Seismic performance of multi-storey light wood frame buildings with hybrid bracing systems

Transcription

1 Congrès annuel 2011 de la SCGC CSCE 2011 Annual Conference Ottawa, ON 14 au 17 juin 2011 / June 14-17, 2011 Seismic performance of multi-storey light wood frame buildings with hybrid bracing systems Andi Asiz and Ying H. Chui Wood Science and Technology Centre University of New Brunswick, Canada Chun Ni and Mohammad Mohammad FPInnovations - Wood Products Division, Canada Ghasan Doudak Department of Civil and Environmental Engineering University of Ottawa, Canada Abstract Lateral load resistance of light wood frame buildings (LWFB) is generally provided by wood based wall panels. From an architectural perspective, the requirement for shear walls may lead to problems with respective to space usage in a building and limit building plans with open concept, which is favoured by consumers, from being adopted. This issue is compounded when the storey height limit is raised. There is a need to provide alternative bracing systems that will provide the required lateral resistance but still allow for open space in buildings. Portal frame systems have been identified by engineers and builders as a viable option to meet this need. However, their application so far has been limited to residential structures up to two storeys high. The main objective of this paper is to investigate the feasibility of using wood or steel portal frame as a substitute for regular wood shear walls in multi-storey LWFB. A computer software SAPWood developed to analyze multi-storey LWFB system and components under dynamic earthquake motions was used. Three to six storey tall buildings were analyzed using layouts that represent situations associated with decisions on utilizing efficient space usage. The reference case was the building having wood based shear walls as the main lateral load resisting system. In the other cases a hybrid approach was utilized for the lateral load resisting systems, and where some of the shear walls are replaced by wood or steel portal frames. All shear wall and portal frame hysteretic properties for both cases and interstorey (hold-down) connection properties were derived from test data and detailed numerical modeling of wall sub-systems. The buildings were subjected to various earthquake ground motion excitations, and each ground motion was scaled until failure in the components or excessive inter-storey drift was reached. Structural performance comparison amongst these cases included: storey drifts versus ground acceleration, failure or collapse behaviour, and force distribution along the shear wall lines and between the shear wall lines. Key words: light-frame construction, lateral load resisting systems, wood shear wall, wood portal frame, seismic analysis, drifts. 1. Introduction In light wood frame building (LWFB) applications, portal frame systems can be introduced to accommodate wider openings that cannot be achieved if regular wood braced (shear) walls are used. The portal frame is normally designed to reach higher lateral load carrying capacity while maintaining vertical load carrying capacity mainly from moment resisting connections. Presently in LWFB practice, there are two types of material used in portal frame systems, wood and steel. Typical wood portal frame consists of GC-128-1

