Impact of Moisture and Load on Vertical Movement of a Simulated Platform Frame Wood Structure

Abstract

Abstract The objective of this study was to assess the relative contributions of wood shrinkage and building dead loads to vertical displacement in the form of height loss for wood-frame structures. Identifying the relative contributions of these factors while monitoring full-sized buildings presents significant challenges; therefore, a laboratory test was conducted under controlled hygrothermal conditions and loading. Two small-scale wood-frame structures were constructed with dimension lumber wall assemblies and floor joists. Both model structures were equipped with vertical movement and moisture content (MC) monitoring systems. The structures were conditioned in a chamber to a MC of approximately 20 %, and then moved into the laboratory environment for drying and loading. Model Structure No. 1 was loaded immediately; model Structure No. 2 was not loaded until after the wood had dried to the equilibrium MC. By using a unique lever system, the applied load simulated a dead load that could be experienced by the bottom floor of a six-story wood-frame building. Shrinkage coefficients were measured using samples cut from the horizontal framing members. The results suggested that the loads “forced” the structures to displace downward. At the driest time in the laboratory, the total movement of model Structure No. 1 reached a maximum of about 19 mm after 10 months, which is comparable to in situ recorded measurements from the bottom floor of a four-story wood-frame building in coastal British Columbia. Shrinkage appeared to account for 70 % of the total vertical movement, with the remaining 30 % contributable to load-induced displacement under the conditions of this test. Model Structure No. 2 showed less vertical movement but more initial displacement than model Structure 1 due to a larger settlement. This study suggested that there may be a need to consider the effect of loads in addition to wood shrinkage when estimating building height loss.