Design method for the elastic bending behaviour of steel-CLT composite beams – The double-γ-method

The potential of composite beams made of a steel girder and cross-laminated timber (CLT) to efficiently extend the use of CLT to spans over 8 m has already been confirmed. Describing the bending behaviour of steel-CLT composite beams proved to be challenging since it is not only determined by the bending deformation but also by the shear deformations of the shear connectors and the transverse layers of the CLT. Based on numerical and experimental results, this article shows the inaccuracy of the existing design methods as the general γ-method according to EN 1995-1-1 Annex B or the sandwich method. Therefore, a new design method is developed, which accurately describes the deformations and stresses of the composite beam under bending loads. This three-part method utilises the basic principles of the γ-method. To consider the shear deformations of the shear connectors and within the CLT, two independent γ -factors are introduced. If necessary, the approach can be extended for an arrangement of shear connectors at large distances or a spatial stress state. Based on extensive investigations on long-term deformations, the double- γ -method is also extended to cover long-term effects. The design method reproduces the numerical and experimental results very well. • Steel-CLT composite beams show complex and unique deformation behaviour • Existing, analytical design methods cannot cover the bending behaviour sufficiently • New Double-γ-method as the first method to easily considers all shear deformations • Great accuracy for all relevant cross-sections and configurations of shear connections • Basis for design for user-friendly normative regulation