Some open issues in the seismic design of bridges to Eurocode 8-2

This paper summarises the ongoing research on the seismic design of isolated and integral bridges at the University of Surrey. The first part of the paper focuses on the tensile stresses of elastomeric bearings that might be developed under seismic excitations, due to the rotations of the pier cap. The problem is described analytically and a multi-level performance criterion is proposed to limit the tensile stresses on the isolators. The second part of the paper sheds light on the response of integral bridges and the interaction with the backfill soil. A method for the estimation of the equivalent damping ratio of short-span integral bridges is presented to enable the seismic design of short period bridges based on Eurocode 8-2. For long-span integral bridges, a novel isolation scheme is proposed for the abutment. The isolator is a compressible inclusion comprises tyre derived aggregates (TDA) and is placed between the abutment and a mechanically stabilised backfill. The analysis of the isolated abutment showed that the compressible inclusion achieves to decouple the response of the bridge from the backfill. The analyses showed that both the pressures on the abutment and the settlements of the backfill soil were significantly reduced under the thermal and the seismic movements of the abutment. Thus, the proposed decoupling of the bridge from the abutment enables designs of long-span integral bridges based on ductility and reduces both construction and maintenance costs

Challenges and opportunities for the application of integral abutment bridges in earthquake-prone areas: A review

Integral Abutment Bridges (IABs) are robust structures without joints and bearings, hence they are less vulnerable to natural and manmade hazards, whilst they require minimal maintenance throughout their lifespan. As a result of these engineering advantages, IABs are appealing to road and railway agencies and consultants. Despite their advantages, IAB design and construction is challenging and the main barriers for extensive use of IABs originate from the interaction between the abutment and the backfill soil. This interaction causes permanent deflections of the backfill soil and enhanced soil pressures on the abutment of passive nature. Under strong earthquake excitations, the response of IABs is strongly affected by the aforementioned interaction. Surprisingly, no agreement has been reached to date in the international literature as to whether this is a beneficial or a detrimental effect. The reasons for acknowledged disagreements in the literature indicates a conceptual gap in IAB design and assessment and it, therefore, requires further investigation.To the best of the author's knowledge, the significance of this interaction in earthquake resistant IABs is dependent on a number of factors, such as the type and intensity of the earthquake, the type, length and condition of the bridge after a number of years of service, the type and height of the abutment, the bridge dynamic characteristics, e.g. stiffness, damping, mass and the type of the backfill soil, among others. Many of these competing and clashing, factors lead to worse or better IAB responses, and this depends on the additional inertia mass of the backfill soil, the additional input motion exerted on the bridge from dual paths e.g. the foundation of the abutment and the backfill soil, the dissipation capacity and stiffness of the abutment and backfill soil. With the aim of better understanding the seismic response of IABs and opine with regard to the importance of the abutment and backfill soil on the seismic response of IABs, a comprehensive state of the art review is conducted in this paper. The review includes all the aspects relevant to the IAB-backfill interaction, with emphasis on IABs subjected to earthquake excitations. The research-based evidence provided here postulates a very complex interaction effect, which may have a positive or negative effect on IAB seismic responses. The evidence gathered also suggests a minimal understanding of the potential benefits of the IAB-backfill interaction, yet a reasonable understanding of the aggravated seismic response due to the same interaction in other instances. The paper includes literature-based evidence and inferences on IAB seismic designs and concludes with the results of an extended numerical study, which was conducted to provide further evidence with regard to the effect of the bridge-backfill interaction on the seismic response and design of IABs. A representative IAB was utilised as the base model and a comprehensive parametric study was conducted varying the abutment type and height, the bridge length and the backfill soil properties. The results are evidence that, indeed, the backfill soil predominantly benefits the bridge as it reduces its bending moments and pier drifts, and which potentially can lead to more economic designs. However, the IAB-backfill interaction is strongly case-dependent and therefore meticulous and detailed modelling of the backfill soil is believed to be important to avoid underestimation of bridge stress resultants and consequent under-designs.•Comprehensive state-of-the-art-review on the seismic response of Integral Abutment Bridges (IABs).•Challenges in earthquake-resistant IABs are discussed with the view of promoting innovation in IAB design and construction.•Opinion, based on new findings, on the benefits and detriments of the bridge-backfill interaction in IABs.•For the analysed IABs, the dynamic bridge-backfill interaction reduced, in most cases, the pier bending moments and drifts.•The backfill soil seems to increase significantly the mass in shorter IABs and also increase the stiffness in longer IABs

