Investigation of the application of high-strength steel in dissipative elements

Abstract

The term High-strength Steel (HSS) is currently used for steel grades with yield strength exceeding or equal to 460MPa. Production of HSS is the result of significant improvements in steel making technologies, achieved in the last decades. The structural application of HSS may reduce member sizes and workload of transportation and construction, thus providing significant economic, environmental and architectural advantages. However its application is still limited while its relevance to seismic design is currently under discussion. The present thesis investigates experimentally and numerically the possible application of HSS in the dissipative elements of the innovative FUSEIS system. FUSEIS consists of a pair of closely-spaced strong columns rigidly connected via multiple dissipative links which may be beams or pins. The system resists lateral loads by developing axial forces in the columns and bending in the links while in case of pin links catenary action also develops. An important advantage of FUSEIS is reparability: in case of a strong earthquake damage is concentrated in the replaceable links, protecting the rest structural members. The experimental investigations include eight large-scale cyclic tests on FUSEIS systems consisting of different types of links (hollow-section and HEA beams and circular pins) and steel grades (S355, S500, S700), conducted in the National Technical University of Athens. Relevant component tests, conducted in another Laboratory, are overviewed. Reference is also made to previous experimental campaigns involving FUSEIS with S235 links. For various reasons, comparison between the different tests is limited. However it can be deduced that HSS specimens exhibited good seismic response despite their reduced ductility (compared to conventional steels). The tested systems are simulated via different models with increasing complexity and suggestions are given in order to approximate their response. Eventually, two case studies are designed according to the Eurocodes: a two-story and a five-story building including FUSEIS systems with S700 beam links. The case studies are subjected to performance-based evaluation, by using non-linear static and dynamic analyses while considering the response of their most critical components. Simulation of material non-linearity in the dissipative elements is based on the aforementioned test calibration. Given that the seismic design of buildings is governed by stiffness demands, the specific application of HSS does not fully benefit from the material’s advantages. The models are assessed at two limit states following two different methodologies, both of which result in the acceptance of their design q-factor (equal to 3.5). Overall, the objective of this study is to explore whether the hysteretic behavior of HSS can be sufficient for dissipative zones, rather than comparing it to conventional steels. Experimental and numerical investigations provided affirmative results although further investigations would be required.