A crucial issue in the design of a mid-rise Cross Laminated Timber (CLT) building under horizontal seismic action, is the definition of the principal elastic vibration period of an entire superstructure. Such vibration period depends on the mass distribution and on the global stiffness of the buildings. In a CLT structure the global stiffness of the buildings is highly sensitive to deformability of the connection elements. Consequently for a precise control of the vibration period of the building it is crucial to define the stiffness of each connections used to assemble a superstructure. A design procedure suitable for a reliable definition of the connection stiffness is proposed referring to code provisions and experimental tests. Discussion addresses primary issues associated with the usage of proposed procedure for numerical modeling of case study tall CLT buildings is reported.
With the rapidly increasing rate of urbanization worldwide especially in high seismic regions, researchers and engineers are seeking cost-effective building systems that are sustainable and can achieve superior seismic performances. Cross Laminated Timber (CLT) is an engineered wood based material which is known as a suitable wood product for tall building construction because of its robustness and enhanced fire performance. However, the traditional CLT platform shear wall lateral system is susceptible to damage at their connections during strong earthquakes, one of the potentially viable solutions to avoid connection damage in strong earthquakes is to implement inter-story isolation system in multi-story CLT buildings. Application of such a system leads to elongated building natural period, shifted lateral displacement demands, and increased amount of damping. This concept has been used in both steel and concrete structures but not yet in any wood buildings. Traditional light-framed wood building has a height limit of 4~5 stories which inter-story isolation is not beneficial or necessary. Only until recently, CLT material enables the construction of tall wood buildings that can fully utilize the benefit of inter-story isolation. The objective of this study is to apply performance-based seismic design methodology for inter-story isolated tall CLT buildings to explicitly target desirable wood building drift and isolation deformation levels. Thus, a generalized Direct Displacement Design (DDD) philosophy for inter-story isolated CLT buildings was outlined to provide different levels of performance against moderate to strong earthquakes. The proposed DDD approach was developed and implemented with two examples of different building heights. The examples include a 12-story CLT building representing mid-rise construction, and a 20-story building representing tall wood construction. The resulted design was validated using numerical simulation with an existing software package validated through large scale shake table tests. It is discovered that for the height range currently planned for wood construction, having just one isolation layer can provide reasonable level of damage mitigation. This study also provides insight into the widely used DDD approach on a fundamental level. A specific study on the uncertainty in current DDD procedures applying to linear and nonlinear structural systems was conducted. The research work result was disseminated as peer-reviewed publications that have been published or in the process of preparation.
Mass-timber has gained popularity in the construction of mid-rise buildings in the last decade. The innovation of constructing tall buildings with mass-timber can be seen in the student residence at Brock Commons built in 2016 at the University of British Columbia. It is the world’s tallest timber hybrid building with 18 stories and 53 meters’ height above the ground level. The building has 17 stories of mass-timber superstructure resting on a concrete podium with two concrete cores that act as a lateral force resisting system for earthquake and wind forces. The mass-timber superstructure of 17 stories took ten weeks whereas the concrete cores were built in fourteen weeks. There could have been a substantial reduction in the project timeline leading to cost savings, if mass-timber was used for the cores. The motivation for concrete cores was driven by the sole purpose of easier approval procedure. The objective of this thesis was to evaluate the possibility to design the Brock Commons building using mass-timber cores. First, the procedure for the approvals for tall timber buildings by understanding the code compliance for Brock Commons is discussed. Then, the actual building with concrete cores is modeled, with the model being calibrated with the results from the structural engineers of record. These concrete cores are then replaced by the same configuration using Cross Laminated Timber (CLT) cores to investigate the structural feasibility of Brock Commons with a mass-timber core. The results presented herein show that Brock Commons with CLT core having the same dimensions and configuration is unstable under seismic loading for Vancouver, BC, as specified by National Building of Canada 2015. However, when the configuration and thickness of CLT cores are changed, the structure can meet the seismic performance criteria as per the code.
