Abstract
There are currently no standards regulating water management for mass timber elements during construction, little knowledge of impacts of moisture exposure (wetting and drying performance, dimensional stability, checking), and few precedents serving as guidelines for monitoring moisture response of mass timber. To address these gaps, a hygrothermal monitoring study was devised to track moisture performance of U.S. made cross laminated timber (CLT) and glulam at a three-story mass timber building. This paper discusses moisture measurements that were collected during the first six months of construction at a CLT rocking shear wall and a timber floor connection. Despite the limited number of structural systems monitored during construction, the distribution and number of sensors in these elements allow to draw some important conclusions. The data confirmed that moisture distribution and wetting/drying rates varied based on local conditions and details (aspect, coatings, connections, etc.), with measurements at an uncoated, north-facing area showing the highest moisture levels (reaching fiber saturation at multiple ply depths and locations). Most locations rarely exceeded 16% moisture content for more than a few months. Certain moisture-trapping details consistently showed higher moisture levels (i.e., above 16%) and poorer drying. Some interior plies continued to show slow increases in MC even after months of drying conditions. These observations suggest preventative approaches implementable in the design (e.g., avoiding moisture trapping details), during fabrication (e.g., localized coating), and construction (e.g., sequencing installation to minimize exposure and allow drying).
Highlights
In North America there has been a recent surge of interest and investment in advancing the use of mass timber construction at the mid- to high-rise scale [1]
While standards have been developed in certain countries for the on-site protection of timbers during construction and in service (e.g., [3,4]), they are as of yet unmanifested in North American practice, where instead there are only suggested guidelines for such matters [5]
The timber structure is comprised of a glulam post-and-beam gravity system, and lateral load-resisting system including hybrid wood/concrete floor diaphragms with cross laminated timber (CLT) decks, and post-tensioned, self-centering, CLT shear walls [40,41]
Summary
In North America there has been a recent surge of interest and investment in advancing the use of mass timber construction at the mid- to high-rise scale [1]. While standards have been developed in certain countries for the on-site protection of timbers during construction and in service (e.g., [3,4]), they are as of yet unmanifested in North American practice, where instead there are only suggested guidelines for such matters [5]. This means that exposure severity will be, in large part, defined by designer/contractor precautions—which will be influenced by awareness of the issue and possible solutions—in addition to the construction system, schedule and site conditions themselves. A moisture mitigation protocol [43] was developed prior to manufacturing the timbers that specified a vapor permeable coating be shop-applied at (1) the edges of all mass timber elements (double-coated), all faces of glue laminated columns and beams, (3) the faces of 4CLT
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