Membrane Roof Renovation at Dresden Central Station

From Dark Tunnels to a Light-Filled Experience Station

At the end of the 19th century, Dresden Central Station stood as a prime example of grand-scale construction, boasting a magnificent steel structure. The station featured a three-aisle steel truss building extending 240 meters in length. The impressive spans included 30 meters for the two outer halls and 60 meters for the central hall, largely covered with glass, which bathed the space in light, much like a greenhouse. This was a building of high aesthetic value and historical significance, though it was severely damaged during World War II. During the GDR era, the steel beams were encased in wood and sealed with cardboard, tar, and slate stones. The result was a very dark building with an unpleasant tunnel-like atmosphere. It wasn’t until after reunification that Deutsche Bahn decided to completely renovate the station, to restore its status as one of Dresden’s landmarks.

Award-Winning Design Featuring a Membrane Roof

For this renovation, the architects Foster+Partners created a design that utilized the historic steel structure as a support while introducing a wave form inspired both by the curvature of the three halls and the ripples of the nearby Elbe River. At that time, the use of textile membranes was highly popular and widely discussed. Many architects, then as now, explored this fascinating material, experimenting with its flexible formability. Even back then, as in the current renovation, the decision was made to use the PTFE-coated glass fabric GF-7000 from the Serge Ferrari Group (formerly Verseidag). The result was a wonderful lightness in the construction, and thanks to the outstanding translucency, a great deal of light was brought into the station. This transformation delighted both users and the client, as it ended years of darkness. Suddenly, the entire station could be illuminated, with light reaching even the most remote corners.

Roof Design Details

To address the specific design requirements, low points were created in the roof where the hall trusses converge—specifically at the "haunch joint," where the bases of the halls meet. Here, the membrane was drawn down deeply to create a series of "funnels." These funnels were integral to the roof's drainage system, channeling water from the edge zones of the central and side aisles.

The new roof was put into service in 2006. However, by the end of the decade, it became evident that the funnel construction was not capable of handling the large amounts of snow and ice that slid into them in an avalanche-like manner. The funnels could not adequately manage and drain the melting snow and ice, leading to significant concerns.

Designing the funnels to handle the self-propelled movement of snow masses had to be reconsidered in the roof renovation concept, independent of the choice of membrane material.

To rule out potential factors for the new design, additional investigations were conducted. For example, there was concern that ice blocks sliding into the funnels could cut through the membrane or that icicles falling from the newly designed skylights might puncture the membrane. To address these concerns, the IF- Group developed a test where sharpened wooden stakes with a diameter of 12 cm were dropped from a height of 6 meters onto the membrane at various impact angles. The membrane, however, sustained no damage beyond minor surface abrasion and could neither be cut nor punctured.

Over the last 20 years, knowledge about the properties of fabric membranes has significantly advanced. Engineering firms with extensive experience in membrane construction are now well aware of the increased demands and challenges involved.

What Did the New Concept Entail?

Point 1: Skylights to Cover the Funnels

Eliminating the funnels by altering the roof's shape was not an option. Therefore, the solution was to cover the funnels. Skylights—essentially "glass eyes"—were integrated into the membrane roof to close off these funnel entrances in an oval, three-dimensional shape. These skylights consist of a steel structure with three girders, which are freely rotatable around their longitudinal axis on the hall trusses to avoid imposing any constraints on the membrane. The membrane is connected at the edges, allowing snow and water to flow onto this glazed surface. Centimeter-wide open joints around the skylight collect the water, and below the skylight, the funnel membrane connects to ensure final drainage. This design effectively decouples the funnel membrane from the load-bearing roof membrane, with the skylight inserted as a separating structure. The funnel membrane no longer serves a structural function but remains a decorative element and drainage layer, made from the same material as the main membrane roof. From the perspective of a traveler on the platform, the appearance is only marginally different from the previous design.

Point 2: New Orientation and Formatting of Membrane Elements

Foster+Partners wanted to avoid any assembly seams, fearing that they would disrupt the flow and diminish the aesthetics. The seams run parallel to the arches, and constructing a crossbeam would have interfered with the aesthetic guidelines. However, if the original large fields had been maintained, it would have been impossible to install these long membrane segments without creases and folds.