Tall wood buildings have been at the foreground of innovative building practice in urban contexts for a number of years. From London to Stockholm, from Vancouver to Melbourne timber buildings of up to 20 storeys have been built, are under construction or being considered. This dynamic trend was enabled by developments in the material itself, prefabrication and more flexibility in fire regulations. The low CO2 footprint of wood - often regionally sourced - is another strong argument in its favour. This publication explains the typical construction types such as panel systems, frame and hybrid systems. An international selection of 13 case studies is documented in detail with many specially prepared construction drawings, demonstrating the range of the technology.
The objective of the current project is to develop a performance-based design process for wood-based design systems that would meet the objectives and functional statements set forth in the National Building Code of Canada. More specifically, this report discusses the fire and seismic performance of buildings, as identified as a priority in a previous FPInnovations report (Dagenais, C. (2016). Development of Performance Criteria for Wood-Based Building Systems).
The wood engineering community has dedicated a significant amount of effort over the last decades to establish a reliable predictive model for the load-carrying capacity of timber connections under wood failure mechanisms. Test results from various sources (Foschi and Longworth 1975; Johnsson 2003; Quenneville and Mohammad 2000; Stahl et al. 2004; Zarnani and Quenneville 2012a) demonstrate that for multi-fastener connections, failure of wood can be the dominant mode. In existing wood strength prediction models for parallel to grain failure in timber connections using dowel-type fasteners, different methods consider the minimum, maximum or the summation of the tensile and shear capacities of the failed wood block planes. This results in disagreements between the experimental values and the predictions. It is postulated that these methods are not appropriate since the stiffness in the wood blocks adjacent to the tensile and shear planes differs and this leads to uneven load distribution amongst the resisting planes (Johnsson 2004; Zarnani and Quenneville 2012a). The present study focuses on the nailed connections. A closed-form analytical method to determine the load-carrying capacity of wood under parallel-to-grain loading in small dowel-type connections in timber products is thus proposed. The proposed stiffness-based model has already been verified in brittle and mixed failure modes of timber rivet connections (Zarnani and Quenneville 2013b).
Cross-laminated timber (CLT) is a type of mass timber panel used in floor, wall, and roof assemblies. An important consideration in design and construction of timber buildings is moisture durability. This study characterized the hygrothermal performance of CLT panels with laboratory measurements at multiple scales, field measurements, and modeling. The CLT panels consisted of five layers, four with spruce-pine-fir lumber and one with Douglas-fir lumber. Laboratory characterization involved measurements on small specimens that included material from only one or two layers and large specimens that included all five layers of the CLT panel. Water absorption was measured with panel specimens partially immersed in water, and a new method was developed where panels were exposed to ponded water on the top surface. This configuration gave a higher rate of water uptake than the partial immersion test. The rate of drying was much slower when the wetted surface was covered with an impermeable membrane. Measured hygrothermal properties were implemented in a one-dimensional transient hygrothermal model. Simulation of water uptake indicated that vapor diffusion had a significant contribution in parallel with liquid transport. A simple approximation for liquid transport coefficients, with identical coefficients for suction and redistribution, was adequate for simulating panel-scale wetting and drying. Finally, hygrothermal simulation of a CLT roof assembly that had been monitored in a companion field study showed agreement in most cases within the sensor uncertainty. Although the hygrothermal properties are particular to the wood species and CLT panels investigated here, the modeling approach is broadly applicable.
Recent architectural trends include the design and construction of increasingly tall buildings with structural components comprised of engineered wood referred to by names including; cross laminated timber (CLT), laminated veneer lumber (LVL), or glued laminated timber (Glulam). These buildings are cited for their advantages in sustainability resulting from the use of wood as a renewable construction material. Previous research has shown that timber elements contribute to the fuel load in buildings and can increase the initial fire growth rate – potentially overwhelming fire protection system and creating more severe conditions for occupants, emergency responders, and nearby properties.
The overarching goal of this project Fire Safety Challenges of Tall Wood Buildings Phase 2 (involving five tasks) is to quantify the contribution of CLT building elements (wall and/or floor-ceiling assemblies) in compartment fires and provide data to allow comparison of the performance of CLT systems against other building systems commonly used in tall buildings.