Grand Inspiration

Nigel Young/Foster + Partners

A team of British and German architects and engineers restored much of the station to its 19-century grandeur, strengthened the structure’s arches and other supports, crowned the building by restoring a glass cupola, and added a distinctly modern touch over the three platform halls: a 30,000 m² prestressed membrane roof. The initial planning for the project began in 1997, underwent various iterations, and was completed in November 2006—in time to help commemorate the 800th anniversary of the founding of Dresden.

After the war Dresden fell under the control of the communist German Democratic Republic (DDR from its German initials)—also known as East Germany. Poorly implemented repairs and inadequate maintenance during slightly more than four decades of DDR rule led to further damage and diminished the station’s ornate appearance. Of greater significance, corrosion and weather-related damage ultimately threatened the stability of the steel arch structure, notes Peter Voland, the head of the structural engineering department of Schmitt Stumpf Fruehauf & Partner (SSF), of Munich, Germany, the lead engineering firm for a recently completed €140-million (U.S.$192-million) refurbishment of the station.

The refurbishment was designed by the London-based architecture firm Foster + Partners, led by the Pritzker Architecture Prize–winning Norman Foster. The project restored much of the station to its previous Wilhelmine-style grandeur, strengthened the structure’s arches and other supports, crowned the building by restoring a glass cupola, and added a distinctly modern touch over the three platform halls: a lightweight, translucent prestressed membrane roof designed by the London and Berlin offices of the international engineering firm Buro Happold. The initial planning for the project began in 1997, underwent various iterations, and was completed in November 2006—in time to help commemorate the 800th anniversary of the founding of Dresden.

The membrane roof, above, is divided into a series of fabric panels that range in width from 5 to 14 m—determined by the basis of the spacing between arches. The roof’s geometry follows a double curvature of high and low points that alternate between ridges, where the membrane rises above the arches, and valleys—called pull-downs by the design team—formed where the roof of the middle hall intersects with the roofs of the side halls. Tensioned cables stretch across the halls in a north–south direction, lifting up the membrane to help generate the form of the roof canopy. Unstressed cables hang loosely beneath the membrane panels to help maintain the stability and continuity of the roof in the event that any panel fails catastrophically or needs to be removed for maintenance or repair. The western facade of the masonry reception building, right, opens onto the middle platform hall, which features filigreed steel arches that rise to a height of 32 m. In every second arch bay, a portion of the new prestressed membrane roof is pulled down to form a conical hopper that collects and drains rainwater from the roof into pipes located inside the arch columns. To accommodate the membrane roof’s horizontal loads of 500 to 600 kg/m—which snow and wind could increase to 1,500 to 2,000 kg/m—the design team added a secondary steel structure atop the arches; the membrane connects directly to this secondary structure, not to the arches.

Schmitt Stumpf Fruehauf & Partner/Ullrich Windoffer

The station remained open during all demolition and construction work but did close for several weeks in August 2002, when, as part of widespread flooding throughout central Europe, the nearby Weisseritz River overflowed its banks and placed much of the facility under approximately 1 m of muddy water. The Weisseritz, which had been diverted in the 19th century so that the station could be constructed, temporarily returned to its original riverbed through the middle hall, notes Voland. Because much of the refurbishment was conducted atop elevated, movable platforms that provided workers with access to the roof-level portions of the arches, the restoration itself was only slightly delayed by the flooding.

A critical success, the refurbishment of the station received the 2007 European Award from the Royal Institute of British Architects (RIBA) and is one of six finalists for this year’s £20,000 RIBA Stirling Prize—awarded by the RIBA and the Architects’ Journal to the architects deemed to have made the greatest contribution to British architecture in the past year. The winner of the prize will be announced on October 6. The project is also one of three finalists for the Institution of Structural Engineers’ Heritage Award for Infrastructure 2007, which will be announced on November 14.

Constructed between 1892 and 1897 at the southern border of Dresden’s downtown region, the station consists of three arched ingot-iron platform halls—designated the north, middle, and south halls—that surround all but the eastern facade of an approximately 4,500 m² masonry reception building. Designed by the architects Ernst Giese and Paul Weidner, it was considered one of the grandest railway stations in Germany before World War I, notes Voland. The reception building featured a 34 m high glazed square cupola atop cruciform arcades. In addition to steel filigrees on the arches in the platform halls, there were large glazed skylights in the barrel-vaulted ceilings above the platforms, cast-iron ornamentation throughout the facility, and decorative steel facades on the longitudinal exteriors of the north and south halls that featured a series of glazed entrance portals with steel mullions.

