Long-term behaviour of steel-timber composite (STC) shear connections

The long-term behaviour of steel-timber composite (STC) connections under sustained loading is studied both experimentally and theoretically. The laboratory work involved applying a sustained load to twelve STC push out test specimens and monitoring the load-slip behaviour of the shear connection (including the interaction between timber, steel beam, and shear connector) over a 16-month period in a Service Class 2 environment in accordance with EC5. The mechanical shear connectors utilised to connect the hot-rolled steel sections to cross laminated timber (CLT) panels were coach-screws, dog-screws, post-tensioned high strength bolts, and high strength bolts located in cementitious grout pockets. In addition to the testing programme, a 3D moisture diffusion analysis considering the effects of temperature was conducted to predict the moisture content of the timber panels. The results of the experiments and diffusion analysis were employed to calibrate a long-term rheological model for the shear connection which considers the effect of the stiffness change caused by variations of the moisture content, creep, mechano-sorption and inelastic shrinkage. The rheological model was used to predict the slip and thereby to calculate the creep coefficient of the STC members over a service life of 50 years. An average creep coefficient of 2.5 for the STC joints tested was obtained. The high strength bolts in grout pocket shear connectors had the lowest creep coefficient of 0.6, whilst the post-tensioned high strength bolts exhibited the highest creep coefficient of 3.9, owing to the loss of post-tensioning.