Structural monitoring and seismic analysis of a base-isolated bridge in Dogna

Introduction. In recent years base isolation has become an increasingly applied structural design technique to protect bridges and buildings from severe earthquakes. Main goal of base isolation is to produce a substantial decoupling of the superstructure from the substructure resting on the shaking ground, minimizing the internal state of stress during an earthquake by increasing the period of the structure and by acting as energy dissipation device (Naiem and Kelly, 1999). This structural solution also has the benefit of giving better distribution of the seismic forces between the various elevation supports. In the specific field of bridges, base isolation techniques were initially confined to long structures, but in several countries – due to more severe seismic codes – base isolation has been recently applied also to small and mediumsize bridges. In this paper, results of an experimental/analytical study focused on the dynamic behavior of a 75m long, reinforced concrete (RC), post-tensioned, based isolated bridge are discussed. The bridge was built in the Municipality of Dogna (Friuli Venezia Giulia), in an area with high seismic activity. The structure replaces an existing bridge – positioned a hundred of meters upstream – that highlighted marked hydraulic inadequacy during an exceptional flood (August 2003). Within the works carried out to restore the Fella river hydraulic regime, the new bridge reduced occupation of the river bed (e.g. single pier support). The mountain environment, moreover, suggested the use of a slender and low-impact structural solution, although with appropriate seismic resistance, hence preferring the base isolation technique (Alessandrini et al., 2009) to traditional supports (e.g. fixed or unidirectional bearings). In May 2007 the bridge underwent an extensive series of static truck-load and harmonic forced vibration tests with low levels of excitation. One of the purposes of the investigation was to verify the reliability of finite element (FE) models to describe the measured dynamic behavior of Dogna bridge. Dynamic data (namely, natural frequencies, mode shapes, and damping factors) can in fact provide meaningful results, if used to improve the FE model of a bridge, enabling for example to properly estimate mechanical properties of materials and boundary conditions. Another important purpose of the present research was to define a baseline model of the bridge for future diagnostic investigation, being this issue is of great importance for the company who manages the local highway network. A long-term plan of maintenance of the bridge, in this sense, could not neglect possible changes of performances of the bearing devices (Bonessio et al., 2012). Even though their response variations have been the goal of research at material and device level, procedures for monitoring the device functionality in service should be in fact properly developed. The periodic removal of isolators from bridge for a laboratory test campaign, for example, would not appear as a feasible approach, due to high economical impact. A more reliable solution, otherwise, could be represented by periodical testing of the bridge with devices in service, to localize and quantify global response changes generated by local degradation of structural components, as well as by isolation devices. Most of the diagnostic methods for damage detection in bridges are in fact based on comparison between the structural responses of the actual possibly damaged state and the reference (undamaged) configuration of the bridge. It follows that, to give a proper interpretation of the damage-induced changes on its dynamic characteristics, it is crucial to have at disposal an accurate knowledge of the undamaged configuration of the bridge, and this was another important goal of the present study. In this paper, experimental and analytical investigations are recalled and summarized from a recent extended research study (Bedon and Morassi, 2014). Harmonically forced tests performed on Dogna bridge are used for extraction of its dynamic properties, via experimental modal analysis (EMA) techniques. An accurate 3D