Projecto Base de uma Ponte Ferroviária com Tabuleiro de Betão Armado Pré-esforçado Executado por Lançamento Incremental

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1 INSTITUTO SUPERIOR TÉCNICO Universidade Técnica de Lisboa Projecto Base de uma Ponte Ferroviária com Tabuleiro de Betão Armado Pré-esforçado Executado por Lançamento Incremental Preliminary Design of a Railway Bridge with a Prestressed Concrete Deck Executed by Incremental Launching Rui Pedro Carrasco Pãosinho Extended Abstract March 2011 i

2 Projecto Base de uma Ponte Ferroviária com tabuleiro em Betão Armado Pré-esforçado Executado por Lançamento Incremental Preliminary Design of a Railway Bridge with a Prestressed Concrete Deck Executed by Incremental Launching Rui Pedro Carrasco Pãosinho IST, Technical University of Lisbon, Portugal Key words: Railway Bridge, Prestressed Concrete Box Girder, Incremental Launching, High-Speed, European standards 1 Introduction This dissertation aims the preliminary design of a railway bridge, with a prestressed concrete box girder deck erected by the incremental launching method. A structural analysis of the construction and in service stages is carried out, together with the required safety verifications. 2 The base case design solution After a careful study of the site constraints, it was decided to choose a seven span deck with five 51 m typical spans, and two 37,5 m lateral spans (Fig. 1). The design solution adopted for the deck was of a prestressed concrete box girder, due to its high stiffness and relative low cost. The 12,3 m width cross section has a constant depth of 3,5 m (Fig. 2). Due to the distance between the two vertical webs of the box girder, the piers have a Y shape at the top, followed by a rectangular geometry down to the foundation. The deck is supported by the abutments with guided sliding bearings, in contrast to the fixed bearings placed between the deck and the piers. In addition, there two seismic dampers were considered between the deck and each abutment due to the high horizontal forces produced during the seismic evens. This design solution produced the best results when compared with the other possibilities of fixing the deck two one of the abutments. In this scenario the transverse forces applied on the bridge are absorbed mainly by the piers, while the longitudinal forces are distributed between the piers and by dampers to both abutments, ensuring that all of the different structural elements can safely withstand the seismic forces. The design solution and construction method are presented, taking into account the various constraints of the project and the materials adopted. The design criteria defined in the structural Eurocodes and the actions are established for both the construction stages and the in service phase. An additional special analysis was performed, evaluating the dynamic response of the deck to the passing of high-speed railway trains as defined in Eurocode 1. It is concluded that the circulation of this type of railway traffic does not represent a particular concern in terms of deck deformation and acceleration. A cost evaluation based on the general definition of the structure included in the design drawings is also presented and compared with a cost evaluation made for a steel-concrete composite deck solution proposed for the same bridge, in the frame of another dissertation. Figure 2 Deck box girder cross section Figure 1 Longitudinal layout for the preliminary design 1

3 Slenderness Ratio Environmentally protected areas where it is not convenient to place temporary structures to support the deck during construction. The implementation of the ILM has many advantages both to the Contractors and the Owners of the project, namely: Figure 3 Pier top frontal and lateral views There are two distinct geological areas, with two different load bearing capacities. On the left side of the longitudinal layout, the soil mechanical properties are quite favorable, presenting STP values of 60, ideal for shallow foundations. On the other hand, the right side of the longitudinal layout displays much lower values during the SPT test at a shallow depth, only reaching an SPT of 60 at a depth of around 25 m. Taking into account these conditions, the decision was made that the E1 abutment and the P1 through P4 piers would have strip footing foundations, while P5 and P6 piers and the E2 abutment would need pile foundations. The materials used in all of the structure are: Concrete class C40/50; Reinforcement rebar s grade A500NR; Prestressing tendons of steel grade A1670/ The Incremental Launching Method used for the Construction of Concrete Decks Taking into consideration the 30 to 35 m distance from the deck to the ground, the deck spans and cross-section, and the not very long length of the deck, the incremental launching method was an appropriate option for the construction of the deck. The incremental launching method (ILM) consists of building the bridge deck behind one or both of the abutments, and by means of hydraulic jacks, pushing it incrementally to its final position. This method is used when the valley below the deck is deeper than 25 m and the spans are between 40 m and 60 m long. It is widely used in Europe due to its competitiveness and overall quality, as well as its capacity to overcome specific constraints, such as: Very deep valleys; Deep and/or wide rivers; Steep slopes where machinery is inaccessible; Reduced construction yard areas; Reduced man-power; Minimal disturbance of the surrounding environment; Increased worker safety, since the majority of the construction is done behind the abutment, on solid ground; Greater construction speed, due to the fact that the deck and piers can be built simultaneously by different specialized teams; Light weight equipment that can be reutilized on other projects of the same method. The design of the cross section is of great importance, since the deck must be stiff enough to withstand the launching process, but at the same time as light weight as possible. The slenderness ratio (relation between the typical span and the deck depth) is a good measure of this performance. Box girder decks usually have a 20 to 25 slenderness ratio, which is the span-to-depth ratio, values that on the ILM drop to a much lower 13 to 17 ratio. Various slenderness and span lengths of concrete deck bridges using the ILM were researched, later being combined in a graph (Fig. 4). The ILM when applied to a prestressed concrete deck, consists of casting in-situ a segment of the deck, generally 50% of the length of the longest span, behind one or both of the abutments, and after the curing process is completed, pushing it forward so that another segment can be cast behind it. For box girders the casting process is generally done in two phases, the first casts the lower flange and the webs, and the second finishes by casting the top flange, this process is done on a weekly cycle. This procedure occurs because the formwork for the top flange has to be supported by the bottom flange and the cooling process of the top flange and webs happens at different paces (Fig. 5) slenderness ratio slenderness ratio (with the use of many temporary towers and/or long steel noses) Length of the longest span L (m) Figure 4 Slenderness ratios for numerous prestressed concrete incremental launched bridges. 2

