pedestrian bridge in the city of logroño

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__________________________________________________________________________________ C/ Hernán Cortés, 19 - 1º Dcha / 39003 Santander * Tfno: (+34) 942-319960 * Fax: (+34) 942-319961 * E-mail: [email protected]

Pedestrian Bridge in the city of Logroño (Spain). Juan J. Arenas - Dr. Civil Engineer Guillermo Capellán - Civil Engineer. Arenas & Asociados Santander, Spain. February 2007 Located in the North part of Spain, Logroño, about 160.000 inhabitants, is the small capital of the La Rioja region, well known because of its high quality and important production of red wine. The city lies along the south bank of the Ebro River, flowing from the West to the Mediterranean, and giving rise to an urban model made up of straight streets arranged as an orthogonal net. The South border of the city has been marked for many years by a peripheral highway that has limited its urban development. Until the City Council decided to jump over the highway and develop the South areas, with the unavoidable requirement of erecting several pedestrian bridges. Logroño being now a red spot for symbolic, modern architecture, the pedestrian bridge of la Cava is the result of a contest launched by the Logroño City Council. The span to be covered by the structure was about 60 meter and the minimum vertical clearance over the highway level being 530 cm. Both ends of the pedestrian bridge were green park areas with enough space for developing the access ramps. The winner proposal of the contest was the engineering firm Arenas & Asociados. The structure was conceived like a steel three dimensional truss, spanning 61 meter over the highway, with a cross section of curved members and triangular shape, with variable height and width. The overall impression transmitted by the truss is that of a one span flexure resisting girder. Materialised with steel profiles and tubes, the new structure recalls us the gothic art shapes. Since the pedestrians walk inside the three dimensional truss, in their relation with the structure enjoys its internal architectural space. The fact of using the lateral curved façades of the structure for creating walls of glass creates a pedestrian space fully protected from the rain, the wind and, very important detail, from the noise of traffic.

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Figure 1: Local plan of the new crossing.

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The pedestrian bridge spans over the urban highway. The right zone is a green park area where two ramps of different slopes are located. At the left one can see the new residential quarters and the wooded hill that allows several ramps to develop. The steel truss has been conceived with a maximum simplicity. In fact it is made up of transverse steel frames longitudinally spaced 360 cm, connected at the vertexes of the triangular shape by 3 main longitudinal members, able to support compressive or tensile forces. The upper vertexes of the transverse frames are connected through a couple of circular tubes 60 cm diameter. For the longitudinal member of the crown we have used 2 symmetrical tubes instead of only one because welded connections with the transverse frames are easier, but also because the duality (2 tubes) reinforces the power of the ideal vertical plan. The steel profiles that connect the lower vertexes of the triangular shape are steel, trapezoidal shape, fully designed members, looking for an optimum connection with the lower end of the frames. We can observe that the set of transverse frames welded to the longitudinal upper tubes and to the lower trapezoidal profiles give rise to a basic structure, able to support longitudinal bending moments but clearly unable to transmit shear forces. This is why a secondary net of small diameter tubes must reticulate the space between each pair of transverse frames. With this light system placed in every one of the two curved façades, and with the floor materialised as a concrete-steel composite cross section, the spatial truss becomes fully structurally efficient. Where each one of the lateral curved façades can be understood like a steel shell structure.

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Figure 2: Elevation, Plan and cross sections of the pedestrian bridge.

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The steel three-dimensional truss shows clearly the variable size of its different cross sections. Simply supported at the left abutment, it is fixed in the right (North) support, giving rise here to a negative bending moment materialised by a couple of forces, where straight, external members are ready to receive them and to translate them to the foundations.

Foto Cámara Oscura Figure 3: View of the bridge looking westwards. At the right side there is the North abutment where the spatial truss is fixed. At the left, the truss is simply supported on a precast concrete wall that contains the artificial hill. Because of the ground organization, we conceived the truss girder as a beam fixed in the North abutment and simply supported at the South one with the width and height of the truss decreasing gently from the North to the South abutments. It is interesting to note that varying the depth of a beam is a usual decision in bridge design with clear structural and aesthetic consequences. But in the case of the Logroño pedestrian bridge, such height variation is accompanied and underlined by the decreasing width of the deck when one walks from the city to the new quarters. The result is that crossing the bridge constitutes a true dramatic experience, because the cross section of minimum size is coincident with the opening of the South abutment, this last frame acting as a threshold of the crossing, when the walker clearly feels that, throughout the bridge, he has been translated to a different and more open urban ambience. As bridge designers, we are usually constrained to constant width designs. The width must be maintained in traffic bridges but in pedestrian bridges a constant width is not at all compulsory. On the contrary, one of the cultural values of an urban footbridge is the contribution to a particular ambience where the pedestrian is invited to go through the bridge. We think that a strong variation of the width and the height of the truss, like the one offered in the Logroño design, creates the wish and appeal people to walk along it.

