The complexity of a flowing line | Interview architects bus bridge

After one and a half years and more than sixty construction phases, the bus bridge in Zwolle is ready. With enthusiasm, the three designers from design and engineering firm ipv Delft explain how the design came about and what sets this bridge apart.

Gerhard Nijenhuis, designer and director of ipv Delft: 'We are very pleased with the final result. Especially since it is a complex project, just because of its location over the railroad. It required not only a clear vision from us as designers, but also real craftsmanship from the executor, BAM Infra. The driving in of the steel main span, the solid pouring of the concrete abutments, with such an S-shaped bridge that is very complicated. Moreover, this has not been done before in the Netherlands.'

Yet it actually looks quite simple....
'That, of course, is the intention. Our goal is always to create an obvious design, as if there is no other way. That only works if you create a clear design and in the elaboration put everything at its service.'

Who asked you to create the design?
Nijenhuis: "In the summer of 2016, Martin Kuis, regional manager of BAM Infra, asked if we wanted to submit a bid design for the tender together with BAM. In the end, we were one of four parties that submitted a design. I remember that there was considerable time pressure, and the tender was also scheduled in the middle of the vacations. In addition, the program of requirements was very compelling: the ramps and the S-shaped route were almost completely fixed. This meant that as a tenderer you could really only use a box girder as the main structure.'

So how did your design differ from the others submitted?
Architect Joris Veerman: "We looked for a bridge that would fit the future atmosphere of the railroad zone. The area now has an industrial railroad character. The municipality wants to transform it into a pleasant green campus where people can work and recreate. We therefore gave the bridge a ceiling of bamboo slats, giving it the appearance of a city park bridge. That idea was called 'a golden touch' by the assessment committee.

How is the bridge structurally constructed?
Veerman: "The steel main span and the concrete abutments are poured together. In this way, the main span forms a single unit both structurally and aesthetically. To reinforce the dynamics of the bridge, the supports were designed in such a way that they appear to be subtly inclined, following the curve of the bridge. Moreover, they are not positioned centrally under the main girder, but eccentrically. This allowed the length of the main span to be reduced. The fact that the bridge then also rests on only one support block per column, instead of the usual two, gives an airy image, almost as if the bridge lies loose on the supports. This was possible by using the torsionally rigid box section, which we needed anyway to make the S-shaped main span. The BAM structural engineer, Tristan Wolvekamp, supported us very well in this.

Were there many parties involved in the realization?
Ronald Rozemeijer, designer: 'Of course, but we mainly worked with three parties: the contractor and clients ProRail and Zwolle, very clearly. It was an intense collaboration, not without discussion with the executor, but friction gives luster. Intensive cooperation is necessary, because in such a complex project as a bus bridge over the railroad, you encounter all kinds of challenges. Only by working together integrally as a team and keeping the goal in mind can you achieve a good result. This also applies to the tender phase, you have to be able to reach agreement as a tender team. Before we reached the final design, we made dozens of variants. Fortunately, within the tender team we quickly agreed on the best variant. When working out the tender design we then managed to maintain clarity.'

So the bridge turned out as you envisioned?
Nijenhuis: "Yes. Despite the pressures and challenges, the clear concept has become a reality. We are delighted with that!