4 be favorable. The adopted solution involves applying uniform compression in both flanges while guaranteeing decompression of the bending moments. The shear force resistance was evaluated with a safety factor of 1,35 to ensure adequate security. Another concern was the position of the casting joints; after being determined that each section would have half the length of the longest span, their position in the final configuration was of great importance. The solution to place the casting joint at quarter span length from the piers ensured that they would more or less be located where the bending moments were equaled zero. Figure 5 Different cooling rates of the top flange and web (Octávio Martins, 2009) A prestressed concrete deck has de advantage of being a cost effective solution when compared with other possibilities, such as a steel-concrete composite deck. On the other hand, the concrete decks higher dead load produces equally higher bending moments and shear forces during launching. To solve this problem, it is generally adopted, either a lightweight steel launching nose fitted to the cantilever end of the deck, which reduces the dead load at the front, or temporary steel piers at half span that reduce the bending moments by the same amount. 4 Actions and Design Criteria The launching of the deck also induces a great amount of friction on top of the piers, causing bending moments at the bottom sections. This event is especially important in tall piers, which due to their height, suffer on a larger scale. Additional small eccentricities of the deck vertical load can also cause large bending moments in the base due to the large dead load of the bridge deck. The in service phase was subject to all verifications in accordance with the structural Eurocodes, using the following design combinations of actions and design checks: The General Combination of Actions: - Seismic combination: ; Ultimate Limit States (ULS) utilizing the appropriate partial factors; Serviceability Limit States (SLS) a) Stress limitation through: i. Characteristic Comb ; All the actions and design criteria are in accordance with the new structural Eurocodes. Beyond the permanent loads of the self weight, super imposed loads and prestressing, the live actions considered in this study are the following: Traffic vertical loads the LM71 load model; Dynamic factor ф; Nosing Force; Actions due to traction and breaking; Actions on non-public footpaths; Geotechnical static equilibrium; Wind actions on the deck and piers; Variations of temperature, shrinkage and creep these three factors were combined in one single equivalent temperature variation; Seismic action quantified in two different directions through the use of the appropriate response spectrums. The launching phase required special attention, due to the uncommon restrictions it put on the entire structure. During launching the deck is subject to cyclic changes in both bending moment and shear force, fact that doesn t allow the use of parabolic final tendons, since their configuration would not always ii. Frequent Comb ; b) Crack control; c) Maximum concrete compression; d) Maximum vertical deflection. 5 Structural Verifications During Launching Using a launching nose with a length of 60% of the longest span (Gohler & Pearson, 2000), the structural analysis of the launching stages yielded the envelope of bending moments presented on Fig. 6. From these results, it can clearly be seen that there exists an area where the bending moments (both negative and positive) are much higher than on the rest of the deck. The area corresponds to the position over a pier, when the front of the deck is in a cantilever, right before reaching the next pier. And the same section, more or less, corresponds to the mid-span position, when only the launching nose is in cantilever. 3

