Mitoulis SA (2014) Uplift of elastomeric bearings in isolated bridges subjected to longitudinal seismic excitations, Structure and Infrastructure Engineering
© 2014 Taylor & Francis Bearings are used to isolate bridge substructures from the lateral forces induced by creep, shrinkage and seismic displacements. They are set in one or two support lines parallel to the transverse axis of the pier cap and are typically anchored to the deck and to the pier cap. This detailing makes them susceptible to possible tensile loading. During an earthquake, the longitudinal displacements of the deck induce rotations to the pier caps about a transverse axis, which in turn cause tensile (uplift) and compressive displacements to the bearings. Tensile displacements of bearings, due to the pier rotations, have not been addressed before and questions about the severity of this uplift effect arise, because tensile loading of bearings is strongly related to elastomer cavitation and ruptures. An extended parametric study revealed that bearing uplift may occur in isolated bridges, while uplift effect is more critical for the bearings on shorter piers. Tensile displacements of bearings were found to be significantly increased when the isolators were eccentrically placed with respect to the axis of the pier and when flexible isolators were used for the isolation of the bridge. The results of this study cannot be generalised as bridge response is strongly case-dependent and the approach has limitations, which are related to the modelling approach and to the fact that emphasis was placed on the longitudinal response of bridges.
Mitoulis SA, Tegos IA (2013) Seismic retrofitting of bridges based on indirect strategies, Assessment, Upgrading and Refurbishment of Infrastructures
An indirect retrofitting scheme for bridges is analytically studied and evaluated. The scheme is based on the reduction in seismic actions of the bridge, namely the displacements of the deck and the bending moments of the piers by utilizing external key walls (barrettes) that participate in the earthquake resisting system (ERS) of the bridge as external supports. Simultaneously, the deck of the bridge is made partially continuous by replacing part of the existing sidewalks by new connecting slabs that are fixed on the existing ones. No strengthening of the existing members of the ERS of the bridge was attempted. The new sidewalk slabs respond as RC structural struts connecting the subsequent simply supported spans of the deck, while sliding on the rest of their lengths. The end spans of the deck are connected with the new key walls (barrettes) constructed behind the abutment. During the bridge service, the part of the RC struts, which are supported by the existing sidewalks, i.e. the unrestraint part of the struts, respond as concrete struts (during expansion of the deck) or ties (during the contraction of the deck). The role of these structural struts is to receive safely the deck constraint movements through their constraint shortening (struts) or lengthening (ties). During an earthquake the movements of the deck are effectively restrained by the external supports namely the key walls. Hence, the displacements of the deck and the resulting loading of the existing piers, bearings and foundations are reduced. The effectiveness of the above retrofitting scheme has been assessed on an existing bridge of Aliakmon River, actually built in the early '90s. The study revealed that this low cost retrofitting scheme can effectively reduce the seismic demand of the bridge.
Mitoulis SA, Manos GC, Tegos IA (2013) Shaking table study of the seismic interaction of an isolated bridge deck with the abutment utilizing small-scale models and numerical simulations, ECCOMAS Thematic Conference - COMPDYN 2013: 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Proceedings - An IACM Special Interest Conference pp. 1054-1063
It has been recognized that an isolated deck develops horizontal displacements of considerable amplitude during a strong earthquake. In this case the possibility of mobilizing the abutments in moderating such large amplitude horizontal response is beneficial for the safety of the structure. Thus, apart from lowering the seismic forces by the low-stiffness isolator units, the interaction between the deck and the abutments in the form of pounding for large horizontal deck response amplitudes aims at limiting through this mechanism excessive horizontal deck displacements. Such a problem was examined at the laboratory of Strength of Materials and Structures of Aristotle University using a small-scale physical representation that retains in a qualitative way the following important features: 1. A relatively stiff steel platform, representing the bridge deck, which is supported on a shaking table by two flexible supports, representing the isolator units; it is subjected to simulated horizontal earthquake motions developing large amplitude horizontal displacement response. 