Determination of fire resistance ratings for glulam connectors within US high rise timber buildings

Multi-storey mass timber buildings constructed with cross laminated timber and glulam are being developed globally. Where engineered limber such as glulam is utilized, the column to beam connections need to be constructed with a fire resistance rating equal to that of the connecting members. The preferred glulam connectors are either a concealed steel plate with bolts and dowels; or a concealed proprietary screw-in sleeve type connector. The fire resistance of connectors for glulam members is an unresolved design issue, as there is no clear methodology to assess their capacity under fire, when the timber is exposed and not clad behind fire protective plasterboard. There is limited fire test data on concealed connectors under shear forces, which is the normal loading condition within a constructed building. Fire test data is also limited on full-size specimens. Correlations developed to date to calculate concealed connector fire resistance have only limited application. A methodology for the design of glulam beam to column connections has been. developed based on an extensive literature review, examining the key issues for connection failure. It has been determined that char rate for the limber at the connection needs to be increased above the normally accepted design values, due to the influence of the steel connectors. Secondly, the reduction in timber strength behind the char layer needs to be accounted for, by including a greater depth of reduced strength and stiffness timber, such that the connection can effectively transfer the applied forces through the timber to the steel connector. The methodology detailed within this paper provides a simple approach to evaluate the limber cover to the concealed steel connector, where the timber strength and stiffness are effective.