BASE CASE DESIGN OF A RAILWAY BRIDGE WITH A COMPOSITE STEEL-CONCRETE DECK EXECUTED BY INCREMENTAL LAUNCHING. Pedro Lindo Guerreiro Madeira Santos

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1 INSTITUTO SUPERIOR TÉCNICO Universidade Técnica de Lisboa P r o j e c t o Base de uma Ponte Ferrovi á r i a c o m T a b u l e i r o Misto Aç o - B e t ã o E x e c u t a d o por L a n ç a m e n t o I n c r e m e n t a l BASE CASE DESIGN OF A RAILWAY BRIDGE WITH A COMPOSITE STEEL-CONCRETE DECK EXECUTED BY INCREMENTAL LAUNCHING Pedro Lindo Guerreiro Madeira Santos Extended Abstract October 2010

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3 Projecto Base De Uma Ponte Ferroviária com Tabuleiro Misto Aço-Betão Executado por Lançamento Incremental Base Case Design of a Railway Bridge with a Composite Steel-Concrete Deck Executed by Incremental Launching Pedro Lindo Guerreiro Madeira Santos IST, Technical University of Lisbon, Portugal 1 INTRODUCTION This dissertation aims to execute the base case design of a structural solution for a railway bridge with a composite steelconcrete deck, executed by the Incremental Launching Method (ILM). Composite plate girder composite steel-concrete decks have been used in the last years in numerous roadways and railway bridges (Figure 1). Their applicability in railway bridges is particularly interesting since they fulfill fundamental requirements needed by this type of structures: lightweight and enough stiffness to unsure railway circulation. Abstract This thesis presents the design of a railway bridge with a steelconcrete composite deck erected by Incremental Launching. Structural analysis is carried out both during construction stages - and for service conditions, evaluating the structural stresses during these stages, at the concrete and steel deck section. Safety verifications for the deck, the piers, the foundations and the abutments are presented during construction and for service conditions. To proceed with this aim the launching method is first described, the design constrains are identified, the structural solution is presented and pre-designed and the structural materials are listed. General design criteria, design actions and combinations are then presented, according with Eurocode standards, as well as, the main design checks for the deck, piers, foundations, abutments, bearing supports and concrete stab connectors. The deck dynamic response for the railway circulation is also computed for the high-speed trains defined according with the Eurocode 1, evaluating the possibility of circulating with these trains over the designed bridge deck. Figure 1 Plate girder composite steel-concrete deck solution for a double track railway bridge [m]. Moreover, the use of ILM is appropriate to this type of deck solution. First, the steel structure is launched into place, and secondly the concrete slab is cast in place or erected using precasted segments. This method requires usually steel beams with constant height, and not as slender as they could be if another type of construction method was adopted. But this characteristic is also very convenient for railway deck, since higher vertical stiffness is required. Finally, cost estimation is performed, taking into account the general layout defined in the structural drawings, being compared with a prestressed concrete deck solution designed for the same bridge. Key words: Railway Bridge, Steel-Concrete Composite Section, Incremental Launching, High- Speed, European standards. 2 THE INCREMENTAL LAUNCHING METHOD In the present design it is necessary to build a bridge in a sloping valley and is not possible the occupation of the land under the deck, even temporary during the construction stages. The incremental launching method in combination with the composite steel-concrete deck solution offers advantages in comparison with -1-