2. The possibility of bridge deck pounding on the abutment was introduced through a connector device that became active after the deck response exceeded a certain amplitude, introducing an initial gap within this connector. Despite the fact that these two basic response mechanisms, flexibility of isolator units and connector force-displacement characteristics, are crude small-scale representations of the actual mechanisms that are mobilized in a prototype bridge deck, the qualitative characteristics of this problems are retained. A number of simulated earthquake tests provided the necessary measured acceleration and displacement response of the model steel platform of the small-scale model and the force-displacement response of the connector and the flexible supports of the steel platform with the shaking table. This was next utilized to validate numerical simulations of this small-scale experimental representation of the bridge-deck pounding problem. By comparing the numerical predictions with the measured response of this small-scale experimental representation of the bridge-deck pounding problem it can be concluded that such numerical simulations can yield quite accurate predictions provided that the force-displacement characteristics of the isolator units as well as the force-displacement characteristics of the mechanism representing the bridge deck-abutment pounding are defined with reason
Mitoulis SA, Tegos IA, Stylianidis KC (2010) Cost-effectiveness related to the earthquake resisting system of multi-span bridges, Engineering Structures 32 (9) pp. 2658-2671
The design of structural systems depends on a wide range of compliance criteria. More specifically, with respect to earthquake resistant bridges, the issue of economy, i.e. the cost-effectiveness, is mostly influenced by the design concept, that is the selection of the optimum structural system, which is related to specific conditions and requirements. The study presents part of the results of an extended investigation program, conducted for Egnatia Odos SA, which manages the major and longest motorway in Northern Greece. The scope of the investigation program was to assess the range of cost of different bridge systems, designed to low and high seismic actions. The present paper focuses on the cost-effectiveness of four different bridge earthquake resisting systems: (a) a seismically isolated bridge, whose deck is supported on all the piers and abutments through low damping rubber bearings, (b) a "semi-integral" bridge, (c) a "quasi-integral" bridge, which was the "reference" bridge, and (d) a "fully integral" bridge, with full-height abutments, which are rigidly connected to the deck. The accommodation of both serviceability and earthquake resistance of bridge systems was studied with the objective of minimizing their structural and final costs. © 2010.
Cui L, Mitoulis S (2015) DEM analysis of green rubberised backfills towards future smart Integral Abutment Bridges (IABs), Geomechanics from Micro to Macro - Proceedings of the TC105 ISSMGE International Symposium on Geomechanics from Micro to Macro, IS-Cambridge 2014 1 pp. 583-588
Integral Abutment Bridges (IABs) are the future jointless bridgeworks supporting the "get-in, get-out stay-out" philosophy of sustainable, low maintenance and resilient bridges. The longer the bridge the more demanding on the design of the abutment and the backfill soil due to the thermal movements of the IA, as both the abutment and the soil should remain elastic to ensure the bridge rideability. A wide field of study is open to abutment and backfill design innovation, as no unified procedures are available in current codes for the design and construction of Integral Abutment Bridges (IAB). In light of this wide field of study, a green material of tyre shreds is introduced to replace the traditional soil for backfills. The use of rubberised backfilling (RubFills) is studied in the current paper to assess its ability of remediation of the serviceability problems of IABs such as the bump at the end of the bridge due to the backfill settlements and the increase in passive pressures. Discrete Element Method (DEM) was adopted to investigate the interaction between different backfill mixtures with rubber materials and the bridge abutment, which is subject to cyclic lateral movement due to temperature changes. Short term and long term performance of the RubFill and bridges were assessed on the basis of: (a) passive pressures on bridge abutment; (b) granular flows and the consequent backfill settlement. The compacted rubberised soil is found to reduce the lateral pressure (46%-53%) and bending moment (48%-56%) built up following cyclic movement of bridge abutment. © 2015 Taylor & Francis Group.
Mitoulis SA, Tegos IA (2010) Connection of bridges with neighborhooding tunnels, Journal of Earthquake Engineering 14 (3) pp. 331-350
A large number of bridges are constructed between tunnels. This co-existence can be developed in order to reduce the seismic actions of bridges, as their end parts can be restrained by the tunnels. This restrain requires the accommodation of the resulting serviceability problems, which are possible to be arranged by means of appropriate approach elements and expansion joints. In the present study, an appropriately configured approach element is proposed with which a semi-connection of the bridge with both tunnels is achieved. This approach slab is designed in a manner to accommodate both serviceability and earthquake resistance of the bridge. The proposed semi-connection of the bridge with the neighborhooding tunnels was proven to be efficient as the parametric investigation showed that the interaction of the bridge with the stiff tunnels can lead to reductions in the seismic actions of the bridge.
