Seismic Design of Composite Moment-Resisting Frames With Cfst Members

Abstract eng:
The main objective of the research presented in this paper is to gauge the benefits of using concrete-filled steel tube (CFST) members from a seismic design perspective. To this end, a large set of composite moment-resisting (MR) frames was defined and its design was performed according to the requirements of Eurocode 8. In order to outline the set of MR frames, the methodology prescribed in FEMA P695 was used, regarding the definition of the archetypes. The basic performance groups were divided according to two seismic areas in Portugal, and considering frames of 3, 5, 8 and 12 stories. The set of 8 archetypes was designed using combinations of the following criteria: a) CFST columns of circular, square and rectangular cross section; b) behaviour factor (q) calculated according to EC8 ductility class DCM, and according to an Improved Force-Based Design (IFBD) procedure proposed by Villani et al. In total, 48 composite frames were designed. The analysis of the obtained design solutions shows that there are strong limitations when commercial square and rectangular cross-sections are used in seismic design of composite MR frames, as the majority of available members violate the EC8 cross-section slenderness requirements for medium ductility class. However, the opposite conclusion is found for circular CFST columns, as the criterion prescribed in the European code is more relaxed. Additionally, the results show that the consideration of a fixed value of the behaviour factor according to EC8 may greatly defeat its effectiveness, as the members become oversized and the purpose of using q, i.e. taking advantage of the inelastic behaviour of the structure, is compromised. However, if the Improved Force-Based Design (IFBD) procedure is adopted, the designer is able to reach a solution that is different and adjusted for every seismic scenario, with reductions in steel weight of the frame averaging 20% in the 48 designed frames. Additionally, it was also found that the use of circular CFST columns is beneficial to the composite MR frame design, as square and rectangular solutions tend to use more steel for the same archetype. Finally, one composite MR frame with circular CFST columns and an equivalent MRF with steel open profile columns were numerically modelled in OpenSees, and their seismic performance was evaluated through incremental dynamic analysis and collapse fragility assessment. The results obtained clearly indicate that composite moment-resisting frames with concrete-filled steel tube columns have a better seismic performance that an equivalent steel-only structural system.

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