International Conference on New Horizons in Green Civil Engineering
Wood structures such as the Wood Innovation and Design Center in Prince George and the UBC Tallwood House, an 18 storey, 53-meter-tall mass timber hybrid building are examples of new and innovative wood structures that encompass new construction techniques, unique materials and novel building practices. Empirical data on the condition of critical components and access to the real-time status of the structure during construction gives Architects, Engineers and Contractors critical information to make informed decisions to either validate or improve the construction plan. Data recorded during the life of the building helps validate the design decisions and proves the viability and feasibility of the design. Methods and practices used to monitor both the moisture performance of prefabricated cross laminate timber (CLT) as well as the vertical movement sensing of the building during and after construction are explored in this paper. Moisture content of the CLT panels has been recorded from manufacturing and prefabrication to storage, through transport and during installation and will continue throughout the service life of the building.
The calculated and expected displacement of the wood columns is scheduled to take several years as the structure settles, however a first-year analysis and extrapolation of the data was conducted. Monitoring during transport, storage, and construction proved that CLT panels were resilient to moisture issues while in the manufacturers storage, but prone to direct exposure to moisture-related problems regardless of the precautions taken on site. Despite construction during typical Pacific Northwest rain, informed decisions were made to ensure the panel moisture content could decrease to acceptable ranges before continuing to secondary construction phases. The moisture trends observed in the building were proportional to the control samples as both were subjected to similar environmental conditions.
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.
A key question about new generation taller wood buildings is how they will perform over time in terms of durability and livability. This project will determine how best to measure these qualities by selecting sensors, determining testing and measurement protocols, and implementing testing assemblies in selected CLT buildings in Oregon. Future research will use the knowledge developed through this project to carry out post-occupancy monitoring, generating valuable new insights into building performance.
The evaluation of damages in large-span timber structures indicates that the predominantly observed damage pattern is pronounced cracking in the lamellas of glued-laminated timber elements. A significant proportion of these cracks is attributed to the seasonal and use-related variations of the internal climate within large buildings and the associated inhomogeneous shrinkage and swelling processes in the timber elements. To evaluate the significance of these phenomena, long-term measurements of climatic conditions and timber moisture content were taken within large-span timber structures in buildings of typical construction type and use. These measurements were then used to draw conclusions on the magnitude and time necessary for adjustment of the moisture distribution to changing climatic conditions. A comparison of the results for different types of building use confirms the expected large range of possible climatic conditions in buildings with timber structures. Ranges of equilibrium moisture content representative of the type and use of building were obtained. These ranges can be used in design to condition the timber to the right value of moisture content, in this way reducing the crack formation due to moisture variations. The results of this research also support the development of suitable monitoring systems which could be applied in form of early warning systems on the basis of climate measurements. Based on the results obtained, proposals for the practical implementation of the results are given.
The research conducted will provide new climatic data which takes into account certain extreme weather events being attributed to climate change to minimize and/or prevent the risk of failure of tall wood buildings and mass timber structures. The project will offer guidance on the design for durability of tall wood building enclosures and fill existing gaps in knowledge about the extent of the effects of the future climate conditions and extreme weather events (e.g. heat waves, rainfalls, wind storms, etc.) on the resistances to deterioration of building materials, air leakage, vapour diffusion, and water ingress.
Project contacts are Shiling Pei (Colorado School of Mines) and Samuel L. Zelinka (Forest Products Laboratory)
This project will generate three benchmark data sets for multistory CLT building moisture performance in different climate zones. Data will include moisture contents at key wood components and high moisture risk locations throughout the buildings. A relatively simple, but fully validated, numerical model for analyzing similar building moisture performance will be recommended. These results will be useful for structural engineers and architects to accurately consider moisture in their design of mass timber buildings.