Editorial

Bridge engineering research and practice continuously pursue better understanding of engineering phenomena and delivery of innovative applications, pushing the boundaries of structural engineering. Among the topics that are included in bridge engineering activities, and that also characterise the topics covered by this Journal, there are several aspects related to innovation and research in the field. These can be interpreted both through theoretical research approaches and through the practical exemplification of innovative and challenging applications. This edition of Bridge Engineering actually reflects this dichotomy, and in fact the reader finds a first group of papers characterised by a greater abstractionism. The first paper is an experimental endeavour backed by a 5-year experimental programme and validates experimentally numerical models used for estimating long-term deflections of reinforced concrete cantilevers. This laboratory study with significant practical applicability, also assesses the influence of heterogeneity on the long-term deflections of reinforced concrete components and aims at alleviating these deflections by making use of compression reinforcement and low-shrinkage concrete (ID38 – Sousa et al. 2020). The second paper studies specific models for the preliminary evaluation of the effect of cables in the stiffness of bridges supported by cables (ID28 – Tan et al. 2020). Equations are developed for preliminary estimates of longitudinal displacement in suspension bridges and the cable-spring effect and its longitudinal restraint stiffness on towers is quantified. Research papers in this edition also include a novel approach to improving the estimation of the local scour depth at bridge piers due to accumulated debris (ID45 – Ebrahimi et al. 2020). The method computes a debris factor based on the dimensions the debris blockage. The method takes one step ahead by applying the methodto a full-scale bridge in the UK that is suspected to have failed as a result of debris-induced scour. On the more practical side of this edition there are contributions describing the most recent applications in the field of bridge engineering, which can represent examples in the field and challenges from which the reader can draw points of interest for new and future applicative developments. This peculiar aspect essentially characterises this journal and may probably be of special interest to professional engineers, which in their activities they have to cope with the implementation of creative and sometimes revolutionary solutions. Some examples are also given in this edition of the Journal, in particular the launch philosophy of the west flyover at Stockley airport junction (ID15 – Beavor et al. 2020), which includes the longest single-span constructed on the Great Western railway since Brunel’s Saltash Bridge in 1859. Between these two opposite but connected poles, in bridge engineering, intermediate studies can be found. For example, the preliminary study of special design works, such as the integral bridge concept herein presented for the third runway at Heathrow airport (ID44 – Sandberg et al. 2020), shown in Figure 1 below. At over 140 m in the total length, the adoption of an integral bridge of this length is in excess of most integral bridges in the UK. This paper presents a comprehensive SSI study to understand the behaviour of this long integral bridge during thermal deck movements

Risk and resilience of bridgeworks exposed to hydraulic hazards

Transportation infrastructure is a pylon for the society and economy, enabling the services and transportation of goods, under normal and emergency circumstances. Bridgeworks act as bottlenecks within road and rail networks and their failures due to e.g. floods, cause disproportionate losses, which are expected to be exacerbated due to climate change. Thus, pinpointing the vulnerabilities and quantifying the resilience of bridges within transportation networks exposed to hydraulic hazards is of paramount importance. However, reliable quantification of risk and resilience of flood-critical bridges is not yet available, as there is a lot of engineering guesswork for qualitative assessments. This paper describes a new integrated framework for the resilience assessment of bridgeworks and networks subjected to hydraulic hazards such as scour, debris flow and hydraulic actions. An overview of the critical hydraulic hazards, and the evaluation of their intensity measures based on regional and site-specific approaches is provided in the paper. The framework also includes vulnerability models for bridges for the evaluation of direct losses, i.e. physical damage, as a means to deliver the quantitative risk assessment (QRA) of the exposed bridgeworks and networks. The second component of the resilience framework is the restoration and reinstatement models, which are expressed by practical restoration times and tasks. Toward this end, this paper summarises an on-going comprehensive survey, which aims to elicit knowledge from experts, in an effort to develop restoration models for bridges exposed to floods. The framework is a useful tool for allocating the resources reasonably toward efficient management and consequence analysis on a network level

