Experimental behaviour of a full-scale timber-concrete composite floor with mechanical connectors

TL;DR: In this paper, a 6-storey office building with light-frame timber walls and platform construction was used to evaluate the performance of the TCC beam under short and long-term loading.

Abstract: The timber-concrete composite (TCC) beam is a construction technique that can be used for upgrading of existing timber floors without the need of demolition. This possibility has been investigated through full-scale tests on a 6-storey experimental building with light-frame timber walls and platform construction, where the existing timber floor for domestic use was upgraded for reuse as an office building. The acoustic flooring was replaced with a 60 mm lightweight concrete slab, connected to the existing joists with inclined shear connectors (SFS screws) to form the TCC floor. The floor and environmental conditions were monitored during the concrete pouring and hardening, and during the application of the live load. Two different types of construction, propped and unpropped, were compared, and an extensive experimental investigation was performed on material components (lightweight concrete, timber, and connection system) with the aim to fully characterise the behaviour under short- and long-term loading. The unpropped floor was then tested to failure under monotonic loading, and two different boundary conditions, namely the actual joist-to-wall joint and a perfectly pinned support, were investigated. An advanced FE model was validated on the test results and used to predict the deflection in the long-term. The composite floor achieved the target stiffness and the design load for satisfying ultimate and serviceability limit states for office loading in the UK. The actual joist-to-wall restraint was characterized by a low degree of fixity, however it produced early longitudinal crack formation in the proximity of the support. The final collapse of the floor as a whole occurred progressively under increasing load after failure for fracture in tension of an individual joist. The higher drying shrinkage of lightweight concrete raised the deflection during concrete curing and hardening.