Structural response of high strength S690 welded sections under cyclic loading conditions

High strength S690 steel offers an attractive solution for use in buildings and bridges, owing to the inherent high strength to self-weight ratio of the material which can lead to significant savings in terms of cost and time. However, take up in practice has been hampered by concerns related to the deterioration in mechanical properties of the S690 steel plates after welding as a result of changes in microstructure. Whilst recent studies have illustrated the high levels of ductility that can be provided by the S690 steel, it is essential to assess and quantify the inelastic cyclic response of welded sections made from the same material under earthquake attacks. This paper therefore presents an experimental investigation into the structural response of high strength S690 welded sections under various cyclic loading conditions. A detailed description of 32 cyclic tests, carried out under two different cyclic protocols with various combinations of target strains and loading frequencies, is provided. Particular focus is given to examining the strength degradation in the material, the number of effective cycles completed before fracture, and the energy dissipation performance. A direct comparsion is also provided between the cumulative cyclic response of the S690 welded sections and their unwelded S690 plate counterparts, based on which the deterioration in behaviour is readily quantified. It is shown that a significant deterioration in strength similarly occurs in both the unwelded and welded S690 sections with the increase in number of cycles, particularly under large target strains. However, in terms of ductility, the number of effective cycles to fracture of the S690 welded sections is found to be considerably lower than for the S690 steel plates, typically by a factor of 2 to 3. Moreover, the energy dissipation densities of the S690 welded sections are shown to be only 41% to 47% of those for the counterparts. Overall, in addition to highlighting the importance of conducting realistic cyclic tests for assessing ductility, the findings provide detailed insights into the structural response of the S690 welded sections in steel structures under earthquake attacks, including the influence of target strains and loading frequencies. The results also enable detailed quantification of the cyclic response for the purpose of numerical modelling as well as for determining reliable ductility criteria.