4 another construction methods and deck solutions. This method of construction requires reduced areas for the construction yard, minimizes the environmental disturbances, increases security speeds up construction, and particularly preserves unoccupied the land under the deck even during construction. When using the incremental launching method (ILM) the steel part of the deck is usually executed behind one of the abutments, where the construction yard is located, being progressively launched in lengths of 25 to 50% of the typical span, until it reaches the opposite abutment. The use of ILM in the construction of steel and composite bridges have increased in the last 20 years, in Europe as well as in Asian countries, even being a relatively expensive method, due to costly equipment and specialized manpower required. However, it is a very effective way to overcome local site constrains and issues that other constructive methods cannot proper solve. The ILM offers significant advantages both to the Owner and to the Contractor, namely: Minimum disturbance of the surroundings of the working areas, including environmentally sensitive areas; Construction yard smaller for erecting the steel structure; Reduced and simpler equipment; Increased security, since most of the construction works are carried out in the construction yard, working therefore directly on the ground and supported at a fixed location; Greater control on the final quality due to pre-fabrication of steel components; Possibility of reducing the construction duration. The ILM may be adapted in several situations, with very difficult site constrains such as: Very deep and sloppy valleys; Deep or wide watercourses; Areas with poor soil inaccessible to machinery; Protected and valuable environmental resource areas. In the vast majority of composite steel-concrete decks only the steel girder is incrementally launched to its final position. At a later stage, the concrete slab is executed, usually concreted in sections of 10 to 15 m, using a mobile slab formwork directed supported on the steel structure already assembled. During the incremental launching operation, the steel structure is placed on a set of temporary bearing supports installed on the piers. At the end of the launching operation, these supports are replaced by permanent pot bearings. The required force to the launching is obtained by a set of hydraulic jacks acting directly on the girders, or, in some cases, applied indirectly using pre-tension cables. In order to reduce the negative bending moments of the steel beams during the launching operations, a front nose may be adopted, which reduces the cantilever length of the deck. Alternatively, temporary supports may be used during the launching stages, usually executed with steel towers in the middle of the piers. Any of these solutions ( or both together in the case of spans over 70 to 80 m) reduce significantly the negative bending moments (and also the positive bending moments), that the main girders of the bridge are subjected during the launching operations. It is always more economical to carry out the launching operations only from one side only (when it is possible due to road or rail inplane constraints), because the launching process is optimized and the steel assembling hard is located in just one place. However, there are some particular situations when this is not possible, for example for the case of in-plan deck road alignments consisting of a straight line and a circular curve. This was for example the case of the Alcântara railway viaduct, in Lisbon, erected by incremental launching partially from pier P7 and form the abutment. Although the steel part of a composite deck is lighter than a prestressed concrete deck executed also by the incremental launching method, some unusual aspects need to be properly evaluated during the construction stages, when using the ILM. For one hand, the effects of the vertical concentrated forces that appear at the bottom flange level, and are transmitted to the relatively slender webs, have to be considered. On the other hand, it is also necessary to evaluate the bending and shear ultimate resistance of the deck cross-section, as well as its global stability due to the effect of bending and torsion, especially for the case of double I -2-

5 girder deck sections, but also for the case of box-girder crosssections (which during construction behave like open sections). the piers and abutments geometry, as well as, the structural materials adopted. It is also necessary to consider the possibility of in-plate webs instability due to the effect of vertical concentrated forces of the support sections. This effect is commonly known as Patch Load effect. Usually, it is recommended to increase the web thickness of the middle span sections, when the ILM is used to deal with the vertical forces induced during launching. This solution increases the overall steel quantity needed. Alternatively, a better solution consists to adopt vertical web stiffeners, shortly spaced at the support deck sections, and more spaced at the rest of the deck sections. This solution increases the critical load of instability of the web panels, without an important quantity of steel. Several authors have studied the Patch Load phenomenon using finite element models. The results show that the webs are not damaged while the Patch Load is less than 49% of their shear strength. Some authors propose that any yielding should be permitted in all sections (Rosignoli, 2002) and (Granath, 1998). Given the constraints it was selected a continuous deck with seven spans, two lateral spans with 37.5 m and five typical interior spans with 51 m (Figure 3). A composite steel-concrete deck was selected, with a cross-section formed by a double plate girder and with a concrete slab on the top (Figure 1). The deck is 3.55 m deep, which corresponded to a slenderness of L/15, for a typical span of 51 m. The materials used for the deck, according with the national and European code designations, are: C40/50 for the top slabs; In order to comply with this safety verification Rosignoli advises increasing the design vertical reactions by 30% to account for possible misalignment of the web from the launch restraints and due to geometric imperfections. After all steel structure has been launched the slab is executed using prefabricated panels or in-situ slab concreting. When prefabricated panels are used only the connections between the panels and the boxes which are concentrate the connectors have to be casted on site. However a most common option consists in executing a cast in-situ slab. When this is the case, the slab is casted in segments, first over the spans, and only after over the support deck section. This procedure prevents cracking the slab over the support sections due to its self-weight. Figure 2 Reinforced concrete deck slab [m]. S355NL for the flanges and webs of the plate girders; and A500NR for the reinforcement bars of the deck slab. Piers and abutments are executed in reinforced concrete, C30/37 and C40/50, respectively, and A500NR. 4 DESIGN CR ITERIA AND DECK STRUCTURAL VERIFICAT IONS The definition of actions and general design criteria are in accordance with the new Eurocodes. 3 THE BASE CASE DESIGN S OLUTION After discussing the site constraints it was decided to design a composite steel-concrete deck, in view of the benefits of this solution for a railway bridge deck. The general layout of the base case design solution involves the selection of the spans, the definition of the typical cross-section of the deck, the definition of First, for the present case of an incremental launching deck construction, the analyses and safety verifications during the most demanding launching stages was performed. According with the Eurocode 4-2 Clause , during all launching stages it was verified that no yielding occurs in the steel plate girders. Secondly, structural analysis was carried out during slab concreting various stages, when applying the finishing works, and -3-