The north and south halls each measure approximately 240 m long. The north hall has a width of 31 m and the south a width of 32 m, and their steel arches rise to a height of 19 m. The middle hall is larger—165 m long and 59 m wide—and its main arches rise to a height of 32 m. The standard bay for the station is created by arches spaced at 10 m intervals, but the arches framing the main entrance portals along the longitudinal facades—just behind and on either side of the reception building—are set 14 m apart and are flanked by 5 m wide bays. Several 8.5 m wide bays and additional 5 m wide bays are located in the portions of the north and south halls that extend along the sides of the reception building.

Before the current refurbishment, the station had undergone various alterations over its 100-plus years. Certain cast-iron components were replaced by brick masonry, for example, and many of the skylight portions of the original corrugated metal roof were covered by timber and felt after World War II, effectively turning the platforms halls into dark caverns. Thus, a first step in the refurbishment involved an extensive investigation of the existing structure to demarcate the original construction and any additions. Because no as-built drawings or original structural designs could be found, the refurbishment team was required to examine the entire building and to conduct a detailed condition survey of the structure, including testing the materials, notes Voland.

Foster’s vision for the project included the removal of components that were not original and, wherever possible, the repair of the original components. “The aim of the refurbishment was to restore the station to its former glory as a light, airy space and to avoid new structures,” says Voland. To that end, the investigations determined that the entire steel arch structure, the main structure of the glazed north and south facades, and the steel-mullioned gable ends of the three platform halls—known as the Hallenschürzen (hall skirts)—were portions of the original structure.

The station’s owner—Deutsche Bahn AG Station & Service, a department of Germany’s national railway system—had initially wished to limit the project to the addition of a new membrane roof. But the investigations indicated that some portions of the original structural system were in such poor condition—owing to corrosion, plastic deformations, and improper repairs—that a general structural refurbishment was required along with the replacement of certain portions of the structure, explains Voland.

In particular, the Hallenschürzen needed to be demolished and replaced by new gable ends. Designed as welded steel structures, the new Hallenschürzen closely resemble the originals but can accommodate the requirements of modern railway equipment and maintenance practices, notes Voland.

The main riveted steel frames of the decorative facades on the north and south hall exteriors were repaired, although the infill panels, as in the refurbishment of the Hallenschürzen, had to be demolished and rebuilt as modern welded glazed grids, says Voland. The original cast-iron columns of the facades were reused in some bays, he adds.

The station’s arches were originally designed primarily to transfer the vertical roof loads—the dead load of the steel structure itself, as well as the timber, glass, and metal sheeting on the roof—to the foundations, which consist of various supports that were constructed of either natural stone or masonry. In several locations, the arches were tied back on top of masonry piers and in the interior masonry facades by long steel anchors so that the tension was superimposed by the weight of the pillars, notes Voland. All of these anchors were heavily damaged by corrosion and had to be demolished and replaced. The cast-iron supports were repaired and rebuilt. But it was not an easy process.

Because the columns of the middle hall arches were located directly in front of masonry walls, it was not always possible to access the rear portions of these steel elements and the masonry walls for restoration and corrosion protection. The solution required the use of hydraulic presses to lift the middle hall arches. The arch columns were then cut and removed for restoration off-site. Where new corrosion protection was applied on-site, the steel members were first covered with dust-resistant metal sheets so that the sandblasting of the existing lead-based protection would not create an environmental hazard. After the original coating was removed, a new corrosion protection consisting of a two-component epoxy and a polyurethane resin base was applied, says Voland.

The centerpiece of the refurbishment was the addition of a 30,000 m² prestressed membrane roof. The 0.8 mm thick glass fiber translucent canopy is coated with polytetrafluoroethylene (PTFE). The membrane material is similar to the 320 m diameter roof of London’s Millennium Dome (see “Fabric Roof Crowns Millennium Dome,” Civil Engineering, May 1999). Although the new roof is considerably lighter than the station’s previous covering—approximately 1.2 kg/m² for the membrane roof, compared with approximately 45 kg/m² for the postwar roof—the membrane system imposed considerable horizontal loads—500 to 600 kg/m—on structural supports that previously had accommodated primarily vertical loads, notes Voland. Snow and wind loads could increase these horizontal forces to 1,500 to 2,000 kg/m, he adds.

Nigel Young/Foster + Partners

The €140-million (U.S.$192-million) refurbishment project repaired the masonry and natural stone features of the approximately4,500 m² reception building, created new space for shops to line the building’s cruciform arcades, restored a 34 m high glass cupola, and installed a movable transparent foil cushion in the cupola to facilitate natural ventilation. Designed by the Pritzker Architecture Prize–winning British architect Norman Foster, the station refurbishment has already received one architecture award and is a finalist for another architecture prize as well as a structural engineering award.

