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Hybrid Wood and Steel System: Overstrength and Ductility

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Presentation on theme: "Hybrid Wood and Steel System: Overstrength and Ductility"— Presentation transcript:

1 Hybrid Wood and Steel System: Overstrength and Ductility
M.A.Sc Student: Carla Dickof Supervisor: Professor Stiemer, UBC; Professor Tesfamariam, UBC Okanagan FP innovations: Erol Karacabeyli Marjan Popovski

2 Project Description Goal:
Analyse and provide guidelines for the design of the hybrid seismic force resisting system, steel moment frames with infill wood shear walls Hybrid System of Interest: Hybridize steel and wood into a vertical seismic force resisting system. Focus on steel moment frames with a wood infill wall system Address material incompatibilities with special attention to hydroscopic properties in wood Provide values for equivalent static seismic design of system For this project I looked at the hybridization of wood and steel into one seismic system. Specifically I was looking at the placement of wood panels within a steel moment frame I was attempting to determine the effect to strenght and stiffness the panels would have Additionally we hoped to provide some estimates at the seismic factors used for static design in most codes

3 Hybrid System: Base Building
Building Plan Frame Elevation We started by creating a test building to analyse. 4x3 bays Analysed one frame with 2D analysis 3 bay frame with exterior bays of 9m and an interior bay of 6m Ground floor height of 4.5m and all other stories 3.65m typical for an office building

4 Hybrid System: Parameters
Options Infill Wall Types CLT shear walls Midply shear walls Ductility Limited Ductility Ductile Storeys 9 6 3 Braced Bays One Bay 2 Bays 3 bays Bracket Properties Gap between infill and steel frame We looked at several parameters The type of infill wall The ductility of the desing of the steel frame 3 different heights of building The configuration of infill bays As well as the gap spacing between the edge of the panel and the steel frame Infill Case 1 Infill Case 2 Infill Case 3

5 Bare Frame Design Steel moment frame to be designed based with NBCC ductility requirements Infill walls to be added and compare the response of the frame and the response of plain wood walls Ductility Type Steel Moment Wood Rd Ro D 5.0 1.5 2.0 1.7 MD 3.5 LD 1.3 The steel frames were designed to meet the criteria in Canada’s steel design code One set of frames was designed to meet the ductility requirements of a highly ductile frame and ther other for a limited ductility frame To allow for a more direct comparison of strengths and stiffnesses both were design for an equivalent static load associated with the ductility of a wood building between bare frames and infilled frames

6 Infill Walls: Midply Shear Walls
Midply walls have higher strength compared to standard plywood shear walls Nails in double shear Nail head does not pull through sheathing Increased nail edge distance Failure of walls occurs through buckling of studs For the infill walls, two types of walls were looked at. The first was midply plywood shear walls These provide a central layer of plywood between 2 studs instead of on the outside of the wall Allows for higher strength and stiffness compared to typical shear walls because the nails are in double shear These were modeled taking in to account as non-linear systems taking in to account the strength and ductility after the peak strength is reached

7 Infill Walls: CLT Walls
Approximated as elastic perfectly plastic with plasticity model Elastic properties determined using composite theory Strength limits determined from product data Plain CLT systems show all deformation in connectors Confinement from surround frame may cause deformation in the panel Parallel to grain Perpendicular to grain ELASTIC PROPERTIES Elastic 7800 MPa 4600 MPa Shear 250 MPa STRENGTH Tension 16.5 MPa Compression 24 MPa Crushing 30 MPa 5.2 MPa The other type of wall considered were CLT walls. CLT walls include both rocking behaviour and shear deformation in the panel The shear in the panel is often ignored as they are so much more stiff than the connectors Within a steel frame bay, even after the connectors fail, the panel will continue to be loaded Little information is available about the failure in in-plane shear of CLT panels A simplified elastic perfectly plastic model was used here with the ultimate strenght of 5.2MPa Further testing is required to determine the true behaviour of these panels Shear and Rocking Pure Rocking Pure Shear

8 Connection between Wall and Frame
Nailed bracket connection developed for CLT walls Bracket behaviour is independent in different directions Confinement also provided along edges of panel to provide confinement using “gap” elements The connections were modeled using the behaviour shown here as developed at FPInnovations Different behaviours were noticed parallel and perpendicular to the grain grain Also, links representing the confinement were modeled around the panels that act in compression only The panels were modeled to sit directly on the steel beams below them A gap of varying size was provided around the sides and the top of the panel

9 Pushover Results Midply walls do not add significant stiffness or strength to the system; CLT infill walls show significant gains over plain steel moment frame Effect of Infill Panel Type: Single Storey Single Bay Frame

