Footbridge over the river Mogent in Montornès del Vallès | Xavier Font

Montornès del Vallès is a municipality of 16,000 inhabitants in the metropolitan area of Barcelona. Montornès Centre is the main settlement, where most of the population —more than 12,000 inhabitants— live. However, a significant part of its residents —some 3,700— live in Montornès North, located just over a kilometre away. The two urban areas are on opposite sides of the River Mogent, which crosses the municipality from east to west. So far, there was only one bridge to cross the river. The main goal of the construction of this new footbridge was to provide a new pedestrian path to reduce the length of the displacements between these two main districts, and at the same time to offer a more friendly route for pedestrians and cyclists with a higher quality experience than the one that resulted from walking along the streets of the industrial estate that stretches between Montornès North and the old bridge over the river.

The client wanted us to provide a new connection with a reduced initial investment, and solutions with low costs of maintenance without giving up providing a proposal with a careful design, which might contribute to improve the landscape quality of this river area. The main design decisions that allowed us to achieve these purposes were the choice of a Corten steel truss deck, very efficient from the structural point of view and without the need to be protected against oxidation; the use of reinforced concrete for the structural elements in contact with the ground; the use of gabion retaining walls for the ramps, cheaper than concrete ones and unattractive to graffiti warriors; the use of synthetic wood timber, which does not require periodic care; and the choice of LED technology for lighting, which provides the necessary levels of illumination with a highly reduced consumption of energy.

Due to the hidraulic requirements asked by the Catalan environmental agency, the main span length of the structure could not be less than 45 meters, and the lower part of the structure had to be between 3.5 and 4.0 meters above the current level of the river banks, which in turn required, in compliance with the current accessibility regulations, the construction of long ramps on both sides of the river. In addition to the hidraulic requirements, another factor that greatly conditioned the design of the project was the presence of three high-voltage overhead power lines.

Concept

In essence, the whole project consists of three large objects: the deck over the river and the two access ramps. The three elements are of a comparable size and our intention was to provide them with similar volumes but highlighting, with an appropriate choice of materials, the contrast between them.

For the main element of the project —the deck— a simple structural solution of a single 47.7 metre long span Corten steel truss girder was chosen. The proposed structure is formed by two longitudinal Warren truss girders linked together at their top and bottom flanges by horizontal Warren trusses. This cross section was conceived in such a way that most of the structural depth of the main girders is set out above the level of the deck paving. With this arrangement, it was possible to achieve the maximum clearance over the river bed with minimum height and, therefore, it allowed us to minimize the length of the approach ramps.

The depth of the girders was fixed inferiorly by the geometric limitation established by the hydraulic factors previously mentioned, and at the top by the minimum safety distance to the cables of one of the high-voltage overhead power line, which passes through the centre of the river.

The decking of the footbridge is placed over the lower horizontal truss so that the users walk inside a rectangular tubular element, with a functional width of 3.0 meters and a free height of 2.7 meters; arrangement that offers a safer feeling against the proximity of the power line cables. The choice of the truss typology provides a great transparency to the crossing and allows pedestrians to fully enjoy the views that result from the necessary elevation of the path. However, with the aim of attenuating the tunnel effect generated by this configuration, which might become somewhat claustrophobic, the upper horizontal lattice is interrupted in wide sections and appears only at its two ends and around mid-span, where, for structural reasons, is more necessary.

The Warren truss, however, is a classical structural typology that is culturally closely associated with past times. Therefore, with the intention of giving the proposal an air of modernity and accentuating an intended effect of controlled chaos, we introduced a marked irregularity to the rhythm of the truss cells of both girders.

The partially covered path along the deck deepens the hierarchy of the space by clearly highlighting the difference between the approaches and the spanning section that flies over the river. From outside, the deck appears as a forceful object, clearly differentiated from the ramps, and big enough to compete with their imposing volume. The contrast between these elements was accentuated by the choice of their main materials —Corten steel for the deck and granite stone for the filling of the gabions on the approaches—. The result is a structure that is shown as a huge object placed in the air, which seems it has no contact with the rest of the work, as if it was levitating; this latter effect was reinforced partially hiding in the shade the supports on the abutments.

Because it is a suburban somewhat marginal area, graffiti and other acts of vandalism are quite common. This fact totally conditioned the choice of some materials, especially the one of the retaining walls of the ramps, which, because they present vertical faces of considerable dimensions, had they presented smooth surfaces, would have been a claim and a great temptation for the warriors of the aerosol spray paint. For this reason we opted for the use of gabion walls filled with granite stone —a very common material in this area— arranged so that they also hide the concrete parapets of the abutments. This type of retaining structures presents very rough and irregular finishing surfaces, on which the painted images appear totally deformed; for this reason they are not so attractive to the graffiti vandals.

For the ramps, a zigzag configuration was proposed, taking advantage of the provision of the prescriptive level landings. Doing this way we intended to avoid the formation of long single slopes, which could therefore invite cyclists and skaters to reach too high speeds. This zigzagging arrangement also has as a result that the retaining walls adopt broken shapes that break the monotony of the long parapets that would have resulted if a more linear solution had been chosen.

The street lighting along the deck is formed by linear warm white color LED lights. These are mounted inside special Corten steel elements, which protect them from possible vandalism, and are fixed and oriented along the axis of the diagonal bars of the main trusses. During the night this arrangement provides a pleasant illumination along the footbridge and highlights the sought irregularity of the structure. Along the ramps the street lighting is solved through lampposts, also with LED technology and the same warm white color temperature.

The choice of the pavements was also made with the intention to emphasize the contrast between the deck and the approach sections. Over the footbridge, synthetic timber —made from recycled wood products that are mixed within a polymeric matrix— was chosen. This provides a warmer and more tactile feeling close to that of natural wood, without, however, the periodic maintenance the latter requires. On the other hand, along the section where the wood decking was installed, a longitudinal one meter wide strip of electrowelded Corten steel grating was placed, which provides a safer passage for the cold winter mornings, when timber becomes extremely slippery. For the ramps, a pavement of concrete slabs was built.

The final word

We believe that, although the design of this project has been strongly conditioned by the site requirements, we have achieved the goal of providing a new connection with a reduced initial investment and solutions with low costs of maintenance, without giving up providing a proposal with a careful design, which enriches the landscape quality of that river area and contributes to improve the quality of life of its users.

Concept design: Alfa Polaris, S.L. Xavier Font

Structural design: Alfa Polaris, S.L. Xavier Font, Cristina Fernández

Client: Montornès del Vallès City Council. Pere Pascual Mariné, Josep Pérez Amatller

Health and safety: Trema Projectes.Andreu Gilaberte, Àngels Salvadó

Site Supervision: Montòrnès del Vallès City Council, assisted by Alfa Polaris, S.L.

Main contractor: Excavaciones y Construcciones Benjumea, S.A. Albert Barrero Gil, Carlos Marín Suárez

Steel fabricator: Grupo Goded. Juan Rosa Hernández, Agustín Macau Farran

Gabion Walls: MUROXS. Joan Planas