Are current robustness design strategies appropriate for large timber structures?

The paper investigates the consequences of using material independent robustness design strategies in large mass timber buildings by looking at codes, research and historical structure failures, followed by a numerical study of material specific accidental scenario on a mass timber frame. Modern engineers turn to timber more and more often as the only construction material with the ability to capture CO₂. Thanks to the development of Engineered Wood Products such as Cross Laminated Timber (CLT) the size of buildings with timber as primary structural system has increased dramatically, meaning that robustness considerations have now become of particular importance. The paper reviews the robustness strategies identified in literature and those available in codes and discusses the important differences between engineered timber and more conventional construction materials. Consequently a finite element analysis modelling study is conducted on an experimental 5-storey CLT frame to investigate how a material specific accidental loading scenario - a compartment fire - compares to a traditional alternative load path analysis with notional element removal. The loss of mechanical strength and stiffness in elevated temperatures well before the point of structure combustion was a focal point of the analysis and hence a choice was made to model the structure with no section loss. The study has concluded that in this model the influence of contained fire on compressive and shear strength utilisation have shown to be comparable to or larger than in the element removal scenarios. However, the governing failure mode across all of the load cases investigated was bending, in which the removal of an element caused largest utilisations of available strengths of timber.