The new Reno bridge on the A1 Milan-Naples Motorway

Transcription

1 The new Reno bridge on the A1 Milan-Naples Motorway Furlanetto, G., Ferretti Torricelli, L., Marchiondelli, A. Structural Engineering Department SPEA Ingegneria Europea s.p.a. Via G. Vida n Milan - Italy INTRODUCTION For a long time Italy has been involved in the upgrading of the highway net work both at National and International level. One of the main strategic works currently in phase of realization is the Variante di Valico, the new highway connecting Bologna to Florence aimed at upgrading the existing A1 Milan-Naples motorway, ring of conjunction between Northern and Southern Italy and crucial point of the European Corridor 1. In such plan, characterized by a particular complexity related to the technical and ecological difficulties in crossing the Appennines, the new Reno bridge is situated. Born from the will of representing a characteristic symbol in a layout in which half of the segment develops in tunnels and just a third part is realized by bridges, the Reno bridge, with its m of length and its peculiar inclined piers, well fits in the morphology of the territory answering to the need of minimizing the interferences with the surrounding natural and anthropic environment. Keywords: concrete, box girder, incremental launching, prestress DESIGN CRITERIA FOR THE NEW RENO BRIDGE The road plan of Variante di Valico has to be inserted in a various and articulated territory, characterized by the presence of the Appennines and by narrow valleys crossed by unstable rivers. Therefore the whole project has to be confronted with a very strict prescriptive framework aimed at protecting the environment, in particular focusing the attention to the hydraulic, geotechnical, acoustic and ecological impacts of the several works. In agreement with such prescriptions, the conceptual design of the Reno bridge has been developed with the objective to obtain the following goals: - to reduce the environmental impact of the structure - to leave the river bed clear from piers - to avoid any trouble to the local road network - to minimize the construction and maintenance costs - to minimize the construction time - to achieve an optimal statical and seismic behavior During the conceptual design phase the following constraints were also to be considered: - the rapid fluctuation of the water level, with frequent alternations of flood and low water regime; - the amplitude of the segment to overpass (about 600 m), running partly in the floodplain zone and partly on the proper river bed, along a curved alignment - the distance from the ground level varying from 10 m to 30 m, with a marked discontinuity on the right side of the river, where this distance suddenly decreases with a rapid slope - the amplitude of the clear span on the river, minimum 130 m length - other restraints imposed by the need to connect the new road alignment respectively with the existing motorway located on the Bologna side and with the alignment of the Monte Mario tunnel, located on the next segment of the new motorway on the opposite side. After some preliminary studies, the new Reno Bridge (Figure 1) consisting of a continuous concrete box girder beam incrementally launched, seemed to present the best performances from the structural and economical points of view, in particular as regard to construction and maintenance costs.

2 Fig. 1 The new Reno bridge DESCRIPTION OF THE STRUCTURE General Aspects The new Reno bridge, shown in Figure 2, consists of a continuous prestressed box girder beam of m length. The segment interested by the structure may be ideally subdivided in a floodplain zone and in a long span zone aimed to overpass the river bed. The organization of the bridge provides a sequence of nine spans of x m length, supported by six typical piers, located in the floodplain zone and by a couple of special piers, characterized by two inclined arms each one, forming a V shape, located in the long span zone. Provisional piers are provided during the launching of the bridge on the long spans. By taking advantage from the particular shape of these atypical piers, the last three spans ( m) are subdivided in five smaller spans having m length, carrying the effective number of the total spans from 9 to 11. Fig. 2 Longitudinal profile (Northern carriageway) As Figure 3 shows, the bridge design presents two separate structures for each carriageway differing just for the planimetric radius of curvature of the alignment, which measures 1400 m for the southern carriageway and 1350 m for the northern one. The altimetric radius of curvature is m. The longitudinal slope starts from about 3.5 % on the Bologna side and ends at about 0 % on the Florence side, while the transversal slope is constant and equal 4.5 %. In order to facilitate the insertion in the next motorway segment, which continues with variable radius of curvature, the span of the lateral cantilevers of the southern carriageway on the Florence side has been made variable, increasing of about 0.5 m the span of the inner cantilever and decreasing the span of the outer. 2