Mitoulis SA, Tegos IA (2011) Two new earthquake resistant integral abutments for medium to long span bridges, Structural Engineering International: Journal of the International Association for Bridge and Structural Engineering (IABSE) 21 (2) pp. 157-161
A wide field of study is open to new abutment configurations and design innovation as no unified procedure is available for the design and construction of integral abutment bridges (IABs). In this framework, an extended state-of the-art review on the configuration of IABs, with emphasis on the European Bridge Engineering, was done and two new integral abutments were studied. The primary feature of both integral abutments is the de coupling of the in-service response of the bridge from the backfill soil and the utilization of the backfill's resistance during earthquake, aiming at reducing the seismic demand on bridges. This objective was achieved by accommodating the in-service constraint movements of the deck through the flexibility of the IAB and via as small as possible clearances. During an earthquake the IABs interact with the backfill soil and reduce the displacements of the deck and thereby the seismic demand on bridge piers and foundations. Abutments will be useful in future design of intermediate to long-span bridges.
Manos GC, Mitoulis SA, Sextos AG (2011) Preliminary design of seismically isolated R/C highway overpasses- Features of relevant software and experimental testing of elastomeric bearings, ECCOMAS Thematic Conference - COMPDYN 2011: 3rd International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering: An IACM Special Interest Conference, Programme
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.
This paper investigates the potential tensile loads and buckling effects on rubber-steel laminated bearings on bridges. These isolation bearings are typically used to support the deck on the piers and the abutments and reduce the effects of seismic loads and thermal effects on bridges. When positive means of fixing of the bearings to the deck and substructures are provided using bolts, the isolators are exposed to the possibility of tensile loads that may not meet the code limits. The uplift potential is increased when the bearings are placed eccentrically with respect to the pier axis such as in multi-span simply supported bridge decks. This particular isolator configuration may also result in excessive compressive loads, leading to bearing buckling or in the attainment of other unfavourable limit states for the bearings. In this paper, an extended computer-aided study is conducted on typical isolated bridge systems with multi-span simply-supported deck spans, showing that elastomeric bearings might undergo tensile stresses or exhibit buckling effects under certain design situations. It is shown that these unfavourable conditions can be avoided with the rational design of the bearing properties and in particular of the shape factor, which is the geometrical parameter controlling the axial bearing stiffness and capacity for a given shear stiffness. Alternatively, the unfavourable conditions could be reduced by reducing the flexural stiffness of the continuity slab.
Nikitas G, Bhattacharya S, Hyodo M, Konja A, Mitoulis S (2014) Use of rubber for improving the performance of domestic buildings against seismic liquefaction, EURODYN 2014: IX INTERNATIONAL CONFERENCE ON STRUCTURAL DYNAMICS pp. 259-265 EUROPEAN ASSOC STRUCTURAL DYNAMICS
Mitoulis SA, Tegos IA (2010) An unconventional restraining system for limiting the seismic movements of isolated bridges, Engineering Structures 32 (4) pp. 1100-1112
An external restraining system with steel piles is introduced under the main objective of the study, which is the enhancement of the earthquake resistance of seismically isolated bridges. This objective is examined through the possibility of the improved seismic participation of the approach embankments, which are able to dissipate part of the induced seismic energy. The seismic participation of the embankments, which are seismically inactive, according to current conceptual design of bridges, is achieved through the extension of the continuous deck slab of the bridge onto the embankments and its restraint by the backfill through steel piles. The serviceability needs of the deck are accommodated by: (a) the flexibility of the steel piles, (b) the looseness of the backfill soil, (c) the partial replacement of the embankment's surface layers by expanded polystyrene (EPS) and (d) the in-service allowable cracking of the continuity slab. A parametric study was conducted and showed that the restraining system can effectively reduce the seismic displacements of the bridge. The proposed technique can be utilized in all bridge structures, and is more efficient in those exhibiting large displacements during an earthquake. Crown Copyright © 2009.
