THE EUROPE BRIDGE, IN PORTUGAL: THE CONCEPT AND STRUCTURAL DESIGN

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1 THE EUROPE BRIDGE, IN PORTUGAL: THE CONCEPT AND STRUCTURAL DESIGN A.J.Reis 1,J.J.Oliveira Pedro 2 ABSTRACT The Europe bridge is a cable stayed bridge with a main span of 186m. A 3D stay cable arrangement is adopted to suspend a composite 3D truss in the deck erected by a precasted segmental scheme. The concept and structural design is described. 1. INTRODUCTION The Europe Bridge, presently under construction in Coimbra, Portugal, is integrated in the national highway IC3 crossing the Mondego River upstream of the existing bridge of Santa Clara. The general layout of the highway system under construction is shown in Fig. 1. Two roundabout are connected through a complex set of viaducts and branches establishing the connection between the main bridge and the urban roadway network. The project was developed taking into consideration the urban nature of the bridge, apart from its highway functional requirements for 6 traffic lanes. From the bridge engineering point of view, the structure under construction has several innovative concepts, the most relevant one of being a cable stayed bridge with a three dimensional composite truss for the deck built by precasted segmental construction. It represents the first Portuguese bridge, design and built with national technology, by a precasted segmental scheme. 1 2 Technical Director of GRID-Consulting Engineers. Professor of Bridges and Structural Engineering at the IST-Technical University of Lisbon Civil Engineer at GRID-Consulting Engineers. Assistant at the Depart. of Civil Engineering at the IST-Technical University of Lisbon

2 Fig.1 The Europe Bridge in Coimbra. Location and connections to the city network. 2. CONCEPTS AND DESIGN OPTIONS For design case to be discussed here it was important to understand in the design competition what were the expectations of the general public and of the Owner. The bridge should not be a simple connecting system between two parts of the city. It should be a part of the city, a pedestrian connection, as well, between the two green parks planned for the river banks, a landmark for the city. Urban bridges shall satisfy aesthetics and functional demands and to be the expression of social, cultural values and technological developments of our era. Other basic concepts taken as well into consideration at the preliminary design were related to the type and levels of highway traffic forecasted, requiring access connections (branches and roundabouts), with the city network yielding additional difficulties for the pedestrian function. Besides, the riversides are green spaces for the city and should be connected by the bridge for the benefit of the pedestrian use. The left hand riverside will integrate in the future residential buildings and public equipments. The bridge may be in the future a public space for viewing the city and enjoying the Mondego River. The bridge, for six lanes of roadway traffic, has a total length of 330,3m between the expansion joints at the transition piers with the approach viaducts. In the right hand side bank, the approach viaducts enter into the city with complex curved branches, including as well a two level avenue. At the preliminary designed for the competition, we have studied 5 solutions, including 3 cables stayed bridges, 1 arch type solution and 1 balanced cantilever box girder solution. The design solutions were developed taking into consideration the following aspects: - Functionality, environmental integration and aesthetics. - The occupancy of the river by bridge piers. - Symmetries, asymmetries and spatial integration.

3 - The pedestrian function. - Transparency, slenderness and harmony. The solution retained, is a cable-stayed bridge (Figs.2 and 3) with a single mast, located in the riverside opposite to the occupied city zone, and inclined at 8º with the vertical. Figure 2 The Europe Bridge in Coimbra Montage. Aerial view. The bridge has a main span of 186m over the river. A 3D arrangement of stays is adopted, axial staying in the main span (double stay cables) and two planes of externally anchored backstays. These two planes, (Fig. 3) opening in the transverse direction, are like a gateway to the city. The deck is a 3D composite truss, made of high strength steel tubes and two concrete slabs prestressed in the transverse and longitudinal directions. The cross section of the deck (Fig. 4) is equivalent to a box girder, 3.70m height, being the lower flange adopted for the walkway. Two helicoidal ramps connect this walkway to the green parks in the riversides. The transparency of the bridge deck is apparent when the bridge is viewed from the city and when the pedestrians walk on the lower flange. The pedestrian and roadway functions of the bridge could be accomplished with this type of solution. The bridge deck is executed by a precasted segmental scheme with the precasting yard located in the right riverside and the precasted segments (3,75m length), weighting a maximum of 150 tons, being transported in the river and lifted to the deck. After the assembly by prestressing, the overhangs of the deck slab are casted in situ and a 2 nd transversal prestressing applied. There is no need to transport precasted segments in the city, avoiding any interference with the operating traffic. With the proposed solution, the following objectives were accomplished: Figure 3 The Europe Bridge. Montage. View from the left side bank.

