Structural Awards 2019

The ‘Kienlesberg Bridge’ / “Kienlesbergbrücke” is a combined tram, cyclist and pedestrian bridge crossing a busy railway site in Ulm, Germany.

The 270m-long crossing is the keystone in Ulm’s new ‘Line 2’ tram network providing a north‐south connection linking the city centre with the science park. The bridge provides a safe and enjoyable new crossing for pedestrians and cyclists while spanning 14 railway tracks.
The undulating appearance of the structure is a well-considered response to the flow of forces within the static system. The sinuous silhouette of the top beams and the distinctive pair of trusses crossing the biggest 75m-long span are an homage to the nearby Neutor Bridge, a listed historic monument built in 1907.
The bridge was opened to the public in September 2018.

Creativity and Innovation: Meeting changing needs and cultural challenges

The entire project faced a significant number of challenges to which the team had to respond in a flexible way.
First, the vicinity of the listed Neutor Bridge demanded a respectful and visually related approach to geometry and details, regardless of the ongoing and unpredictable changes of external requirements.
Second, the bridge crosses a major rail network spanning 14 tracks, which required constant coordination with the rail authority Deutsche Bahn for site access and temporary track closures.
Third, the bridge crosses the portal of a high-speed railway project, which was being constructed simultaneously with the Kienlesberg Bridge. Bridge construction and especially its launch needed to be orchestrated in relation to the tunnel construction both in the planning phase and during execution.

Structural Design

The entire bridge was designed as a continuous steel beam, subdivided into 5 spans ranging between 34 and 75m, which proved to be most economical and most suitable for a launched construction. Due to the heavily constrained site, including live railway and a major building site, launching proved to be an appropriate method of construction.
The main structure is a trough with two continuous steel box girders on both sides of the tram deck and an additional foot- and cycle-path cantilevering off the side of the south beam. The depth of the longitudinal main girders varies to match the stress diagram responding directly to the span lengths. The outcome is a sinusoidal wave with maximum heights above the columns. The southern beam has greater depths as a result of carrying an additional weight of the pedestrian deck. To avoid a visually heavy appearance as well as to save material, the two 5m tall waves supporting the main span have been dissolved into a pair of transparent trusses. Apart from structural advantages, in this way the geometrically complex bridge receives a central focal point that introduces a visual order and clearly defines the main span of the structure.
The intermediate 4 pairs of columns that support the bridge over its length are not arranged in lines perpendicular to the bridge axis but react to the extremely constrained situation at the railway land below. All columns are slender, 600mm diameter steel tubes, with a rigid joint at the bridge beams. This semi-integral approach allowed to minimise the number of bearings.

Construction

The structure of the main crossing was prefabricated in short segments, assembled on an elevated scaffolding at the south abutment and launched above the rail tracks.
Being a common building procedure for bridges of this length, the Kienlesbergbrücke has become a precedent nevertheless, as its curved east section has complicated the entire launch process. The launch has happened in three stages. First, the five straight spans of the bridge were launched elevating three meters above the bridge’s final position. Second, the remaining six segments of the curved span were added in the location of the south abutment. In the third phase, the entire structure was gradually lowered onto its bearings at the abutments and its intermediate spans were welded to the steel columns establishing a semi-integral structure.

Excellence: Team Work; working with local partners

The interdisciplinary work between KREBS + KIEFER and Knight Architects began as early as the competition stage where the team jointly developed a concept beginning with napkin sketches and hand calculations on site.
In scheme and detailed design stages the team members communicated daily, ensuring a structurally optimal design without compromising the original architectural ideas.
During construction the design team was supported by the client who provided the site supervision and ensured that any variation of the design by the contractor would be authorised by the design team. This led to a high amount of flexibility without compromising the original design intent.

Elegance and good detailing

The bridge design had to meet ambitious architectural standards set by the local planning council.
Being a large structure in a prominent location, it had to respond appropriately to the context in which it was set as well as provide a sufficient level of comfort and safety for its non-motorised users.
The variable height of the bridge beams and their sinuous flow add a sense of rhythm to the long structure. It mitigates the impact of the complex horizontal and vertical alignment and, in a playful way, speaks to the different span lengths and the underlying structural concept. The highly dynamic vertical movement of the top chord generates an enjoyable experience for the cyclist, as different views towards the city are being hidden and revealed while passing by.
For the pedestrian, the bridge deck has been locally widened at the areas of the truss beams.

This is a highly sympathetic new pedestrian, cyclist, and tram bridge. The structure reflects the enormous benefits of interdisciplinary work between engineers and architects.

The sinuous silhouette is derived from the natural distribution of bending moments, and optimised for a longitudinal launching process.

At the same time, it is an architectural homage to the nearby 110-year-old wrought-iron Neutor Bridge. The judges admired how the graceful, economic, and efficient appearance belies the complexity of the site and irregularity of the support conditions.

Notably, the structure of the main crossing was prefabricated in short segments, assembled on an elevated scaffolding at the south abutment and truss adjacent.

This three-stage launch procedure was complicated by the curved east section but well managed by the engineers.