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Foto Cámara Oscura

Figure 4: Inside night and day views of the bridge from the North abutment.

Figure 5: Inside view of the end of the steel truss in connection with the North abutment. Fixing the spatial truss at one support means resisting in this cross section a negative bending moment, in the same way than a typical continuous girder does. But the gothic shape of the truss cross section makes it necessary to provide the particular structural members that make it possible to resist such negative moment. The bending moment is equivalent to a couple of horizontal forces, placed at the crown of the frame and at the deck level. The first one is the tensile force, transmitted by the upper tubes. The second force is the sum of two equal, compressive forces transmitted by the lower trapezoidal girders to the concrete platform that, with an interesting spatial organisation, constitutes the North abutment of the structure.

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As one may see in the drawings and photos, such North abutment is composed by a two tier, reticulated, concrete and steel structure, conceived for taking the above named forces from the steel truss and for transmitting them to the foundations level. It is not easy to describe such a particular spatial structure, but one can understand that it is the result of a search for a maximum clarity of the spatial flow of forces, and, at the same time, for the aesthetic quality of the resulting members and joints. The three dimensional model here included is enough expressive of the architectural interest that can be derived from an engineer’s pure conception.

Figure 6: Model of the steel truss and the North abutment.

Figure 7: The North abutment.

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The spatial truss is fixed, resists a negative bending moment and must transmit to the abutment the resulting forces. The axial steel member which stretches the upper tubes of the truss supports a tensile force that is derived by an inclined axial steel member (also under a tensile force) and by two transversely inclined steel struts that, under a compressive force transmit them to the deck of the abutment.

Figure 8: Joint in the North abutment. The steel compressive strut transmits to the concrete deck the inclined, compression force arisen from the negative moment of the truss. As engineers trying to design on a base of rationality (structural efficiency) and of aesthetic quality, it is important to explain the reasons, beyond the purely structural, of the choice of the cross section variation. Up to the point where we are able to rationalize our choice, we can say that:

1. The pedestrian bridge connects the new South quarters to the old city. Of course, the inhabitants of the North quarters will use the pedestrian way for accessing the South, but t it seems clear that the centre of gravity of the urban entourage is now and will be placed in the future in the North park. To fix the structure in the North abutment, with the corresponding concentration of structural members, seems then fully reasonably.

2. A driver going along the peripheral highway detects, by the volume variation of the truss, where the main activity of the city is located.

3. The structural members that fix the truss, able to resist the horizontal tensile force transmitted by the crown tubes to the North abutment, herald the whole structure and, instantly allow us to understand the flexure mechanism of the truss.

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Figure 9: Glazing of the lateral façades of the steel spatial truss. The shapes of the structure become subtle under the glass. The transverse gothic frames and the longitudinal members at the crown and bottom define a clear, though unfinished, construction. The reticulated thin tubes in the lateral façades transform the basic truss into a complete structure. It is interesting to note that the good rule of providing bridges with a curve upwards profile, is here reinforced by the horizontal, transverse curvature derived from the variable shape of the transverse frames. The resulting summary is structural force trough geometrical lightness.

Foto Cámara Oscura

Figure 10: Maximum integration of the urban elements: Bridge, structural steel members of the abutment, reticulated concrete structure, ramps, trees and park. Glazing provides the truss ….

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Figure 11: Night view of the ground level of the North abutment. Arch shapes provide vertical anchorage fort the steel axial member placed in the first floor and, at the same time, a direct way for transmitting horizontal forces. In the North abutment there are two curved ramps that allow people to go up and down for accessing the footbridge, offering slopes of 6 percent and 8.8 percent. They are placed in a green park and we have taken care of their insertion among the existing trees. For the design of ramps we have counted with the collaboration of Arrizabalaga Architects, with plentiful results. Because the footbridge is intended for the use of a population located along a sizable park, the two ramps allow us to a better distribution of the citizens walking the bridge.

Figure 12: The two ramps at the North abutment with different slopes. The design of steel supports has tried to harmonize with the park trees, and has integrated the lamps.

