Volume 51, Issue 2 - February 2016


Around the Bend: Düsseldorf’s Kö-Bogen Shows Curved Glass Possibilities

Glass is only as flat as the façade wants it to be.

That seemed to be the approach to the Kö-Bogen, or the “King’s Bow,” mixed-use commercial complex in the heart of Düsseldorf, Germany. The large-scale office and retail complex connects the city’s central park and historic commercial center and consists of two 85-foot high buildings and a connecting bridge.

Designed by New York architect Daniel Libeskind, the buildings sport large façades of natural stone and curved glass elements.
“The façade is intricately patterned: horizontal from some vantage points, vertical from others, and conceived so that the arrangement of stone and glass panels and aluminum louvers express a homogenous reading,” reads a description from façade contractor Lindner Group of Arnstorf, Germany.

Murphy Façade Studio, based in Ireland, assisted the architect in the design phase. The scheme, as described by Murphy, “incorporates the use of fully bespoke factory fabricated twin skin [unitized] panels with integrated venetian blinds in the upper three office floors of the building.”

The lower three retail floors incorporate large-format glazed unitized panels. “The [unitized] panels also contain flat and curved travertine natural stone, including operable stone vents, from the Tivoli quarries outside Rome,” according to information from Murphy.

The total façade area spans more than 160,000 square feet, with more than 86,000 square feet of flat insulating glass and another 23,000-plus square feet of curved and bent glass produced by Saint-Gobain Glassolutions Objektcenter Döring Berlin. These elements measured up to 8.6 feet by 18.4 feet in different radii, some bent more sharply than others. The project also includes 24 different geometric “buckled” shapes.

Mock-up design of the geometric shape was done well in advance, as the bent glass producer worked on technical solutions for the desired “kinks” in the glass.

Insulating glass spacers used for the curved and bent insulating glass units had to accommodate a variety of different requirements, which provided a unique challenge for Edgetech Europe, a subsidiary of Quanex Building Products. Additionally, various tests were done that involved bending the glass with different coatings to the desired geometry.

Lindner designed the façade system and did drawings for the project, as well as both static and building physics testing.

“The fixing of the glass panes without any mechanical fixation is not regulated in Germany,” according to information from Lindner. “Lindner performed countless calculations and testing ... to get the approval of the building authorities.”

The project team came up with the solution of “two straight legs and different bending angles in convex and concave design,” according to Carsten Kunert of Saint-Gobain Glassolutions.

“The façade blends into the building’s extraordinary design, which is a mixture of straight and wavy lines,” according to Lindner’s website. “Behind the interaction of the almost white, roman travertine and large-scale glass panels, a skillfully hidden modular structure can be found.

“… Due to the wavy shape of the structure, [many] angles can be found in the mullion-transom construction. In order to create identic frame views both inside and outside of the building, a special fulcrum was chosen for the profile connection.”

According to Christoph Rubel, European technical manager of Edgetech Europe, the softly curved shape of the building contains a variety of angles in the post- and beam-construction.

“The challenge is to receive the widths both the same on the inside and the outside. [To do so] a special fulcrum had to be used and the outer glass bar had to be made in different widths,” he says. “The glass façade is designed with a load-bearing bonding from both sides, so that the outer bonding was used as mechanical safety and thus could have been approved by the building authorities.”

Rubel says the extent to which curved and thermally kinked double insulating glass was used on the project was unique.

“Only through intensive collaboration between all parties involved and the expertise gained through many projects over the years, a real project like this with unique panel geometries could have been achieved,” he says. “In addition, the consequent development and upgrading of equipment and process had made the thermal bending of the highly sensitive coatings in small radii possible.”

Challenges posed by the complicated design carried over from the pre-construction phase to the installation. Proper scheduling was critical.

Because the project featured so many unique façade elements, logistics were very important during the installation, as Lindner had to make sure that the correct element was on site at the right time.

—Nick St. Denis

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