Seismic Design of Steel Frames with Protected Connections

Abstract

The present paper is devoted to the seismic design of steel frames constituted by multistep I-shaped cross-section beam elements. The proposed design problem formulation is aimed at protecting the connections among beams and columns. In particular, reference is made to beams welded at their ends to appropriate steel plates connected by bolts to the columns. Therefore, the protection against brittle failure of the beam end sections is ensured by appropriate constraints of the optimal design problem. A useful comparison is made between the adoption of the so-called Reduced Beam Sections (RBS) and the use of multistep beam elements. In particular, the RBS approach here considered is the well-known dogbone technique consisting of reducing the width of the beam cross-sections in correspondence with suitably located beam portions, while the typical multistep beam element is constituted by a factory-made I-shaped uniform piecewise profile. To perform the necessary comparison, reference is made to a three-story, two-span plane steel frame constituted by elastic, perfectly plastic material and subjected to static and seismic loads. The load conditions and the relevant combinations have been imposed in compliance with the Italian structural code. The frame is first studied as constituted by European standard steel profiles on sale, and the related design is obtained using the optimization tool contained in SAP2000 software. A linear dynamic analysis is performed to determine the response of the frame. Later, the same frame, either equipped with dogbone and constituted by multistep beam elements, subjected to serviceability load conditions, is studied in terms of inter-story drifts and beam deflections. The geometry of the multistep beam elements is obtained by the solution to the proposed optimization problem. Furthermore, a nonlinear static analysis is performed to evaluate the capacity curves of the same frames. The results obtained for the frames equipped with the described different devices, compared with those related to the original frame, provide very interesting information on the sensitivity of the seismic response of the structure, showing the full reliability of the multistep beam element approach.