The Europe Bridge in Portugal: concept and structural design

Transcription

1 Journal of Constructional Steel Research 60 (2004) The Europe Bridge in Portugal: concept and structural design A.J. Reis a,b,, J.J. Oliveira Pedro a,b a GRID-Consulting Engineers Ltd., Av. João Crisóstomo 25, 3 v, 1050 Lisbon, Portugal b Civil Engineering Department, Instituto Superior Técnico, Lisbon, Portugal Abstract The Europe Bridge is a cable-stayed bridge with a main span of 186 m. A 3D stay cable arrangement is adopted to suspend a composite 3D truss in the deck erected using a precasted segmental scheme. The concept and structural design are described. # 2003 Elsevier Ltd. All rights reserved. Keywords: Urban cable stayed bridge; 3D stay cable arrangement; Precasted segments; Composite deck 1. Introduction The Europe Bridge, presently under construction in Coimbra, Portugal, is integrated to the National Highway IC3 across 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 roundabouts are connected through a complex set of viaducts and branches, thereby 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 of six traffic lanes. From the bridge engineering point of view, the structure under construction has several innovative concepts, the most important one being that of a cable-stayed bridge with a 3D composite truss in the deck built by using the method of precast segmental construction. It represents the first Portuguese bridge, designed and built based on the national technology by a precasted segmental scheme. Corresponding author. Tel.: ; fax: address: grid@grid.pt (A.J. Reis) X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi: /s x(03)

2 364 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Fig. 1. The Europe Bridge in Coimbra. Location and connections to the city network. 2. Concepts and design options For the design case to be discussed here, it is important to understand the expectations of the general public and of the owner in the design competition. 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 at the river banks, besides being a landmark. Urban bridges should satisfy aesthetics and functional demands and should be the expression of social, cultural values and technological developments of our era. Other basic concepts also taken into consideration at the preliminary design stage were related to the type and levels of highway traffic forecasted, requiring access connections (branches and roundabouts) with the city network posing additional difficulties for the pedestrian. Besides, riversides are green spaces of the city and should be connected by the bridge for the pedestrian s benefit. The left-hand riverside will integrate residential buildings and public institutions in the future. The bridge may also be a public place for viewing the city and enjoying the Mondego river. For six lanes of road traffic, the total length of the bridge is m between the expansion joints at the transition piers with the approach viaducts. On the righthand bankside, the approach viaducts enter into the city with complex curved branches, including a two level avenue as well.

3 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) At the preliminary design stage of the competition, we studied five models, including three cable-stayed bridges, one arch type solution and one balanced cantilever box girder solution. The models were developed taking the following aspects into consideration: Functionality, environmental integration and aesthetics. The occupancy of the river by bridge piers. Symmetries, asymmetries and spatial integration. The pedestrian function. Transparency, slenderness and harmony. The model chosen 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 8 v vertically. The bridge has a main span of 186 m 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, 3.70 m in height, with the lower flange adopted as a 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 pedestrians walk on the lower flange. The pedestrian and roadway functions of the bridge could be accomplished with this type of model. Fig. 2. The Europe Bridge in Coimbra. Montage. Aerial view.

4 366 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Fig. 3. The Europe Bridge. Montage. View from the left-side bank. The bridge deck is executed by a precasted segmental scheme with the precasting yard located on the right riverside and the precasted segments (3.75 m length), weighing a maximum of 150 tons, transported in the river and lifted to the deck. After assembly by prestressing, the overhangs of the deck slab are cast in situ and a second transversal prestressing is applied. There is no need to transport precasted segments in the city, avoiding any interference with the operating traffic. With the proposed model, the following objectives were accomplished: Functional compatibility for the highway traffic and the pedestrian use of the bridge. Configuration, by using the bridge, of a new urban space relating the two parts of the city. Fig. 4. The Europe Bridge: cross-sections.

5 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Bridge typology contributing to a monumental impact an expected landmark for the city. Design and construction technology, keeping the traffic transportation systems permanently in operation. 3. The structural system The bridge is a cable-stayed bridge with a main span of 186 m and side spans of 45 and 50 m on the left-hand riverside and 46 m on the right-hand riverside, as shown in Fig. 5. The deck is supported by neoprene teflon pot bearings allowing unidirectional movements at piers P2 P4. At pier P1, the deck is built in, transferring all the longitudinal forces induced by the stay cables. This makes P1 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 backstays balances 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 1:0 m 2. At pier P3, the deck can slide over the pier, which extends by a mast up to about 85 m 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, measuring 3.70 m. The lower flange is a longitudinal prestressed slab with a width of about 10 m and the upper flange a transverse and longitudinal prestressed slab of 30 m width. The thickness of the lower flange is, for the typical sections, 0.30 m; for the upper flange, a variable thickness is adopted between 0.20 m at the tip of the cantilever and 0.93 m at the connection with the transverse diagonals. The concrete adopted for the deck is C45/55 and the steel reinforcement A500NR ðf syk ¼ 500 N=mm 2 Þ. The transverse prestressing of the upper flange has Fig. 5. Longitudinal cross-section and foundations.

