Special Issue on Seismic Resistant Timber Structures

In the last decade, there has been a renewed interest in timber buildings around the world, owed in part to the naturally optimal structural properties of timber and to the environmental advantages of building with timber. Several midrise buildings have been constructed, including a nine-story building in London, a nine-story building in Milan, and a 10-story building in Melbourne. A 14-story building is being constructed in Norway, and even taller timber buildings have been designed in Sweden, Austria, and Canada. Timber is a lightweight material and, as such, performs particularly well in moderate to intense earthquakes. However wood itself is not ductile and can have brittle failure modes, such as splitting, if not addressed properly within the seismic design procedure. A large body of research has been completed, and a number of projects are under way worldwide on the seismic behavior of timber structures and buildings. Full-scale shake table tests have been conducted in Japan, the United States, and Europe on an array of designs. Extensive component and joint testing has also been completed, and advanced numerical models developed. An effort has been made to implement this new knowledge in codes of practice, such as the Eurocode and the New Zealand and the Canadian standards.

This special issue brings together papers from the key researchers all over the world working on the seismic resistance of timber structures. It includes a forum paper and 17 technical papers presenting research carried out in the United States, Canada, Italy, Germany, Slovenia, China, Japan, and New Zealand. Five topics are addressed: (1) light-frame timber buildings, (2) retrofit of existing light-frame timber buildings, (3) cross-laminated timber buildings, (4) innovative rocking and hybrid heavy timber walls, and (5) alternative bracing systems.

Four papers investigate the seismic behavior of light-frame timber structures. Bahmani and van de Lindt present the results of an experimental investigation into the behavior of contributions from wall finishes including wood structural panels, gypsum wall board, stucco, and horizontal wood siding. They do this by combining either two or three layers experimentally to identify if the hysteretic models can be directly superimposed or if a new combination rule is needed to allow sheathing layers to be modeled on top of one another. Swensen et al. examines the possibility of increasing the strength and stiffness of gypsum wallboard fasteners for better performance of light-frame buildings in earthquakes. They conclude that adhesives provide the best alternative and provide a comprehensive examination of the response and damage progression in their wall tests. Tian et al. present the results of a numerical retrofit in a slow three-story hybrid test program that focused on retrofit at the bottom story only. Test data are presented, and it is concluded that the results provide an excellent ground story-only retrofit due to the energy dissipation devices’ ability to reduce peak deformations in the bottom story while preventing damage propagation to upper stories. Seim et al. present the results of experimental monotonic and cyclic tests carried out on light-frame wall subassemblies and sheathing-to-framing connections using gypsum fiber boards (GFB) and oriented strand boards (OSB) as sheathing material. A numerical model was calibrated on the experimental results and used to estimate the behavior factor. The authors conclude that GFB can be used as sheathing material in earthquake prone areas in the same way as OSB if the requirements regarding the minimum thickness and detailing of the connections are met.

Two companion papers investigate the retrofit of existing light-frame timber buildings known as soft-story buildings as part of the NEES-Soft Project. Bahmani et al. examined the design of the representative building, the retrofit methodologies, and the numerical validation of the retrofit procedures. Van de Lindt et al. presents the full-scale shake table test results of the four-story light-frame wood building with four different retrofit types featuring two different retrofit methodologies. The retrofits included a ground story-only procedure and a performance-based seismic retrofit procedure, which engaged all building levels. While the two retrofit methodologies had different performance objectives, both were found to be effective and to satisfy their respective goals.