3 Fig. 3 Planimetric view of the new Reno bridge The materials used for the realization of the bridge are characterized as follows: Concrete: Bridge deck: Class C35/45 MPa, with w/c ratio 0.5; the minimum cubic strength for the first stage prestressing has been fixed at 35.0 MPa; Typical piers: Class C30/37 MPa, w/c ratio 0.45; Special piers (core): Self compacting concrete Class C35/45 MPa for the core filling; Abutments: Class C30/37 MPa, w/c ratio 0.45; Foundations: Class C30/37 MPa, w/c ratio 0.45; Steel: Special piers (outer shells): Rolled steel Class S355 for the outer shell and internal trusses; Ribbed bars: Mild steel type FeB 44K (fy/ft 430/540 MPa); Prestressing tendons: Strands? 0.62 of harmonic steel type St 1670/1860 MPa. Bridge Deck The bridge deck consists of a continuous post-tensioned unicellular box girder. The total width of the deck, equal to m, accommodates a roadway of m wide and two lateral curbs 0.50 m wide, equipped with security guardrails. The cross section of the deck is constant along all the bridge; special diaphragms are located in correspondence of the pier axes and the prestressed tendon anchorages. The box section (Figure 4.a) has total height of 4.50 m and net width of 8.90 m, with lateral cantilevers of the top slab 3.40 m long. The thickness of the top and bottom slab is 0.25 m, increased respectively to 0.65 and 1.0 m near the joints with lateral webs, which are 0.45 m thick The transversal diaphragms are formed by a bottom crossbeam and two lateral uprights, derived by the thickening of the bottom slab and the lateral webs for a length of 1.20, 1.80 and 4.40 m respectively on the typical piers (Figure 4.b), the abutments (Figure 4.c) and the connection with the V-shaped piers (Figure 4.d). 3

4 Fig. 4 Typical cross section and particular diaphragms Prestressing System The deck is longitudinally prestressed using post-tensioned tendons formed by strands? 0.62 made of harmonic steel type St 1670/1860 MPa. The prestressing system is articulated in two stages: a preliminary (1 st stage) prestressing activated during the construction phase and a final (2 nd stage) prestressing set after the completion of the launching. Both these prestressing are realized by adopting post tensioned tendons formed by strands? 0.62 made of harmonic steel type St 1670/1860 MPa. First Stage Prestressing The first stage prestressing (Figure 5.a), progressively activated after the casting of each segment, is formed by 14 tendons made by 19 strands each, stressed at 4047 kn. It runs in the top and bottom slab (8 and 6 tendons respectively) on a straight alignment characterized by barycentric axial resultant. The typical length of each tendon is 90 m, with the exception of the tendons located on the first and last segment, and covers the length of two typical segments. In every joint section, before launching, 8 or 6 tendons are stressed alternatively, whilst the remaining ones pass through the cross-section and will be sheathed and stressed in the subsequent construction stage of the next segment. Each tendon is stressed at the end located on the opposite side of the launching direction and linked with the preceding tendon by means of special coupling joints. The total axial force provided by the first stage prestressing is about kn, resulting in a constant compressive stress along the concrete section of about 4.40 MPa. Final Stage Prestressing The second stage prestressing (Figure 5.b) is carried out after the completion of the extrusion of the whole deck and the realization of the pier/deck connection of the atypical piers. It is formed by tendons with curved profile running along the webs of the box girder. The eccentricity of these tendons varies in order to counteract the bending moment due to external loads. The tendons are stressed at both ends, where the anchorages are located in special thickening of the webs, inside the box girder. The continuity of the second stage prestressing along the beam is allowed by the overlapping of the tendons in the anchorage zone. 4

5 Fig. 5 Layout of prestressing tendons in the cross-section: (a) 1 st stage; (b) 2 nd stage For the typical spans (Figure 6.a) 2+2 tendons made by 22 strands are provided, with a stressing force of 4686 kn/unit. The prestressing of the long span zone (Figure 6.b) is formed by 5+5 tendons made by 31 strands, with a stressing force of about 6600 kn/unit. The transition between the two zones is gradual: 2+2 tendons made by 31 strands overlap in continuity with the 2+2 tendons of the last 45 m span, and, in a separate thickening located at about the third of the 69 m span, the remaining 3+3 tendons made by 31 strands are stressed. The maximum length of the 31 strands tendons is 135 m. (a) (b) Fig. 6 Longitudinal layout of prestressing tendons : (a) in typical spans; (b) in long span zone Supports As mentioned above, the support system of the bridge is globally formed by two abutments, six typical piers and two atypical V-shaped piers, rigidly connected with the deck. Typical Piers The typical piers (Figure 7), characterized by height variable from 12 to 16 m, present cellular cross section, with dimensions 9? 3 m and thickness 0.3 m. The top diaphragm, 1.50 m thick, has the same plan dimensions of the pier cross section. Atypical V-shaped Piers As symbol of the whole design, the two special piers of the long span zone, 24 and 25.5 m high respectively, present a V shape (Figures 8) and are formed by a pair of inclined arms, having centroidal axis convergent toward the foundation level, and inclined with an angle of about 42 with respect to the vertical axis. The arms are doubled into two beams formed by a steel shell filled with self compacting concrete. Each steel shell has elliptical cross section with dimensions of the principal axes of 3.5 and 1.8 m respectively and thickness of 12 mm. The shell is preliminary bolted at the base of the foundation plinth and at the top of the 5

