Evaluation of Keyed-Through Tenon Joints for Timber Frame Construction

Keyed-through tenon (KTT) joints are timber frame joints consisting of a key placed through the tenon, creating bearing against the back side of the mortise. This joint allows greater flexibility in key sizing compared with typical pegged mortise and tenon joints. However, no experimental testing is available to estimate the strength capacity of these joints. This paper measured the strength and stiffness of KTT joints considering differences in wood species, protruding tenon length, and number of keys. A tensile load was applied to a KTT joint similar to previous testing conducted on pegged mortise and tenon joints. Shorter tenon lengths produced brittle failures of the tenon, whereas longer tenon lengths produced ductile bending and crushing of the keys. Yield load values using the 5% offset method could not be determined for the shorter tenon length samples, but the ultimate load values could be used for yield load. Two keys were approximately twice as strong as a single key because of the increased bearing area and the number of tenon shear planes for load transfer. KTT joints with two keys and 11-in. (279-mm) protruding tenons were recommended from this experimental study. Results of KTT joints compared with previously tested pegged mortise and tenon joints with two 1-in. (25-mm) diameter pegs demonstrated that KTT joints with two keys produced stiffer and stronger joints with maximum load values 41% greater than previously tested pegged joints of similar composition. Size limitations and protection of the protruding tenon from damage are important considerations in the design of KTT joints in structures. (C) 2014 American Society of Civil Engineers.