Bridges of The Future Being Designed by COMBRI Project

The TECNALIA Technological Corporation have taken part, through its Construction Unit, in the European research project, "Competitive steel and composite bridges by innovative steel plated structures - COMBRI", the goal of which is to make advances in design for the bridges of the future.

The COMBRI research project and the dissemination project "Valorisation of Knowledge for Competitive Steel and Composite Bridges - COMBRI", the latter being an adjunct of the former, have come together to issue the COMBRI Design Manual in order to improve the competitiveness of steel and steel and concrete composite bridges through helping to implement the most recent methodologies in structural design in the European Union using Eurocodes. Both projects are financed by the European Commission's Research Fund for Coal and Steel programme.

The COMBRI project has focused specially on the verifications of the plate buckling of steel plated structures and, thereby, on the contents of the following Eurocodes: EN 1993-1-5 "Plated structural elements", EN 1993-2 "Steel bridges" and EN 1994-2 "Composite bridges".

The COMBRI Design Manual is divided into two parts, to provide a more clearly organised document for its practical application:

• Application of Eurocodes. This describes the different national background of participating members of COMBRI countries on the application and interpretation of Eurocode calculation methods. With the goal of providing information in the most useful manner, this is presented applied to two types of composite bridges - twin-girder and box-girder - using practical examples that include references to the Eurocodes methodologies.
• State of the art and conceptual design of composite, steel-concrete bridges. This part describes the main typologies corresponding to the participating countries in COMBRI - Belgium, France, Germany, Spain, and Sweden. These typologies reflect the current practice in these countries of using applications both for conventional bridges as well as for other, more novel ones. Also dealt with are a number of solutions arising from R+D projects, improvements applicable to steel and composite bridge design are discussed and the possibilities and limitations of the methodologies currently included in the Eurocodes outlined.

The most outstanding results of COMBRI are the following:

• The use of hybrid girders: the redesign of a S355 steel box girder, proposing a S460 and S690 steel hybrid girder, gives a reduction in cost of material of 10% in spans and 25% at the piers.
• The recommendation of double composite action: using composite action in the bottom flange at the piers, where compression is acting, is a competitive solution despite the greater complexity of design required.
• The rationalisation of the use of transverse stiffeners on the webs: the cost in terms of labour requires its minimisation and, as an alternative, the use of longitudinal stiffeners and simpler constructive details. It is shown that longitudinal stiffeners are not the most competitive solution for web depths of less than 4 m.
• The design of simpler diaphragms: while not consuming a lot of steel, from an economic viewpoint, it is important to reduce hours of fabrication through eliminating components, possibly transverse stiffeners also.
• The design for using the launching technique for bridges: the resistance to a patch load has been studied, improving on the Eurocode EN- 1993-1-5 design methodologies and enabling longer loaded lengths and, thereby making the launching of bridges with prefabricated concrete slabs possible.
• The methodologies of Eurocode EN-1993-1-5 for the flanges, principally the bottom flange of the box girder cross-section, can lead to unsafe results for low rigidity solutions and, so, minimum rigidities are recommended for stiffeners and the use of large trapezoidal stiffeners provide two stiffened lines for the same effort of welding required for a single open section stiffener and its torsional rigidity provides an increase in critical stress.
• The results of the COMBRI research are presented in the final report in such a manner that they also serve for enhancing the Eurocode EN 1993-1-5.

Finally, within COMBRI it has been developed the software EBPlate to determine the elastic critical buckling stresses for their application in the design of bridges.