Modelling the load-carrying capacity of reinforced concrete slab bridge: With a focus on slabs constructed with inverted T-beams

Short-span bridges in Denmark are often constructed as reinforced concrete slab bridges. Many of these bridges are more than 40 years old and were designed with traffic load models, which do not cover the load level from modern heavy transport. In order to ensure that the bridges can carry today's heavy transport with the required level of safety, it is necessary to either replace/strengthen these bridges or to document that the existing bridges are able to carry higher loads than what they were designed to carry.

The latter requires refined assessment methods that exceed the complexity of simple design methods and utilise the full capacity of the whole structure. Limit analysis based on the assumption of rigid plastic materials has shown to be a relatively efficient manual method to determine the load-carrying capacity of reinforced concrete slabs in bending.

In recent years, finite element limit analysis based on the lower bound theorem has shown to be an even more efficient method to determine the load-carrying capacity of slabs automatically no matter the complexity of the slab and loads. In this thesis, developments within the field of modelling the load-carrying capacity of reinforced concrete slab bridges are presented.

An essential and initial part of modelling the load-carrying capacity accurately is to apply load models that reflect the actual loads. Thus, a survey of extremely heavy vehicles, which is the governing load for assessment of the load-carrying capacity, is presented. A framework for finite element limit analysis (FELA) of slabs that accounts for shear-moment interaction is presented.

The framework is based on the lower bound method and can be implemented with general optimisation solvers. The conic yield criteria for slabs in bending are combined with different shear-moment limitations. It is demonstrated how different interactions affect the load-carrying capacity of slab bridges and how failure due to shear limitations can be identied with the collapse mechanism.

A layer model for slabs with shear reinforcement is developed based on a mechanical model. The layer model accounts for shear-moment interaction and is implemented effciently with second-order cone programming. In the thesis, a particular type of slab bridge is considered, which is constructed with precast inverted T-beams.

Such bridges have, in the main research project, been tested, and evidence of structural behaviour similar to that of solid slabs is found. Two test series of slab strips from existing bridges are examined and numerically investigated. It is shown that the precast beams in the slab strips fail due to diagonal cracking if the bond strength of the construction joints is insufficient.

Furthermore, a layer model within the FELA framework is developed which accounts for the construction joints in bending.