Thus, to support the new roof, the refurbishment featured a conversion of the entire longitudinal structural system. All of the existing purlins were removed and in their place new steel bracing—consisting of square pipe purlins and cross bracing—was added to every second bay to create 10 m wide trusslike structures between these arches. In this way the loadings of the membrane’s longitudinal forces were reduced. These bays were linked by sheets of the membrane to accommodate thermal movement, Voland adds. Because the membrane forces on the end bays—where wind loads can be significant—were not balanced, as in the middle bays, the project also strengthened the main vertical portals of the gable ends by reinforcing the existing steel bracing, installing additional bracing, and adding reinforced-concrete panels to the foundations.

Michael Cook, Ph.D., CEng, Buro Happold’s London-based director of structural engineering, compares the horizontal tension in the membrane roof to the pull on the edge of a trampoline. “Dealing with horizontal forces was a very challenging aspect of the project,” says Cook. “A normal roof just provides great weight.”

The refurbishment team initially questioned how much resistance the existing arches would provide, in particular, whether they would be flexible enough to accommodate potential changes in the membrane under load, says Cook. To ensure that the membrane flexed as required, the team designed a secondary steel structure to sit atop the arches; the membrane is connected directly to the secondary structure, not to the arches, explains Cook. Vertical and horizontal loads are transferred from the membrane to the top chords of the arches through this secondary structure, which consists of pairs of tubular elements approximately 114 mm in diameter with 8 mm thick walls. Balanced longitudinal stresses and the high unbalanced horizontal loads are transferred to all arch bays at either end of the platform halls. These end bays were strengthened laterally with additional cross bracing to provide horizontal stiffness; the additional struts and cross wires were designed to mimic the station’s original bracing.

“Later on, it turned out that the arches were actually laterally very soft and did not put up the resistance that we imagined they would,” Cook says. This enabled Buro Happold to stiffen the secondary structure. “But separating the membrane and the arches was still useful because it allowed us to do clever things to let in light,” he adds.

The membrane itself transmits approximately 13 percent of daylight, significantly reducing the need for artificial lighting inside the platform halls. Of greater importance, the use of the secondary steel supports alters the geometry of the membrane sufficiently to provide space for a series of 67 elliptical glazed skylights at the top of the barrel-vaulted membrane sections. These skylights—which would not have been possible if the membrane had been attached directly to the simple steel boxes of the arches—provide “extra clarity and a glimpse of the sky,” notes Cook. Moreover, artificial lighting from the platform halls reflects off the underside of the canopy at night, “creating an even wash of illumination, while from the outside the whole structure radiates an ethereal silvery glow,” notes a description of the refurbishment from a Foster + Partners press announcement.

Divided into a series of fabric panels that range in width from 5 to 14 m—determined on the basis of the spacing between arches—the roof’s geometry follows a double curvature of high and low points. Ridges where the membrane rises above the arches give way to valleys—called pull-downs by the design team—formed where the roof of the middle hall meets the roofs of the side halls. In every second arch bay, the pull-down points form a conical hopper that collects and drains rainwater from the roof into pipes located inside the arch columns. The double tubes of the secondary supports connect to the hoppers via a series of elliptical pipe rings.

In the bays without hoppers, a series of highly tensioned cables—called flying cables—stretch across the halls in a north–south direction, connecting the ends of the glazed skylights in the middle hall to the ends of the skylights in the north and south halls. These flying cables lift up the membrane on either side of the hoppers to help generate the form of the roof canopy. “It’s that lifting and pulling of the membrane against itself that gives you the tension” for the roof geometry, explains Cook.

The tension of the flying cables also generates significant additional horizontal forces in the arches—approximately 135 kN, says Voland.

A second set of tensioned cables—outlining the openings at the top of the hoppers’ cone—is designed to transfer the snow loads that fall on the membrane roof. Although Dresden is not a city of exceptionally harsh winters, whatever snow does fall will drift into the hoppers, building up considerable weight and generating high stresses on the membrane, says Cook. The snow cables strengthen the membrane locally, connecting one arch to another by a contoured diversion around the top of the hoppers to transfer the snow load into the arches.

A third set of cables hangs loosely between the arches and beneath the membrane panels, connecting to the braced bays at either end of the platform halls. Although this untensioned system does not contribute to the roof structure under normal conditions, the cables act as a replacement “skin” to transfer horizontal loads and maintain the stability and continuity of the membrane in the event that any membrane panel fails catastrophically or needs to be removed for maintenance or repair, explains Cook.

Along the longitudinal facades of the side halls, the membrane is fixed continuously to a square tube structure. The latter follows the curve above the 14 m wide entrance portals and is reinforced horizontally by a sloped steel tube arch, notes Voland.

