In general, the design of existing composite bridges is based on an elastic model of the section resistance. Within the span, under the effects of positive bending moments, the use of plastic resistance in design is authorized under certain conditions. Over the intermediate supports, because of local instabilities of the compressed metal elements, only the elastic design is permitted. Experimental work has shown, however, that composite sections nevertheless have a certain ductility under negative bending moments. The idea of taking into account this restricted ductility when verifying the structural capacities of beams was recently studied. It was highlighted that the ductility depends on several parameters related to the shape of the cross section and the material properties, and that a simple model makes it possible to estimate the rotation capacity of these composite sections easily and in a conservative way. An analysis method, based on the rotation capacity required to enable a certain redistribution of the bending moments, was developed to take into account the available but limited rotation capacity near the supports. The purpose of the current research work will be to widen the application of this method to cover all composite bridges, regardless of span length or section configuration. This implies a certain number of finite element calculations, and the presentation of the results in a manner that will be usable in practice. The benefits of the method are simplicity and the elimination of the complications associated with elastic design, as well as a more realistic evaluation of the true bridge capacity. This method, applied to existing bridges designed elastically, will enable better exploitation of their plastic reserve and will allow a more objective judgement of their structural adequacy in the context of durable bridge use.
The goal of the research is to place at the disposal of practitioners a new method of checking slender composite bridges. This method makes it possible to calculate the effective capacity of the bridge by taking into account the limited ductility of the slender composite sections in the intermediate support regions. The innovation of the method stems from the inclusion of the negative moment region’s previously ignored limited rotation capacity in modelling the bridge behaviour and verifying its structural capacity. In particular, this new method will enable the realistic estimation of possible reserve capacities for existing bridges. The method will also make it possible to envisage and quantify the effectiveness of simple reinforcements of the metal structure close to the intermediate supports to increase the capacity of a given bridge.
The following methods will be used:
- Development of elements necessary for the practical use of the method (taken into account the different forms section of composite beams and scope bridges using finite element calculation) and provision of these elements in a form usable in practice (graphics and / or equations).
- Illustrations of the application of the method to the means of examples of existing structures.
- Enlargement of the method on practical ways to increase the bearing capacity of the bridges.
The work program is provided according to the following plan:
- Description of the details of the new method in a simple and accessible for verification practice (18%)
- Calculation and drafting of rotation functions available - shear forces (13%)
- Calculation model control and drafting of rotation functions available - stiffeners (type, position, rigidity) (12%)
- Calculation and preparation of the required rotation functions - moment redistribution (7%)
- Calculation and preparation of the required rotation functions - in span curves (13%)
- Calculation and preparation of the required rotation functions - shrinkage and temperature (12%)
- Calculation and preparation of the required rotation functions - litters (12%)
- Choice of examples and numerical applications and final report writing (13%)
This new method of verification makes it possible to carry out a calculation of the structural safety which is preciser and closer to the real behavior. The advantages resulting from this method are numerous and they are related to the plastic design of the structures:
- the loading history, the visco-elastic behaviour of the concrete and other imposed deformations can be neglected at ultimate limit state,
- when designing new bridges, the engineer can more efficiently optimize the cross sections over the support and in the span,
- during the verification of existing bridges, originally designed by means of an elastic resistance model, the new method of verification allows to calculate a larger bearing capacity than the one based on the elastic model. This additional bearing capacity can still be increased by placing a longitudinal stiffener on the webs of the steel girders near the intermediate supports. The new method also makes it possible to determine the increase in resistance due to this stiffener.