5 problem, temporary prestressed stay cables composed by only 2 strands were positioned from the top of one pier to the foundation of the previous one (Fig. 8). By doing this, a bending moment was created, that counteracted the one produced by the friction generated during the launching operations. Figure 6 Deck bending moments envelope during launching [knm] In the design of the prestressing, two bending moments situations were considered; the first of the maximum bending moments (both negative and positive) and the second of the maximum bending moment excluding the area where the peak values occurred. It was quickly realized that it was unfeasible to prestress the entire deck to overcome a peak value of bending moment. The adopted solution consisted in prestressing the deck to overcome the more regular values, adding external temporary prestressed bars, on a 16 m strip, to counteract the peak values. The prestressing chosen solution is presented in Fig. 7. Figure 8 Temporary stay cable schematic for piers P4 and P5 6 In Service Structural Verifications Using the previously defined combinations, an analysis of the service phase yielded the following results: Figure 7 Cross section featuring the chosen prestressing solution during launch Coupled straight tendons were chosen, each consisting of 19 strands of 0,6``. Six tendons were applied in the bottom flange, and eight on the top flange, while twenty 50 mm diameter prestressed bars were assigned externally in the center of the top flange to overcome the additional negative bending moment near the front nose. An alternative launching solution using temporary steel piers at the middle of the spans and suppressing the launching nose was also analyzed, but was discarded since it required more coupled straight tendons than the adopted solution and the cost of the steel piers surpassed that of the launching nose. During construction, the ultimate limit states (ULS) either for the bending moments or the shear force were verified by a wide margin in comparison with the ultimate resistance values. The bearing friction during the launching process was also examined in some detail. The maximum friction observed would crack the base of the two tallest piers, P4 and P5. To solve this The values obtained from the frequent combination were used in the verification of the serviceability limit state of decompression, while the most conditioning characteristic combination was utilized in the verification of the cracking Service Limit State. A quick analysis of the obtained bending moments shows that these surpass those of the launching phase, requiring the use of additional prestressing. The adopted prestressing solution consists of both external tendons and mid-span section internal tendons tensioned after launching operations end. The tendon schematic is as shown in Fig. 9. 4

6 0 37,5 49,5 88,5 100,5 139,5 151,5 190,5 202,5 241,5 253,5 292,5 304,5 equilibrium, the foundation resistance and the reinforced concrete checks of important parts of the abutments. Figure 9 In service external and internal added prestressing Vsd Vrd1 The external prestressing consists of 4 tendons with 22 0,6`` strands, while the additional internal prestressing is composed by 4 tendons with 19 0,6`` strands. This prestressing scheme proved to be extremely effective, although the average compression applied to the deck cross section is very high, about 7 to 8,3 MPa. The ULS verification was made using the following combination: Graph 2 Deck shear force at ULS [kn] 7 Deck Behavior for High Speed Circulation The following graph presents the comparison with the resistance bending moment s values, being visible that the deck bending safety is assured Graph 1 Deck acting and resisting bending moments [knm] The same combination was used for the shear force ULS, producing the following graph. The shear reinforcement was evaluated using this data in accordance with EN to guarantee shear safety. In service conditions, for the verifications of the piers and foundations, the most conditioning combinations were the seismic comb and the wind plus LM71 comb. All code verifications were successfully assured, guaranteeing structure safety. The abutment stability was assured, as well as their foundation safety. The abutments safety verifications involved the High speed railway bridges are an ever-growing occurrence, especially in recent years due to their energy and economical stability, environmental and mobility concerns for the future. With the implementation of this new kind of transportation in Portugal in the near future, dynamic analysis like the one presently conducted are of growing importance. According with the EN the structure response is function of several parameters needed for a dynamic analysis of high speed railway traffic, these are: Speed of railway traffic; Span length; Structure mass; Natural frequencies of the whole structure; Number of axels, their load and relative spacing; Structure damping; Vertical irregularities in the track; Vehicle mass and suspension characteristics; Vehicle imperfections; Existence of ballast. According to the EN 1990 A , the maximum vertical acceleration on a ballast track is of 3,5m/s 2, for 10 different load model trains (High Speed Load Models type A - HSLM-A) travelling at different speeds. In this dynamic study several time history analysis were performed between the traveling speeds of 40 m/s and 120 m/s (144 km/h to 430 km/h). For these ten different HSLM-A the dynamic deck response was evaluated during time. The maximum vertical acceleration occurs in the lateral spans, with a value of 1,562 m/s 2, less than half of the maximum permitted value, which proves the deck responds well to the circulation of high speed trains. 5

