Novel Lightweight Timber Composite Element: Web Design in Shear and Compression

The paper examines the build-up and some aspects of the mechanical behavior of a novel lightweight timber composite called the Keel-web element. The element is a double-skinned composite similar to a multiple box beam element. The flanges consist of finger jointed lumber chords arranged and glued in parallel. The name-giving specific characteristic of the element consists of the multiple S-shaped webs made of plywood or OSB, resembling ship keels, glued in between the flanges. The element, which can be produced in a fully automatized process, as straight or cambered, with lengths of up to 35 m, has recently obtained a German technical building approval. The build-up will first be examined, and afterwards, the engineering design approach for the shear force and compression capacity, partly following Eurocode 5, is shown.

In order to analyse in greater detail the stability and nonlinear bending of the pre-curved webs at end supports, the load deflection behavior of the webs at an increasing support force is studied by 2

nd

and 3

rd

order beam column and plate theory. Additionally, the bending stresses introduced from the manufacturing process of the S-shaped webs which are subject to relaxation, must also be considered. With increasing loads, the out of plane web displacements are restrained by the adjacent webs, leading to a reduction of the free web height. Solving the differential equation for a fixed-end beam, valid results for small deformations can be calculated, while a nonlinear finite element simulation is performed in order to consider large deformations and contact boundaries. It is shown that the stressstate of a 3D shell model with contact simulation of the webs provides a good estimate of the experimental load capacities.