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.
The use of mass timber structural products in tall building applications (6–20 stories) is becoming more common around the world including North America. A potential concern is the environmental wetting of mass timber products during construction because such products may dry out more slowly than light-frame structural lumber, and wood, as an organic material, is susceptible to deterioration at elevated moisture contents. In order to better understand the moisture conditions present in high rise timber constructions, a long-term moisture monitoring program was implemented on an eight story, mixed-use, mass timber framed building in Portland, Oregon. The building was monitored with an array of moisture meters to track moisture content throughout the building’s construction and operation. This paper presents data covering a period just over one year starting from the manufacture of crosslaminated timber (CLT) panels. Hygrothermal properties of CLT samples of the same type used in the building were measured in the laboratory, and wetting and drying experiments on representative CLT samples were conducted. Simulated moisture contents using a one-dimensional hygrothermal model compared reasonably well with laboratory experiments and building site measurements.
International Conference on New Horizons in Green Civil Engineering
April 25-27,2018. Victoria, Canada
This paper presents preliminary findings from an ongoing research program instrumenting CLT buildings to measure wood moisture content. An overview of the research program is presented along with data from first year of moisture monitoring in an 8-story building in Portland, Oregon. This project measures the wood moisture content throughout the construction cycle, including the fabrication, shipping, staging, and erection of the panels. These preliminary field measurements can help characterize moisture changes in CLT during construction and guide the construction of future CLT buildings.
Mass timber products have shown promise as an innovative alternative to conventional framing systems for use in tall wood buildings, but this new trend in design and construction poses concerns for the long-term durability of the products. A major challenge that classically faces timber products is the threat of moisture-induced mold and decay fungi, which are a heightened concern in mass timber buildings exposed to the environment for extended duration during construction. Consequently, it is important to understand the hygric and thermal (hygrothermal) conditions that mass timber products can experience in multi-story constructions and to be able to quantify the behavior of the products for their suitable design and implementation. An eight-story mass timber building located in Portland, Oregon was chosen for this study and was instrumented for moisture content monitoring through its production, construction, and in-situ use. Record breaking precipitation levels occurred during the building’s construction and while dimension lumber and glulam products subsequently dried to acceptable levels, cross laminated timber products (CLT) dried more slowly. These measurements have an observed bias and the decay risk for the products is inconclusive. Samples of CLT used in the building were characterized for hygrothermal properties and integrated into WUFI, a simulation software, for analysis of the building. The software showed limitations for correctly simulating the behavior of CLT in isolated lab experiments and therefore a re-calibration was performed for accurate simulation. Preliminary on-site simulation results provide a decent approximation of observed data despite its high variance, but drying rate predicted by the program is lower than what was measured.