Optimal design of concentrically braced steel frames in moderate earthquake areas

Abstract

The purpose of the present contribution is to summarize the results achieved in the frame of the research project MEAKADO funded by the Research Fund for Coal and Steel of the European Commission. The goal of this project is to develop specific design procedures for steel and steel-concrete composite structures in regions characterized by a low to moderate seismic activity, with an appropriate reliability level. The intention is to find an optimal balance between safety and economical concerns. Two parallel ways are proposed in this prospect. The first one is based on the exploitation of dissipative phenomena that are commonly not explicitly considered while the second one consists in investigating the possibilities of adequately adjusting the requirements imposed by existing seismic codes to better suit with the reality of moderate seismic risk. The objective of the proposed research action is to come out with an intermediate way of designing in which reduced but controlled amount of ductility is accounted for, providing thus the necessary safety with respect to uncertainties on the seismic action, but where the local ductility and structural homogeneity requirements are less stringent than designing according to Eurocode 8-Ductility Class Medium principle, in order to focus on intermediate values of behavior factors. These requirements should be tuned according to the actual seismicity level of the area. The research program focuses on moment-resisting frames and concentrically braced frames. It comprises a combination of experimental and numerical studies and should result in proposals formulated according to a pre-standard format in the perspective of further revisions of the design codes. The project focuses exclusively on the two main structural types for ordinary steel structures, i.e. concentrically braced frames (CBF) and moment resisting frame (MRF). A very special focus is put on CBF as being the very most commonly realized in practice (80 to 90% of the steel structures really built are laterally braced by CBF at least in one direction) and as the most constrained by current Eurocode 8 rules due to the combined requirements on diagonal truss bars slenderness and homogeneity of overstrength all over the height of the structure.