Force based design guideline for timber-steel hybrid structures : steel moment resisting frames with CLT infill walls

Provincial code changes have been made to allow construction of light wood-frame buildings up to 6 storeys in order to satisfy the urban housing demand in western Canadian cities. It started in 2009 when the BC Building Code was amended to increase the height limit for wood-frame structures from four to six. Recently, provinces of Quebec, Ontario and Alberta followed suit. While wood-frame construction is limited to six storeys, some innovative wood-hybrid systems can go to greater heights. In this report, a feasibility study of timber-based hybrid buildings is described as carried out by The University of British Columbia (UBC) in collaboration with FPInnovations. This project, funded through BC Forestry Innovation Investment's (FII) Wood First Program, had an objective to develop design guidelines for a new steel–timber hybrid structural system that can be used as a part of the next generation "steel-timber hybrid structures" that is limited in scope to 20 storey office or residential buildings. The steel-timber hybrid structure incorporates Cross Laminated Timber (CLT) infill walls in the steel moment resisting frames. This structure is aimed to couple strong, ductile steel moment resisting frames with lighter and stiff CLT infill walls. L-shaped steel bracket connectors were proposed to connect the steel frames to the CLT panels. Thorough experimental studies have been carried out on the seismic behaviour of the bracket connections at UBC and FPInnovations for the past four years. These connections are bolted to the steel frame and nailed to the CLT infill walls. Moreover, the provided brackets ensure full confinement between the structural elements and energy dissipation under intense seismic action. The National Building Code of Canada (NBCC) allows an Equivalent Static Force Procedure (ESFP) design method to be used with appropriate overstrength and ductility factors for the seismic design of structures. However, NBCC (NRC 2010) does not have the appropriate overstrength and ductility factors to design the proposed hybrid structure. Thus, in this report, overstrength and ductility factors were quantified analytically. A robust finite element model of the hybrid structure that accounts for the CLT panel and frame interactions was developed in OpenSees (Open System for Earthquake Engineering Simulation) (Mazzoni et al. 2006) and used for the analytical investigation. Initially for buildings designed with an Rd = 2 and Ro = 1.5, 18 different hybrid buildings were modeled and subjected to monotonic static pushover loading by varying the following modeling variables: building height (1, 3, 6, 9, 12, 15 and 20 storey), CLT infill configuration (one-bay infilled and two-bay infilled), connection bracket spacing (800 mm), and ductility class (ductile D and limited ductile LD). In order to have a non-conservative and economical design, 3-, 6-, and 9-storey hybrid buildings were designed using Rd = 4 and Ro = 1.5. A nonlinear static pushover analysis has been performed to validate the overstrength factors of the hybrid buildings under consideration. In order to check the FEMA P695 (FEMA 2009) acceptable failure probabilities and collapse margin ratios, Nonlinear Time History Analysis (NLTHA) and Incremental Dynamic Analysis (IDA) were carried out using 60 ground motion records. Ground motions were selected and scaled for the city of Vancouver by considering site class C of NBCC 2010 (NRC 2010). Due to the complexity and the contributions of sub-crustal and subduction type earthquakes to the total seismic hazard, the traditional FEMA P695 (FEMA 2009) was not utilized in the ground motion selection and scaling. Therefore, a new ground motion selection criteria that specifically incorporates the seismicity of Vancouver, Canada, were utilized for this project. In the IDA, conservative collapse criteria have been followed to define the dynamic instability of the building. In this approach, structural hardening was only considered for interstorey drift values less than 10% and the lowest spectral acceleration value was considered as a limit state point. Analysis was done by a high performance computational method using 200 clusters of computers at The University of British Columbia research computing service center. The results show that the presence of CLT infill walls significantly affects the systems overstrength value, by sacrificing ductility. From this research, it can be concluded that an overstrength factor of Ro = 1.5 and a ductility factor of Rd = 4 showed acceptable and economical design of the proposed hybrid structure.

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