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Steel energy dissipators can be combined with mass timber in integrated seismic lateral force–resisting systems to achieve designs with enhanced seismic performance and sustainability benefits. Examples of such integration include the use of mass timber post-tensioned rocking walls equipped with steel energy dissipation devices. This study proposes a solution using buckling-restrained boundary elements (BRBs) with mass timber walls detailed to pivot about a pinned base. This design allows the walls to rotate with minimal flexural restraint, distributing drift demands more uniformly with building height and reducing crushing damage at the wall base. Experimental quasi-static cyclic tests and numerical simulations were used to characterize the first- and higher-mode behavior of a full-scale three-story building featuring a mass timber gravity system and the proposed mass timber-BRB system. Under first-mode loading, the specimen reached 4% roof drift ratio with stable hysteretic behavior and a nearly uniform story drift profile. While residual drifts were nonnegligible due to the lack of self-centering, analytical estimates indicate realignment is likely feasible at the design earthquake level. Under second-mode loading, the specimen exhibited near-linear behavior with high stiffness. Experimental results were corroborated with numerical simulations for the isolated gravity frame, first-mode-like, and second-mode-like loading protocols. It is expected that results from this study will facilitate greater use of mass timber seismic lateral force–resisting systems.
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Estimating first and higher-mode effects for the design of rocking mass timber walls with controlled overturning moments
To address functional recovery after earthquakes, there is growing interest in developing enhancedperformance seismic-resisting systems. Rocking walls, featuring a base gap-opening mechanism and designed to remain essentially elastic above the base, have demonstrated their potential in various construction materials, including mass timber. If combined with steel energy dissipators, the resulting hybrid steel-mass timber rocking walls have emerged as a promising seismic-resisting system. This study focuses on Post-Tensioned Mass Timber Rocking Walls supplemented with Buckling-Restrained Brace (BRB) boundary elements and builds upon findings from experimental programs funded by the National Science Foundation (NSF) and the United States Department of Agriculture (USDA). The rocking mechanism, controlled by the BRBs and the Post-Tensioned (PT) rods, provides self-centering behaviour, reducing the potential for residual drifts and improving post-earthquake repairability. An estimating method for higher-mode loading profiles is proposed and applied to a six-story archetype, which was tested at the Large High Performance Outdoor Shake Table (LHPOST) at the University of California San Diego (UCSD) in January 2024 as part of the NHERI Converging Design Project. The estimating method is practically formulated to facilitate the implementation in design procedures.
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- Award ID(s):
- 2120683
- PAR ID:
- 10632044
- Publisher / Repository:
- 18th World Conference on Earthquake Engineering (18WCEE)
- Date Published:
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract This paper presents a computationally efficient numerical model for predicting seismic responses of post‐tensioned cross‐laminated timber (CLT) rocking wall systems. The rocking wall is modeled as a simple linear beam element with a nonlinear rotational spring at the base. The model is primarily intended for preliminary design and assessment of multistory buildings using this particular lateral system. A method was developed to determine the nonlinear rotational spring parameters by considering the dimension of the CLT wall panel and post‐tensioned steel rods and energy dissipating devices’ contributions. The proposed model was validated by comparing the simulated results with the responses from a series of shake table tests of a full‐scale two‐story building with CLT rocking walls. The numerical results show reasonable agreement with the shake table test results considering the simplicity of the model.more » « less
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Mass timber is a sustainable option for building design compared to traditional steel and concrete building systems. A shake table test of a full-scale 10-story mass timber building with post-tensioned mass timber rocking walls will be conducted as part of the NHERI TallWood project. The rocking wall system is inherently flexible and is expected to sustain large interstory drifts. Thus, the building’s vertically oriented non-structural components, which include cold-formed steel (CFS) framed exterior skin subassemblies that use platform, bypass, and spandrel framing, a stick-built glass curtain wall subassembly with mechanically captured glazing, and CFS framed interior walls, will be built with a variety of innovative details to accommodate the large drift demands. This paper will describe these innovative details and the mechanisms by which they mitigate damage, provide an overview of the shake table test protocol, and present performance predictions for the non-structural walls.more » « less
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Blass, Hans (Ed.)Wood buildings in North American has been predominantly constructed using light-framed wood systems since early 1900’s, with only limited exception of heavy timber construction in some non-residential applications. This situation is likely to change in the future with the growing acceptance of mass timber construction in the region. In fact, a number of mass timber buildings have been constructed in recent years in the U.S. and Canada, including low- to mid-rise mixed-use buildings (e.g. UMass Student Center, T3 building) and tall towers (e.g. Brocks Commons at UBC). Most of these buildings utilized cross laminated timber (CLT) or nail laminated timber (NLT) floors and heavy timber framing systems to support gravity loads, and a non-wood lateral system such as concrete shear walls or a braced steel frame to resist wind and seismic loads. Although CLT material and glulam products have been recognized in the U.S. and Canada (IBC (2018) and NBCC (2015), there is currently no mass timber lateral systems in the U.S. and only one system (platform style panelized CLT shear wall) in Canada that is currently recognized by the building codes. As a result, special design procedures and review/approval processes must be followed for any building intended to use a mass timber lateral system. There is a need to promote codification of mass timber lateral systems in order to help further develop mass timber building market in North American. At the time of this paper, there has been an on-going effort to devel-op seismic design parameters for panelized CLT shear walls in the U.S. (ref) following the FEMA P695 procedure for platform construction. The other lateral system that at-tracted significant attention and research resources is post-tensioned CLT rocking wall system, which has the potential to be applicable to balloon framed low-rise to tall wood buildings. This paper will focus on recent research development on CLT rocking wall system in the U.S. and the effort to develop a seismic design procedure for this system for inclusion in the NDS Special Design Provisions for Wind and Seismic (SPDWS)(2008). While the expensive and time consuming process of the FEMA P695 process would provide the ability to use the equivalent lateral force method for design purposes, this path is not part of the discussion included here.more » « less
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null (Ed.)Nonlinear time history analyses were conducted for 5-story and 12-story prototype buildings that used post-tensioned cross-laminated timber rocking walls coupled with U-shaped flexural plates (UFPs) as the lateral force resisting system. The building models were subjected to 22 far-field and 28 near-fault ground motions, with and without directivity effects, scaled to the design earthquake and maximum considered earthquake for Seattle, with ASCE Site Class D. The buildings were designed to performance objectives that limited structural damage to crushing at the wall toes and nonlinear deformation in the UFPs, while ensuring code-based interstory drift requirements were satisfied and the post-tensioned rods remained linear. The walls of the 12-story building had a second rocking joint at midheight to reduce flexural demands in the lower stories and interstory drift in the upper stories. The interstory drift, in-plane wall shear and overturning moment, UFP deformation, and extent of wall toe crushing is summarized for each building. Near-fault ground motions with directivity effects resulted in the largest demands for the 5-story building, while the midheight rocking joint diminished the influence of ground motion directivity effects in the 12-story building. Results for both buildings confirmed that UFPs located higher from the base of the walls dissipated more energy compared to UFPs closer to the base.more » « less
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