Tegou SD, Mitoulis SA, Tegos IA (2010) An unconventional earthquake resistant abutment with transversely directed R/C walls, Engineering Structures 32 (11) pp. 3801-3816
An analytical investigation is performed aiming at identifying the applicability and the seismic efficiency of an unconventional abutment, which restrains the seismic movements of the bridge deck. The abutment consists of the extension of the deck slab of the bridge onto transversely directed R/C walls with which the, so-called continuity slab, is monolithically connected. The restraining walls play the role of an additional horizontal and relatively flexible support of the deck of the bridge. The design of these restraining walls is based on two criteria referring to on one hand the accommodation of the in-service induced longitudinal movements of the deck and on the other hand on the earthquake loading of the walls. The walls are constructed in a concrete box-shaped substructure, which replaces the conventional wing-walls and retains the backfill material. The foundation of the abutment is checked and found to have adequate resistance against sliding and overturning. The proposed abutment was attempted to be implemented in a precast I-beam bridge. The study showed that the abutment can achieve a desirable control of the seismic movements of the deck and therefore reduces the seismic actions of the bearings, the piers and their foundation. The restraining effect of the abutment is also significant even in stiffer bridge resisting systems. © 2010 Elsevier Ltd.
Argyroudis SA, Mitoulis SA, Pitilakis KD (2013) Seismic response of bridge abutments on surface foundation subjected to collision forces, ECCOMAS Thematic Conference - COMPDYN 2013: 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Proceedings - An IACM Special Interest Conference pp. 860-878
Bridges are important components of the roadway and railway networks, as they must remain operational in the aftermath of the seismic event. Permanent movements of the backwall and the backfill soil and rotational deformations of the abutment-backfill system are well known failure modes that potentially may incite deck unseating mechanisms. However, only a few studies dealt with the modeling of deck-abutment-backfill pounding effect. In this framework, an extended parametric study was conducted on a simplified abutment-backfill analytical model. A typical seat-type abutment was analyzed using 2D nonlinear FE model in Plaxis. Simultaneously, a refined abutment-backfill model was built in commercial software SAP2000 in view to highlight significant parameters of the interaction aiming at identifying the effect of collisions on anticipated damages of the abutment. The assessment of the deckabutment-backfill response was performed on the basis of longitudinal maximum and residual movements and rotations of the abutment that may affect both the integrity and the postearthquake accessibility of the bridge. SSI effects due to the interaction of the deck with the abutment and the backfill soil were considered; analyses showed that large seismic movements during an earthquake and permanent movements of the abutment are deemed to put in danger the abutment itself, the integrity of the end spans and finally the accessibility of the bridge. Comparison of different seat-type abutment models in Plaxis and SAP2000 revealed that modeling of bridge abutments with emphasis on the geotechnical design should be properly made. Poor design assumptions may have a serious impact in the assessment of the response of the abutment-backfill-bridge system.
Bridges are important components of the transportation network that should maintain mobility and accessibility even after severe earthquakes. The current design philosophy of earthquake-resistant bridges requires the disastrous seismic energy to be dissipated in hinges that are formed in the piers, while the deck should remain essentially elastic. However, postearthquake restoration of damaged piers is challenging, time-consuming, and causes traffic disruptions. In this context, this paper proposes a novel resilient hinge (RH), that is cost-effective and has minimal damage during earthquakes. The RH is a versatile substructure that dissipates energy through the yielding of easily replaceable steel bars, thus offering rapid restoration times. It is designed to have recentering capabilities because a number of steel bars remain primarily elastic. Numerical models of single-column piers with the proposed hinge were studied and compared with conventional reinforced concrete piers to investigate the efficiency of the design. It was found that the piers with RHs exhibit a significant reduction in residual drifts when compared with the ones of the conventional piers. Application of the proposed philosophy in irregular bridge models enables a more rational and even distribution of ductility requirements along the bridge piers.
Abutments are not considered to participate strongly in the earthquake resisting system (ERS) of Eurocode-based designed bridges. However, previous studies showed that seat-type abutments can reduce effectively the seismic actions of bridges, especially when the openings at the expansion joints accommodate only the serviceability movements of the deck. Alongside, a wide field of study is open to new abutment configurations and innovation, as no unified procedure is available for their design and construction. In this framework, a new earthquake resistant abutment with high capacity wing walls is proposed and analytically investigated. The proposed abutment decouples the in-service response of the bridge from the backfill soil by small clearances at the expansion joints, which separate the deck from the abutment. During an earthquake the bridge movements are restrained by the high capacity wing walls and the backfill soil. The seismic performance of the new earthquake resistant abutment is evaluated by utilizing a benchmark bridge, whose design was based on Eurocodes, which has a relatively expensive isolation system with lead rubber bearings and dampers. Two alternative design schemes that utilized the seismic restraining effect of the proposed earthquake resistant abutment were re-designed and compared to the benchmark on the basis of seismic resistance and cost-effectiveness. The comparative results showed that the seismic participation of the proposed abutment with the backfill soil reduces effectively the seismic demand of the re-designed bridge schemes. Accordingly, the initial and the final bridge costs are effectively decreased, showing that the proposed unconventional design is a reliable scheme for future designs of bridges in earthquake-prone areas. © 2014 Elsevier Ltd.
