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Summary

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 monitoring showed that the wood inside the building reached an average moisture content (MC) of about 5% in the winter heating seasons and about 8% in the summer, from an initial MC of about 13% during the construction. Glulam columns were extremely stable dimensionally given the changes in MC and loading conditions. With a height of over 5 m and 6 m, the two glulam columns monitored on the first floor showed very small amounts of vertical movement, about 2 mm (0.04%) and 3 mm (0.05%), respectively, over a period of about three years and a half. Assuming the two monitored columns are representative of the other columns along the column line, the cumulative shortening of the six glulam columns along the height of the building would be about 12 mm (0.05%), not taking into account deformation at connection details or effects of reduced loads on upper floors. The CLT wall was found to be also dimensionally stable along the height of the building. The measurements showed that the entire CLT wall, from Floor 1 to Floor 6, would shorten about 19 mm (0.08%). The PSL transfer beam had a reduction of about 12 mm (1%) in the depth, i.e., along the building height. The CLT floor panels also showed considerable vertical movement of about 5 mm (3%) in the thickness direction. All the differential movement was expected and taken into consideration in the design and construction of the building. In terms of the roof performance, two locations, both with a wet concrete layer poured above the plywood sheathing, showed wetness during the construction but continued to dry afterwards. The satisfactory drying performance can be attributed to the interior ventilation function designed for the roof assemblies by integrating strapping between the sheathing and the mass timber beams below.

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