The solution was to rotate the cutting direction of the membrane strips—initially parallel to the trusses—by 90 degrees for two reasons:

The membrane's warp direction, which is stronger, was aligned with the direction of snow removal, i.e., from truss to truss. Previously, some fields had to be reinforced with cables because they were aligned in the weft direction, which has about 15% less tensile strength. With the new orientation, cable reinforcement was no longer necessary, and there are no longer any seams from truss to truss, which could weaken the tensile strength of the membrane structure.

The individual membrane strips are now generally limited to about 10 meters, rather than running 30–40 meters parallel to the trusses before the next seam. Ralf Dinort explained: "A fabric initially represents just a flat plane. The shape is defined by the cutting and assembly. The more pieces I use to create this three-dimensional surface, the more accurately I can replicate this 3-dimensionality. This was hardly possible with the original roof, where only at the end of a very long cut—along the warp—could some material be removed to approximate the shape of the saddle surface. This approximation of the anticlastic membrane shape had to be distributed along the length, or else fabric distortions would occur. This can only be achieved through strong shearing, comparable to a thread pattern arranged at right angles that deforms into a rhomboid shape. We made the membrane strips smaller and arranged them crosswise. Although this was not exactly what Foster+Partners had originally envisioned, we were able to convince them that this original concept needed significant revision."

As a result, the individual fields between two hall trusses were divided into six segments, meaning that each field now contains five assembly seams as well as the additional funnel decoupling.

The outcome is much smaller assembly and transport units, which greatly reduce the risk of creasing.

Recommendation for the Same Material: GF-7000 by Verseidag / Serge Ferrari

Once the essential conditions were established for successfully planning and implementing the roof structure, the question arose of whether a new material should be considered given the complete replacement. The IF Group recommended using the original membrane material, GF-7000, for this demanding renovation, as it had proven its high quality over time. Ralf Dinort noted: "We work with various laboratories and have repeatedly conducted relevant tests for different projects, even developing our own testing methods. For example, a flexometer test was developed during a thesis, allowing us to test a wide range of materials, including a very informative double-fold test. The glass-PTFE fabric GF-7000 from Verseidag performed the best, especially regarding resilience against folds. It was clear to us that this material should be recommended again for the replacement roof. Furthermore, we have experienced Verseidag and the Serge Ferrari Group as partners who work closely with the engineering firm to develop viable details and improvements. They are always receptive to solving problems, initiating research, or conducting special tests to confirm calculation bases. These important factors in the collaboration lead to optimal solutions for the client. Another important factor was the high transparency in material production, which was evident from the beginning during visits to the laboratory or production at Verseidag-Indutex under real conditions, where every detail could be scrutinized."

The decision to use the original material also supported an early project start, as only confirmation and supplementary tests were required for the previous ZiE and UIG (internal approval processes of Deutsche Bahn) created in 2000/2001, ensuring their continued validity. This allowed the use of existing data and maintained compliance with the extensive, 500-page fire protection concept, which would have needed a complete overhaul if a new material had been chosen.

Fire Protection Requirements During Construction

The membrane roof was required to meet a corresponding fire protection certification, which was fully achieved by the GF-7000 fabric from the Serge Ferrari Group with a B-s1, d0 certification.

An essential aspect of the fire protection concept was also ensuring safety during the construction phase. The fabric was evaluated in the assembly situation, in combination with a platform net (a safety net installed 1 meter below the membrane) and an additional dust protection net 50 cm below. For the planned 3-4 year construction period, the smoke extraction issue needed to be clarified, as the overall layer package had to be considered. In this combination, real fire tests were conducted, demonstrating that all requirements were met with the GF-7000 textile fabric. Whether used alone or in combination with other materials, the fire protection concept shows that the membrane requirements are a consistent theme, with much of the concept tailored to them.

Strict Quality Management for Membrane Production and Assembly

A critical element in avoiding creases and maintaining material resilience during production, transport, and assembly of the membrane roof elements was a comprehensive and very strict quality control program, which represented a change from the original construction. Two independent laboratories were commissioned: DEKRA in Stuttgart by the contractor, Pfeifer Seil- und Hebetechnik GmbH, and ELLF in Essen by the client, Deutsche Bahn.

The construction contract's quality management concept includes a series of material tests.

Additionally, Deutsche Bahn conducted another series of tests, simulating long-term behavior under various stress levels in load chambers. Both undamaged material and material with different types of damage were examined in these tests, focusing on the long-term behavior of pre-damaged material.

To ensure consistent quality across all construction phases, which span from 2022 to 2025, fabric samples from each production batch of the membrane are taken and subjected to extensive testing procedures.