6 provisional piers, and acts as a formwork during the casting of the core. The shell is also internally strengthened by means of longitudinal ribs and steel trusses. After the hardening of the inner concrete core, from the statical point of view the cross section is considered as a composite steel concrete cross section. Longitudinal reinforcing bars are provided at the base and at the top of each pier. After the completion of the launching of the deck, the arms are rigidly connected to the bridge with? 36 steel bars using coupling sleeves let in the concrete of the bottom slab. Abutments The two abutments, realized in reinforced concrete, are respectively 11.5 and 4.0 m high (Figure 7). The elevation is formed by a transversal thick wall and two lateral walls. The abutment located on the Bologna side is characterized by a thicker elevation and two additional longitudinal walls, on which are installed the launching devices during the construction stage. Fig. 7 Typical piers and abutments Fig. 8 V-shaped piers Foundations The ground foundation is characterized by a top layer of filling material, having poor characteristics, with thickness variable from 7 to 12 m, under which a layer of medium to dense gravel has been found. Considering the good characteristics of this base formation, direct foundations were chosen for all the substructures, providing a treatment of the top layer with jet grouting columns. For the typical piers a rectangular plinth with dimensions 14.5 x 9.0 x 2.0 m has been provided. The foundations of the two V-piers of the long span zone have circular shape with diameter of 20 m and thickness of 2.50 m. The two abutments are based on a rectangular plinth with thickness of 2 m and plant dimensions (width x length) of 16.5 x 22.0 m and 16.1 x 22.0 m respectively. In addition, for the foundations located near the river bed, a perimetrical ring of jet grouting columns is provided. Same treatments have been realized also for the provisional piers. Restraint Devices The frame formed by the two V-shaped piers of the long span zone and the deck constitutes the horizontal fixed point of the whole structure. Two horizontal sliding bearings, allowing longitudinal and transversal movements and characterized by a maximum load bearing capacity of about kn are provided on the remaining supports. These devices are located near the web axes of the box girder, at a transversal distance of 6.90 m. The transversal restraint is made by a longitudinal sliding guide, with a transversal load bearing capacity of 1600 kn, located at the centre of the box girder. In the final stage, the static scheme of the structure is therefore of a mixed type. After the launching of the bridge deck, the supports of the long span zone are rigidly fixed at the bottom of the box girder, realizing in this way a frame. The supports of the typical spans statically realize, instead, the vertical and transversal restraints only, leaving free the movement along the longitudinal direction. 6

7 CONSTRUCTION PROCESS The incremental launching method is one of the highly mechanized erection methods used in bridge construction. The method consists of manufacturing the superstructure of the bridge by sections in a prefabrication area behind one abutment. Each new unit is concreted directly against preceding one and, after it has hardened, the resultant structure is moved forward by the length of one unit (Figure 9). Fig. 9 Launching process The construction process of the Reno bridge may be synthesized in the following steps: - realization of abutments and typical piers - realization of the provisional piers - realization of the inclined arms of the V-shaped piers - launching of the bridge deck and tensioning of the first stage prestressing tendons - realization of the rigid connection between bridge deck and V-piers - tensioning of the final stage prestressing tendons - dismantling of the provisional piers. In the following, the main steps of the construction process are presented, focusing the attention on the longitudinal launching of the deck and the realization of the special piers of the long span zone. Longitudinal Launching of the Bridge Deck The girder has been subdivided into 14 segments with a typical length of 45 m, with the exception of the first and last segment, which are and m long respectively. The weight of the typical segment is about kn. In the launching stage provisional piers are provided in the middle of the long spans, reducing in this way the maximum span length (93 m) to the typical span (45 m). The provisional piers are formed by four circular reinforced concrete piles with diameter of 1.50 m. The head of these piers are made by a rigid steel frame, equipped with the launching bearings. Piers of the same type have been provided to support the inclined arms of the V shaped piers during the launching of the bridge; at the top of these piers a ribbed steel structure has been placed in order to rigidly connect the top of the arms to the top of the concrete pier. Also this structure is fitted with temporary sliding bearings (Figure 10. a). In order to reduce the wide fluctuation of the bending stresses during launching, a 32 m steel launching nose, formed by two I beams, has been rigidly fixed at the end of the first segment using 52? 40 harmonic steel bars tensioned at the force of about 1000 kn/unit (Figure 10.b). (a) (b) Fig. 10 Launching bridge deck: (a) Provisional piers; (b) Launching nose 7

