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.
Two of the major topics of interest to those designing taller and larger wood buildings are the susceptibility to differential movement and the likelihood of mass timber components drying too slowly after they become wet during construction. The Wood Innovation and Design Centre in Prince George, British Columbia provides a unique opportunity for non-destructive testing and monitoring to measure the ‘As Built’ performance of a relatively tall mass timber building. Field measurements also provide performance data to support regulatory and market acceptance of wood-based systems in tall and large buildings. This report covers vertical movement and roof moisture performance measured from this building for about three and a half years, with sensors installed during the construction.
The report first describes instrumentation. The locations selected for installing displacement sensors for measuring vertical movement comprised of the following: glued-laminated timber (glulam) columns together with cross-laminated timber (CLT) floors on three lower floors; a glulam column together with a parallel strand lumber (PSL) transfer beam on the first floor; and a CLT shear wall of the core structure on each floor from the second up to the top floor. Sensors were also installed to measure environmental conditions (temperature and relative humidity) in the immediate vicinity of the components being monitored. In addition, six locations in the timber roof were selected and instrumented for measuring moisture changes in the wood as well as the local environmental conditions. Most sensors went into operation in the middle of March 2014, after the roof sheathing was installed.
The Wood Innovation and Design Centre (WIDC) in Prince George, British Columbia, with 6 tall storeys and a total height of 29.5 m, provided a unique opportunity for non-destructive testing and monitoring to measure the ‘As Built’ performance of a relatively tall mass timber building. The mass timber structural system consists of glulam columns and beams with cross laminated timber (CLT) floor plates and shear walls. Vertical movement of selected glulam columns and CLT walls and the moisture content of the innovative mass timber roof were monitored as these components are unique to mass timber buildings. Indoor temperature and relative humidity conditions were also measured. The mass timber CLT and glulam elements are susceptible to longer-term differential movement as they slowly dry after manufacturing and construction. The paper describes instrumentation and discusses the measurement results for two years following the topping out of the structure.
The monitoring indicated that the wood inside the building could reach a moisture content (MC) close to 4% in the winter in this cold climate, from an initial MC of around 13% during construction. Glulam columns were dimensionally stable in the longitudinal direction given the MC changes and loading conditions. With a height of over 5 m and 6 m, respectively, two glulam columns directly measured by sensors each showed vertical movement below 3 mm (i.e., 0.04%). The cumulative shortening of the six glulam columns along the height of the columns (24.5 m) is expected to be approximately 11 mm. This did not take into consideration any potential settlement or deformation at connections between glulam columns, or effects of reduced loads on the top two unoccupied floors. The CLT wall panels were also dimensionally stable along the height of the building, with cumulative vertical shrinkage of about 19 mm (i.e., 0.07%) from Level 1 to Level 6. In contrast, the 5-ply CLT floor slabs made up of wood in radial and tangential grain shrank in thickness by about 5 mm (3.0%) on average. With regards to the performance of the mass timber roof, the CLT roof panels started out dry and remained dry due to the robust assembly design and the dry indoor conditions. In one area the plywood roof sheathing was initially wetted by the application of a concrete topping below a piece of mechanical equipment, it was able to dry to the interior within a few months. Overall the monitoring study showed that the differential movement occurring among the glulam columns and the CLT wall was small and the mass timber roof design had good drying performance.
The Tallwood House project was intended to advance the design and
manufacture of mass timber products in Canada and demonstrate
that mass timber is a viable structural option for mid-rise and
high-rise buildings. The use of mass timber and engineered wood
products in high-rise construction is becoming more common
around the world leading to a growing interest in the performance of
mass timber over time.
This report describes the performance of the mass timber structure
in Tallwood House, between September 2017 and August 2019,
based on measurements of the moisture content in the prefabricated
CLT floor panels and the displacement of the vertical structural
system. It is intended to initiate discussions on the performance of
mass timber structure elements during building occupancy and lead
to further research that can explore the influential factors.
