Abstract
Connections are often the weakest link in timber structures so that their failure during fire exposure may result in local or total collapse of a building. The failure modes are complex and poorly understood due to wood’s orthotropicity, the temperature and moisture dependency of wood’s properties, and the presence of metal fasteners that complicates the stress distribution and changes the process of heat transfer into the timber element. In this study, we investigated the coupled physics of heat and mass transfer in a test specimen made from spruce, with a steel fastener inserted in a pre-drilled hole. It imitates a central slice of a timber-to-timber connection exposed to the elevated temperature of 523 K from one side. The heat induced redistribution of the hygroscopic moisture in wood was quantified and visualized by means of neutron radiography. The temperature gradient was measured inside the specimen, at different distances from the heated surface. As the temperature increased in wood, moisture was displaced downstream of the heat source, resulting in a zone with increased moisture content ahead of the drying front and beneath the steel fastener. This wet front occurred where the temperature was below the water evaporation point. It was shown that the steel fastener affected the transport of heat and moisture within the test specimen. The occurring phenomena during the transient state may play a critical role in the embedment strength of wood and therefore influence the load-carrying capacity of timber connections in fire.
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Acknowledgement
The experiments were carried out at the NEUTRA beamline of the Paul Scherrer Institute and EMPA Laboratories for Materials Science and Technology, Switzerland. The authors would like to acknowledge the support of Daniel Heer from EMPA for sample preparation and of Jan Hovind from Paul Scherrer Institute NEUTRA beamline for maintenance of neutron imaging soft- and hard-ware.
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Gilani, M.S., Hugi, E., Carl, S. et al. Heat Induced Desorption of Moisture in Timber Joints with Fastener During Charring. Fire Technol 51, 1433–1445 (2015). https://doi.org/10.1007/s10694-014-0416-3
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DOI: https://doi.org/10.1007/s10694-014-0416-3