Five papers investigate the seismic behavior of cross-laminated timber (CLT) buildings. Pei et al. present a state-of-the-art examination of research carried out on CLT buildings, but more pointedly go into depth on what research is needed for implementation in high-seismic regions of the United States and around the world. Their objective was to lay the foundation for enabling tall resilient CLT buildings in the United States. Popovski and Gavric present the results of quasi-static monotonic and cyclic tests carried out on a two-story full-scale model of a $6.0\times 4.8\mathrm{m}$ CLT house. Parameters, such as direction of loading, number of hold-downs, and number of screws, were varied in the tests. The CLT structure performed according to the design objectives, with failure mechanisms due to shear of nails in the brackets in the first story as a result of sliding and rocking of the CLT wall panels, and no global instabilities detected even after the attainment of the peak resistance. Yasumura et al. present the results of an experimental cyclic test carried out on a 5.7-m high, $6.0\times 4.0\mathrm{m}$ CLT building loaded with additional masses on the roof to simulate the behavior of a three-story building. Two different types of panels were tested: narrow $1.0\times 2.7\mathrm{m}$ and wide $6.0\times 2.7\mathrm{m}$ wall panels. The elastically designed buildings were found to have an actual failure load 60–80% higher than the design load, with an excellent seismic behavior. Sustersic et al. investigate the influence of the wall geometry, vertical load level, friction, connection stiffness, strength, and ductility on the seismic analysis of multistory CLT buildings. The load-carrying capacity of a four-story case study building is calculated via nonlinear static pushover analysis and assessed using displacement-based design. The wall geometry has a significant effect on the behavior factor $q$, which reduces from three to two when the width of the wall panel increases from 2 m to the total length of the building. The friction may have a beneficial effect; however, the authors suggest that its influence is conservatively neglected. Po’siè et al. present a numerical study conducted on a seven-story CLT building equipped with a linear translational tuned mass damper (TMD) placed on the top of the building. TMD parameters (mass, stiffness, and damping) were designed using a genetic algorithm technique by optimizing the structural response under seven recorded earthquake ground motions compatible, on average, with a predefined elastic spectrum. Several comparisons between the response of the structure with and without TMD subjected to medium-intensity and high-intensity recorded earthquake ground motions are presented, demonstrating the effectiveness of these devices to reduce the notoriously high drifts and seismic accelerations of these types of structures.

Five papers investigate the seismic behavior of innovative rocking and hybrid heavy timber walls. Zhang et al. investigated the force reduction factors, specifically the ductility factor Rd, of a novel hybrid structural system proposed for buildings up to 12 stories tall. In this system, heavy timber panels act as shear walls and are connected to each other and to perimeter frames through steel beams. The Rd factor was evaluated by limiting interstory drift in nonlinear time history analyses to an acceptable limit of 2.5% drift with 90% probability of nonexceedance. Based on the analyses presented, the authors recommend a ductility factor of 5.0.

Kramer et al. investigate the performance of a hybrid rocking CLT wall with a new alternative energy dissipating solution that allows self-centering. They apply the concept of steel buckling restrained braces (BRBs) with a milled steel portion of the brace inside of a grouted steel pipe, which substantially reduced damage to the wall during testing. Loo et al. examined the seismic response of a numerical model developed using the results of a series of wall tests with slip-friction connectors. Residual drifts are significantly reduced, and numerical analyses suggests that inclusion of such devices in the bottom level of a multistory wall unit would allow the structure to recenter following an earthquake. Sarti et al. present two papers focused on the seismic behavior of heavy timber walls made of laminated veneer lumber prestressed with unbonded tendons. In the first paper, the results of quasi-static experimental tests carried out on a series of two-third-scale posttensioned walls with alternative arrangements and combination of dissipaters (both internal and external mild steel tension-compression yield devices) and posttensioning force are discussed. High level of dissipation was obtained with negligible residual displacements and reduced damage to the wall element. In the second paper, an alternative configuration of the rocking/dissipative wall system, based on the use of a column-wall-column posttensioned connection, was investigated. The paper presents the design, detailing, and experimental testing of a two-thirds-scale wall specimen of this alternative configuration. Different wall configurations are considered in terms of posttensioning initial force as well as dissipation devices layout. The experimental results confirm the excellent seismic performance of the system with the possibility to adopt multiple alternative configurations.

Two papers investigate the seismic behavior of innovative alternative bracing systems. Xiong and Liu compare the experimental monotonic and cyclic behavior of four different systems used to brace a one-story, one-bay post, and beam glulam construction with bolted connections. The X-brace and the K-brace systems provided markedly better elastic stiffness but lower ductility. The frame with a knee-brace system showed increased elastic stiffness compared with the simple frame system and higher ductility compared with the frame with the X-brace and the K-brace system. The use of light-frame construction within the frame performed well in terms of elastic stiffness and ductility. Pozza et al. present the structural characterization of a novel hybrid shear-wall system obtained by coupling standard timber light-frame construction with an external precast reinforced concrete slab screwed to the wooden frame. The structural performance under monotonic and cyclic loading of such hybrid shear wall was experimentally investigated demonstrating a good ductility. A possible value for the behavior factor was estimated by investigating the seismic response of a reference building realized with the innovative hybrid shear walls using a numerical model.