Towards a combined probabilistic/consequence-based safety approach of structural glass members
Delft University of Technology, Faculty of Architecture, the Netherlands
Glass is generally considered to be an unsafe material to use in building structures. To limit the risk associated with structures, Dutch and European codes focus on the probabilistic approach. It will be shown that this is insufficient for structures in general, and for glass structures in particular. Much more than common structural materials like steel and reinforced concrete, glass is highly susceptible to a wide range of incidents causing glass breakage. Hence, the question should not just be "when will the glass break?" (probabilistic approach), but also "what will happen when it does?" (consequence-based approach). Analysis of projects realized in practice show that structural engineers are indeed aware of this. They use varying strategies, like laminating and providing alternative load paths, to limit the consequences in case of glass failure. However, such strategies are applied to varying extent, depending on the experience and opinion of the engineer, the location, the structure type, and probably on many more factors. The premises and a priori requirements applied by either engineer or governing bodies with regard to the consequences of glass failure, are not formulated explicitly, nor in building codes nor in literature. At best, they remain implicit. Therefore, an explicit combined probabilistic/consequence-based safety approach is proposed for structural glass members. The consequence-based part of such a combined approach sets specific requirements to the residual strength of structural glass members for a certain period of time, at prescribed levels of damage. The concept of Member Consequence Classes (MCC) is introduced, in order to be able to differentiate requirements for members based on their role within a structure and the function and accessibility of the structure at hand. The presented approach allows for an open discussion on the required level of residual strength at varying levels of damage. Furthermore, it provides a basis for comparing design options in terms of safety and it allows for the development of standard structural glass members with a proven level of safety. Finally, because it sets requirements for the application of structural glass in any (building-) structural application, it provides a fundamental basis for the development of structural glass beyond the existing small scale glass structures like entrances, pavilions and conservatories.
Key words: Safety, risk, probability, consequence, damage, residual strength