DETAIL Project of the Month — February 2008
Refurbishment and Renovation of Dresden’s Central Station
Architects: Foster + Partners, London
Structural engineers: Schmitt Stumpf Frühauf & Partner, Munich; Büro Happold, London
The concept for the refurbishment and renovation of Dresden’s train station called for treating the existing structure with the utmost respect, reinforcing the impact it makes, and introducing forward-looking elements to represent modern-day rail travel. Daylight now streams into the train shed through the 30,000-square-metre translucent membrane roof of PTFE-coated, glass-fibre fabric. At night, artificial light reflects off the internal surfaces, and the roof emits a shimmering glow. The renovation work was carried out while the train station was in operation, which had previously been unheard of for a membrane of this size.
Dresden’s is the third largest train station in Germany’s eastern states — after Berlin and Leipzig. It was designed by Ernst Giese and Paul Weidner in the late nineteenth century. The historic vestibule building is adjoined by a 60-metre-wide central train shed (for trains which originate and terminate in Dresden), and two 32-metre-wide lateral sheds with raised tracks (for trains which continue through the station). During the post-war era, the shed roofs were sheathed in wood boarding, allowing very little light to enter the space.
All additions and interventions were removed to make it possible to experience the building as intended by the original architects. The existing iron structure was refurbished and reinforced: additional steel members were incorporated so that both the new membrane and the fine detailing of the historic steel structure are highlighted. The linear structural arch system was originally only intended to distribute the vertical roof loads. But due to the non-coplanar, pre-tensioned stresses on the membrane, considerable horizontal stresses arise: a geometric, secondary structural system was inserted as “adapter” for the membrane.
The existing structure was subdivided into segments consisting of two or three arches, each linked to horizontal framing and stiffened. The membranes — which vary in size from 10x5 m to 10x14 m — are attached to the secondary structure’s cylindrical steel sections which are situated atop the arches. Employed in pairs, the cylindrical steel sections direct the membrane stresses, via steel-section struts, to the upper chord of the existing girder.
Above the arches’ vertexes the distance varies between the cylindrical sections, allowing for lens-shaped skylights. Between the central shed and the lateral sheds, at every other arch, the membrane is pulled down to form cones. In addition to facilitating the drainage of the roof surface, the cones alter the curve longitudinally, contributing to the overall structural soundness.
At the remaining arches the membrane is directed along suspended steel cables from the ends of the skylights in the central shed to the lateral sheds. Each arch truss’s upper chords are bound together with a back-up cable so that the strains are not too severe on the existing structural system in the event that a membrane is temporarily compromised.
Because the simple, economical membrane clamps do not allow post-tensioning, it was necessary to manufacture the membrane with extreme precision with regard to size and shape, and to painstakingly plan the installation process. That included taking into consideration the changes which occur in the membrane once three-dimensional stresses have been applied. The renovation and restructuring measures also included the vestibule building’s 34-metre-high cupola. It was restored and updated: now an ETFE-film pillow is suspended beneath the glass-covered cupola.
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