2 wood structural panel overlapping a header to form a moment resisting connection (Fig 1). Hence, the wood portal frames can resist high lateral loads for a small width 400mm to 600mm (16 to 24 ) to height ratio relative to the regular shear walls. Depending load application, wood portal frame with clear span of 1.8 m to 5.5m (6 to 18 ) and wall height of 2.44m to 3.05m (8 to 10 ) can be designed using double or triple 2x12 (38mm x 284mm) lumber or structural composite lumber (SCL) headers. So far the application of the wood portal frames has been limited to one and two storey high residential buildings that are designed mostly using prescriptive based approach (IBC, 2006). For engineering design, the lateral load carrying capacity of wood portal frame can be calculated based on critical strength of one of the following options: a combination of nailed moment connection and base (anchored) connection divided by the portal frame height; lateral capacity of sheathed nailed connections; and shear strength of wood based panel, whichever governs (Martin et al, 2008). Due to similarity in lateral response under cyclic load, structural performance comparisons have been made between wood portal frames and regular wood panel shear walls. Based on test results, some of the portal frames are comparable and even perform slightly better in term of the strength and stiffness relative to those of regular wood shear walls with slightly larger width-to-height ratio (Martin, 2003). However, unlike the regular wood shear walls, no mechanics based method is yet available to determine lateral deflection of portal frame. Steel portal frame used in LWFB typically consists of a beam and two columns that are bolted or welded to form rigid (moment) connections (Fig 2). The frame resists lateral loads primarily through bending in the beam and columns. Due to availability of various sizes of steel members (e.g. AISC section), virtually any steel portal frame can be designed to accommodate span and loading needs in LWFB. Standard mechanics based method is readily available to determine lateral strength and associated drift of steel portal frame under lateral and gravity load. Few proprietary steel frame products are also available in the market, e.g. Simpson Strong-Tie (2010). The steel portal frame is primarily applied in the first storey of single or multi-storey buildings and can be used in new or retrofitted buildings that have either larger opening in windows or entry ways or garage front, tuck-under parking, great rooms, etc. The main reason behind this is that it is relatively straightforward to anchor steel frame to concrete foundation rather than to wood diaphragm components in platform-based LWFB. In this study, the feasibility of extending the application of wood or steel portal frames in LWFB up to six storeys is investigated. Fig 1: Wood portal frame (APA, 2002) Fig 2: Steel portal frame (Simpson-Strong Tie, 2004) 2. Structures analysed The building configurations analysed in this study were established based on possible substitution of regular wood shear walls with alternative portal frame systems to achieve wider openings and larger spaces. Three building layouts were analyzed as shown in Table 1. The first layout is the reference case in which the shear (braced) wall lines were composed of only regular wood shear walls. The second layout is a mix of portal frame systems and regular wood shear walls, and the third one has the wood shear wall completely replaced by a single portal frame in a shear wall line. Based on this layout and design information (described in the next paragraph), it was obtained that 5.49m (18 ) open space can be designed for the mix portal frame and regular shear walls, and 9.8m (32 ) open space can be utilized for GC-128-2

3 the single steel portal frame assuming 1.22m (4 ) width for the columns. Three- and six-storey buildings assuming uniform layouts from the first storey to roof were analysed. No wood portal frame was used in the 6-storey buildings due to inadequate lateral strength particularly at the lower storeys. All buildings were arranged to have three shear wall lines in the Y-direction and two shear wall lines in the X-direction. When portal frame is used, it is located in the interior shear wall line in the Y-direction only since the focus of this study is on investigating the portal frame behaviour relative to the reference case. Storey height is assumed to be 2.74m (9 ). In total, seven structures were analysed: four cases for the 3-storey buildings and three cases for the 6-storey buildings. The presence of plasterboard finish (gypsum wall board) was not considered in the analysis. For design purposes, the unit floor weight plus dead load for all buildings was taken as1.44 kn/m 2 (30 lb/ft 2 ). The buildings were designed in Site Class D and PGA=0.47g. Based on this design information, the seismic coefficient (V/W) is 0.135g, where V is the base shear force and W is the total weight of the structure. The base shear is distributed inversely proportional to the height of the building and shear forces are distributed to the walls using tributary-based approach. Assuming the use of 11.1 mm thick OSB sheathing for all wood shear walls, edge nail spacing can then be specified. For the 6-storey structures, 10d common nails (length=76.2 mm) spaced at 50 mm o.c. were used to connect the wall sheathing to the framing for 1 st, 2 nd, and 3 rd storeys; 8d common nails (length=63.4 mm) spaced at 75 mm o.c. were used for 4 th and 5 th storeys, and spaced at 150mm o.c for 6 th storey. For the 3-storey structures, 8d common nails were used and they were spaced at 50mm, 75mm, and 150 mm o.c. for 1 st, 2 nd, and 3 rd storeys, respectively. The sheathing-to-framing nail information was used to generate the hysteretic wall properties. It was assumed that hold-downs were used at each wall end, with 25.4 mm-diameter bolts applied at 5 th - 6 th storeys, and 32 mm-diameter bolts for 1 st - 4 th storeys. For the 3-storey building, based on the shear load and available ultimate strength data (Martin, 2003), 600mm (24 ) wood portal frames with 5.49m (18 ) double-2x12 lumber headers can be used to replace 3.66m (12 ) wood shear walls from 1 st to 3 rd storeys (see Table 1). As another alternative, 5.49m (18 ) steel portal frames that can sustain similar lateral load (and gravity load) can be designed and used in combination with the 3-storey structures using either standard sections or proprietary products. Table 2 summarizes steel members used for the 3-storey and 6-storey buildings. It should be noted that for mix of lateral load resisting systems, the lateral load distributed to the portal steel frame was multiplied by the ratio of R between wood and steel (6.5/3.5=1.86) since the seismic load calculation was based on wood (ASCE, 2005), where R is the ductility factor. Table 1: Analyzed structures (1 = m) Reference Portal frame 1 Portal frame 2 Layout 1 Layout 2 Layout 3 Steel portal frame-1 (clear span=18 ) GC-128-3

