Fire resistance of unprotected cross-laminated timber (CLT) floor assemblies produced in the USA

Fire concerns are one reason for building code restrictions against tall wood buildings in US. Despite growing body of empirical data on fire performance of cross-laminated timber (CLT) generated in Europe and Canada, lack of full-scale tests performed on structural CLT "made in the USA" is often quoted as a barrier for approval of massive timber in tall buildings in the US. The goal of this project was to contribute towards removing this barrier by testing the fire resistance of three full-scale unprotected CLT floor assemblies fabricated by two US-based manufacturers. The assemblies represented two species groups (SPF and DF-L), and two adhesive systems (PUR and MF). All assemblies ma ASTM E119 standard qualifying criteria for 2-h fire rating in loaded condition. While char fall-off was observed in assemblies bonded with PUR, the differences in mean char rates in the first two layers between assemblies bonded with PUR and MF were smaller than the variability of char rates within individual assemblies. Statistical significant difference was observed only for cumulative char rate for the entire time of exposure. The effect of softening of the PUR bonds was most apparent as an accelerated rate of assembly deflection in the late stage of the tests. The unprotected half-lap joints provided adequate barrier against transmission of ha gases and flames through the assemblies.