When the new arch bridge was first proposed, the design assumed that the construction would be carried out using temporary towers and temporary stays to support the arch cantilever. Joint-venture contractor Ferrovial-Agroman/Vias y Construcciones proposed an alternative construction method, eliminating the towers and using temporary diagonals instead for support, and as a result, redesigned the whole structure, introducing the use of high-strength concrete for the arch and piers, and steel for the deck. The revised design was carried out by the technical division of Ferrovial-Agroman.

The cross-section of the arch is designed as a high-strength concrete (75MPa) box to minimise its size and weight, resulting in a box only 6m wide by 3m deep. The walls are extremely slender, with webs 250mm thick and 200mm on the webs and slabs. These thicknesses increase by 50mm on the three segments next to the springing of the arch, where the stress is greater. The arch piers are also made of high strength concrete which minimises the size and self-weight of these units too. The bridge deck is composite steel and concrete; the steel part consisting of two box beams 1m deep and 400mm wide, made of S355 JR steel; the concrete part is 12m wide and a variable thickness concrete slab of up to 260mm, which gives the camber to the carriageway.

Construction of the deck abutments started in June 2001 with excavations being carried out for the foundation slabs, which were then concreted in order allow the rock anchorages to be drilled. These were designed to support the free cantilever during the construction process and were built by subcontractor Kronsa. Twenty steel cables exerting an ultimate force of 10,000t/m on each abutment made up each rock anchorage. The cables were installed in four planes with lengths ranging from 30m up to 35m, the last 20m of which was anchored back into the underlying rock. In addition, tests were performed on the anchorages to measure adherence and creep.

Once the construction of the anchorages was complete, the abutments were finished and the arch access viaducts built. Extremely complex excavations were carried out on both slopes in order to build these viaducts, which extended to the arch abutments. In April 2002, once the south abutment of the arch had been concreted, the travellers were assembled and the construction of the arch, using cantilever construction supported by temporary diagonals, began. This process had four distinct phases.

The first phase was to build the arch section - the length of arch between adjacent piers - and this was made up of segments concreted on site while supported by temporary stays. Each arch section was divided into four segments, each about 6m long. Each segment was concreted in cantilever using a special formwork unit, which was supported by the previously-cast segment and temporary stays. Once each segment had hardened, the cantilever formwork was moved forward by built-in hydraulic jacks and wheels, to the position of the next segment, 6m further on. The formwork was positioned to take into account any future deflections it may undergo, and it was connected to the edge of the previous pier. Reinforcement was added, the interior and exterior formwork was closed, and the entire segment was concreted in a single phase. State-of-the-art superplasticiser additives were included in the concrete to create a workable mix that could fill the entire section in a single go.

Once all four segments of the section were finished, a new temporary diagonal was installed and the previous ones were adjusted. These diagonals were made up of 15mm-diameter steel cables - 12 or 15 depending on the section - with an ultimate force of 26tm each. The cables were distributed in four planes, two on each side of the arch section. This configuration was chosen so that if a cable should fail due to the prestressing jacks being incorrectly operated or for any other reason, there would still be sufficient support available. In fact the system was proved to work properly during the construction project, when a number of strands came loose because the wedges were not properly hammered in. Because of the fact that there were four cables, it was easy to change the members and wedges that were suffering from this problem, by releasing one cable at a time.

Once the diagonal cables had been installed and stressed by subcontractor Tecpresa, the construction of the next pier column would start. Columns were built using climbing formwork in 4m raises.

The final part of the cycle was to place the steel deck section with the help of a crane. Steel members were delivered over the deck sections that had already been built, lifted by the travelling crane which then slewed 180 degrees to place them above the new arch section. Firstly the pier cross-beam was placed, then the longitudinal beams and finally the cross bracing. The joints of all these steel members, and their connections with the previous deck sections were designed to be prestressed with high-strength bolts in order to make the deck-assembly process easier and quicker.

The greatest stresses on most of the members in this structure occur during the construction process, therefore the materials chosen had to be suitable for that phase of the work - a fact that required in-depth analysis of the construction method. The free cantilever method is analysed by using a variable-depth lattice beam where the deck is represented by the upper member of the beam, the arch by the lower member, the piers by the struts and the cables by the diagonal.

The basic concept of a lattice - bar triangulations that work in compression or tension - demonstrates that the upper member works in tension, the lower in compression, the struts also work in compression and the diagonals work in tension. Whether the diagonals are in tension or compression depends on the direction in which they run; on the Los Tilos arch they work in tension, acting as a classic Pratt beam. Hence steel was chosen for the deck in order to best resist tension; the arch was made of high strength concrete in order to resist compression and to minimise the weight, which were the same reasons why the columns were also made of high resistance concrete. The diagonals, which work in tension, were made of high resistance steel because they work perfectly under tension and have the benefit of being cheaper than rolled steel beams - an important factor since they are only temporary.

It is vital in a lattice that all the members undergo elongations of equal magnitude, otherwise second-degree bending moments may occur if the knots are built-in. In this case, however, the concrete piers and the steel deck are not as flexible as the high resistance cables, so all undergo differing degrees of elongation. With nothing to correct this, the structure would deform, bending each frame, and acting as a Vierendeel beam. Hence the diagonals have to be re-stressed to prevent the frames bending and prevent excessive deflections in the cantilever. To allow for this, the construction sequence for each section also included a re-stressing operation in which hydraulic jacks were applied to the diagonals to correct their elongation.

Naturally, concrete arch structures undergo long-term deformation during the first few years due to shrinkage and creep, in addition to their instantaneous elastic deflection. The main deformation in such an arch is a shortening in its length, measured during the execution as a result of its axial effort compression. Where the arch has fixed supports at both ends, as is the case at Los Tilos, this loss in length generates undesirable bending moments. Therefore, before carrying out the final concreting to close the crown of the arch, the two halves must be jacked apart to compensate for the predicted shortening that will occur over time. Four 800t/m hydraulic jacks were used on the Los Tilos arch, and were placed in the 1.12m gap between the two halves. A force of approximately 1500t/m was required to jack the sides apart by just 160mm.

The final step was to place the precast concrete slab for the deck, before completion and finishing operations such as the installation of the security fence and handrail could be carried out. The handrail was specially designed with to ensure that children could not fall or climb on it and also to hinder potential suicides which could be a problem on such a high bridge. The arch was inaugurated and opened to traffic at the end of last year. Client for the project was the local authority Gobierno de Canaries and the national Ministerio de Fomento.

Santiago Fadón is technical director of Ferrovial-Agromán, José Emilio Herrero is head of the bridge section, and Juan José Sánchez and Marcos Sánchez are senior civil engineers in the bridge section of Ferrovial-Agromán

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