ABSTRACT
Structural health monitoring (SHM) of bridges and, in particular, the development of efficient and reliable damage detection methodologies have been given a huge attention over the last three decades, by both the scientific community and bridge owners. Damage detection techniques involve the comparison of the structure’s response obtained in the monitoring stage with that of a baseline condition. The damage detection process generally involves five increasing knowledge steps ranging between, in a first stage, signalizing the occurrence of damage and, in a last stage, assessing its effects on the safety and lifecycle of the bridge. Therefore, detailed assessment and characterization of the in-service behaviour of the structure is paramount for successfully completing the damage detection process.
With the aim of applying sound damage detection approaches based on the quasi-static component of moving-load responses in concrete bridges, this paper is devoted to the analysis of the in-service behaviour of these structures under moving-loads. A reduced-scale laboratorial model of a reinforced and prestressed concrete frame, subjected to a moving-load, was chosen as the case study. Although the laboratorial structure is not a scaled down model of a bridge, the authors believe that its response is representative of the behaviour of typical full-scale frame concrete bridges. Furthermore, it is noteworthy that smeared cracking might be observed along the frame’s beam, which is typical in concrete bridges.
The structure under analysis was subjected to an extensive set of experimental tests, which comprised the observation of the structural response during the passage of a moving point-load, both forward and backwards, for different loads and under different prestress force scenarios (Cavadas & Figueiras 2014). That study revealed that, even though the pre-existing cracks remain entirely under compression (thanks to the applied prestressing forces) during the passage of the moving load, the structure exhibits a non-linear behaviour. This behaviour has important implications in the context of the application of damage detection techniques to cracked beam structures. On the one hand, it implies that the experimental influence lines become dependent on the direction of moving loads. This fact was evident in the experimental tests, where different influence lines for displacements, rotations and strains were obtained for forward and backward movement directions. On the other hand, the non-linear behaviour also implies that the beam’s deformations due to a constant point load will change if the prestressing force is modified.
In this context, the main purpose of the present paper consists in the discussion and justification of the nonlinear behaviour observed in the laboratorial model. For that purpose, detailed finite element analyses were conducted. Nonlinear constitutive models were adopted to simulate the cyclic behaviour of cracked reinforced concrete structures. Special attention was devoted to the bond-slip behaviour at the interface between the reinforcing bars and the surrounding concrete, and also to the cyclic relationship between the local crack width and the concrete stress normal to the crack. The nonlinearity of concrete in compression was not considered because the structure is subjected to reduced concrete compressive stresses. The supports deformability was also considered in the analyses. Finally, it could be demonstrated that the observed nonlinear response is due to two main reasons: (i) the hysteretic behaviour of cracked reinforced concrete elements, which occurs even when pre-existing cracks are entirely under compression; (ii) the supports deformability. Even though the results and discussions presented in this paper were obtained for a reduced scale laboratorial model, the phenomena under analysis can be found in real structures and therefore, this subject is relevant in the context of the application of damage detection techniques to real bridge structures.