4 - functional compatibility for the highway traffic and the pedestrian use of the bridge. - configuration, by the bridge, of a new urban space relating the two parts of the city. - bridge typology contributing to the monumental impact an expected landmark for the city. - design and construction technology, keeping the traffic transportation systems permanently in operation Figure. 4 The Europe bridge: cross sections. 3. THE STRUCTURAL SYSTEM The bridge, is a cable stayed bridge with a main span of 186m and side spans of 45 and 50m in the left riverside and 46m in the right riverside, as shown in Fig.5. Fig. 5 Longitudinal cross section and foundations.

5 The deck is supported by neoprene teflon pot bearings allowing unidirectional movements at the piers P2, P3 and P4. At the pier P1, the deck is built in, transferring all the longitudinal forces induced by the stay cables. This makes P1 as an abutment where the anchorages of the backstay cables are located. The overall equilibrium of P1 is as shown in Fig.6. The horizontal component of the forces of the back stays balance partially the horizontal force induced by the deck. Only the difference of these forces is transferred to the foundation by the system of barrets 6.0 x 1.0m 2. Fig.6 Equilibrium of permanent forces for the Europe bridge. At the pier P3, the deck can slide over the pier, which extends by a mast up to about 85m above the ground. The deck is a continuous composite truss girder a box girder type with a depth, between the external faces of the flanges, of 3.70m. The lower flange is a longitudinal prestressed slab with a width of about 10m and the upper flange a transverse and longitudinal prestressed slab of 30m width. The thickness of the lower flange is, for the typical sections, 0,30m; is the upper flange a variable thickness is adopted between 0,20m at the tip of the cantilever and 0,93m at the connection with the transverse diagonals. The concrete adopted for the deck is the C45/55 and the steel reinforcement A500NR (f syk = 500N/mm 2 ) The transverse prestressing of the upper flange has a maximum of is 1300 kn/m(effective prestressing). The longitudinal prestressing in the upper and lower flanges is variable along the bridge and is made of internal prestressing cables, generally between 7 and 12 strands 15mm each, and prestressing bars 32mm diameter. The upper and lower flanges are interconnected by the steel diagonals with rigid connections at the nodes, as described in the next section. The stay cables with a three dimensional arrangement, as previously referred, are made of independent 15mm strands, and are 7,5m distance apart along the deck at the main span. There are two stay cables at each section of the main span, 31 to 50 strands each, arranged in two parallel planes about 0,80m distance apart and anchored along the axis of the bridge. The back stage cables, from 37 to 91 strands, are arranged in two planes, anchored at the foundation of pier P1 and the top of the mast.

6 (a) (b) Fig. 7 Masts: Longitudinal and cross sections. The mast has a variable box cross-section with overall external dimensions of 2,3x6,93 m at the base and 3,89x4,50m at the top as shown if Fig.7. The upper part is a steelconcrete composite structure, where the steelwork is connected to the concrete walls by 22mm shear connectors. The steel part of the mast was designed to resist to the anchorage forces for the stay cables. The foundations of piers P2, P3 and P4 are made of 2,0m bored piles as shown is Fig.5 the maximum depth reached by the piles is about 25m. 4. THE CONSTRUCTION SCHEME The bridge deck was designed to be built by a precasted segmental scheme, with epoxy joints and segments of 3,75m length, as shown in Fig. 8. To reduce the maximum weight of the segments to 1500 kn as well as transverse differential shrinkage effects, the width of the segments was limited to 20 meters. The remaining part of the deck width, are the transversal cantilevers (overhangs), which are concreted after erection of the precasted segments.

7 Fig. 8 Model of the precasted segments. The 1 st and 2 nd spans, i.e. P1-P2 and P2-P3, were erected by using formwork supported from the ground; the main span is erected by cantilever, using a derrick to lift the precasted segments from the barge. After erecting a 3.75m segment, the longitudinal prestressing is applied and a new 3.75m segment is erected; the stay cables are installed every two segments. The segments are casted in a precasting yard, and are moved by a crane. The segments are matched casted with epoxy joints. After erection of a certain number of segments at the main span, the transversal cantilever of the deck is concreted and the while progressing with the installation of the stay cables at the main span. This allows to take advantage of the longitudinal forces induced by the stay cables to induce compressive stresses at the cast in situ part of the deck. 5. STRUCTURAL STEELWORK The steelwork of the Europe Bridge is the steel structure of the deck and the steel structure included in the upper part of the mast. The deck, as previously referred, includes a 3D truss made of high strength tubes in steel S460NH (EN10210) CHS type with 298mm diameter and variable thicknesses between 12,5mm and 50mm. The steel diagonals are connected at the joints by steel plates S420NL (EN10113) as shown in Fig. 9. Three typical details are adopted as shown in the figure and in the model. All the connections are by welding. The diagonal forces are transferred between adjacent segments by shear and compression at the special devices as shown in Fig. 9 at the level of the lower flange.

