Lucas King, M.Eng. 2012
Dr. Rodrigo Mora
Keywords: wood-frame construction, vertical movement, laboratory experiment
Accommodating differential movement in multi‐storey wood‐frame buildings is becoming more important and also challenging as the limits of wood‐frame construction are pushed to higher levels. The goal of this research project is to better understand the impacts of moisture content and applied load on the dimensional changes of structural wood components and assemblies used in standard wood‐frame platform construction. As a part of a FPInnovations research project, this work involved monitoring vertical displacement of two small scale wood‐frame assemblies under the ambient temperature and relative humidity conditions at FPInnovations’ laboratory in Vancouver. Two structures were built to represent the bottom floor of a typical wood‐frame platform constructed building (including bottom plates, studs, floor joist and top plates). Efforts were made to build these two structures identical to each other using end matched SPF No. 2 lumber and were built in accordance with current national building codes. Small end‐matched lumber samples were also prepared to measure the shrinkage coefficient of each major member used in these two structures.
The two small scale floor/wall assemblies were built and conditioned in a high humidity chamber to allow wood to reach approximately 20% moisture content (MC) (on an oven‐dry basis). 20% was selected as the initial MC target because recent field monitoring conducted by FPInnovations shows that during wood‐frame construction in Vancouver throughout the winter, wood members exposed to the outdoor climate average approximately 20% MC (FPInnovations, March 2012). Once the initial MC target had been reached, the two structures were instrumented with moisture pins and displacement sensors, removed from the conditioning chamber and placed in FPInnovations Wood Engineering laboratory. An 11,500lb load was applied to Structure 1 using a lever system, representing a typical load on the load bearing walls of a bottom floor of a modern 6‐storey wood‐frame building. It was planned that Structure 2 was going to be loaded in the same way after the structure reached the equilibrium moisture content in the lab. The purpose was to observe the displacements during the drying process without loads and then during the loading period without drying. Once removed from the high humidity conditioning chamber, the two structures began losing moisture (drying out) with an expected equilibrium moisture content (EMC) of approximately 10% in the laboratory’s controlled environment. Vertical displacements at various locations within both of the structures were monitored as wood components approached laboratory EMC. Due to time constraints, this report only covers the first phase of the testing, i.e. the testing period before Structure 2 is loaded.
The second phase of this research experiment will involve loading Structure 2 with equal weight as Structure 1. Since MC will remain relatively constant during the second phase, measurements made in structure 2 are expected to reflect vertical displacement caused by loads, not shrinkage of wood. The total vertical displacement in both structures should represent a combination of wood shrinkage, bedding in movement, elastic compression and time‐dependent compression (creep). Up to now a greater amount of vertical displacement was observed in Structure 1 than in Structure 2 due to the combined effects of induced loading and shrinkage. Although the impacts of shrinkage and load cannot be accurately quantified at this point of the experiment, results do indicate that displacement due to shrinkage far exceeds displacement due to elastic deformation. This research project suggests that to reduce differential movement in multi‐story wood‐frame construction, proper protection of wood against moisture during material storage and construction is critical. Design measures may also be taken to reduce the amount of horizontal solid wood elements within load bearing walls, such as by using engineered floor joists instead of lumber floor joists in mid‐rise wood frame construction.