Mechanical safety assessment for reinforced concrete columns based on ultimate limit surfaces

Abstract

It is proposed to assess the mechanical safety of key structural elements under accidental conditions using an ultimate limit surface formed by the joint consideration of local strength criteria for each type of ultimate internal force, without analyzing a single critical cross-section. Such a surface is constructed for vertical loadbearing elements (columns and pylons) of rectangular cross-section subjected to combined loading. Specifically, cases of eccentric compression with bending moments in the two principal planes are con-sidered, as well as the combined action of axial compressive force, bending moment, and torque induced by the specific features of load application. The calculations are performed in a dynamic formulation accounting for both physical and geometric nonlinearities. A plasticity-based damage model is adopted for concrete, while an elastoplastic model with a bilinear stress–strain diagram and no hardening are used for the reinforcement. The dynamic additional loading is modeled by an impulsive load, the magnitude and duration of which are determined approximately based on the law of conservation of momentum. Such a load simulates an accidental action, for example, the impact of a falling body on a deformable slab at the joint interface with the element. Based on the constructed dynamic ultimate limit surface, an assessment of mechanical safety is performed, which includes not only ensuring the absence of total failure but also accounting for the risk of material losses under various accidental scenarios. Practical aspects of the assessment and the risk calculation algorithm are demonstrated using examples of elements made of normal-weight concrete reinforced with welded frames.