6 thereby for the live loads and temperature gradients design During the launching operation structure very high negative Figure 3 General longitudinal layout of the Base Case design solution [m]. actions. It was assumed all span deck sections are concreted in a single stage, and after the support regions were executed in 12 consecutive stages. Therefore, the slab execution was subdivided into 13 stages in order to consider all stages of concreting and then to assign stresses to the section that was in fact to resist, until slab was concreted only the steel section, thereafter the composite steel-concrete deck section. An evaluation of in-service stresses and strains was carried out for the introduction of each new action, and considering the actual deck section (only steel or composite steel-concrete deck section). bending moments are applied on the deck. However, the two plate girders are connected with an in-plane and vertical stiffness system that assures the lateral stability to flexion-torsion effects. However, during launching lower flanges are subjected to high compressive forces. To evaluate the resistance of the flanges paragraph 3 of clause from EN 1993 was used, according to which the compressive force is obtained by N Ed =M Ed h, being M Ed the applied bending moment and h the vertical distance between the flanges. During the incremental launching also a local verification was performed on the steel plate girder webs, evaluating the effect of a concentrated vertical load introduced by the supports - "Patch- Figure 4 General procedure for "Patch-Loading" steel webs safety verification. -4-

7 Loading Effect". The model-checking local resistance of the web loaded by a concentrated load in its plane is defined in Section 6 of EN , being outlined in the Figure 4 diagram. Also the piers are particularly requested during the launching deck stages, since the deck can induce significant horizontal forces on the top of the piers due to friction at the support devices. Thus, it was also necessary to properly evaluate these forces and their effects on the piers, verifying that no cracking occurs at the base pier s sections. To minimize the frictional coefficient Teflon layers were adopted at the interface between the deck and the secondary effects of shrinkage and creep shall be taken into account. In fact, in a composite beam, where the concrete is restrained by the steel section, there are two distinct effects of shrinkage of the slab. The first effect is a set of auto-balanced longitudinal stress tension and compression in the slab on the beams, the second is a positive bending moment is generated due to the imposed deformation being eccentric from the center of gravity of the composite section in a hyperstatic structure. Figure 5 Deck steel beams flange plant and web stiffeners. temporary supports placed at the top of the piers during launching operations. This solution allowed the use of friction coefficients between 2% and 6%. For the present case, there were no major difficulties in verifying the cracking security of the piers as a result of friction forces, since the weight of steel deck structure being launched was relatively low. In the longitudinal analysis, deck composite cross-section proprieties were evaluated, being obtained homogenized all steel sections. This procedure depends on the nature of the action, setting up short-term actions (action variables), long-term (permanent actions), and actions over time (typically the effect of shrinkage of the slab). Each of these three situations corresponds to different widths of the slab, being the reduction made in the elastic modulus of the slab which was considered for each action. When the structure is isostatic the curvature generated by eccentric shrinkage of the slab does not result in stresses, leaving only the longitudinal stress, called the primary effect of shrinkage. In the slab cracked regions, these stresses are void. In non-fissured areas the stresses resulting from primary effects of slab shrinkage, have opposite signs to the permanent and variable actions, and therefore it usually a safe procedure to not take into account this effect in check security. In hyperstatic structures however, the positive curvature of the resulting shrinkage introduces vertical deformations are incompatible with interior support conditions, which gives rise to reactions of self-support balanced and positive bending moments on the deck. The reactions are generally positive on the interior support and negative support in the end. These stresses are usually referred as secondary effects of shrinkage. Creep and shrinkage effects were simulated using and equivalent negative temperature applied to the non-cracked slab deck sections. Clause (7) of EN 1994 referred that primary and In the ULS, unlike the ELS, the applied bending moments can be added and considered applied at the composite deck section, because near collapse a redistribution of stresses between the two materials occurs, in each cross-section. According to EN 1990, for -5-

