Disproportionate Collapse Resistance of Timber Beam-column Connections

Abstract

Disproportionate collapse occurs when a structure collapses globally as a result of localised initial damage, such as element removal. Multi-material beam-column connections in timber post-and-beam buildings provide deformation capacity, continuity, and ductility necessary to form alternative load paths in the event of such initial damage. Accordingly, these alternative load paths can avert disproportionate spread of the initial damage, i.e. increase robustness. With more and more such sophisticated connections being introduced into the industry, it has become imperative to comprehensively understand their strength and deformation capacities along with failure modes under such extreme situations, especially to design for robustness, develop design guides, and for optimisations. However, very limited experimental and numerical studies are available in the literature. This project, thus, aims to investigate the disproportionate collapse resistance of modern beam hanger type steel-timber beam-column connections to inform robustness design guidance for engineered timber post-and-beam buildings. First, suitable constitutive models were proposed for timber to realistically mimic its behaviour under compression, tension, and shear. Next, a computational modelling approach capable of performing complex simulations of metal-timber connections that include material softening, damage, non-linearities, a large number of contacts, and diverse loading conditions was developed. A novel empirical method to obtain true stress-strain relations of ductile metal components in connection assemblies using computer vision techniques was also introduced in order to accurately simulate their post-necking behaviour. Then, two custom-designed knife-plate beam hanger type steel connectors were tested experimentally and numerically under shear and moment-dominated loading until failure to assess their performance under large deformations. Finally, the disproportionate collapse resistance of timber column and beam two-bay sub-frames composed of those two connectors was examined experimentally and numerically under different rates of middle-column removal scenarios, including sudden removal. The tested two custom-designed beam hanger connections display ductile shear and moment responses with large rotational capacities, enabling significant structural capacity to be retained without failure during large deformations. Column removal experiments demonstrate that the sub-frames can form alternative load paths through catenary action in the event of a loss of column. However, their capacities are insufficient to solely resist the accidental limit state floor loads calculated as per Eurocode. Moreover, the axial forces developed are 90% to 174% larger than the Eurocode recommended tie force requirement. In addition, rotational stiffness, average maximum rotation prior to loss of sub-frame vertical load capacity, and peak moment capacity of the connections under combined moment-axial-shear forces resulting from column removal were found to be significantly lower than those under flexure-only forces. Furthermore, the sudden removal of a column demands about two times larger vertical load resistance compared to the applied load to prevent progressive failure, indicating a 50% reduction in the load-carrying capacity of sub-frames. The numerical models developed and the proposed constitutive models yield promising results that agree well with experiments. The empirical true stress-strain relations determined using the proposed method exhibit a more accurate simulation of post-necking behaviour of metals. The computational modelling approach developed, including the constitutive models, is an effective tool that enables realistic prediction of pre- and post-failure behaviour of complex metal-timber connections through a variety of simulations. The proposed empirical method using photogrammetry allows the determination of post-necking true stress-strain relations of metal components used in complex or multi-material assemblies to inform numerical modelling. Experimental studies provide a valuable understanding of the performance of knife-plate beam hanger-type timber connections to improve robustness design codes and guidelines, particularly addressing the axial force (tie force) capacity requirement, rotation capacity requirement, the effect of combined forces on the moment and rotation capacities of connections, and the influence of sudden dynamic loss of a column.