To erect the new roof, the membrane panels were stretched into position using large ratcheting equipment and then bolted to the secondary steel supports via small metal plates that had been welded along the upper edges of the supports’ double tubes. Each plate had to be welded at a precise angle to conform to the complicated, varying angles of the membrane as it was stretched over the arches, Cook adds. Other, more complicated options for fastening the membrane panels to the secondary structure had been considered, including an adjustable screw connection, he notes. But the refurbishment team chose to use bolts after being assured by the fabric’s manufacturer—Skyspan (Europe) GmbH, of Rimsting, Germany—that the panels would stretch sufficiently to be pretensioned into position.

Buro Happold worked closely with SSF to design the membrane roof through a process of physical modeling, form finding, and computer analysis using a proprietary software system of Buro Happold’s called Tensyl. Erection of the membrane roof began in February 2001 with the removal of the postwar timber and felt roofing. Temporary supports were required during the erection of the membrane to resist the fabric’s stresses and prevent excessive loading on the arches, notes Cook. Once the membrane was installed and prestressed, the temporary struts were removed.

From an aesthetic standpoint, the membrane roof opens up an impressive vista within the platform halls. “The hard roof used to come all the way down around the arches, down in the valleys” created by the arches and the roofline, notes Cook. “So within the station you could see only the shape of the arch you were under. The other arches were rendered invisible by the valley line, which was very low. But with the membrane, not only does it provide light but it also raises the valleys higher.”

This created an arch-based pattern in the lengthwise direction, explains Cook. “So if you’re standing in the station now you not only can appreciate the nearest arch but looking through you can see the others. It connected the whole roofscape together,” he explains.

On the longitudinal facades of the platform halls, the refurbishment team removed some of the sandstone face bricks that had been used after World War II to cover damage to the sandstone columns. In certain cases, the evidence of nonstructural wartime damage to these columns was left intact as a historical reminder, notes Voland. During the refurbishment, the Deutsche Bahn design office also demolished and reconstructed the platform in the north hall—a separate project that needed to be coordinated with SSF’s construction sequencing, notes Voland.

In the reception building, SSF worked closely with Foster + Partners to repair the masonry, the natural stone features, and all roofs in that structure. Most significant of all, the refurbishment team restored the square cupola located directly above the intersection of the 70 m long central concourse and the 50 m long intersecting concourse. The cupola’s original glazing—covered by the postwar repairs—was replaced and a new, movable transparent foil cushion was installed beneath the cupola to facilitate natural ventilation. The movable foil seals in the heat during cold weather and permits hot air to escape during warm weather.

The original waiting rooms in the reception building have been converted to a travel center and restaurant area, and space has been created for shops to line the concourses.

As part of the refurbishment, the station will now serve Germany’s InterCityExpress (ICE) high-speed trains, which are much longer than the trains that previously used the station. In early designs, the refurbishment team considered extending the membrane roof by as much as 200 m beyond the current end points of the side halls to accommodate the ice trains—with some “amazing geometries and beautiful curves” once the roof was free of the restraints imposed by the existing arches, notes Cook. But financial factors have countermanded those plans for now, notes Voland.

Perhaps the greatest challenge during the project involved integrating modern materials into the historically important features of the 19th-century structure, notes Voland. The new roof, in particular, could have ended up as an unattrac-tive or intrusive element that was simply added to the existing station. “But we were keen on merging both structures in a light and simple way,” Voland explains.

Norman Foster echoed a similar theme during the ceremonies last November that marked the completed renovations. “Our redevelopment of Dresden Station is a true celebration of the nineteenth-century original through the means of our times,” Foster declared. “The dramatic roof structure has been specially engineered to rest comfortably on the original station arches—revealing the fine historic detailing while flooding the space below with natural light, reducing energy consumption, and reinventing the station for the twenty-first century.”

Project Credits
Owner: Deutsche Bahn AG Station & Service
Architect: Foster + Partners, London
General planner and structural engineer: Schmitt Stumpf Fruehauf & Partner, Munich, Germany
Structural engineer for the membrane roof: Buro Happold, London and Berlin
Project managers: AYH Homola GmbH & Co. KG, Dresden, Germany (membrane roof), and Kaiser Baucontrol Ingenieurgesellschaft mbH, Dresden, Germany (reception building)
General contractor: ARGE Dywidag und Heitkamp, Dresden, Germany
Subcontractor for membrane roof: Skyspan (Europe) GmbH, Rimsting, Germany
Mechanical and electrical engineers: Schmidt Reuter & Partner, Dresden, Germany (membrane roof), and Zibell Willner & Partner, Dresden, Germany, Ingenieurbüro Steinigk, Cottbus, Germany, and Kompetenzzentrum Gebäudeautomation Krause, Erfurt, Germany (reception building)
Lighting consultant: Speirs and Major Associates, Ltd., London
Engineering review: Prof. Ruehle, Jentzsch & Partner GmbH, Dresden, Germany