Recently, mid-rise heavy hybrid-wood and concrete post and beam constructions have been identified as one of the next generation of building systems in Canada. Differential shrinkage is a normal phenomenon in structural systems built using multiple construction materials. In reinforced concrete, steel bars are normally designed to carry stresses induced by concrete shrinkage or differential shrinkage between the steel and concrete. In timber structures, however, shrinkage is greater than in concrete and mainly occurs due to the reduction in moisture content. Therefore, any changes in relative humidity at unsealed surfaces of wood elements may cause movement.
The project is divided into two parts, with Part I dedicated to the differential movement in heavy post and beam wood-concrete mid-rise buildings, while Part II is focused on differential movement in light frame platform mid-rise systems. This report presents information related to Part I of the project. In this study, a 6-storey hybrid wood and concrete post and beam building constructed in Québec City was instrumented. Since the building has reinforced concrete shear walls mixed with a timber post-and-beam structure, differential movement between the two systems was identified as an issue to consider. Several posts and beams were instrumented with potentiometers and relative Humidity and Temperature probes to assess the differential movement between the wood structure and the concrete core. The monitoring process started in December 2009 and will be ongoing for the coming few years.
Results from monitoring the heavy timber building in Québec over a one-year period indicated that the movement caused by moisture changes is minimal. Moreover, analysis of the monitoring data so far has revealed that overall movement is showing some signs of stabilization with limited localized changes after a year following construction and occupancy. Collected data also showed that the indoor conditions recorded were stable for temperature but showed considerable levels of fluctuation for relative humidity.
A calculation methodology could be adopted for the estimation of the total movement in a mid-rise post and beam heavy timber building based on shrinkage properties of wood, expected moisture gradient in service and number of joints in the vertical and horizontal structural members of the building. More research is needed, however, to validate the proposed method.