EXTENDING THE SERVICE LIFE AND CURRENT REPAIR PERIOD BY INCREASING THE EARTHQUAKE STABILITY OF MULTI-SPAN BRIDGES

ABSTRACT

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 (cost-effectiveness) is mostly affected by the basic concept of their design, namely the selection of the optimum structural system for specific conditions and requirements. The present study focuses on the cost-effectiveness of four different bridge systems: (a) a fully-integral bridge with an innovative full-height integral abutment, (b) a quasi-integral bridge, which was the “reference” system, (c) a semi-integral bridge, and (d) a “floating deck” bridge, whose deck is supported on piers through bearings. The accommodation of both the in-service and the seismic requirements of bridge systems were studied with the objective of minimizing structural and final costs.

АННОТАЦИЯ

Проектирование структурных систем зависит от широкого спектра критериев соответствия. Если говорить более конкретно о сейсмостойких мостах, то вопрос экономичности (рентабельности) в основном зависит от базовой концепции их проектирования, а именно выбором оптимальной конструктивной системы для конкретных условий и требований. В данном исследовании основное внимание уделяется экономической эффективности четырех различных мостовых систем: (а) полностью цельный мост с инновационной полновысотной интегральной опорой, (b) квазиинтегрированный мост, который являлся «эталонной» системой, (c) полуинтегрированный мост и (d) мост с «плавающим настилом» палуба которого поддерживается на стойках через подшипники. С целью минимизации конструктивных и конечных затрат была изучена совместимость как эксплуатационных, так и сейсмических требований к мостовым системам.

Keywords: conceptual design; structural cost; semi-integral; earthquake resistance; serviceability; maintenance.

Ключевые слова: концептуальное проектирование; стоимость конструкции; полуинтегральный; сейсмостойкость; эксплуатационная пригодность; техническое обслуживание.

Introduction.

The design of bridges is mainly governed by their prevailing function, which is their ability to withstand traffic loads. The selection of the optimum structural system is influenced by site-specific conditions, traffic requirements and structural methods, [1]. However, the seismic response of bridges is also often proven to be decisive for the resisting system and critical for individual members, which are intensely stressed, during earthquake.

It is well known that the design of bridges, as that of structures, should comply with specific serviceability, as well as structural requirements imposed by the implemented structural method. Further to these, the basic design parameters are:

• safety, including earthquake efficiency;
• serviceability;
• economy (of construction and maintenance);
• aesthetics.

The criteria referring to safety and serviceability are determined by provisions set out in codes, [2], [3], which constitute the current state-of-practice of bridges. The cost assessment for a particular bridge can be carried out on both the basis of experience and according to the studies of similar design cases. In any case, cost depends on the basic concept of the design, namely the resisting system that has been selected. The basic concept of design not only includes the general idea of the total structure, but also the idea of the structural elements and the configuration of structural details. In the light of this, the basic concept of design is the most important, the most interesting and also the most creative part of the work of the Civil Engineer. An apt basic concept minimizes difficulties in the design study and also in the construction. Considering that it is possible nowadays to calculate and implement almost any concept, there are many people wrongly assume that the more details that are examined, the more complete that design will be, even if the concept is weak.

However, apart from the successful concept, the details should also be considered, since taking them in isolation and comparing them with the overall result gives the impression of insignificance. Nevertheless, dealing correctly with most of the details is not a detail but rather can lead to perfection, according to Michelangelo. The selection of the cross-sections of concrete structural elements according to the economy criterion is also included among the details, on the condition that this selection satisfies the aforementioned safety requirements [3].

The fact that the local economy does not always serve the total economy of construction due to the complex interaction between structural members under seismic loading should not be disregarded, i.e. any increase in the dimensions of the cross-section of the piers for the sake of local economy generally relieves the expansion joints and the bearings. On the other hand, the resulting decrease in the fundamental period of the system leads to an increase in seismic actions, while at the same time reduces the value of the shear span ratio of the pier (αs), which in turn reduces the value of the behavior factor q, [3], and, therefore, in another way increases the magnitude of seismic actions.

