Timber-concrete composite beams

Abstract

In this paper an easy-to-use design model for timber-concrete composite beams is discussed. The model is applicable for computer simulations as well as for hand calculations. A research programme was started in 1992 in co-operation with the University of Karlsruhe, to study the loadbearing capacities of timber-concrete composite beams that are subject to bending. This research programme also included shear tests, creep tests, Monte Carlo simulations on floor systems and short term tests on a platelike timber-concrete structure, that are not included in this publication. The load-slip characteristics of three different connector types were determined and thirty bending tests, ten for each connector type, were carried out on beams constructed with these connectors. The bending test-specimens failed due to combined bending and tensile failure of the timber, that is near knots or at a fingerjoint. Depending on the configuration of the beam and behaviour of the connectors, other phenomena could occur first. These phenomena however never initiated total collapse of the beam. Although timber beams normally exhibit brittle failure in the tensile zone, the composite beams showed a plastic behaviour before total collapse occurred. This behaviour was caused by plasticity of the connectors. Hardly any plasticity was observed at the 5-percentile characteristic strength values for single T-beams and systems, when timber representing the Dutch strength class K17 was modelled. An elastic calculation model thus proves to be correct for most timber-concrete composite beam configurations, provided that timber beams of ordinary strength classes have been installed. This observation is no longer valid if gIulam or timber from the highest strength classes is used. It shifts the characteristic strength values upwards and plasticity of the concrete compression zone or plasticity of the connectors occurs before the timber beam with characteristic strength collapses.