4 Steel portal frame-2 (clear span=32 ) Reference Wood portal frame (clear span=18 ) Reference Steel portal frame-1 (clear span=18 ) Steel portal frame-2 (clear span=32 ) Table 2: Steel members used (AISC, 1999) Building 3-storey 6-storey Storey Portal frame 1 Portal frame 2 beams column beams column 1 W12x16 W14x22 W18x35 W16x26 2 W12x16 W12x19 W18x35 W18x35 3 W12x16 W12x14 W18x35 W18x W14x22 W16x26 W18x40 W18x W14x22 W16x26 W18x40 W18x35 6 W14x22 W12x16 W18x40 W16x26 3. Structural modelling Structural modeling of the buildings was conducted using SAPWood software capable of simulating the three-dimensional seismic response of LWFB through a degenerated two-dimensional planar analysis (Pei and van de Lindt, 2007). Major numerical input for SAPWood software includes diaphragm coordinates, shear wall geometry (locations, lengths, and height), shear wall properties, hold-down properties, masses for the diaphragms and walls, and time-domain earthquake ground motions in the orthogonal directions. The model assumes that all diaphragms in this building are rigid. In SAPWood, each diaphragm movement was represented using 3 degrees-of-freedom, i.e. two translations and one rotation about vertical axis. Each shear wall spring element can be calibrated to reflect the hysteretic response under lateral cyclic load including strength and stiffness degradation, and pinching effect (Fig 3- left). Table 3 gives selected response parameters (initial stiffness K 0, resistance force parameter F 0, pinching residual resistance force F 1, slip at maximum restoring force X u, and stiffness factors r 1 -r 4 ) for the shear walls and wood portal frame. For the wood shear walls, the hysteretic properties were derived based on the numerical modeling option available in SAPWood software using nailing geometry information (spacing and length/diameter) and wall length, whereas for the wood portal frame, the properties were extracted directly from test data (APA, 2010). For the steel portal frame, bi-linear spring elements were used in the model (Fig 3-right). Since at the moment there has not been test data available, the initial stiffness was estimated based on linear elastic analysis and the post yield stiffness was assumed to be 15% of the initial stiffness (Aoki and Ikeda, 2006). GC-128-4