8 Construction Yard The construction yard is located about 30 m behind the abutment on the Bologna side and it consists in a prefabrication area and in a casting area. The prefabrication area is reserved to the assembling of the steel reinforcement cages (Figure 11.a). After assembling, the reinforcement cage forming the webs and the bottom slab is lifted with a traveling crane and translated longitudinally in the casting area. The casting area is formed by a 50 m movable outer formwork, fitted on two fixed rails on which the segment slides during the launching stage; the sliding surface in direct contact with the concrete is protected with steel plates which slide together with the segment during launching (Figure 11.b). (a) (b) Fig. 11 Construction yard: (a) Prefabrication area; (b) Casting area (Insertion of the steel cage) Launching Equipment The total weight of the launched structure is about kn. The pushing equipment is dimensioned in order to supply a total pushing force of kn, calculated on the basis of a maximum design friction coefficient (5% on the sliding supports and 8% on the casting rails) and the effect of the longitudinal slope. Actually, the maximum total pushing recorded by the monitoring system during launching never exceeded kn. With the exception of the launching of the first and last segment, the total vertical reaction at the abutments oscillates between and kn. The launching equipment consists of two launching devices located at the top of the abutment (Figure 12.a). Each device is formed by four lifting jacks with total capacity of 8000 kn and vertical stroke of 20 mm, sliding on a steel plate, and two pushing jacks with total pushing capacity of 7000 kn. The maximum stroke of the launching devices is 800 mm. At the top of the lifting jacks a steel plate fitted with flexible blades allows, together with the vertical reaction, the transmission of the horizontal pushing force by friction. For launching the first and last segment, during which the vertical reactions drop to very low values, the transmission of the horizontal force is helped by steel tendons anchored to the bridge. The maximum lifting at the jacks was fixed in 5 mm, in order to avoid too high bending stresses due to the imposed deformation. An electronic control system was installed to prevent any unintentional excessive lifting of the structure. The evolution of pushing and lifting pressures is also continuously checked and recorded during the progression of the launching process (Figures 12.b-c). 8

9 (a) (b) (c) Fig. 12 Launching equipment: (a) Hydraulic jack; (b) and (c) Electronic control system Working Schedule The typical working cycle for the construction of a single segment may be summarized as follows: - cleaning of the formwork and greasing of the fixed rails - insertion of the steel cage of bottom slab and webs - positioning of the tendons - casting of the bottom slab - positioning of the inner scaffolding of the webs, made by wooden panels - casting of the webs - positioning of the top slab inner scaffolding, and of the prefabricated reinforcing cage of the top slab - casting of the top slab - hardening of the concrete - launching of the segment The typical working cycle took a minimum of one week. Structural Analysis for the Launching Process The analyses carried out investigated the behavior of the structure during construction, the service life and under a seismic event. Due to the complexity of the construction sequence, a great effort was made to especially investigate the global and local behavior of the structure in the various construction stages. A step by step analysis of the launching of the whole beam has been carried out. In particular, the fluctuation of shear and bending stresses along the deck has been investigated, with the plotting of envelopes of maximum and minimum stresses in every section with the progression of the launching (Figure 13). The evolution of the lifting and pushing forces at the launching devices (Figure 14) has been also evaluated in order to dimension the devices and to make possible a direct check of the behavior of the structure in site. Here, the following loading conditions have been considered: - self weight of the box girder - dynamic amplification to the self weight effects during launching stage - linear temperature variation between top and bottom slab - lifting of the launching jacks - unintentional variation of the top level among different piers - unintentional level variation between the two bearings of the same pier In addition, for the dimensioning of the pushing jacks, the following loadings have been considered: - friction effects on sliding bearings, casting rails and on transversal sliding restrains - effect of the opposing longitudinal slope 9