8 the ULS verifications the following two load combinations were considered: Calculations have shown that the first combination was critical for positive moments and the second combination for negative In composite steel-concrete deck bridges stresses are limited in service to ensure that, in normal operation, there are no situations of yielding and cracking, besides those are already assumed in the bending moments. Quasi-permanent combination, used for concrete slab mid-span compressive stress verification, corresponds to:. models adopted in the elastic analysis performed. The Eurocodes 2, 3 and 4 thus establish stress limits for structural steel, concrete reinforcement, to be checked on service conditions: Concrete: Clause 7.2(3) of EN refers, for a quasipermanent combination of actions, the maximum compressive stress in the concrete must be 0,45 f ck ; and Clause 7.2 (102) EN , refers for concrete exposure classes XD, XF XS, that the compressive stress must be less than 0,6 f ck for the characteristic combination of actions. 5. PIERS AND ABUTMENTS D ESIGN The piers are strongly constraint by the geological and geotechnical conditions that determine the dimensions and the foundation solutions. The design of piers and deck-to-piers connection strongly determines the piers structural behavior. Adopting fixed bearings, the effects of uniforms temperature variations and horizontal railinduce forces on the piers. Reinforcement: Clause 7.2.2(4) EN refers for the characteristic combination, the stress in steel reinforcement should be less than 0,8 f sk., but when taking into account also the action of imposed deformations, the same clause increases the limit value to 1,0 f sk. Structural Steel: Clause 7.3(1) of EN refers, for the characteristic combination of actions the stresses are limited to f y / γ Mser with γ Mser = 1,0. It is also important to assess how the infrastructure piers and abutments resists to the seismic forces. The design "more classical" was to consider a fixed abutment that would absorb almost all of longitudinal seismic forces. However, for the present case this design leads to very high stresses in the fixed abutment. Therefore a different design approach was adopted, using damper devices on both abutments. This procedure allowed the deck to be fixed to all piers, and thus piers absorb a fraction of the longitudinal seismic forces. This design concept can be used in regions of moderate and high To the evaluation of stresses in service, the combination of action to be considered cannot be done by adding the diagrams of bending moments as actions are applied in different sections of only steel or composite steel-concrete (and those with different homogenization). Thus, a step by step procedure of evaluating stresses resulting from each separate action is required, making the overlap of successive effects in terms of stresses, and verifying security by comparing the resulting stresses with the limits assumed for each material. The characteristic combination (equation 6.14b of EN 1990) This leads to the seismic activity, according with clause (1) of EN1998-2, providing that the structure has a ductile behavior, which means it Mrd χ y χ u ,001 0,002 0,003 0,004 Figure 6 Piers moment / curvature relationship. χ following three cases: -6-

9 has forms of energy dissipation when subjected to intense seismic actions. This is achieved by ensuring that the structure generates several plastic hinges on the deck support infrastructures. Piers and abutments have the required rotation capacity prior to the rupture occurrence as it is shown for the pier base section in Figure DECK BEHAVIOR FOR HIGH SPEED CIRCULATION Deck dynamic response for the crossing of several high speed railways as defined in part 2 of EC 1, was evaluated, with the aim of verifying the possibility of such trains to circulate in the design structure. The dynamic effects on railway bridges have been under intense The span length; The structure mass; The structure natural frequencies and its vibration modes; The number of axles, its spacing and their corresponding load; The mass vehicle and the suspension characteristics; The structure damping; The track irregularities; The vehicle imperfections; and The existence of ballast. According to EN1990 A , for ballasted tracks the deck vertical acceleration shall not exceed 3,5 m/s 2, for 10 different high speed load model trains (HSLM-A) travelling at different speeds over the deck. Several time history analyses were Figure 7 Vertical deck accelerations [m/s 2 ] for the 10 HSLM-A s travelling at line speeds between 40 and 120 m/s. study, and will continue to be in the future due to the growing need of mobility, combined with an environmental, energy and economic sustainability. In the near future, this transportation will be adopted in Portugal, which means the design of new railway bridges will probably include the evaluation performed here. EN includes a chapter devoted to dynamical phenomena including the resonance effects. It also mentions the factors that influence the response of a structure to a variable action in time: The overload speed; performed considering these 10 trains travelling from 40m/s to 120 m/s (from 144km/h up to 430 km/h), being verified that (Figure 7): The vertical acceleration is higher at the lateral ending span; but It does not exceed the 3,5 m/s 2 limit. -7-

