Seismic Performance of High Strength Steel Building Frames

摘要

In steel building frames under seismic action, the members designed to remain elastic during an earthquake are responsible for the robustness of the structure and prevention of collapse, being characterised by high strength demands. On the other hand, seismic resistant building frames designed as dissipative structures should allow the development of plastic deformations in specific members and locations.In the present work, the framing solution studied is the one obtained by combining two different steel grades: mild carbon steel (MCS) used in dissipative members and high strength steel (HSS) used in non-dissipative “elastic” members. The current seismic design rules, at least in Europe, do not cover the specific configuration of such ‘Dual-Steel’ structures. Therefore, a comprehensive parametric study devoted to investigate the seismic design and performance of EN1998-1 compliant dual-steel Moment-Resisting Frames (MRF), Concentrically Braced Frames (CBF) and Dual-Concentrically Braced Frames (D-CBF) is presented and discussed in this dissertation.The overall seismic performance has been analysed through static and dynamic nonlinear analyses against three limit states: damage limitation (DL), severe damage (SD) and near collapse (NC). The investigated parameters cover both, geometric and mechanical variables, as the type columns, span length, number of storeys and spectral shape. The comparison between dual-steel structures with those entirely made of MCS showed that: i) in order to fulfil the codified drift requirements and to limit the stability coefficients, the same shapes for members should be used for both structures for the MRFs, but there is a reduction in both, weight and cost for the CBFs and D-CBFs using HSS, which proves it is efficient in economic terms, ii) a similar performance can be recognized in both, dual steel and single grade steel structures; iii) In all examined structural typology, the behaviour factors obtained from incremental dynamic analyses for SD limit state were smaller than the used in the seismic design. These results suggest the need to calibrate the behaviour factors given by EN1998-1.The analyses have shown that the use of HSS in EN1998-1 compliant MRFs is effective in providing overall ductile mechanisms with limited plastic demand, due to the large design overstrength. For the braced frames, the use of the HSS in the non-dissipative members ensured that plastic hinges occurred in the dissipative structural elements with large brace ductility demand, mainly for the braces in compression. In addition, the beams from thebraced bay plays an important role in the seismic performance of these structural systems and is concluded that the use of HSS in beams of braced bays is not advisable.