Optimisation of steel-concrete composite high speed railway bridges

In all civil engineering practices, the goal is to produce effective and safe structures, minimising material usage while conforming to all relevant safety and comfort criteria. To achieve materially efficient bridges, it is of the utmost importance to investigate and optimise various design alternatives and compare them. In this paper, a vast array of simply supported steel-concrete composite railway bridges for high speed railway traffic are optimised using a Genetic Algorithm. Different cross-sections, steel grades, and span lengths are analysed to achieve the lowest possible steel mass. Four different cross-sections are examined: double I-girders, double box girders, a semi box girder with a joint lower flange, and a box girder with inclined webs. Steel grades from S235 to S460 and span lengths from 20–70 m are used in the optimisation. As constraints, Eurocode criteria for accelerations, the ultimate, and serviceability limit states, fatigue, beam end rotations, and beam deflections are used. Web slenderness is limited to avoid web breathing. The results indicate that I-girders produce the cross-section with the lowest mass. The most decisive criteria are accelerations for span lengths below about 50 m and the serviceability limit state stress check above about 50 m. In all cases, cross-sections with very slender webs render the lowest steel mass. • Future designs of steel-concrete composite high-speed railway bridges. • Optimisation is performed to minimise steel usage in the bridge superstructure. • Results indicate that I-girders are the most materially efficient option.