10 Pushover Results Initial stiffness is due the combined stiffness of the moment frame, and the connections between the infill wall and the CLT frame Second linear stiffness is present once gap is close and frame is directly loading the CLT walls, As the connection yields the confinement continues to contribute significantly until the frame begins to fail Effect of Gap Size between Infill Panel and Frame: Single Bay Single Storey Frame

11 Pushover Results Ductile vs Limited Ductility The shape of the pushover curve is similar for infilled ductile and limited ductility frames Ductile frames do not maintain the flat, post yield behaviour For systems with 2 or 3 infilled bays, the ductile frame shows the panel yielding around the same time as the frame Comparatively the limited ductilit frame shows the panels yielding prior to the frames This could allow for more redundancy in the system Effect of Moment Frame Ductility: 3 Storey Steel for all Infill Configurations

12 Pushover Results As the number of stories increasing, the post yield behaviour gets worse The manor of yielding does not change between frame systems Effect of Number of Storeys: Limited Ductility Steel Moment Frames for all Infill Configurations

13 Pushover Results 3 Storey Frame 6 Storey Frame 9 Storey Frame
Ductile Limited Ductility For ductile frames, the frame consistently yields before the panle begins the fail For limited ductility frames, the panel begins to fail at or before the frame begins to yield in the 3 and 6 storey cases All cases showed a significant increase in strength of the system with the addition of infill panels. The yield strength of the frames increased with each panel that was added in a fairly linear manner The yield strength of the panels was much more consistent regardless of the number of infill panels added. Higher panel yield strengths for the systems with only one infilled bay could be attributed to more significant rocking in these shorter panels In all cases the addition of Because the frame is the gravity resisting system, it is preferable for the frame to remain undamaged and the panel to absorb energy through failure Comparison of Frame and Panel Yield for all Frames and Infill Configurations

14 NBCC Seismic Factor Definition
Overstrength (Ro or Ω) Ductility (Rd or µT) Overstrength is the ratio of the design load to the ultimate load of the system Looking at the innate overstrength in this type of system, the design load is taken as the load at first yield Ductility is the ratio of the displacement at the ultimate load to the displacement at failure Failure is taken as an 80% reduction in strength after the ultimate load has been acheived according to FEMA P695 In determining the overstrength and ductiliy, the FEMA P695 guidelines were use The over strength is generally taken fromt eh maximum load achieved over the design load; instead we used the yield load The ductility was taken as the poust ultimate deflection over the deflection at ultimate. This correspondes well with the canadian code standards

15 NBCC Seismic Factors Ductility Factor for all Frames
The ductility is significantly decreased for the ductile moment frames The ductility is consistent, or even increased compared with a limited ductility bare moment frame As you can see, the ductilities are all well below the design ductility allowed for ductile moment frames Here, the ductility for a limited ductility moment frame, which coincides with the ductility factor subscribed for CLT walls by FP Innovations, is fairly consistent with the initial ductility factors found for the infilled system Ductility Factor for all Frames

16 NBCC Seismic Factors Overstrength Factors for all Frames
The overstrengths does not show large variability compared to the bare frame. Here you can see that the overstrength, especially for taller systems, also corresponds better with an overstrength of 1.7, associated witha limited ducitlity moment frame, compared to the 1.5 for a ductile moment frame, Overstrength Factors for all Frames

17 Future Work FEMA P695 guidelines for dynamic analysis
Future work will consider a dynamic analysis of the system following the FEMA P695 guidelines This will allow further refinement of the seisic design factors A design guideline for the strength and stiffness impact of infill walls is also needed Perhaps most importantly, testing of the infill system is required to confirm the model FEMA P695 guidelines for dynamic analysis Partial Incremental dynamic analysis 22 ‘Far-Field’ ground motions

18 Acknowledgements Our supporters at NewBuildS through NSERC and Canadian Steel Institute of Steel Construction Thanks to everyone at FPInnovations, with special thanks to Dr. Popovski and Prof. Karacabeyli, industrial advisors to the project Special thanks to the supervisors Dr. Stiemer and Dr.Tesfamariam from the University of British Columbia Acknowledgements to UBC grad students: Yalda Khorasani, Mathieu Angers, Hassan Pirayesh, Carla Dickof, Caroline Villiard, Benedikt Zeisner. I would like the thank our supporters are NewBuildS through NSERC, as well as the Canadian Institue of Steel Construction. Special thanks to everyone at FP Innovations who have been so helpful as well as my supervisors Dr. Stiemer, and Dr. Tesfamariam, and all the other students who have worked on the project. Any questions

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