On the collapse evaluation of existing RC bridges exposed to corrosion under horizontal loads
Introduction
During the last decades, the intensification of environmental pollution caused a significant increase of the corrosion phenomena in reinforced concrete (RC) bridges [1], [2], [3], [4], [5], [6], [7] that led to numerous collapses, especially in those structures not subjected to an adequate maintenance. Considering the different kinds of RC structures degradation, one of the most common and important ones is the steel rebars corrosion caused by carbonation effects [8], [9], [10], which strongly influence the capacity of the structure to resist both static and dynamic loads, such as seismic actions. Several researches have proposed different types of solutions for this issue, such as the use of new concrete mix designs [11], [12], [13], [14].
Focusing attention on the Italian motorway network bridges, it is possible to observe that the majority of these bridges were built during the 1960s and 1970s and, therefore, now they should be subjected to several maintenance operations in order to guarantee high safety levels. Moreover, during the last years, the Italian Civil Protection Agency has paid particular attention to the seismic vulnerability assessment of these structures, also through the development of specific guidelines providing methods for risk indices evaluation [15], [16], [17].
Different approaches have been proposed to evaluate the collapse mechanisms of the piers of existing RC bridges under seismic loads, in order to define the correct maintenance or retrofitting interventions during time, achieving an optimization of the bridge management process [18], [19], [20], [21]. Despite such developments, several issues remain open, mainly concerning the correct evaluation of the piers collapse mechanism of the bridges that do not present a predominant vibration mode, as in the case of multi-span bridges with frame piers and elastomeric bearings [22]. In these cases, in fact, the use of standard pushover analyses can lead to an overestimation of the structure capacity because such approach neglects the effects of the higher order vibration modes in the activation of the collapse mechanism. This aspect is even more evident in presence of corrosion effects, which may reduce, to a large extent, the stiffness and strength of the piers.
Considering the above-mentioned issues, the authors present an efficient simplified procedure for the collapse evaluation of existing RC bridges exposed to corrosion under horizontal loads. The procedure is characterized by a low computational effort and is based on the implementation of a simplified 3D finite element (FE) model of the bridge: the structural elements are modelled using beam elements and the effects of the corrosion due to carbonation are considered through the steel rebars cross area reduction. The non-linear material behavior is taken into account by placing suitable plastic hinges located at the ends of the bridge piers.
Moreover, in this study, special attention is focused on the correlation between the corrosion effects on RC bridges and the seismic performance of the structure expressed in terms of proper risk indices related to the failure of the first structural element, considering both ductile and brittle mechanisms and three different scenarios of corrosion levels.
Section snippets
Finite element models and collapse mechanisms
In this work, FE models of RC bridges were implemented through the software code MIDAS Civil [23]. In order to limit the computational efforts, the deck, the piers and the pier caps were modelled using beam elements, while the elastomeric bearings were represented through elastic links. The beam elements were connected to the bearings by means of rigid links (Fig. 1). The abutments were considered as perfect restraints where the bases of the bearings connecting the deck to the abutment are
Corrosion effects
The corrosion effects on RC members due to carbonation are considered through the steel area reduction of the longitudinal and transversal reinforcements. Carbonation phenomenon occurs when carbon dioxide, widely present in the air, penetrates into the concrete and starts reacting with hydroxides, such as calcium hydroxide, generating carbonates. This chemical reaction reduces the pH value of the pore solution below 8.5, resulting in an unstable passive film protecting the surface of steel
Analysis method
Simplified procedures based on non-linear static pushover analyses may represent an effective and practical tool for the seismic assessment and retrofitting of existing RC structures [34], [35]. On the other hand, as already explained, multi-span bridges with frame piers and elastomeric bearings do not present a predominant vibration mode. The use of standard pushover analysis can lead to an overestimation of the structure capacity because such an approach neglects the effects of the higher
Application to two case studies
The previously described procedure has been applied to two motorway bridges, characterized by frame piers, located in Northern Italy and built between the 1960s and 1970s. The FE models of the considered bridges are presented in Fig. 6, while Table 3 reports the soil type and peak ground acceleration (PGA) considered for the seismic vulnerability assessment: the site parameters are determined according to [25] for the different sites where the bridges are located.
Table 4 reports the main
Conclusions
This paper has presented an efficient procedure for a preliminary estimate of the collapse mechanism of multi-span RC bridges exposed to corrosion effects due to carbonation and subjected to horizontal loads. The procedure was applied to two RC motorway bridges characterized by frame piers and located in two different zones of Northern Italy. In order to reduce the computational effort, simplified FE models of the bridges under study were implemented through beam elements with appropriate
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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2022, Developments in the Built EnvironmentCitation Excerpt :Highly corroded bridge piers can be subjected to non-uniform corrosion with corrosion pit which was not considered in the simplified FEM model. Rebar corrosion significantly affects the structural behavior of RC structures (Berto et al., 2009; Crespi et al., 2020; Crespi et al., 2022; G.M. et al., 2018) and it is important to consider mechanical damages caused by corrosion products and related cracking in the surrounding concrete (Coronelli and Gambarova, 2004). In the simplified FEM model, corrosion substance and corrosion induced cracking were considered when the corrosion ratio is more than 5%.