Structural Behaviour in Concrete Frame Corners of Civil Defence Shelters Non-linear Finite Element Analyses and Experiments

Sammanfattning: From a safety point of view it is important that a concrete structure, apart from necessary load capacity, also is able to show ductile behaviour that allows redistribution of forces so that a local failure not lead to total collapse of the structure. A structure's ability to exhibit such behaviour is highly dependent on the reinforcement detailing of the joint connections between its independent members. Accordingly, to obtain sound structural behaviour, the joints should ideally be as strong as the structural members connected to them and show ductile behaviour in the ultimate limit state. Even though fulfilling these requirements, the reinforcement detailings in frame corners previously prescribed in the Swedish Shelter Regulations for the design of civil defence shelters were difficult to apply correctly. Therefore, a simpler method, by which all reinforcement bars are spliced within the corner region, was proposed and evaluated.

To examine the effectiveness of the new detailing when subjected to a positive (opening of the corner) or a negative (closing of the corner) moment, a combination of a literature survey, static full-scale tests and static non-linear finite element analyses has been carried out. The main parameters varied were the reinforcement detailing and the reinforcement ratio, and it was found that the new alternative is suitable to use. However, since a civil defence shelter is designed to withstand transient loading, such as blast waves from a nearby explosion or the impact of falling masses from a collapsing building, this also has to be taken into consideration.

When subjected to such loads, the response of a structure may be quite different from that of static loading. The peak load for transient loads is often several times higher than the "equivalent" static load used in the design of the shelter. Further, if the load is applied fast enough, it is possible that part of the structure will still "not be aware" of the loading when another part reaches failure. Therefore, non-linear finite element analyses based on explicit integration were carried out to increase the knowledge of how a structure behaves when subjected to such transient loads. It was found that even though the global structural behaviour may differ widely there are still large similarities in the local behaviour between a static and a transient load case. The results imply that the conclusions made in the previous static studies are valid also for a transient loaded structure.

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