Extending the application of integral frame abutment bridges in earthquake prone areas by using novel isolators of recycled materials

Integral Abutment Bridges (IABs) are jointless structures without bearings or expansion joints, which require minimum or zero maintenance. The barrier to the application of longspan IABs is the interaction of the abutment with the backfill soil during the thermal expansion and contraction of the bridge deck, i.e. serviceability, or when the bridge is subjected to dynamic loads, such as earthquakes. The interaction of the bridge with the backfill leads to settlements and ratcheting of the soil behind the abutment and, as a result, the soil pressures acting on the abutment build-up in the long-term. This paper provides a solution for the aforementioned challenges, by introducing a novel isolator that is a compressible inclusion (CI) of reused tyre derived aggregates (TDA) placed between the bridge abutment and the backfill. The compressibility of typical tyre derived aggregates was measured by laboratory tests and the compressible inclusion was designed accordingly. The CI was then applied to a typical integral frame abutment model, which was subjected to static and dynamic loads representing in-service and seismic loads correspondingly. The response of both the conventional and the isolated abutment was assessed based on the settlements of the backfill, the soil pressures and the actions of the abutment. The study of the isolated abutment showed that the achieved decoupling of the abutment from the backfill soil results in significant reductions of the settlements of the backfill and of the pressures acting on the abutment. Hence, the proposed research can be of use for extending the length limits of integral frame bridges subjected to earthquake excitation

Preliminary design of seismically isolated R/C highway overpasses- Features of relevant software and experimental testing of elastomeric bearings

The preliminary design of seismically isolated R/C highway overpasses is the tar-get of a software based on the current design provisions of Eurocode 8 (Part 2) as well as on engineering decisions included in the expert system. The features of this expert system, which is aimed to facilitate the design of a highway overpass by isolating its deck with the inclusion of elastomeric bearings, are presented and discussed. For such an upgrade scheme a number of successive checks is necessary in order to select an optimum geometry of the bearings. The developed software includes a series of checks provided by Eurocode 8 (Part 2), in order to ensure the satisfactory seismic performance of the selected upgrade scheme. In doing so, the software accesses a specially created database of the geometrical and mechanical character-istics of either cylindrical or prismatic elastometallic bearings which are commercially avail-able; this database can be easily enriched by relevant data from laboratory tests on isolation devices. The basic assumptions included in the software are (a) modeling the seismic re-sponse of the bridge overpass as a SDOF system, and (b) only the longitudinal direction re-sponse is considered; it is common practice for seismically isolated bridge systems to restrain the transverse movement of the deck by stoppers. Moreover, the results form a number of tests performed in the Laboratory of Strength of Materials and Structures of Aristotle Univer-sity, verified the quality of the production process of a local producer of elastomeric bearings subjecting production samples to the sequence of tests specified by International Standard ISO 22762-1 (2005). Strain amplitudes larger than 250% resulted in the debonding of the elastomer from the steel plating. Artificial aging resulted in a small increase of the axial (ver-tical) stiffness and a small decrease of the shear (horizontal) stiffness of the tested bearings. More specimens must be tested to validate further these findings

Multiple hazard fragility analysis for granular highway embankments: moisture ingress and scour

Fragility functions express the probability that an asset exceeds some serviceability or limit state for a given level of environmental perturbation or other loadings, to which the asset is subjected. They are important components in the quantitative risk analysis of infrastructure exposed to natural hazards and they have typically been derived for structural assets. It is relatively difficult to derive fragility functions for geotechnical assets, such as highway or railway slopes and embankments, due to their inherent heterogeneity. In this paper, a generic granular highway embankment is modelled using the finite element method, considering various groundwater profiles and scour depths at the toe to quantify the deformation of the road surface. A probabilistic assessment of the magnitude of deformation and the groundwater level and scour depth is undertaken to derive fragility functions for the prediction of damage to assets exposed to these multiple hazards. The process of fragility function derivation is explained, uncertainty values are derived, and various regression methods are undertaken. This study is a first attempt to provide a basis for the prediction of slope deformation, and hence of damage, due to moisture ingress and scour, which can be aggravated by climate change. This can be used for the assessment of existing assets, and the design of new ones in the pursuit of more resilient transport networks, as well as for other assets such as levees, dams and other similar earthworks, with some limitations