Overstrength for enhancing the robustness of structures

In the present paper, a methodology able to increase the robustness of a frame struc- ture is proposed. This methodology focuses on the structural behaviour of the key elements, which are endowed with a certain overstrength that can be employed to enhance the capacity of the whole structure to withstand a specific hazard scenario. The “loss of a column” can be an example of the possible scenarios that a robust structure should withstand. The additional resistance due to overstrength can be adopted to reduce direct local damage due to accidental events (local strengthening) and to prevent the propagation of local damage to a dispropor- tionate extent (progressive collapse resistance). Furthermore, this structural measure guaran- tees the formation of alternative load paths and, thus, the capability of the structure to redistribute the loading due to a local damage among the remaining structural components. The paper aims at evaluating the role of overstrength in the design (or in the retrofit) of struc- tural members and in the development of several collapse resisting mechanisms of the whole structure. Those considered for framed structures are: compressive arching, flexural and ten- sile catenary action developed in the beams adjacent to the initial damage, as well as membrane actions developed in the supported floors. In addition, the influence of the portion of structure adjacent to the area directly affected by the damage is considered. The procedure proposed in the present paper suggests a strategy that is suitable for a prelimi- nary robustness-oriented design, involving specific solutions in terms of redundancy, local re- sistance and mechanical properties of the structural members.