Seismic assessment of a hybrid light wood-frame structure connected to a balloon-type CLT core
Date
2023-08-01
Authors
Eini, Ariya
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Abstract
Light wood-frame structures are the most common type of construction for residential and low-rise buildings in North America. The 2015 edition of the National Building Code of Canada has increased the height limit for light wood-frame construction from 4 to 6 stories. With the increase in building height, the design of light wood-frame structures may be more likely governed by inter-story drift under wind and seismic loads. To reduce the inter-story drift, a hybrid system, consisting of CLT cores and light wood-frame structures, is proposed in this study. Given the lack of design guidelines on the performance of hybrid wood structures under seismic loads, it is imperative to gain a comprehensive understanding of their performance under earthquake load that includes the hybrid performance, as well as performance of individual sub-structure and inter-structure connections.
The first part of this study focuses on assessing the influence of energy dissipation due to different pinching levels on the seismic performance of individual sub-structure, i.e., a light wood-frame shear wall system, over a wide range of fundamental periods. The study revealed that structures with periods less 0.7 s are more susceptible to the effects of hysteresis loop pinching than long-period structures. The residual strength of pinching loops has a greater influence on the seismic performance than the stiffness of the pinching loops. Hysteretic energy dissipation derived from standard reversed-cyclic tests can provide a better understanding on the seismic resistance of timber structures. However, the hysteretic energy under a seismic event at near-collapse stage neither agrees with quasistatic cyclic test’s energy dissipation nor is well correlated to the maximum seismic capacity of the structure.
In the second part of this study, monotonic and reversed-cyclic tests were conducted on the connections between the two subsystems in the proposed hybrid building. The CLT core and light wood-frame structures were connected on the floor level with self-tapping screws (STSs) inserted at 45°, 90°, and mixed angles (45° and 90°). Results show that 45° STSs connections had high stiffness but low energy dissipation, while 90° STS connections had high energy dissipation but low stiffness. Mixed-angle Connections had significantly higher ductility and energy dissipation compared to connections with STSs only inserted at 45° or 90°.
The final part of this study investigated the effect of STS connection ductility on the seismic performance of a hybrid building consisting of light wood-frame shear walls and a balloon-type CLT core. The National Building Code of Canada requires the lowest seismic force modification factors (RdRo) of the subsystems be used for the design of a hybrid building if the subsystems are rigidly connected, which may be conservative if a ductile connection is used. Therefore, a numerical study was conducted to determine the possibility of utilizing higher RdRo values for the design of the proposed hybrid structures. Hybrid models were developed where the two subsystems were connected using STS inserted at 45°, 90°, and mixed angles (45° + 90°). Pure light wood-frame structures and pure CLT structures were also analyzed as reference cases. One-, four- and six-storey archetypes were designed with trial RdRo factors. The OpenSees software was used to develop a 2D numerical model for each archetype. The RdRo of the five analyzed cases was evaluated following the Canadian Construction Materials Centre guideline and using the 22 FEMA P695 far-field ground motions. The results show that Rd = 2 and Ro = 1.5 are acceptable for cases of pure CLT structures, and hybrid structures connected using STS inserted at 45° and 90°. The hybrid buildings connected using STS inserted at mixed angles (45° + 90°) can be assigned with Rd = 2.5 and Ro = 1.5. The archetypes designed with Rd = 3 and Ro = 1.7 are deemed satisfactory for pure light wood-frame structures.
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Keywords
Light wood-frame shear wall, CLT core, Incremental Dynamic Analysis, Seismic performance, Seismic force modification factors, Hybrid lateral load-resisting system, Self-tapping screw connection, Cross-Laminated Timber