Vertical movement of wood frame buildings has become an important consideration in recent years with the increase of building height in Europe, North America, and Asia up to 6-storeys. This movement is composed of wood shrinkage and load-induced movement including initial settlement and creep. It is extremely difficult to identify the relative contributions of these components while monitoring full size buildings. A laboratory test was therefore designed to do this under controlled environmental and loading conditions. Two identical small-scale platform frame structures with dimensional lumber floor joists were designed and constructed, with built-in vertical movement and moisture content monitoring systems. The two structures were first conditioned in a chamber to achieve an initial moisture content (MC) about 20% to simulate typical MC on exposed construction sites in wintertime in Coastal BC. After the two structures were moved from the conditioning chamber into the laboratory environment, using a unique cantilever system, Structure No. 1 was immediately loaded to measure the combined shrinkage and deformation in the process of drying. Structure No. 2 was not loaded until after the wood had dried to interior equilibrium moisture content to observe the shrinkage and load-induced movement separately. The load applied on the two structures simulated a dead load experienced by the bottom floor of a six-storey wood frame building. The vertical movement and MC changes were monitored over a total period of six months. Meanwhile, shrinkage coefficients were measured by using end-matched lumber samples cut from the plate members of the two structures to predict the shrinkage amounts of the horizontal members of the two structures.
The results suggested that a load must be applied for movement to “show up” and occur in a downward direction. Without loads other than the wood weight, even shrinkage could show as upward movement. Monitoring of Structure No. 1 appeared to separate the contributions of wood shrinkage, initial settlement (bedding-in movement), and creep reasonably well. The entire movement amount reached about 19 mm after six months, which was comparable to the vertical movement measured from the bottom floor of a 4-storey wood-frame building in BC. Shrinkage accounted for over 60% of the vertical movement, with the other 40% contributed by load-induced movement including initial settlement and creep (when elastic compression was neglected); the magnitude of creep was similar to the initial settlement amount. Structure No. 2 showed less vertical movement but an increased settlement amount at the time of loading, indicating the presence of larger gaps between members when the wood was dry (with an estimated MC of 11%) before loading. Depending on construction sequencing, such settlement should occur with increase in loads during construction and can therefore be ignored in design. However, this test suggested that there may be a need to consider the impact of creep, in wet climates in particular, in addition to wood shrinkage.
This laboratory test will be maintained for a longer period to observe any further vertical movement and the relative contributions of shrinkage and creep. Similar tests should be conducted for structures built with engineered wood floor joists, given the fact that most mid-rise platform buildings use engineered wood floor joists instead of lumber joists.
Project contacts are Shiling Pei at the Colorado School of Mines and Samuel L. Zelinka at the 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.
Two of the major topics of interest to those designing taller and larger wood buildings are the susceptibility to differential movement and the likelihood of mass timber components drying slowly after they are wetted during construction. The Wood Innovation and Design Centre in Prince George, British Columbia provides a unique opportunity for non-destructive testing and monitoring to measure the ‘As Built’ performance of a relatively tall mass timber building. Field measurements also provide performance data to support regulatory and market acceptance of wood-based systems in tall and large buildings.
This report first describes instrumentation to measure the vertical movement of selected glulam columns and cross-laminated timber (CLT) walls in this building. Three locations of glulam columns and one CLT wall of the core structure were selected for measuring vertical movement along with the environmental conditions (temperature and humidity) in the immediate vicinity. The report then describes instrumentation to measure the moisture changes in the wood roof structure. Six locations in the roof were selected and instrumented for measuring moisture changes in the wood as well as the local environmental conditions.
The objective of this study was to assess the potential effects of climate change on the moisture performance and durability of massive timber walls on the basis of results derived from hygrothermal simulations. One-dimensional simulations were run using DELPHIN 5.9.4 for 31 consecutive years of the 15 realizations of the modeled historical (1986–2016) and future (2062–2092) climates of five cities located across Canada. For all cities, water penetration in the wall assembly was assumed to be 1% wind-driven rain, and the air changes per hour in the drainage cavity was assumed to be 10. The mold growth index on the outer layer of the cross-laminated timber panel was used to compare the moisture performance for the historical and future periods. The simulation results showed that the risk of mold growth would increase in all the cities considered. However, the relative change varied from city to city. In the cities of Ottawa, Calgary and Winnipeg, the relative change in the mold growth index was higher than in the cities of Vancouver and St. John’s. For Vancouver and St. John’s, and under the assumptions used for these simulations, the risk was already higher under the historical period. This means that the mass timber walls in these two cities could not withstand a water penetration rate of 1% wind-driven rain, as used in the simulations, with a drainage cavity of 19 mm and an air changes per hour value of 10. Additional wall designs will be explored in respect to the moisture performance, and the results of these studies will be reported in a future publication. View Full-Text