The load-carrying capacity assessment of existing road bridges, is a growing challenge for civil engineers worldwide due to the age and condition of these critical parts of the infrastructure network. The critical loading event for road bridges is the live load; however, in earthquake-prone areas bridges generally require an additional seismic evaluation and often retrofitting in order to meet more stringent design codes. This paper provides a review of state-of-the-art methods for the seismic assessment and retrofitting of existing road bridges which are not covered by current design codes (Eurocode). The implementation of these methods is presented through two case studies in Croatia. The first case study is an example of how seismic assessment and retrofitting proposals should be conducted during a regular inspection. On the other hand, the second case study bridge is an example of an urgent assessment and temporary retrofit after a catastrophic earthquake. Both bridges were built in the 1960s and are located on state highways; the first one is a reinforced concrete bridge constructed monolithically on V-shaped piers, while the second is an older composite girder bridge located in Sisak-Moslavina County. The bridge was severely damaged during recent earthquakes in the county, requiring urgent assessment and subsequent strengthening of the substructure to prevent its collapse.

The assessment of the existing road bridges as a part of bridge management networks has been a subject of several European research projects and actions in the last few decades. Bridge management includes a series of activities, interconnected in order to achieve optimal balancing of required costs, potential risks and overall bridge performance. This paper discusses a valuable link between a specific indicator (traffic load information calculated using Bridge Weigh-in-Motion measurement data) and the corresponding structural performance (reliability of existing bridge). It is shown that implementation of weigh-in-motion measurements in the probabilistic assessment of existing road bridges can reveal hidden bridge reserves and predict bridge reliability development over the required lifetime. Consequently, such approach can yield an unrestricted use of the bridge over a much longer remaining service life. Broader advantages lie in an improved road network management, road bridges in particular, and in a more sustainable development of infrastructure network and greater satisfaction of road users and owners.

The assessment of existing road bridges as parts of infrastructure networks is required in consideration of their deterioration and age. Advanced monitoring and management tools are mainly used for landmark bridges while the decision making process for small to medium bridges, which constitute the majority of the bridge network, mainly relies on condition assessment based on experience and conservative analysis related to design codes. In the analysis of the load-carrying capacity of theses bridges, loads imposed by the passing traffic are predominant due to their variable nature and level of uncertainties. The research presented in this paper outlines the value of additional traffic data, collected with both traffic counters and Weigh-in-Motion (WIM) method in the scope of assessment procedures for these bridges. Adequate processing of collected traffic data is crucial for subsequent extrapolation of maximum load effects on a particular bridge over a certain period in time. By taking into account all related costs, the purpose of this paper is to prove the benefits of employing traffic load monitoring data in structural assessment and subsequent decision-making process in service life management of bridges.

In 2020, Croatia was struck by two catastrophic earthquakes, resulting in more than 50,000 damaged structures. The majority of these are masonry buildings, but there are a number of reinforced concrete structures that suffered moderate to extensive damage. In this paper, the seismic condition assessment and upgrading of existing RC structures are presented with a case study building in Zagreb. The assessment procedure includes initial visual inspection, rapid preliminary evaluation, detailed in situ measurements, and non- and semi-destructive methods. New technologies were applied and followed by numerical modeling and verifications. Strengthening proposals are made that respect owner needs and the needs for the energy retrofitting of the existing RC building. As the integrated approach should be respected in the renovation of existing buildings, this case study can represent an example of good practice in the process of seismic and energy retrofitting.

In December 2020, a strong earthquake occurred in Northwestern Croatia with a magnitude of M_L = 6.3. The epicenter of this earthquake was located in the town of Petrinja, about 50 km from Zagreb, and caused severe structural damage throughout Sisak-Moslavina county. One of the biggest problems after this earthquake was the structural condition of the bridges, especially since most of them had to be used immediately for demolition, rescue, and the transport of mobile housing units in the affected areas. Teams of civil engineers were quickly formed to assess the damage and structural viability of these bridges and take necessary actions to make them operational again. This paper presents the results of the rapid post-earthquake assessment for a total of eight bridges, all located in or around the city of Glina. For the assessment, a visual inspection was performed according to a previously established methodology. Although most of the inspected bridges were found to be deteriorated due to old age and lack of maintenance, very few of them showed serious damage from the earthquake, with only one bridge requiring immediate strengthening measures and use restrictions. These measurements are also presented in this paper.