The resistant system of a bridge includes the deck, the piers and the abutments with their foundations, and the accessories (bearings and joints). It is known that, from the elements of the structural system, the deck, even in the case of a monolithical connection with the piers, does not seem to be adversely affected by earthquake. The predominant criteria for the design of the deck result from serviceability requirements; hence the seismic combination of actions is not critical loading, even for the bottom reinforcement of its supports in areas of high seismicity. Interest in the present report focuses on the design of abutments, piers and their foundations, bearings, expansion joints and backfills, in cases where the embankments participate during earthquake. An extensive study considering the minimum cost for structural members of earthquake resistant bridges is given in [4]. At this point, it can be noted that the structural method, which is implemented for bridge construction, is cost-effective.

During the last decade, the state-of-the-art and practice for R/C bridges worldwide is related to the construction of systems that are as monolithical as possible, [5], [6], as well as to the development of new technology bearings and damping devices, i.e. viscous dampers, which are interjected between the deck and the piers or the abutment, [7]. The parallel progress in these two fields of modern, earthquake-resistant Bridge Engineering, although they may seem rather competitive, in fact leads to design where both monolithical connections and seismic isolation are used in a complementary way. An example of such a dual approach is the case of bridge systems whose deck is continuous and connected to the central piers in a monolithical way while supported on the other end-piers through high damping bearings, offering both relative economy and efficiency.

In integral bridges, the in-service distress of the piers and abutments can be adequately arranged by checking the in-service cracking of those elements according to the code’s provisions. However, the monolithical connection of the deck with the end-piers and the abutments is not always possible in bridges of great total length. A movable stub-type abutment, which consists of a short wall-like abutment-pile cap and a pile row of H-steel piles, provides a structural solution which can arrange the in-service constraints. In addition semi-integral hinged abutments, which consist of two segments interconnected by a dowel, are suggested for longer integral bridges, [8]. In Europe, fullheight abutments are usually implemented, [9], whose wall-like abutment is often pinned on a spread foundation, [10]. Apart from the aforementioned in-service and durability problems of integral abutments, the in-service and the seismic response of the approach embankments also constitutes a current field of study. The wedging of soil behind the abutment results in the long-term build-up of soil pressures, namely the “ratcheting effect”, [11], [12]. Horvath has proposed a combination of structural techniques in order to minimize the in-service distress of the abutments and its approach fills: (a) on the one hand, the approach fills are reinforced by geosynthetic tensile reinforcement to create a mechanically-stabilized-earth (MSE), and (b) on the other hand, a compressible inclusion EPS is interjected between the abutment and the MSE, which plays the role of the desired “expansion joint”. The response of the aforementioned systems, which consists of the reinforced backfill and the compressible inclusion (EPS), was experimentally tested in a full-height abutment by Potzl and Naumann, [13]. The aforementioned experimental study is considered to be of great importance as the configuration of the backfill’s reinforcement as well as the distribution of the earth pressures behind the abutment is investigated. The experiments, conducted in Germany, also prove the recent interest in the construction of integral bridges in Europe.

On the other hand there are “floating deck” bridges, whose deck is supported on piers through bearings, which, due to their serviceability advantages, make them the most commonly-implemented structure in bridge engineering. The bridge designer can choose [14], [15] between: (a) full base isolation, which leads to a significant increase in the period and, as a result, in the deck displacements, or (b) the use of high damping bearings or the simultaneous use of bearings and viscous dampers. Usually, the transverse movement of the isolated deck is restrained by stoppers, in which capacity design is applied.

The present study investigates the two extreme cases: (a) the fully monolithical, namely the integral bridge, which enhances seismic resistance, and (b) the “floating deck” bridge, which favours serviceability. Two more intermediate cases were also investigated, one with sliding bearings and one with low damping rubber bearings. The later two cases were considered to be the compromise between the extreme cases described above. The four different bridge configurations are extensively discussed below, as far as their cost-effectiveness and earthquake resistance are concerned. Serviceability and maintenance are also addressed.

Description and modeling of designed alternative bridges.