7 Figure 10 Vertical deck accelerations for the 10 HSLM-A trains traveling at speeds between 40 and 120 m/s 7,8% Deck 8 Cost Evaluation 15,8% 40,8% Infrastructure A cost evaluation based on the general definition of the structure in the drawings was conducted, and compared with the cost of a steel-concrete composite deck solution for the same bridge also utilizing the ILM. This evaluation was made by multiplying the foreseen material quantities by their unit costs. Pie charts were elaborated illustrating the different costs associated with each section of the bridge and each component of the deck. 35,5% Figure 11 Bridge cost division Miscellaneous Construction Process The estimated total cost of the bridge is , or in other terms, 1035 /m 2 of deck overview area. The steel-concrete composite deck bridge had an estimated cost of , more or less a similar value, even though the deck cost represented a much higher 64,5% of the overall cost. The cost difference is made up by the infrastructure, which in the present design is of a bigger portion due to the high dead load of the concrete deck. 33,0% 22,7% Concrete Formwork 17,0% Rebar Prestressing 27,3% Figure 12 Deck cost division 6

8 9 Conclusions 10 References From the preliminary design of this prestressed concrete bridge built through the incremental launching method, the following conclusions can be taken: i. The use of a launching nose is more favorable than temporary steel piers; ii. Even though the deck is very stiff, it is equally heavy, which requires the use of a great quantity of prestressing; iii. The use of internal straight tendons during launching proves to be a good solution; iv. During launching there is a 16 m area where the bending moments are higher than everywhere else on the deck; because it is an isolated situation, 20 high yield prestressed 50 mm external bars were added to guarantee decompression; v. The passage of the deck over P4 and P5 during launching creates high bearing friction forces that required the use of prestressed stay cables, to prevent cracking form occurring in the base sections of these piers; vi. The in service loads are higher than those during the launching stages, which requires the use of additional prestressing made up of external cables and internal span cables; vii. The final prestressing solution led to ultimate resistance values that were higher than the active ULS values; viii. In the resistance to seismic actions, various possibilities were envisaged, and the most effective was chosen. It consisted in applying seismic dampers in both abutments, since a solution with only one fixed abutment yielded too greater forces; ix. This solution allowed all the piers to have fixed bearings, since the center of rigidity is more or less at the center of the deck; x. The high speed circulation analysis, with velocity s ranging from 144 km/h to 430 km/h and the regulation high speed load models, is not governing the design in terms of forces or deflection when compared with the combined action of two LM71 freight trains; xi. The maximum vertical acceleration observed during the circulation of 10 different HSLM-A was of 1,562 m/s 2, less than half of the maximum allowed value of 3,5 m/s 2 ; xii. The cost evaluation leads to a total cost of , which corresponds to 1035 /m 2 of deck overview area; 40,3% of the total cost is due to the cost of the deck, from which 33% corresponds to the cost of only the prestressing; xiii. This cost evaluation reveals to be more or less the same as the one obtained for a steel-concrete composite deck solution for the same bridge. Even though the prestressed concrete deck has a much lower cost, the cost difference between the two designs is made up by the cost of the infrastructure, which is of a bigger portion due to the large dead load of the concrete deck. CEN Eurocode 0 - Basis of structural design CEN Eurocode 1 - Actions on structures - Part 1-4: General actions CEN Eurocode 1 - Actions on structures - Part 1-5: General actions CEN Eurocode 1 - Actions on structures - Part 2: Traffic loads on Bridges CEN Eurocode 2 Design of concrete structures Part 1: General rules and rules for buildings CEN Eurocode 7 - Geotechnical design - Part 1: General rules CEN Eurocode 8 - Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings CEN Eurocode 8 - Design of structures for earthquake resistance - Part 2: Bridges Regulamento de Segurança e Acções para Estruturas de Edifícios e Pontes. PORTO EDITORA Junho Prof. Manfred Theodor Schmid - A Construção e o Lançamento de Pontes pelo processo dos segmentos empurrados, Rudloff Industrial Ltda Accessed on 11th March VIADUC DES BERGERES - Accessed on 24th July Octávio Martins - Modelo virtual de simulação visual da construção de pontes executadas por lançamento incremental 2009 [Dissertação de Mestrado]. Reis A. J. - Pontes. Folhas da Disciplina. AEIST Association Française de Génie Civil - Guide des ponts pousses, Presses de l école nationale dês Ponts et chaussées Bernhard Gohler, Brian Pearson Incrementally Launched Bridges. Wiley, Rosignoli Marco - Bridge Launching Parma, Italia: Thomas Telford Ltd, Rosignoli Marco Prestressing Schemes for Incrementally Launched Bridges Journal of Bridge Engineering, May VSL International Ltd. The Incremental Launching Method in Prestressed Concrete Bridge Construction April