5 Table 4 summarizes the steel portal frame properties used in the analyses reported herein. The strength of the steel portal frame under lateral load (V u ) can be calculated based on standard plastic design allowing only the beam near the moment connection to develop plastic (collapse) hinges with the supports or bases modeled as fully pinned. However, in practice some slips might occur at the supports and this could impact the portal frame stiffness and yield deformation values. Although it was not used as major input in SAPWood software (current version), the lateral strength of the steel portal frame was needed to check whether failure was initiated in this component. For the hold-down and wood surrounding it, bi-linear spring model with different stiffness value in tension and compression was assumed. The axial stiffness value of wood stud in compression was 14.0 kn/mm, and that of the hold-down in tension was 10.7 kn/mm and 5.4 kn/mm for 1 st - 4 th storeys and 5 th - 6 th storeys respectively. 1,2: backbone curves 3: unloading path 4: transition path 5: uploading path force r 1 K 0 F 0 K 0 1 r 4 K 0 4 X u K p r 2 K 0 3 F 1 4 drift r 3 K 0 K 0 : initial stiffness r 1 -r 4 : stiffness factors for loading path F 0 : backbone tangent intersection F 1 : loading path intersection K p : uploading stiffness as function of α, and β and drift Fig 3: Hysteretic properties for wood shear wall/wood portal frame and steel portal frame Bldg Storey Shear wall length Table 3: Wood shear wall and portal frame properties K 0 (kn/mm) F 0 (kn) F 1 (kn) X u (mm) r 1 r 2 r 3 r st st * st Note: *wood portal frame properties used in the 3 storey building. α=0.8 and β=1.1 for all shear walls and wood portal frame GC-128-5

6 Table 4: Steel portal frame properties Building 3-st 6-st Storey Portal frame 1 Portal frame 2 K 0 K y D y V u K 0 K y D y V u (kn/mm) (kn/mm) (mm) (kn) (kn/mm) (kn/mm) (mm) (kn) Seismic analyses Seismic performance parameters can be quantified using non-linear static (pushover) analysis with inverted-triangular load pattern of the lateral static equivalent earthquake load. Important parameters, including maximum base shear force and ultimate drift, can be determined in the pushover curve which is typically shown in term of drift ratio versus seismic coefficient (V/W). The drift ratio is defined as the ratio between interstorey drift over shear wall height, V is the base shear and W is the total weight of the structure. Fig 4 shows the pushover analyses of the 3- and 6-storey structures. Referring to Table 2, ref indicates the reference structure, steel1 denotes the steel portal frames with 18 (5.49m) clear span mixed with the regular shear walls, steel2 denotes the steel portal frames with 32 (9.75m) clear span, and wood denotes the wood portal frame used only in the 3-storey structure. Based on the current designs, it can be seen from the pushover curves that the reference structure has slightly larger ultimate base shear than the other structures. Or quantitatively the reference structure has higher overstrength factor than the others, where the overstrength here is defined as the ratio of the maximum base shear force to design base shear (ATC, 2009). If the ultimate drift is taken as the displacement at 20% strength loss (0.8V max ), the steel portal frames exhibit higher ultimate drifts relative to the reference case. However, their ductility ratios are about the same, where the ductility here is defined as the ratio drift at yield over ultimate drift. Also, the steel portal frames show slightly higher post-peak base shear relative to the reference structure. This is not surprising since steel is expected to undergo hardening response well after yielding is reached. It can also be seen in the case of 3-storey structures that the wood portal frame can be designed to perform similarly with the steel portal frame. V/W ref steel1 steel2 wood drift ratio (%) V/W ref steel1 steel drift ratio (%) (a) 3-storey structures (b) 6-storey structures Fig 4: Results of pushover analysis GC-128-6

