Robustness Requirements for the Design of Plane Frames

As part of a safe design, which guarantees the stability of the structures against exceptional events and mistakes in the design or construction phase of the manufacture, it is necessary to study the robustness of structures. European and international regulations include, among the structural verifications, those of functionality [operating limit state (OLS)], those at ultimate conditions [ultimate limit state (ULS)], and the verification of the robustness toward exceptional actions. The robustness is generally determined though finite-element analyses of structures able to include nonlinearity and large displacements. No simple calculation methods or rules of design for robustness are available at present. The problem was here addressed by investigating in the context of a simple structural typology, which is that of a plane frame with columns fixed at the foundation and beams fixed or hinged at the ends subject to statically applied vertical loads, as a representative portion of single-story buildings for industrial use. For the definition of the degree of robustness (or structural integrity), simplified analytical models were developed to study the postelastic behavior of the structure or to construct, through simple analytical relationships, the pushdown curve including the effects of material, geometric nonlinearity, and large displacements from which to draw synthetic parameters for the evaluation of structural robustness. In addition, nonlinear static analyses were also conducted with the finite-element calculation software SAP2000, which can predict the static and dynamic behavior of plane or spatial frames subjected to large displacements. Analyses were carried out considering both the effects of geometric nonlinearities and those due to the inelasticity of the material. The analyses carried out highlighted that the structures examined develop an ultimate behavior strongly dependent on the dimensional relationships of the elements that compose them. Specifically, it was obtained that the dimensional ratio of the sections, attributable to the choice of a specific structural element, involves a different response in terms of bearing capacity and ultimate displacement; from this, it follows that only in some cases the choice of the structural elements satisfying SLE and SLU states also satisfy robustness requirements. The relationship in length of the main members (beams and columns or truss and columns) and that of the rigidity and the plastic moments entails keeping the structural typology unchanged, with different collapse mechanisms with different levels of structural strength.