Pere Alfaras Calvo Paul van Hagen Razvan Capra
The paper aims to discuss the advantages and disadvantages of the double composite section in high-speed railway bridges from the design, construction and maintenance point of view. The paper is centred around the differences between single and double composite sections.
Steel composite construction has been extensively adopted in the last 60 years for small to long span bridges (up to 200m).
In composite bridge decks, the steel structure works together with a concrete slab to resist flexural stresses and to control deflections. These decks work efficiently in mid-span regions where the top concrete slab deals with the compression forces whilst the steel structure resists the tensile forces from the positive bending moments.
The efficiency is lost over the intermediate supports where hogging bending moments develop. In these regions, the concrete slab tends to crack leading to an increase in reinforcement to control the tension forces. Similarly, the steel structure below requires additional stiffeners to prevent local or global instability under the compression forces.
To improve the structure’s effectiveness, the double composite section is proposed for the intermediate supports regions. The section adds a concrete slab to the lower (compression) flange of the superstructure.
This results in savings in structural steel, reduction in cross frame requirements and in gains from the additional bracing at midspan due to the bottom slab. The advantages go further, and additional benefits can be seen for high-speed rail bridges: improved dynamic response (increased damping, heavier deck, enhanced torsional stiffness) and limited deck end movements due to the increased stiffness; all leading to an increased passenger comfort and a more economical design.
The disadvantage is that all of these benefits come with an added stage during construction, casting the bottom slab, and increase loads transferred to the substructures.