7 To investigate collapse behaviour, non-linear dynamic analysis was performed using various ground motions that were scaled to increasing intensity until the structure reaches a failure point. The intensity of ground motion causing failure in the structural models is defined as the point on the ground motion versus drift curve having a nearly horizontal slope but without exceeding a peak storey drift of 7% in any lateral load resisting system of a model (ATC, 2009). In the numerical analysis, this is normally indicated by numerical instability in the calculation shown by unreasonably higher structural periods or excessive drifts (lateral dynamic instability). However, as will be shown later, this situation will not be fulfilled as the steel portal frames were assumed to have a bi-linear stiffness in the model. Hence, the steel portal frame was assumed to have exhibited failure or collapse when the component strength of the steel portal frames was reached. In this study, 9 ground motion records were applied at each structure with Peak Ground Acceleration (PGA) ranging from 0.12g to 0.56g and duration from 22 to 44 seconds (PEER, 2000). A 5% damping ratio was assumed for all structures. Figs 5 and 6 show the results of each structure plotted in term of PGA versus maximum storey drift ratio. In each structure, each line is formed by a given ground motion with associated PGA scaled to increasing intensities. Differences between lines reflect differences in the response of the structural model when subjected to different ground motions. PGA causing structures to collapse (PGAc) and collapse margin ratio (CMR) were also shown in Figs 5 and 6. Here PGAc is defined as median collapse value (i.e. PGA at 7% drift ratio), and CMR is defined as the ratio between PGAc and PGA-design (ATC, 2009). As was mentioned in the earthquake design process, PGA-design=0.46g was used. It should be noted that the more ground motions used in the analysis, the more accurate the determination of CMR is. However, since the focus in this study is the performance comparison between various lateral load-resisting systems in buildings, it is believed that 9 ground motions were adequate, and no further probabilistic collapse analyses (e.g. cumulative distribution function) were performed. The results shown in the 3-storey structures indicate that the wood portal frame has comparable CMR relative to the steel portal frame system (steel2). In the 6-storey structures, the steel portal frames (steel1 & steel2) have higher CMR than that of the reference case. Again, this is due to the assumed postyielding behaviour (with no assigned ultimate load parameter in the model) developed in the steel portal frames. Checking the forces near collapse developed in the steel portal frames showed that they are still below the lateral strengths shown in Table 4, indicating that the beam, column and the moment connection are still intact after several cycles of ground motion. If other failure mechanism such as slip that leads to failure developed at the supports, then the capacity of the portal frame would be governed by this case. For the structures with the mix lateral systems in one braced wall line (steel1 and wood), the lateral force within the line was distributed to the wood shear wall and portal frame based on the respected stiffness, both in the linear and post-yield regimes. This is due to modelling assumption that was based on nonlinear spring element arrangement. Also, the lateral force between the brace wall lines were distributed based on the stiffness and this is in accordance with the modelling assumption of rigid diaphragm behaviour. PGA c =1.65g CMR=3.59 PGA c =1.59g CMR=3.46 GC-128-7

8 (a) Reference (ref) PGA c =1. 50g CMR=3.26 (b) steel portal frame (steel1) PGA c =1. 50g CMR=3.26 (c) steel portal frame (steel2) (d) wood portal frame (wood) Fig 5: Results of non-linear dynamic analysis of 3-storey structures PGA c =1. 10g CMR=2.39 PGA c =2. 25g CMR=4.89 (a) Reference (b) Steel portal frame 1 (steel1) PGA c =2. 51g CMR=5.46 (c) Steel portal frame 2 (steel2) Fig 6: Results of non-linear dynamic analysis of 6-storey structures GC-128-8