8 (a) (b) Fig. 9 Model of the steel structure of the deck At the upper flange, the transference forces between adjacent precasted segments are made by the concrete contact surfaces. Shear keys are adopted at the concrete surfaces, at the upper and lower flanges of the precasted segments. 6. DESIGN CRITERIA AND STRUCTURAL ANALYSIS The design criteria adopted, were based on the national code or actions and structural safety (RSA, 1983), or the action code for reinforced and prestressed concrete structures (REBAP, 1983) and or the Eurocode 3 for the design of steel structures (ENV ). A limit stare design format was adopted, which for the ultimate limit states yields the fundamental combination of actions m m G ik + γ q Qik + ψ oj Q jk (1) j= 2 γ i= 1 where G ik characteristic values of the permanent action Q ik characteristic value of the basic variable action gi

9 Q jk characteristic values of the remaining variable actions The partial safety factors are γ gi 1.0 or 1.35 γ g for prestressing unfavourable effects γ q and the reduction coefficients ψ o are the ones established in the Portuguese code RSA. The live loads include two track loadings (600 kn each) or the uniform live loads of 4 kn/m 2 and 50 kn/m as predicted in RSA. Live loads on the walkways (lower flange) were taken according to RSA as 3 kn/m 2 or 20 kn. The uniform thermal actions were taken as ± 15ºC in concrete elements and 35ºC or 25ºC in the steel elements. In what concerns the thermal differential actions in the deck, a combination value of 5ºC between the upper and the lower flange was adopted. Thermal differential actions between the mast and the deck, as well as, between the stay cable and the deck and pylon, were considered. The seismic actions were considered according to the national code, on the basis of the response spectrum predicted in RSA for zone C and with a 5% damping coefficient. For the wind actions, the national code was adopted to quantify the wind speeds. The aerodynamic coefficients were obtained from a wind tunnel test on a sectional model at a 1/62 scale. The upper and lower flanges in the model were made of fibber glass and the diagonals were modelled by cupper tubes with 4,8mm diameter. The total length of the model, for the wind tunnel test, was 450mm. The scale for the wind speeds was 1:5 and the scale of vibration frequencies was 12,4:1. The lowest vibration frequencies in the structure are associated to the bending vertical vibrations and torsional vibrations of the deck at frequencies of 0,51Hz and 0,71Hz. In the wind tunnel tests, the drag, lift and moment aerodynamic coefficients were obtained, defined respectively from the forces (kn/m). 2 D = C D Wh ; FL = C L Wh and M C M Wb (2) F = where W = 1/2ρ U 2 (N/m 2 ) is the wind dynamic pressure, ρ being the air unit mass, h and b are respectively the reference height and width which in the prototype are h=4,3m and b=30m. The maximum drag coefficients C=1,7 were obtained for angles of attach α=4º; the average value of the drag coefficient for α=0, is about 1,0. This value was adopted for the static equivalent wind analysis of the structure. The lift coefficient varies between 0,45 and 0,30 for 4º <α<4º. In what concerns the aerodynamic stability, this was investigated in the wind tunnel tests, as well as with a simplified calculation based on the CECM-ECCS Recommendations for the wind effects on Constructions as well as on the British Design Rules for Bridge Aerodynamics. In the wind tunnel tests for aerodynamic stability the wind speeds were varied between 72 km/h and 270 km/h, as referred to the prototype, with angles of attach of 4º, -2º and 4º. No indication of any type of aerodynamic instabilities was detected in the tests. The static and dynamic structural analyses were based on two finite elements models, one for the erection phase and the other for the service phase of the bridge. Four types of nonlinearities were taken into consideration the catenary effects in the stay cables, the

10 second order effects in the masts, nonlinearity of the foundation of pier P1 and the time dependant effects of the concrete according to the model code CEB-FIP. The local effects in the deck were considered on the basis of a finite element model including shell elements for the flange and 3D bar elements for the diagonals. With respect to the structural design of the deck, the basic design criteria for serviceability limit states were the decompression limit state, the crack width limitation as well as the deformation and vibration limit states. Being a precasted segmental deck, no tension was allowed in the joints between segments for the rare combination of actions including the rare values of the live loads and the thermal gradients in the deck. In what concerns the steelwork of the superstructure, the diagonals were designed according to EC3 as beam-columns. ACKNOWLEGMENTS: Thanks are due to BEG-Bureau d Etudes Greisch, responsible for the checking of the design of the Europe Bridge, as well as, by all the contributions given for the detailed design of the steelwork and contribution for the studies for the erection phase.