Mitoulis SA (2013) Bridges with fixities and bearings vs isolated systems, ECCOMAS Thematic Conference - COMPDYN 2013: 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Proceedings - An IACM Special Interest Conference pp. 845-859
Seismic isolation exhibits a breakthrough in contemporary bridge engineering. The principal of isolation is to protect the bridge piers, by either reducing their seismic actions or through the increase in the damping of the structure. However, there are bridges in which the seismic loading of piers is not effectively reduced when using seismic isolation, and hence the use of expensive and expendable isolators can be avoided. The ineffectiveness of seismic isolation with typical elastomeric bearings was observed in bridges with tall piers. As such the piers can be connected with the deck through rotation-free connections, such as fixed bearings or stoppers, while their seismic loading is not significantly increased. A parametric study is conducted with alternative isolated bridge-models to identify the necessity of piers' isolation against longitudinal seismic actions. Bridge-models with bents of variable heights ranging from 5m to 30m and cross sections ranging from flexible to stiff bent-types were analyzed. All bridge-models were re-analyzed considering that shear keys placed on the piers restrict the longitudinal deck displacements. The adequacy of the piers was checked against longitudinal and transverse seismic actions. The analyses for two levels of the seismic action indicated specific bridge design cases that can utilize both rotation-free pier-to-deck fixities and bearings, while the bridge remains essentially elastic.
Kalfas KN, Mitoulis S, Katakalos K (2017) Numerical study on the response of steel-laminated elastomeric bearings subjected to variable axial loads and development of local tensile stresses, Engineering Structures
Steel-laminated elastomeric bearings are isolation devices which are used extensively in buildings, bridges, nuclear power plants and other structures. Accurate modelling of the behaviour of these devices is of great importance, as the integrity of isolated structures relies heavily on their response. For many years, steel-laminated bearings were designed based on the assumption that they are subjected to compressive and shear
loads, as a result of the dead and the horizontal loads, i.e. wind and seismic loads, acting on the structure. It is only very recently that tensile stresses in bearings were studied, as it was observed that local and global tensile stresses might be developed in bearings under seismic excitations. Most importantly, tension within the elastomer might cause local cracks or, in extreme cases, rupture of the elastomer, which might lead to the loss of support of isolated structures. Yet only a few studies exist in the international literature with regard to response of these devices under combined axial and shear loads. The aforementioned gap in the knowledge and the identified rupture of the elastomer of bearings under tensile loads during recent earthquakes comprised the motivation for this research. In this context, this paper examines the response of steel-laminated elastomeric bearings under cyclic shear and variable axial loads and aims to better understand their behaviour with emphasis
placed on the tensile stresses within the elastomer, their stiffness and dissipation capacity. Extensive numerical research was conducted with ABAQUS and the Ogden hyperelastic model was used for modelling the
elastomeric material. The analyses showed that steel-laminated elastomeric bearings exhibit local tensile
stresses, which alter significantly their stiffness and damping ratio. Most importantly, significant tensile stresses
within the elastomer were observed locally, even when the bearings were subjected to a combination of shearing
Mitoulis S (2016) Some open issues in the seismic design of bridges to Eurocode 8-2, Challenge Journal of Structural Mechanics 2 (1) pp. 7-13
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.
Mitoulis SA, Tegos IA, Stylianidis KC (2013) A new scheme for the seismic retrofit of multi-span simply supported bridges, Structure and Infrastructure Engineering 9 (7) pp. 719-732
There are two alternative strategies that a designer may adopt and combine when faced with the retrofitting of a bridge: (a) the increase in the capacity or (b) the reduction in the actions of the structure. In this article, a new scheme, based on the second strategy, is proposed for the retrofit of existing multi-span simply supported (MSSS) bridges. The reduction in the actions of the bridge was mainly achieved by utilising an external restraining system consisting of I-shaped steel piles driven in the backfill soil and a slab that is the pile-cap of the piles. The restraining system was preliminarily designed and assessed in an existing MSSS bridge system, whose deck slab was made continuous. The existing and the retrofitted bridge were analysed by means of non-linear dynamic time history analysis and their response was compared in terms of serviceability and earthquake resistance performance. The study showed that the retrofitting scheme enhanced effectively the earthquake resistance of the existing bridge. © 2013 Copyright Taylor and Francis Group, LLC.
