Stergios A. Mitoulis

A parametric study on the axial behaviour of elastomeric isolators in multi-span bridges subjected to horizontal seismic excitations

Abstract 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.

A knowledge-based software for the preliminary design of seismically isolated bridges

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.

Uplift of elastomeric bearings in isolated bridges subjected to longitudinal seismic excitations

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.

Seismic response of bridge abutments on surface foundation subjected to collision forces

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.

EVALUATION OF THE STIFFNESS AND DAMPING OF ABUTMENTS TO EXTEND DIRECT DISPLACEMENT-BASED DESIGN TO THE DESIGN OF INTEGRAL BRIDGES

Abstract. There is an urgent need for maintenance-free transportation infrastructures worldwide. Integral Abutment Bridges (IABs) are robust structures, without bearings or expansion joints that require zero or minimum maintenance. The challenge to the assessment of existing and the application of IABs is the dynamic interaction between the bridge and the backfill soil. In many cases, this interaction is misinterpreted due to the inherent non-linear behavior of the soil during the so-called in-service interaction, which modifies drastically the stresses within the backfill soil under daily displacements of the abutment. A step towards the better understanding of the seismic response of IABs is the evaluation of the resistance of the abutment, which depends upon the geometry of the abutment, the properties of the soil and the successive interactions, i.e. quasi-static, under thermal expansion and contraction of the deck, or dynamic, when the bridge is subjected to seismic excitations and/or breaking loads. Towards this end, this paper attempts to: (a) interpret the condition of the abutment and the backfill soil at the onset of the dynamic excitation based upon the antecedent in-service interaction of the components and (b) to evaluate the stiffness and the damping properties of existing and/or representative integral abutments under dynamic loads to extend the Direct Displacement-Based Design (DDBD) to the design of integral bridges. A typical geometry of the integral abutment and typical backfill soil is investigated based on 2D fully coupled FE simulations adopting a visco-elasto-plastic stress model for the soil (coupled approach) under static and dynamic loads.

Shaking table study of the seismic interaction of an isolated bridge deck with the abutment utilizing small-scale models and numerical simulations

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 reasonable accuracy for the prototype bridge.

A new scheme for the seismic retrofit of multi-span simply supported bridges

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.

Connection of Bridges with Neighborhooding Tunnels

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.

Seismic Performance of Novel Resilient Hinges for Columns and Application on Irregular Bridges

AbstractBridges 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 ef...

Preliminary Design of Seismically Isolated R/C Bridges—Features of Relevant Expert System and Experimental Testing of Elastomeric Bearings

Seismically isolated R/C bridges is the target of an expert system based on the current design provisions of Eurocode 8, Part 2, which aims to facilitate their preliminary design. The expert system and the developed software includes a series of checks provided by Eurocode 8 (Part 2), in order to ensure the satisfactory seismic “optimum” performance of the selected isolation scheme. In doing so, the software accesses a specially created database of the geometrical and mechanical characteristics of commercially available cylindrical or prismatic elastomeric bearings, that can be easily enriched by relevant data from laboratory tests on isolation devices. The basic assumption included in the software is modeling the seismic response of an isolated bridge as a S.D.O.F. system. The features of this expert system are presented and discussed. Moreover, results from a number of tests are also included, indicative of the quality control procedure, specified by International Standard ISO 22762-1 (2005).