On the collapse evaluation of existing RC bridges exposed to corrosion under horizontal loads

Highlights

• An efficient procedure for the collapse mechanism evaluation of existing RC bridges under horizontal loads is presented.

• Three different scenarios of corrosion levels are considered.

• A multimodal pushover analysis approach is adopted.

• The results obtained for the different corrosion levels are discussed considering brittle and ductile failure mechanisms.

Abstract

The structural safety and durability of existing reinforced concrete bridges can be considered as one of the current most important research topics in structural engineering, especially after the numerous collapses that have occurred in recent years. Considering the Italian context, it is important to highlight that most of these structures were built in the 1960s and 1970s and, consequently, the effects of materials degradation phenomena have reduced their vertical and horizontal load-bearing capacity.

This paper presents an efficient procedure for the collapse mechanism evaluation of existing reinforced concrete motorway bridges under horizontal loads, considering the corrosion effects due to carbonation through a simplified model that takes into account the steel rebars reduction and applying a multi-modal pushover analysis approach. In this study, three different (slight, moderate and high) corrosion scenarios are considered for two existing reinforced concrete bridges characterized by frame piers. The results of the numerical analyses, which consider both brittle and ductile failure mechanisms, highlight a significant decrease of the seismic capacity of the structures as the corrosion level increases, especially for brittle collapse mechanisms that are strongly affected by the bridge maintenance condition. Such a procedure allows identifying the first structural elements reaching the collapse, providing useful information for the maintenance of existing bridges in terms of repair and strengthening interventions.

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.