The “reference” bridge, Fig. 1(a), has total length of L=34.0+4x43.0+34.0m=240m. The deck of the bridge is continuous and connected to the piers in a monolithical way and is supported on the abutments on sliding bearings. The transversal movement of the deck is restrained by stoppers. The box girder superstructure, Fig.1(e), has a total transverse width of B=13.5m. The piers are wall-like columns, Fig. 1(f), and their cross sections are rounded for the sake of aesthetics. The bridge is founded on Ground Type B and the PGA (ag=0.16g) corresponds to the Eurocode’s Seismic Zone II. The importance factor adopted is equal to γΙ=1.30. The behavior factors adopted were equal to qx=3.5 and qy=2.7 -due to the lower value of the transverse shear ratio αs of the piers- for the longitudinal and for the transverse direction correspondingly. The “reference bridge”, namely the quasi-integral bridge, can be characterized as an intermediate case between the integral and the “floating deck” bridge.

In the present investigation, three more alternative bridge systems were examined aiming at determining the cost-effectiveness and earthquake resistance of these systems as compared with the “reference” bridge. The four bridges, including the “reference” case, Fig.1(a), are shown in Fig.1(b),(c) and (d). All bridge systems had the same geometry -cross section of the deck, Fig.1(e), and pier, Fig.1(f), and the same pier heights. The only differences in the alternative bridges in Fig.1(b),(c) and (d) resulted from the re-design of the systems, which conserve the seismic performance level of the “reference” bridge. As far as concerns the philosophy of the bridge alternatives in Fig.1 selection it can noted that: (i) The alternative of Fig.1(b) represents “floating deck” bridges, which are the most commonly-implemented structure in bridge engineering, aims at the minimization of the in-service distress of the piers and the substructure. Structural methods of precast and incremental launching result in “floating deck” bridges. Semi-integral bridges, like the one illustrated in Fig.1(c), are often selected in the case of multi-span earthquake resistant bridges in which the prestress of the deck induces restraints of the monolithical connections. Finally, the fully-integral system of Fig.1(d) constitutes, as mentioned in the introduction, the current state-ofthe-art in bridge engineering.

The “floating deck” of the bridge, Fig.1(b), was supported on each pier through two bearings, whose dimensions were 700x600x75(45). The selection of the bearings was itself a case study of design, as 12 different isolated bridges were analysed. It is noted that the scaling of the bearings height did not result in the expected distribution of bearings -allocation of small height bearings on central piers and of bearings with a large thickness of rubber on end-piers which are mainly distressed in-service. The above note is attributed to the irregularity in the height of the piers.

The semi-integral bridge of Fig.1(c) is a compromise of serviceability and earthquake resistance requirements. Such bridge systems were implemented in Egnatia Motorway (Krystallopigi and Greveniotikos bridge). The monolithical connection of the deck with the central piers allows the use of a q-factor greater than 1.0 and the end-piers and/or the short piers are protected against in-service constraint movements of the deck, as the latter is supported on them through laminated elastomeric or sliding bearings. In the present investigation, the resultant semi-integral system has six spans and the deck is supported on piers P1 and P4 through laminated elastomeric bearings, Fig.1(c). It is noted that for the longitudinal design earthquake, the piers were considered to dissipate part of the induced seismic energy through hysteretic behavior namely qx=3.5, whereas qy=2.7 was used for the transverse direction.

In Fig.1(d), the longitudinal section of the fully-integral bridge is illustrated. In the resultant integral system all the piers and the abutments are connected to the deck in a monolithical way. The integral full-height abutment, with shallow or deep foundation, has been studied in previous investigations, [16], [17], as far as its earthquake efficiency is concerned. The height of the abutment is habut.=10m and the width is also wabut.=10m. The small thickness of its web and its appropriate reinforcement ensure that the abutment responds in an elastic manner in-service, while the expansion joint provided between the web and the stiff wing-walls prevents their undesirable participation inservice. The layer of EPS-geofoam behind the abutments web and the PVC reinforcement of the backfill soil, which constitutes a self-supported fill, namely mechanically-stabilized-earth, separate in-service the abutment from the backfill while the seismic participation of the system abutmentbackfill is provided by the proper selection of the thickness of the EPS. The backfill’s reinforcement fundamentals and techniques can be found in current codes, [18], [19], or even in state-of-the-art reports, [20]. The behavior factor of the fully-integral bridge was used qx=3.5 for the longitudinal design earthquake, whereas the low transverse shear span ratio of the abutment αsy=1, leads to the inevitable qy=1.