Mitoulis SA (2012) Seismic design of bridges with the participation of seat-type abutments, Engineering Structures 44 pp. 222-233
Abutments are not only earth-retaining systems as they also participate to the earthquake resisting system (ERS) of the bridge, under certain design considerations. Current research mainly focuses on the assessment of the performance of integral abutment bridges, while only a few studies dealt with the design of bridges with seat-type abutments accounting for their seismic contribution. Along these lines, a comparative study on seat-type abutment bridges was performed. The scope of the study was to identify possible differences in their seismic response affecting significant design parameters that are the displacements of the deck and the bending moments of the piers. The study employed three real bridges of variable total lengths, openings at the expansion joints, backfill models and moderate to strong earthquake excitations. Non-linear dynamic time history analysis was performed. The study showed that the strong participation of the abutment and the backfill soil can reduce effectively the seismic demand of bridges. However, attention should be given in bridges with tall piers, whose seismic forces can be increased under certain design conditions. © 2012 Elsevier Ltd.
Manos GC, Mitoulis SA, Sextos AG (2012) A knowledge-based software for the preliminary design of seismically isolated bridges, Bulletin of Earthquake Engineering 10 (3) pp. 1029-1047
Seismic design of isolated bridges involves conceptual, preliminary and detailed structural design. However, despite the variety of commercial software currently available for the analysis and design of such systems, conceptual and preliminary design can prove to be a non-straightforward procedure because of the sensitivity of bridge response on the initial decisions made by the designer of the location, number and characteristics of the bearings placed, as well as on a series of broader criteria such as serviceability, target performance level and cost-effectiveness of the various design alternatives. Given the lack of detailed design guidelines to ensure, at this preliminary stage, compliance with the above requirements, a "trial and error" procedure is typically followed in the design office to decide on the most appropriate design scheme in the number and location of the bearing systems; the latter typically based on engineering judgment to balance performance with cost. To this end, the particular research effort aims to develop a decision-making system for the optimal preliminary design of seismically isolated bridges, assumed to respond as single degree of freedom (SDOF) systems. The proposed decision-making process is based on the current design provisions of Eurocode 8, but is complemented by additional criteria set according to expert judgment, laboratory testing and recent research findings, while using a combined cost/performance criterion to select from a database of bearings available on the international market. Software is also developed for the implementation of the system. The paper concludes with the application, and essentially the validation of the methodology and software developed through more rigorous MDOF numerical analysis for the case of a real bridge. © Springer Science+Business Media B.V. 2011.
Mitoulis SA, Tegos IA (2010) An external restraining system for the seismic retrofit of existing bridges, 9th US National and 10th Canadian Conference on Earthquake Engineering 2010, Including Papers from the 4th International Tsunami Symposium 6 pp. 5023-5032
An unconventional retrofitting measure is proposed for existing bridges, which has the ability to reduce the seismic actions. This is mainly achieved by an external restraining system consisting of IPE-steel piles driven in the existing backfill soil and a restraining slab. The slab interacts with the deck slab of the existing multi-span simply supported (MSSS) bridge system and transmits part of its seismic actions to the piles and therefore to the backfill soil, which has the ability to dissipate part of the induced seismic energy. The proposed unconventional restraining system was implemented and analytically assessed in an existing MSSS bridge system. The study showed that the unconventional restraining system has the ability to reduce effectively the actions and to enhance the earthquake resistance of the existing bridge.
