Proposal to Account for Concrete Shrinkage and Environmental Strains in Design of Timber-Concrete Composite Beams

Timber-concrete composite beams are statically indeterminate structures where a concrete topping is connected via shear connectors to a timber beam. Because effects such as drying shrinkage of concrete, variations of timber moisture content, and environmental temperature cannot freely occur, self-equilibrated stresses (eigenstresses) and additional deflection are induced in the composite beam and may reduce the structural safety at ultimate and serviceability limit states, respectively. The paper presents a simplified design approach suitable for implementation in codes of practice to account for these effects, which are all transformed into uniformly distributed loads to be combined with gravity load. The moisture content variations to consider in design are tabled based on climatic region, size of the timber section, type of exposure, and protective coating. These variations may become significant particularly for the deflection of composite beams with narrow timber sections exposed to outdoor sheltered conditions. Environmental variations and drying shrinkage of concrete are quite influential on the design, particularly when this is governed by deflection control at a serviceability limit state. Composite beams with solid timber slabs and stiff connection, and composite beams with narrow timber section exposed to outdoor conditions were found to be particularly sensitive to these environmental effects.