10 7. COST ESTIMATION Finally, in the frame of the work, a cost estimation was performed, taking into account the general layout of the designed bridge. The total cost was finally compared with the cost of a prestressed concrete deck solution designed to the executed also by ILM. The cost estimation was obtained by the product of two values, the measurements based on the drawings and its correspondent unit cost. Measurements were calculated for following major materials and works: 4,1% Preparatory work and special foundations including piles deep foundations; Landfills; Formworks areas; Concrete volume; Reinforcement bars; Structural steel and connectors; Deck construction equipment s; Miscellaneous (includes deck finishing, pot bearings, expansion joints, damper devices, and other finishing works of the bridge). The estimated total cost of the bridge works is 4,223,272, broken down by deck materials, infrastructures (piers and abutments), deck construction equipment s and miscellaneous (Figure 8).The total cost per deck square meters is 1041 / m 2, which correspond to a relative relatively high figure, typically obtained in railway decks. In fact, the study carried out in parallel for the same bridge with a prestressed concrete deck resulted in a total estimated cost of 4,184,600, just 1% less that the composite solution here presented. 17,5% 13,8% 64,5% Figure 8 Bridge cost repartition. Deck Materials Constructive Processes Infrastructures Miscellaneous 8. MAIN CONCLUSION S A base case design for a railway bridge with a composite steelconcrete deck was presented, adopting the construction by incremental launching. From this study a set of conclusions must be referred to: (i) The designed composite steel-concrete deck with two plate girders verify safety during the launching stages without the need for intermediate supports or a launch nose; (ii) Concreting the slab on steel girder leads to a stress distribution that penalizes the steel girders and reduces the stresses installed in the slab; (iii) Thus, in service, the stress distribution determines the need for greater thicknesses in the flanges of steel girders; (iv) The deck ultimate resistance is fixed by the interior support section, and its resistance is determined by the maximum allowed stain at the bottom flange steel; (v) The deck span sections have a good reserve of resistance, which is shown by the in service lower stress levels and a bending ultimate resistance 52% higher than the design bending moment; In the detailed design it is possibly to optimize the plates thickness in these sections, to reduce the total amount of structural steel; (vi) The study of the interaction between the high-speed (144 to 420 km/h) railway real live-load and the bridge deck, shown that this dynamic actions do not introduce stresses or deformations greater than those obtained for the actions of freight train, LM71, which continues to be mandatory in the design; (vii) However, the high-speed railways HSML-A introduce vertical accelerations on the deck, namely on the lateral end span, which is estimated to be slightly lower than the acceptable limit of 3.5m/s 2 ; (viii) The design option for a fixed abutment was abandoned, given the high seismic force transmitted to the fixed abutment, in favor of a solution with seismic damping devices in both abutments, which allows a better distribution of the seismic forces between the piers and the abutments; this solution allowed all piers to be fixed to the deck, using pot-bearings; (ix) The bridge total cost is estimated in , corresponding to 1041 /m 2 of deck in-plan area; 64.5% of this value corresponds to the deck cost; (x) A parallel study conducted for a prestressed concrete deck for the same bridge, and executed also by the incremental -8-

11 launching method, obtained a total estimated cost of 4,184,600, which corresponds to less than 1% compared to the proposed composite deck solution. REFERENCES [1] Reis A. J. Pontes Metálicas e Mistas - FUNDEC [Book] [2] Rosignoli Marco Bridge Launching [Book]. - Parma, Italia : Thomas Telford Ltd, [3] Rosignoli Marco Creep Effects During Launch of the Serio River Bridge [Book Section] // Concrete international. Design & construction / Book Author Institute American Concrete [4] Virtuoso F. Vigas de alma cheia [Book] [5] CEN Eurocode 0 - Basis of structural design [Book] [6] CEN Eurocode 1 - Actions on structures - Part 1-4: General actions [Book] [7] CEN Eurocode 1 - Actions on structures - Part 1-5: General actions [Book] [8] CEN Eurocode 1 - Actions on structures - Part 2: Traffic loads on [Book] [9] CEN Eurocode 3 - Design of steel structures - Part 1-1: General rules [Book] [10] CEN Eurocode 3 - Design of steel structures - Part 1-5: Plated [Book] [11] CEN Eurocode 3 - Design of steel structures - Part 2: Steel Bridges [Book] [12] CEN Eurocode 4 - Design of composite steel and concrete structures - Part 2 [Book] [13] CEN Eurocode 7 - Geotechnical design - Part 1: General rules [Livro] [14] CEN Eurocode 8 - Design of structures for earthquake resistance - Part 1: General rules, seismic actions and rules for buildings [Livro] [15] CEN Eurocode 8 - Design of structures for earthquake resistance - Part 2: Bridges [Livro]