Mitoulis SA, Tegos IA, Malekakis A (2013) Experimental research on the capacity of bridge shear keys, ECCOMAS Thematic Conference - COMPDYN 2013: 4th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Proceedings - An IACM Special Interest Conference pp. 1025-1039
Conceptual design of bridges has evolved rapidly during the last ten years and new, efficient and cost effective design schemes have been introduced in practice. Use of shear keys, as an active seismic link, is not prohibited in current codes. However, the major concern of having the shear keys damaged one-by-one due to their asynchronous participation still remains an open issue. As such, shear keys are typically used to prevent potential span unseating. Shear keys, also known in European literature as seismic links or stoppers, are stub RC structural elements. A capacity design procedure is provided for these elements, to safeguard the support of the deck. Design of shear keys engineering is an open issue in contemporary bridge. Current state-of-the-art deals with the efficient use of reinforcements, while practitioner engineers dealt with the seismic role of these elements and have proposed different materials for the design of stoppers and/or different reinforcement materials, since sacrificial shear keys can respond as structural fuses to limit the demand of the piers. Shear keys, whether they receive seismic actions or not-the last referring to the case in which keys are utilized to avoid the common unseating of the spans-have peculiar response and unconventional reinforcement requirements due to their loading. Simultaneously, their geometry and size is restricted due to bridge's esthetics. Hence, stoppers are relatively small and stub concrete "blocks", which are expected to receive reliably and safely large pounding forces. In this framework, two alternative reinforcement layouts with transverse hairpin bars were assessed. The efficiency of the proposed reinforcement was assessed by comparing the above rebar with the state-of-practice shear key reinforcements. The required hairpin reinforcement ratio was then evaluated through an analytical procedure that accounted for the relation between the reinforcement hairpin ratio vs the capacity of the shear keys. The procedure indicated the most appropriate reinforcement ratio for a required capacity of the stopper. The study proposes the reinforcement of the stoppers with additional diagonal rebar. Conclusions are drawn based on the analytical models and the experimental campaign.
Mitoulis SA, Tegos IA (2010) Restrain of a seismically isolated bridge by external stoppers, Bulletin of Earthquake Engineering 8 (4) pp. 973-993
The current design of seismically isolated bridges usually combines the use of bearings and stoppers, as a second line of defence. The stoppers allow the development of the in-service movements of the bridge deck, without transmitting significant loads to the piers and their foundations, while during earthquake they transmit the entire seismic action. Despite the fact that stoppers, which restrain the transverse seismic movements of the deck, are used frequently in seismically isolated bridges, the use of longitudinal stoppers is relatively rare, mainly due to the large in-service constraint movements of bridges. The present paper proposes a new type of external longitudinal stoppers, which are installed in stiff sub-structures-boundaries, aiming at limiting the bridge seismic movements. The parametric investigation, which was conducted in order to identify the seismic efficiency of the external stoppers, showed that the interaction of the bridge with the stiff boundaries can lead to significant reductions in the seismic movements of the bridge. Serviceability is appropriately arranged in the paper by expansion joints and approach slabs. © 2010 Springer Science+Business Media B.V.
Manos GC, Sextos AG, Mitoulis S, Geraki M (2010) Software for the preliminary design of seismically isolated R/C highway overpass bridges, 9th US National and 10th Canadian Conference on Earthquake Engineering 2010, Including Papers from the 4th International Tsunami Symposium 3 pp. 1756-1765
The features of an expert system, developed for the pre-design of highway overpass R/C bridges, are presented and discussed. This system is implemented into a software and is aimed to facilitate the seismic upgrading of an overpass by isolating its deck with the inclusion of elastomeric bearings. The preliminary design of such an upgrade scheme is the target of this software based on the current design provisions of Eurocode 8 (Part 2) as well as on engineering decisions included in the expert system; it can also be extended easily to comply with alternative design provisions. The developed software is connected with a database of typically used steel laminated rubber bearings and relevant laboratory test results and it performs a series of checks according to Eurocode 8, in order to ensure the satisfactory seismic performance of the selected upgrade scheme. The parameters that are addressed within this software as independent variables are: the geometry of the overpass, the number of bearings at each deck support, the level of seismic action and the characteristics of the bearings (i.e. their geometry and shear modulus). The final selection of the bearing scheme (in terms of number of bearings and bearing dimensions at each support) is based on a costbenefit criterion aiming at optimizing structural performance at minimum cost. The methodology proposed for the preliminary design of seismically isolated overpasses and the software developed were validated through more rigorous dynamic analyses employing multi-degree of freedom numerical simulations of realistic bridge overpasses.
