Modelling progressive collapse of timber buildings and its applications

Abstract

The primary function of buildings is to provide safety and shelter from the environment and a functional space for human activities. To ensure their safety, design standards provide guidance on the minimum requirements and how they can be fulfilled. In the vast majority of cases, this approach is successful and we as a society witness relatively few failures and collapses of buildings. Nevertheless, some buildings may fail to provide safety and shelter and instead become a hazard to their occupants. Current design standards consider plausible loading scenarios from the intended usage, environmental loads, and accidental loads. However, abnormal loading scenarios from human errors and malicious actions are not considered directly. Because of the increasing frequency of malicious actions and abnormal environmental loads, the societal losses are likely to increase in concurrence with the amount of collapses. To account for abnormal loading scenarios, design standards use threat-independent robustness measures. Robustness refers to the ability to maintain a function after sustaining damage, or the insensitivity to initial damage. The insensitivity can be interpreted that the final consequences should not be disproportionately large to the initial damage. In buildings, it means that an initial damage should not lead to a disproportionately large collapse. However, most robustness measures in design standards were developed for reinforced concrete and steel buildings and may be detrimental if directly applied to buildings comprising other materials, such as timber. Because of the immaturity compared to buildings comprising traditional building materials, their robustness properties are largely unknown. In combination with the recent increase of modern timber buildings, this poses a large risk potential. Therefore, research on timber-specific robustness and implementing results into design standards is time critical. However, this requires the analysis of progressive collapse, which may not be trivial. Progressive collapse can be defined as the spreading of failures in a chain reaction and often requires complex and computationally expensive models. For instance, collapse is associated with phenomena like large deformations and the inelastic behaviour of materials. It is also a highly dynamic process and may involve hysteresis, changing damping properties, and impact loading from falling debris. Despite the challenges, a model capable of analysing progressive collapse can be used to make risk-informed decisions about robustness measures in design standards to maintain and increase the safety of buildings. To this end, a modelling framework for the progressive collapse of timber buildings was developed. It uses a combination of finite element analysis, analytical equations, and results from impact loading tests. The modelling framework can simulate the progressive collapse of timber buildings with members following an orthogonal grid subjected to arbitrary initial damage scenarios and includes features, such as element erosion, debris impact loading, connection hysteresis, and incremental equivalent viscous damping. With some adjustments, the modelling framework can be extended to include other materials. The modelling capabilities were demonstrated on a multi-storey timber building subjected to various column-loss scenarios in Switzerland. For the column-loss scenarios, the corresponding final consequences associated with the loss of materials and life were quantified. They showed that the material losses were inconsequential compared to the loss of life. The consequences were used to assess the risk of the building when considering human errors and malicious actions. Based on this, most of the available resources should be used to reduce human errors. Besides these applications, risk and damage-based methods to quantify the robustness were compared and their practical implications were discussed.