Assessment of existing bridges according to the new Italian guidelines – A case study

This study focuses on the assessment of an existing bridge according to the procedure prescribed by the Italian Building Code (NTC18) and the new Italian Guidelines, which have been specifically developed for existing bridges. International standards have also been used as reference (Eurocode, Model Code and AASHTO). For the first time, the Italian Guidelines specify the detailed criteria to be followed during the assessment process of an existing bridge not only from the seismic point of view but also with regard to the traffic loads. The study covers all the aspect of the assessment process: investigation phase, analyses and verifications for the static and traffic load, seismic loads. The aim of this study, that uses a specific case study, is to investigate the different peculiarities of the assessment of existing bridges and to highlight potential critical aspects of the existing codes. Often, the information regarding existing structures is limited (missing or incomplete design drawings and reports). For this reason, the first step is to investigate the structure through survey of geometries and construction details, tests on materials and detection of defects or damage. All this process is oriented to the best knowledge of the structure. Then, a 3D finite element model of the bridge is built to perform the analysis aimed at evaluating the behavior of the bridge and the acting internal forces under static and dynamic loading. In order to find the situations that are usually more relevant for the verification of the deck, the static and traffic load analysis, are performed. Eigenvalue and response spectrum analysis are also performed, which are the reference point and the most commonly used analysis for the seismic design and assessment of structures. In addition, performing nonlinear static analysis of the piers, was possible to take into account their nonlinear behaviour and compare the results with the ones obtained with the linear dynamic analysis. Focus is also given on shear verification of post-tension beams, in which the amount of transverse reinforcement is usually low, and piers, for which a certain level of ductility is required during seismic events. Verifications are performed at Ultimate Limit States in term of strength for the deck elements and for both strength and ductility for the piers. As expected, the analyses highlighted the importance of the knowledge of geometries and material characteristics, in order to achieve realistic results. High relevance is assumed by the modelling of defects and damage. For this reason, the low level of maintenance and thus the possible high level of deterioration, could be relevant and significantly affect the safety level of the structure. In addition, there is a difference between the traffic load considered during the design of the structures and the one required nowadays by the new codes (also for assessment), that could be more demanding. In general, old existing structures are not designed with a high level of ductility (low reinforcement ratio, no capacity design, etc.) and often, the joints are not adequate with the displacement required by the seismicity of the region. This leads to secondary problems that could also affect the main behavior of the structure. The structure assessment is a delicate subject susceptible to many details and require multiple analysis with different assumption, also aiming to reduce the level of epistemic uncertainty. For example, different properties of the bearing device can be considered to take into account their variation due to the low level of maintenance.