9 5. Discussion One of the important design procedures in multi-storey LWFB is checking the lateral deflection (drift). The current drift calculation is based on semi-empirical equation that incorporates deformation caused by bending, shear, anchorage and nail slip induced in wood-based shear wall (CSA, 2009). When regular wood shear walls are combined with portal systems in a braced wall line, superposition principle of load at each lateral load resisting system can be used in that braced line. This implies that the stiffness of each lateral load system is needed to estimate the drift, since the force based on the above numerical analysis result is distributed based on the stiffness. As in the wood shear walls, iterations are needed in the calculation until the result converges. For the steel portal frame, standard mechanics based derivation (Fig 7) or simple numerical modelling can be used to estimate the initial stiffness, whereas for the wood portal frame, test data or numerical modelling can be used. Note: is a function of the portal frame geometry and section property Fig 7: Steel portal frame structural analysis As for the seismic performance factors (overstrength, ductility, displacement amplification factors) in a hybrid system, existing design procedure such as that given in ASCE-07 (2005) can be used. In principle, the parameters used to calculate the lateral load design shall be referenced (scaled) to the least ductile system. For example, for wood panel shear walls (R=6.5) and the steel (ordinary) moment frames (R=3.5), the design shear would be based on the R=3.5 in the direction of lateral load. In LWFB that is normally constructed using platform-frame construction, the building wall is not continuous vertically because it is disrupted by the floor framing (platform). This condition also applies to the portal frame system, i.e. its columns are not continuous. This could result in high force concentration in the portal frame supports, as portal span gets longer. To avoid excessive vertical deformation (e.g. crushing of the wood), rigid supports or bases are required. This can be implemented by designing special blocking that can spread the forces in addition to the adequate anchoring system. 6. Concluding remarks In general, it can be concluded that it is feasible to mix portal frame systems with regular wood shear walls of multi-storey LWFB provided that resistance and load design (as well as serviceability) requirements are fulfilled. Existing wood panel portal frame (e.g. APA portal frame), which is currently limited up to two-storey residential structures, can be applied in multi-storey LWFB building up to three storeys, depending on the lateral load demand. For steel based portal frame, any configuration can be designed to meet any load demand. Furthermore, other types of portal frame system that can meet structural design requirements can be designed and mixed with LWFB. The challenge would be on detailing connections or strengthening system around the area of portal frame base or supports. Also, feasibility of using continuous (balloon) multi-storey steel frame construction that can be combined with regular LWFB platform based construction needs to be investigated, since higher ductility factor can be designed for this type of moment frame connection. Future numerical analyses of hybrid LWFB will include investigation on diaphragm flexibility and torsional effects. Also, mixing different bracing systems between storeys needs to be studied to see whether there is a sudden jump in the lateral building stiffness (i.e., effect of relative stiffness). Laboratory test and detailed numerical models of mix shear wall system will need to be conducted to study the interaction behaviour including the lateral load distribution. GC-128-9

10 Acknowledgments The authors greatly acknowledge the financial support provided by NSERC under the NEWBuildS research network. Thanks are also extended to FPInnovations for providing test results of wood portal frames, to Dr Shiling Pei of South Dakota State University for providing the latest version of SAPWood software, and to Dr. Borjen Yeh of APA The Engineered Wood Association for also providing test results of wood portal frames. References AISC (1999), Manual of Steel Construction Design, Load and Resistance Factor Design, American Institute of Steel Construction, 3 rd ed, Chicago, USA. Aoki, H., Ikeda, K. (2006), Hysteresis model of low-rise steel frame building and its seismic performance, Steel Structures, 6: APA (2002), 24 wood portal frames under cyclic load - unpublished test results, APA - The Engineered Wood Association, Tacoma, WA, USA. ASCE (2005), ASCE7-05: Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, Reston, VA, USA. ATC (2009), Quantification of Building Seismic Performance Factors, Report No FEMA P695, Report Prepared for the Federal Emergency Management Agency by Applied Technology Council, Washington D.C., USA, 421p. CSA (2009). CSAO86-09: Engineering Design in Wood. Canadian Standards Association, Mississauga, Ontario, Canada. ICC (2006), 2006 International Building Code, International Code Council, Inc, Washington, D.C., USA. Martin, Z., Skaggs, T.D., Keith, E.L., Yeh, B. J. (2008), Principles of mechanics model for wood structural panel portal frames, ASCE Structures 2008: Crossing Borders, Vancouver, Canada, 10p. Martin, Z. (2003), Cyclic evaluation of APA Sturd-I-Frame as wall bracing, APA Report T , The Engineered Wood Association, Tacoma, WA, USA, 21p. Pei, S. and van de Lindt, J.W. (2007). Seismic Analysis Package for Woodframe Structures Version 2.0: UsersManual for SAPWood for Windows, Colorado State University, Fort Collins, USA, 72p. PEER (2000). PEER Strong Motion Database, Pacific Earthquake Engineering Research Center, Berkeley, CA, USA, Simpson Strong-Tie (2004), Strong Frame TM Ordinary Moment Frame C-SF10, Product Specification. GC