Numerical Modelling of Timber Building Components to Prevent Disproportionate Collapse

Abstract: An increasing number of multi-storey buildings are being constructed with engineered wood products, such as glulam or cross-laminated timber (CLT). Multi-storey timber buildings can be safely designed for foreseeable loads, but knowledge is limited concerning their ability to survive unforeseeable events, e.g. accidents, natural disasters or terrorism. Multi-storey buildings with many occupants are required to be able to resist a disproportionate collapse due to an unexpected event. Collapse resistance consists of three lines of defence: I) decreasing the probability of the event, II) decreasing the structural vulnerability and III) increasing the structural robustness. The focus of the present thesis is on defence lines II and III, since they can be affected by engineering considerations.Robustness requires the availability of alternative load paths (ALPs) after an initial structural damage, e.g. the removal of an element. The activation of an ALP, e.g. catenary action, usually happens as the result of a larger displacement than that for which the components are designed, and with the participation of the surrounding structure. Physical tests of removal scenarios are expensive and they are often unable to represent realistic building situations. Numerical models can replace physical tests, e.g. by introducing parameter variations or changed boundary conditions, and can deliver an insight into the underlying mechanisms. Vulnerability depends on the ability of individual components of the structure to withstand loads greater than their intended design loads. To reduce vulnerability, so-called key elements can be be made overly strong. If the uncertainty concerning the material properties is high, e.g. for timber, both nominally stronger and larger amounts of material are required, resulting in inefficient material utilisation. Automated strength grading of sawn timber can narrow the uncertainty, but, even with the current technologies, the variations in the graded material remain large.The predictive power of computerised models for sawn timber offers a great potential for integration with traditional strength grading based on testing combined with statistical models. So far, surface data of sawn timber has been used for numerical models, but X-ray computed tomography (CT) scanning equipment now being installed in sawmills has made it possible to measure the inner structure of logs. Using CT data could make it possible to develop high-fidelity numerical models for predicting the mechanical properties of sawn timber, possibly even before sawing, and this could reduce the uncertainty for structural components and enable the production of high-strength timber. However, attempts to develop CT-based models for timber have been scarce.The objective of the work presented herein was to advance the research front regarding the prevention of disproportionate collapse in multi-storey timber buildings. The work has focused on numerical modelling aspects and on subsystems and components, rather than on entire buildings. The goals were: 1) to describe the state of the art regarding the prevention of disproportionate collapse and its application in timber buildings, 2) to develop models to identify and quantify the ALPs in subsystems and components of CLT buildings, and 3) to develop models of sawn timber based on X-ray CT scanning data, to reduce the uncertainty regarding the mechanical properties of the timber.For goal 1, the literature was reviewed and a survey was conducted among practitioners and researchers in the field. The results provided an extensive overview of the topic and the status quo in the industry, and identified a scarcity of guidelines for multi-storey CLT buildings.For goal 2, non-linear finite element (FE) models were developed for quasi-static pushdown analyses. A study of a platform joint first validated some modelling assumptions. The ALPs in single storeys in a corner bay of an 8-storey CLT building were then studied after the removal of bottom-storey walls. In subsequent parameter variations, the full bay was studied in dynamic analyses. The results identified six different ALPs, which were dependent on the connection capacities and the shear capacity of the floor panels, and indicated that collapse was likely after a double wall removal, but unlikely after a single wall removal. Furthermore, the ALPs in a platform-type CLT floor system were studied in parameter variations of calibrated FE models. The results showed how three different ALPs can develop, depending on the storey, the floor geometry and the connectors.For goal 3, a method was developed for the generation of continuum and FE models from CT scanning data of sawn timber, in which the knots, pith and local fibre orientations were reconstructed. The models gave realistic impressions and they could predict the bending stiffness, strength and initial failure location for Norway spruce sawn timber. The predictions improved, if the eigenfrequency of the sawn timber was also considered for modelling.