In Fig.2(a),(b),(c) and (d), the stick models of the alternative bridge systems are illustrated. These models correspond to the bridge systems presented in Fig.1(a),(b),(c) and (d). In Fig.2(a), the model of the “reference” bridge as well as the translation and rotation stiffness of the piers’ deep foundation is shown. The same stiffness values were used as in the other bridge models. In Fig.2(b), the model of the “floating deck” bridge as well as the translation and rotation stiffness of the couples of the 700x600x75(45) bearings are given. The stiffness values of the bearings were determined according to [14]. Similarly, in Fig.2(c), the model of the semi-integral bridge as well as the stiffness of the selected bearings are presented. Finally, Fig.2(d) illustrates the model of the fully-integral bridge, in which the backfill participates passively during earthquake. The backfills’ resistance was modeled by linear spring elements -one spring per meter of the abutments height. The stiffness of the linear springs took into account: (a) the total unilateral passive resistance of the backfill soil according to CalTrans, [21], [22], and (b) the thickness of the EPS layer. The modeling of such a strongly non-linear response was achieved through successive approximation analysis aiming at a target displacement of the abutment which should be equal to the pre-assumed deformation of the EPS-backfill. The stiffness of the backfill is determined by the tangent stiffness of the backfill, which is defined by the aforementioned target displacement. It is noted that the increased stiffness of the reinforced backfill was not taken into account, which is a conservative design selection.

All the re-designed bridge systems were analyzed with the FE commercial code SAP 2000, [22], and modal response spectrum analysis was implemented.

Results.

The present study investigated and re-designed four different bridge systems, including the “reference” quasi-integral bridge. The aim of the study was to determine the structural cost alteration while the seismic performance of bridges was the same according to the provisions of the current earthquake resistant codes, [2], [3]. The serviceability requirements of the structural elements and the deck were adequately arranged. In Fig.3, the increases and decreases in the cost of each re-designed structural member as well as the total increase or reduction in the structural cost are given. From this figure it can be deduced that the structural cost for the “floating deck” bridge minimizes as the reduction for this design alternative is 4.8% less than the “reference” case. The cost is increases as the monolithical connections increase.

Conclusions.

In the present study, four different bridge design alternatives were investigated as far as concerns their cost-effectiveness concerns. The bridge alternatives offer the same level of seismic performance, according to the provisions of the current European earthquake resistance codes. The study included bridge systems whose selection followed an escalation of monolithical connections. More specifically: (a) a “floating deck” bridge, (b) a semi-integral bridge, (c) a quasi-integral “reference” bridge and (d) a fully-integral bridge were designed and evaluated for costeffectiveness. The investigation took into account the serviceability requirements of the resultant bridge systems as well as the case-dependent design needs. The investigation reached the following conclusions:

Figure 1. Longitudinal sections of the bridge systems compared due to their cost-effectiveness: (a) the “reference” real bridge located in the Arahthos-Peristeri region, (b) the “floating deck” bridge, (c) the semi-integral bridge, (d) the fully-integral bridge and (e & f )cross sections of the deck and the pier

Figure 2. The model of: (a) the reference bridge, (b) the “floating deck” bridge, (c) the intermediate robust bridge and (d) the integral bridge.

Figure 3. The increases and decreases in the cost of each re-designed structural member and the total increase or reduction in the structural cost in € (euro).

A strong influence of the in-service distress of the end-piers is observed. The constraint movements of the fully-integral and the quasi-integral bridge piers proved to be the critical design parameter, which is also reflected in the foundation design [23].

The “floating deck” bridge alternative seems to be competitive to the structural cost of integral systems. However, the high maintenance cost of such bridge systems is underlined as an inevitable disadvantage of the “floating deck” bridge [24].

The conclusions of the present investigation bring to light the problem of the conceptual design of multi-span bridges and, by extension, their cost-effectiveness and earthquake resistance. However, the conclusions drawn cannot be generalized, i.e. in the case of major geometric bridge alteration concerning the total number of spans and/or the heights of the end-piers.

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