Modelling Alternative Load Paths in Platform-Framed CLT Buildings : A Finite Element Approach

Abstract: Timber has become more popular as a construction material during recent years.Engineered wood products, such as glued laminated timber and cross-laminated timber, have enabled the construction of multi-storey buildings.%The reason for this development may lie in the abundant availability of the material with economical and environmental advantages and the development of refined engineered wood products, such as glued laminated timber and cross-laminated timber.Tall buildings with many occupants need to resist a disproportional collapse in case of unexpected exposures, e.g. accidents or terrorism.Structural robustness can improve the collapse resistance of a building.The literature about robustness is comprehensive concerning concrete and steel buildings, but is rather limited regarding timber.A robust building can mobilise alternative load paths in the structure after the removal of bearing building components.Alternative load paths rely primarily on the connections between components.For timber buildings, few investigations exist to evaluate the alternative load paths after a removal.%There is no procedure for a removal aAnalyses usually do not take into account non-linear effects which could influence the capacity of alternative load paths, such as damage of single fasteners, friction, timber crushing and brittle failure.In particular, the alternative load paths in platform-framed cross-laminated timber buildings are not well understood.The goals of this thesis are toi) review the concept of robustness in general and determine the state of the art concerning timber buildings in particular,ii) develop a method to analyse the alternative load paths in a platform-framed CLT building taking into account relevant non-linearities,iii) use the method to elicit the alternative load paths in a building after a wall removal, and iv) study the effects of probabilistic variations of model parameters.The thesis first introduces tall timber buildings and then presents a summary of structural robustness in a collapse resistance framework.The summary includes established analysis methods and specific considerations for timber, whereof a detailed review is provided in Paper I.Paper III additionally provides results of a survey on contemporary practices of professionals around the world concerning robustness.The next chapter describes an 8-storey example building made of platform-framed cross-laminated timber and introduces the modelling approach for an alternative load path analysis after a wall removal.The approach is based on the finite element method with the commercial software Abaqus.The deterministic part of the approach includes a non-linear static pushdown analysis of single storeys in a bay and elicits the alternative load paths and their capacity.Finite connector elements in the model substitute single fasteners including their elastic, plastic, damage and rupture behaviour.The 3D models of the walls and floors account for timber crushing, brittle failure and contact friction.A simplified non-linear dynamic model of the entire bay uses the pushdown results as inputs and evaluates the collapse progression among storeys under a sudden element removal.The probabilistic part of the approach models the uncertainty of the input parameters of the dynamic model by sampling their values from a probability distribution under a Monte Carlo analysis, to evaluate the probability of a collapse.Paper II applies the approach for a simple pushdown of a single storey and Paper IV applies the pushdown and the dynamic analysis to elicit the alternative load paths after a single wall removal.The thesis shows furthermore the results of a corner removal in the example building and of the Monte Carlo analysis for the single wall and corner removal.The developed approach could identify the alternative load paths, determine their capacities and estimate the probability of a collapse, given a wall removal.The approach could be used to classify various removal scenarios in platform-framed multi-storey cross-laminated timber buildings specifically and predetermine design solutions which could provide a desired level of robustness.The approach could be generalised for multi-storey timber buildings of various construction types.