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Figure 13: The two curved ramps of the North abutment. The design of the South abutment is central to the urban quality of this project. The contribution of Arrizabalaga Architects has been the proposal of an earth hill of elliptical shape that allows developing several ramps with greater or lesser slope. The hill composing by itself a green decoration and a noise protector for the inhabitants of the South quarters.

Foto Cámara Oscura Figure 14: End portal frame of the bridge over the South abutment. This is the cross section of the truss of minimum size. The increasing width and height of the truss when people walk to the North transmits them the feeling of accessing through the footbridge the historic quarters of the city.

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Foto Cámara Oscura Figure 15: Night view of the bridge from the South hill and ramps. The inclined end frame of the truss acts as a kind of cantilever that prompts people to go up the hill and to walk the bridge.

Foto Cámara Oscura Figure 16: View of the South hill before tree plantations. The earth construction provides the bridge with horizontal stability. As we have tried to reach, there is a direct contrast between the lightness of the truss and the fill earth construction.

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Method of Erection The construction techniques used during the erection of the new footbridge are mainly the following: - Construction of the concrete access ramps using temporary falsework, together with the definitive

steel piers. - Construction of the steel structure of the main span on the ground. Erection of the main span using

cranes. Pouring of the concrete slab once fixed the North abutment, using steel sheets composite deck system.

The basic construction phases of the process can be seen in the following figure.

Figure 17: Elevation of the main construction phases for the main span.

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Figure 18: Construction on yard of the steel structure of the main span.

Figure 19: Detail of the steel structure during construction.

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Figure 20: Erection of the main span using cranes at night time. The light weight steel structure of the main span is chosen from the very start to be erected in one piece using cranes. In this way the construction of its skeleton is carried out without any disturbance to the traffic. Other than the glazing and the final touches of protective paint, only the pouring of the concrete slab is carried out over the existing road, but even this is carried out without any falsework, with the composite deck steel sheet system. Erection operation is one of the key moments, in which an enormous weight of two hundred tons is lifted and placed in position with tolerances of millimetres.

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Detailed design The construction project of such a structure involves a great amount of detailing and engineering. All through this process, carefully aesthetic design is applied not only to general shapes and outlines, but to each and every element. Such is our commitment, and it can be seen in some of the examples following, in which the connection of the different elements and their relationship is solved both from a technical side and aesthetical point of view.

Figure 21: Definition of one of the steel arched frames. Concrete-steel connection using nelson stud connectors.

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Figure 22: Detail of joints between diagonals and arrival to the main tubular section.

Figure 23: Photos of the joint loading test carried out in the University of Cantabria, in Santander.

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Figure 24: Detailed design of the concrete triangular frame. Geometry

Figure 25: Reinforcement of the triangular frame. Connection to steel structure using steel bars.

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Figure 26: Design of the steel piers in the access North ramps, including lighting elements. Structural, architectural and urban elements are taken into account in the same design effort.

Figure 27: The glass covering creates a protective skin for the footbridge. Lateral service walkways are placed to allow glass cleaning and replacement.

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Structural behaviour The formal interest of the proposed solution is only comparable to its structural adaptation, as it is seen in

the analysis models developed.

Figure 28: Views of the 3D Finite element model used in the analysis.

The main elements the structure is made out are:

- Steel transversal arches: They give the bridge its tubular form, contribute to transversal stability and

hang the deck from its supporting structure. “H” Section shape.

Figure 29: Typical steel transversal arch connecting top and bottom main girders.

- Main longitudinal girders: With double tubular section at the top, and two diamond sections at the

bottom, they form the main supporting elements of the structure. As top and bottom chords of the truss

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they support the bending moments through axile stress. The main span having a fixed end and a hinged

end, the top chord is under tension at the North Abutment at its high point, and under maximum

compression close to mid span section, being exactly the opposite in the bottom chords

Figure 30: Top and bottom main girders typical sections.

- Diagonals: The diagonal elements with a tubular section they form the lattice or spatial net that transmits

the stress between the top and bottom chords of the spatial truss. They cross at pinned joints in the

confluence with the transversal steel arches.

Figure 31: Compression (red) and tension forces in the main top and bottom chords of truss.

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Figure 32: Compression (red) and tension forces in the diagonal, in a crossed arch and suspension effect.

Figure 33: Compression (red) and tension forces in the fixed end of the structure with inclined struts, and

central stay.

pedestrian bridge in the city of logroño

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