Keywords: twin girder, bracings, twin box section, cracked analusis, fatigue, redundancy

Two-girder braced allowing to improve the redundancy.Two-girder with lower slab allowing to get advantage from the box-girder behavior.

In steel bridge building, the Eurocodes are introducing new methods to check the structural behaviour. Some of these new ideas have already been introduced  in the French codes since a few years ago :

• Crack control methods in the composite bridge decks
• Fatigue verifications
• Resistance of beams under concentrated launching loads

Generally, it seems that composite bridges needed some collective research, to renew the structural forms and to introduce new designs, beyond the the twin girder.

The research was carried in two directions so as to answer the two main questions : how to design decks to resist higher torsional effects without cost increase? How to limit the ecenomic impact of more steel due to design codes evelutions.

Because of the wide range of possible solutions : (span length and distribution, roadway), the research work tried to rationalize the possibilities in order to cover a large spectrum of medium span bridges. Two and four lanes decks were chosen on the four types of decks shown in figures 1 ?| 4.

The ?s simplified twin girder ?s  seems to be the most promising solution. The design is simpler than the traditional twin girder. More ecenomy can be obtained with less cross beams. Fatigue resistance seems to be the weak point, because they are considered to be non redundant structures. Resaerch on the steel material and connections with a rigourous inpection and maintenance policy could reduce this risk.

Box sections look promising, but appear less competitive economically. A twin girder can be transformed into a box section with lower bracings or a lower concrete slab. But the design and fabrication become more complex, whereas the modern trend has been towards more simplification.

Fig. 1  Simplified twin girder

Fig. 2 : Simplified Twin box section

Fig. 3 : Twin girder with bracing

Fig .4 : Twin girder with lower deck

Twin girder with lower bracing have more redundancy. They are quite common in railway bridges. The lower slab is more economical as it reduces maintenance and has higher torsional stiffness although a higher weight.

A hybrid composite twin girder  with a concrete slab in hogging moment zone and near the abutments is an interesting solution. The concrete slab participates in the global resistance and prevents lateral instability of the lower flange in compression. The extra steel weight remains below 10 % while the length of the lower slab is reduced by more than 60 %

Eurocodes EC 1 loads lead to a slight increase of the steel quantity for the unbraced twin girder because of the increase in bending moment due to one sided heavy loading. The lower concrete deck is the most convenient structure for EC loads.