Following a 2012 design competition won by engineers Krebs+Kiefer in a consortium with Knight Architects, the client, Stadtwerke Ulm, initiated bridge project in an extraordinarily complex construction context: the new bridge had to carry a pedestrian and cycle lane, alongside the two tram tracks over a busy rail intersection run by Deutsche Bahn. In addition, the tram line runs very close to the Albabstieg Tunnel portal, which is currently under construction as part of the Stuttgart–Ulm line.
An interdisciplinary cooperation between German Engineers and British Architects led to an exemplary design showing how structural design, geometric requirements and construction programme in close presence to the historic, 1907 built Neutor Bridge, could result in a structure in which construction and design complement one another in a coherent way. Ulm – with the world’s highest church tower and the Ulm School of Design – is now another structural landmark richer.
Conditions and restraints: geometry and site
The horizontal and vertical alignment of the 270 m long crossing were heavily determined by the situation on the ground. The south-east end of the line follows the Neutor Bridge embankment, then curves through 30° to the west to make a straight crossing approximately 70 m further down the line. Existing railway tracks made an equidistant or symmetrical placement of supporting piers impossible. The site constraints involved an elevated railway track at the low eastern end of the bridge. The clearance envelope here, which is close to the bridge’s soffit, eventually defined the bridge’s slope as well as its soffit appearance. The pedestrian deck is located on the south side of the bridge – facing the historic centre with the famous Minster – which added to the complexity as the cross section would be highly asymmetric (Fig. B).
Structural design: form follows function
Early in the design stage it became clear that the bridge had to be longitudinally launched, which ruled out a structure below deck. Instead an asymmetric trough was chosen, defined by two continuous, longitudinal beams which frame the tram envelope. A pedestrian deck is attached to the southern beam as a free cantilever.
The main beams undulate in their depth, crudely following the natural bending moment curve of a longitudinal beam. This not only increases transparency, but also creates a visual reference to the Neutor Bridge located just next to the new crossing. Increased spans at the bridge centre and the additional load of the 4– 6 m wide pedestrian deck resulted in the two main girders of the southern beam having to be significantly deeper. In order to increase transparency, these two girders have been dissolved into two arched trusses which became the focal points of the design (Figs. B, C).
Construction: everything has its purpose
The bridge starts at an embankment right at the eastern abutment of the Neutor Bridge. The concrete structure serving as the abutment of the new bridge crosses a service road, therefore it forms a rigid concrete frame counting as span one. From the abutment, the main bridge, a five-span orthotropic steel plate deck is supported on two hollow, airtight welded longitudinal girders. The entire structure is semi-integral: it sits on four pairs of slender tubular steel columns which are welded to the superstructure and firmly bolted to the pile caps of the foundation providing a rigid, moment transmitting connection in each case (Fig. D).
Construction: precision in a tight corner
Assembly took place near the first span on a platform over the railway tracks. The prefabricated superstructure segments delivered on site, welded into larger sections – including a launching nose - and launched in eleven incremental steps. Due to limited vertical space the entire superstructure was assembled 3.5 m above its final position (Fig. E). After the launching was complete, the steel bridge was supported on stacked steel billets (Fig. D). By sequences of lifting the bridge, reducing the height of the stack of steel billets and then lowering the bridge, it was possible to arrive at the design levels to allow the bridge superstructure to be finally welded to the tubular pier heads.
An interdisciplinary approach
The requirements and restrictions on the Kienlesberg Bridge and the need for it to fit in with the urban context all contributed to the specialness of the final structure. Close by and to the west there was the Neutor Bridge, to which the design made explicit reference from the very beginning. Architects and engineers worked closely together during the whole design to optimise the structure and the architectural concept of the geometry.
