quality into a Performance-based Framework for Wood Shearwall Selection, to appear in Proceedings: ASCE Structures Congress 24, Nashville, TN. SEAOSC (2), Report of a Testing Program of Light-Framed Walls with Wood-Sheathed Shear Panels, Final Report to the City of Los Angeles Department of Building and Safety, Structural Engineers Association of Southern California and Department of Civil and Environmental Engineering, UC-Irvine, December.