10 Pushing force vs. reaction force during launching (1) 9500 Rh (kn) (2) (3) Feed (m) (1) Pushing Force (2) Reaction Force (3) Rear Nose Force Fig. 13 Typical bending moment envelope Fig. 14 Evolution of forces at launching A separated detailed analysis has been carried out in order to optimize the dismantling sequence of the provisional piers, and to investigate the evolution of the bridge deck deformations. Realization of the Atypical V-piers Positioning of the steel shafts The steel shells of the V-piers are positioned using two cranes. Each steel element, which is 25 m long, is initially lifted and put into vertical position. Subsequently, the steel shaft is slipped down fitting two steel drive-tubes fixed in the pier foundation as well as the longitudinal reinforcement curtain emerging from the foundation (Figure 15). Fig. 15 Positioning of the steel shafts Casting of the Concrete Core Once positioned and fixed the steel shells with bolts at the base and at the top, the casting of the concrete core starts, subdivided into four stages, in which the concrete level increases about 6 m each, for a total of about 30 m 3 /cast. The longitudinal reinforcement at the top is positioned before the casting of the last phase (Figure 16). The use of self compacting concrete notably speeded up the construction procedure of the piers, because there was totally no need of vibration, nor the need to create int ermediate openings for the tube of the casting pump, that was directly positioned at the top of the pier. 10

11 Fig. 16 Casting of the concrete core of V-piers Realization of the Pier-Deck Connection After the launching of the whole deck, the connection of the V-piers to the bridge has been realized by means of a pulvino, made by a thick concrete plate having dimensions 9.6 x 6.0 x 1.5 m. The plate holds the reinforcement of the connection, which has to sustain great bending and shear stresses. Particularly due to the high shear stresses, the perfect closure of the interface between deck soffit and pulvino extrados was of great importance. For this reason the deck soffit was properly prepared, after the bridge extrusion by washing and brushing. The realization of the connection has been made in two stages. In the first stage the casting of the thick concrete plate of the pulvino is made by using self compacting concrete. In order to achieve the total closure of all the openings at the interface between pulvino and deck, in a second stage, the interface section has been injected with a very fluid cement mortar. The injection was made through a network of PVC valved tubes, installed on the deck soffit before the casting, under a pressure of about 5 bar (Figure 17). Fig. 17 Joint between deck and V-piers Dismantling of the Provisional Piers The construction of the bridge is completed with the realization of the pier/deck connections and the tensioning of the final tendons. Consequently, the provisional supporting system can be dismantled. At this aim, the provisional piers located in the middle of the main spans were decompressed by removing the temporary bearings with hydraulic jacks. The provisional piers located under the V-piers were decompressed with a preliminary partial demolition by using a non explosive agent causing the progressive cracking of the top of each concrete pile. During this phase, the steel head were suspended at both sides with cranes. At the end of the decompression, the concrete shaft was demolished with a hydraulic breaker hammer (Figure 18). 11

12 Fig. 18 Dismantling of the provisional piers CONCLUSIONS This paper presented the Reno bridge recently realized along the new Italian motorway called Variante di Valico, connecting Bologna to Florence. The bridge is formed by a curved continuous prestressed concrete box girder deck subdivided in nine spans for a total length of m. In the first six spans, of typical length equal 45 m, the deck is simply supported by reinforced concrete vertical piers. For the remaining three spans, for a total length equal = 315 m, the bridge consists of a framed system where the deck is clamped to two V-shaped piers, each one formed by four inclined arms converging in the same foundation. Each arm is realized by an outer steel shell with elliptical cross-sectional shape, properly strengthened by internal ribbed trusses and filled by reinforced concrete. The design criteria which motivated the choice of this framed part of the bridge mainly emerged from the need of leaving the river bed clear from piers and, more generally, of minimizing the interferences with the surrounding natural and ant hropical environment. Beside to satisfy the non structural constraints, the adopted solution contributes to realize optimal bridge performance from both the static and seismic points of view. Despite of the simple layout of the structural scheme, the V-piers solution gives a remarkable aesthetical quality to the structure and represents a characteristic symbol of the whole bridge. GENERAL DATA Location: A1 Milan Naples Motorway: Sasso Marconi La Quercia Segment Client: AUTOSTRADE per l Italia s.p.a. Rome (Italy) Project Manager: Arch. Michele Donferri Mitelli Architectural and Structural Design: Structural Engineering Department SPEA Ingegneria Europea s.p.a. (Autostrade Group) Milan (Italy) Engg. Guido Furlanetto, Lucio Ferretti Torricelli, Alessandra Marchiondelli Construction Supervision: SPEA Ingegneria Europea s.p.a. (Autostrade Group) Milan (Italy) Eng. Umberto Pelosio Contractor: TOTO Costruzioni Generali s.p.a. Chieti (Italy) Construction Site Director: Eng. Massimo Maiani 12