6 368 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Fig. 6. Equilibrium of permanent forces for the Europe Bridge. a maximum of 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 of 15 mm each, and prestressing bars of 32 mm 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 3D arrangement, as previously referred, are made of independent 15 mm strands, and are 7.5 m apart along the deck at the main span. There are two stay cables at each section of the main span, strands each, arranged in two parallel planes about 0.80 m 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. The mast has a variable box cross-section with overall external dimensions of 2:3 6:93 m at the base and 3:89 4:5 m at the top as shown in Fig. 7. The upper Fig. 7. Mast: longitudinal and cross-sections.

7 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) part is a steel concrete composite structure, where the steelwork is connected to the concrete walls by 22 mm shear connectors. The steel part of the mast was designed to resist the anchorage forces for the stay cables. The foundations of piers P2 P4 are made of 2.0 m bored piles as shown in Fig. 5. The maximum depth reached by the piles is about 25 m. 4. The construction scheme The bridge deck was designed to be built by a precasted segmental scheme, with epoxy joints and segments of 3.75 m 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 m. The remaining part of the deck width is the transversal cantilevers (overhangs) which are concreted after erection of the precasted segments. The first and second 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 3.75 m segment, longitudinal prestressing is applied and a new 3.75 m segment is erected; the stay cables are installed at every two segments. The segments are cast in a precasting yard, and are moved by a crane. The segments are match casted with epoxy joints. After erection of a certain number of segments at the main span, the transversal cantilever of the deck is concreted 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. Fig. 8. Model of the precasted segments.

8 370 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Structural steelwork The steelwork of the Europe Bridge is the steel structure of the deck and of the upper part of the mast. The deck, as previously referred to, includes a 3D truss made of high strength tubes of steel S460NH (EN10210) CHS type with 298 mm diameter and variable thicknesses between 12.5 and 50 mm. 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. 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. Fig. 9. Model of the steel structure of the deck.

9 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) Design criteria and structural analysis The design criteria adopted were based on the national code of 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 state design format was adopted, whose ultimate limit states yields the fundamental combination of actions: " # X m c gi G ik þ c q Q ik þ Xm w oj Q jk ð1þ i¼1 j¼2 where G ik is the characteristic values of the permanent action; Q ik is the characteristic value of the basic variable action; Q jk is the characteristic values of the remaining variable actions. The partial safety factors are c gi ¼ 1:0 or1:35, c g ¼ 1:2 for prestressing unfavourable effects; c q ¼ 1:5, and the reduction coefficients W 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 to be 15 v C in concrete elements and 35 or 25 v C in the steel elements. In what concerns the thermal differential actions in the deck, a combination value of 5 v C between the upper and the lower flanges 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 fiberglass and the diagonals were modelled by copper tubes of 4.8 mm diameter. The total length of the model, for the wind tunnel test, was 450 mm. 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 0.51 and 0.71 Hz. In the wind tunnel tests, the drag, lift and moment aerodynamic coefficients were obtained and defined, respectively, from the forces (kn/m). F D ¼ C D W h ; F L ¼ C L Wh and M ¼ C M Wb 2 ð2þ where W ¼ 1=2 q U 2 (N/m 2 ) is the wind dynamic pressure, q being the air unit mass, h and b are the reference height and width, respectively, which in the prototype are h ¼ 4:3 m and b ¼ 30 m.

10 372 A.J. Reis, J.J.O. Pedro / Journal of Constructional Steel Research 60 (2004) The maximum drag coefficients C ¼ 1:7 were obtained for angles of attachment a ¼ 4 v ; the average value of the drag coefficient for a ¼ 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 4 v < a < 4 v. 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 and 270 km/h, as referred to the prototype, with angles of attachment of 4 v, 2 v and 4 v. 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 second order effects in the masts, nonlinearity of the foundation of pier P1 and the time dependent 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 as beam columns according to EC3. Acknowledgements Thanks are due to the Bureau d Études Greisch (BEG), who were responsible for the checking of the design of the Europe Bridge as well as to all the contributions received for the detailed design of the steelwork and for the studies for the erection phase.