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 excitations
Abutments are not considered to participate strongly in the earthquake resisting system (ERS) of Eurocode-based designed bridges. However, previous studies showed that seat-type abutments can reduce effectively the seismic actions of bridges, especially when the openings at the expansion joints accommodate only the serviceability movements of the deck. Alongside, a wide field of study is open to new abutment configurations and innovation, as no unified procedure is available for their design and construction. In this framework, a new earthquake resistant abutment with high capacity wing walls is proposed and analytically investigated. The proposed abutment decouples the in-service response of the bridge from the backfill soil by small clearances at the expansion joints, which separate the deck from the abutment. During an earthquake the bridge movements are restrained by the high capacity wing walls and the backfill soil. The seismic performance of the new earthquake resistant abutment is evaluated by utilizing a benchmark bridge, whose design was based on Eurocodes, which has a relatively expensive isolation system with lead rubber bearings and dampers. Two alternative design schemes that utilized the seismic restraining effect of the proposed earthquake resistant abutment were re-designed and compared to the benchmark on the basis of seismic resistance and cost-effectiveness. The comparative results showed that the seismic participation of the proposed abutment with the backfill soil reduces effectively the seismic demand of the re-designed bridge schemes. Accordingly, the initial and the final bridge costs are effectively decreased, showing that the proposed unconventional design is a reliable scheme for future designs of bridges in earthquake-prone areas.
Steel-reinforced high damping natural rubber (HDNR) bearings are widely employed in seismic isolation applications to protect structures from earthquake excitations. In multi-span simply supported bridges, the HDNR bearings are typically placed in two lines of support, eccentric with respect to the pier axis. This configuration induces a coupled horizontal-vertical response of the bearings, mainly due to the rotation of the pier caps. Although simplified and computationally efficient models are available, which neglect the coupling between the horizontal and vertical response, their accuracy has not been investigated to date.
In this paper, the dynamic behaviour and seismic response of a benchmark three-span bridge are analysed by using an advanced HDNR bearing model recently developed and capable of accounting for the coupled horizontal and vertical responses, as well as for significant features of the hysteretic shear response of these isolation devices. The results of the analyses shed light on the importance of the bearing vertical stiffness and how it modifies the seismic performance of isolated bridges. Successively, the seismic response estimates obtained by using simplified bearing models, whose use is well established and also suggested by design codes, are compared against the corresponding estimates obtained by using the advanced bearing model, to evaluate their accuracy for the current design practice.
Interest in integral abutment bridges (IABs) from the industry has increased in recent years. IABs are robust bridges without joints and bearings and hence they are durable and virtually maintenance free; moreover, the resulting cost-saving associated with their construction is significant, a fact that makes IABs appealing to agencies, contractors and consultants. However, their use in long-span bridges is limited by the complex soil?structure interaction. Thermal movements, horizontal loads and dynamic actions are transferred directly to the backfill soil, leading to settlements, ratcheting effects, high earth pressures and deterioration of the backfill soil. The longer the integral bridge the greater the challenge, as movements are increased. This paper provides an extended review of the techniques used in the international literature and in practice to alleviate the interaction between a bridge abutment and the backfill. Subsequently, the performance of an innovative isolation system for IABs using recycled tyres as a compressible inclusion is studied using detailed numerical models of a representative three-span IAB. The proposed isolation scheme was found to be an effective and sustainable method to isolate the structure from the backfill soil, reducing the pressures experienced by the abutments and the residual vertical displacements of the backfill soil.
This article investigates the response of Integral Abutment Bridges (IAB) when subjected to a sequence of seasonal thermal loading of the deck followed by ground seismic shaking in the longitudinal direction. Particular emphasis is placed on the effect of pre-seismic thermal Soil-Structure Interaction (SSI) on the seismic performance of the IAB, as well as on the ability of various backfills configurations, to minimize the unfavorable SSI effects. A series of two-dimensional numerical analyses were performed for this purpose, on a complete backfill-integral bridge-foundation soil system, subjected to seasonal cyclic thermal loading of the deck, followed by ground seismic shaking, employing ABAQUS. Various backfill configurations were investigated, including conventional dense cohesionless backfills, mechanically stabilized backfills and backfills isolated by means of compressive inclusions. The responses of the investigated configurations, in terms of backfill deformations and earth pressures, and bridge resultants and displacements, were compared with each other, as well as with relevant predictions from analyses, where the pre-seismic thermal SSI effects were neglected. The effects of pre-seismic thermal SSI on the seismic response of the coupled IAB-soil system were more evident in cases of conventional backfills, while they were almost negligible in case of IAB with mechanically stabilized backfills and isolated abutments. Along these lines, reasonable assumptions should be made in the seismic analysis of IAB with conventional sand backfills, to account for pre-seismic thermal SSI effects. On the contrary, the analysis of the SSI effects, caused by thermal and seismic loading, can be disaggregated in cases of IAB with isolated backfills.