Quantifying charring and temperature rise of gaps in glulam beam-to-column connections

Abstract

Mass timber construction often has low tolerances due to offsite manufacturing and fabrication. However, due to dimensional tolerances of the natural material as well as the shrinking and expansion due to moisture changes, gaps typically form between members. These gaps can be up to 6 mm and current standards and codes require fire caulking of the gaps regardless of their sizes. In addition, prescriptive char models indicate that significant charring can occur within these gaps thereby reducing the load-carrying capacity of the structural components and providing a potential pathway for fire integrity failures. For all these reasons, experimental investigations of structural systems with intentional gaps are needed to develop guidelines for mass timber buildings. To address this gap, three unprotected T-shape glue laminated beam-column connections with two connections per specimen were fire tested in a gas fired furnace. Each specimen was exposed to the ASTM E119 standard fire for one hour. Each test used concealed beam hanger connections with average gap sizes of 1-, 3-, and 6-mm. Char depth measurements and temperature data were used to quantify the impact of gap size on char penetration and temperature rise in mass timber connections. Connections with gap sizes greater than 1.8 mm had an increase in char depth yet temperatures at the concealed connection remained outside the heat affected layer. As gap size increased, char penetration at the connection increased, with surface charring of the full width of the connection when gap size exceeded 6 mm. Despite the increasing char penetration for gap sizes up to 8 mm, a fully exposed surface at the connection face, analogous with two-dimensional charring, was not observed in any test. Concealed connection temperatures did not exceed an average temperature rise above 139°C for gaps less than 6 mm. Char depth at the connection gap was accurately predicted by one dimensional gap char factors established in the 2024 Fire Design Specification yet underpredicted by factors in the upcoming prEN1995-1-2:2025.

Citation

Garrett; Wyatt; Fischer; Erica; Hasburgh; Laura. 2026. Quantifying charring and temperature rise of gaps in glulam beam-to-column connections. In: Proceedings of the 14th International Conference on Structures in Fire, Queen's University / York University, Canada: 563-572.