Glass cladding with ATEx for Strasbourg Station
Jean-Claude Brehm, project operation manager at SEELE, explains that the shape of the large glass roof is designed as a portion of a "pseudo-Torus" defined by the revolution of a curve about an axis inclined at 17° from the vertical. Despite this spectacular, geometrically complex and undeniably impressive form, the objective was to preserve the view from the forecourt onto the historic façade erected by German Railways in 1883. "The steel structure was designed to look as lightweight as possible: a series of tierods was arranged basically like the spokes of a wheel to provide transverse bracing for the primary structure arches made of built-up sections. And the "Fink" trusses that form the secondary structure composed of a compression member and tension tierods, have a limited visual impact while enabling greater flexibility for connection details." He adds that in this quest for aesthetics and transparency, "curved glazing could not be hot formed because relief and texture would have been formed on the glass by the action of rollers."
A revolutionary process
Jean-Claude Brehm’s team developed production processes and design notes necessary for bending of hardened glass, with the assistance of Patrick Henault from Socotec and Michel Cossavella for CSTB in the framework of a Technical Experimental Assessment. The two panes were cold force-fitted onto the templates, including the insert. It was only after this forming that the assembly was put into an autoclave to make the 6-mm thick glass bond to the insert, with a long-term bond. The result was smoother glazing and an insert released from the residual stresses and defects that are always possible during hot bending (risk of bubbles forming or delamination). Michel Cossavella tells us that there were many unknowns and variables, particularly because the insert included a solar protection film sandwiched between 2 PVB (polyvinyl butyral) layers. "In addition to the solar protection film and a low emissivity layer protecting the glass roof from a greenhouse effect, the double silk screen printing applied to some glazing can modify aging and resistance", he believes. Beyond adjustment and construction precision complexities related to the curved geometries of the set of building elements located in a seismic zone, the main difficulty was to collect all data about the behavior of glass products. This is why CSTB and SEELE carried out many tests, particularly including a simulation of the bond of the different lamination components for a 10-year period, or their performance in air, water and wind and under shocks.
CSTB Lighting experts studied the impact of lighting systems before all this was done. Michel Garcia tells us that the objective was to design and optimize sources, the nature and power of lighting based on a model provided by AREP. The objectives were to evaluate the real perception of the former historic station façade through the large glass roof; to limit glare and to protect night vision on the station forecourt for an observer inside this glass roof. Very quickly, the powerful PHANIE software developed by CSTB engineers was used to calculate spectral illumination that, once translated into RGB color, provided synthetic images accurately reflecting reality with an unequalled rendering quality. Michel Garcia continues "Due to PHANIE that is capable of treating complex architectures, we were able to correct some luminances and other caustics by modeling for all types of weather, under a sunny or cloudy sky, under natural lighting or artificial lighting". He tells us that "The funnel-shaped natural lighting device was adjusted due to this study of sunlight reflected on the stainless steel plate of the elliptical shaped rooflight located above the bus arrivals in the basement of the station forecourt".
Client: SNCF, Station Development Division, and Strasbourg Urban Community
Project Manager: SNCF Infrastructure Management (DAAB)
Design and engineering offices: AREP and RFR
Construction of glass roof and manufacturing of laminated glass: SEELE
Inspection office: SOCOTEC
Delivery: October 2007
• Surface made of curved glass: about 4000 m²
• Glass roof dimensions: 120m long, 20m wide (average), and 25m high
• Primary structure: arches (composed of 5 tangent circle arcs with radii of 12 to 36 m), columns with a tubular section articulated at the bottom and top, tierods placed around the arch in the form of bicycle wheel spokes (for transverse stability of the glass roof); 9m grid
• Secondary structure: "Fink" trusses (composed of a compression member and tension tierods) connecting the primary structure as a portion of a circle, and a series of tensioned solid round bars between the intersection nodes of primary and secondary structures (longitudinal stability): 4.50m grid
• Tertiary member: parallel to the primary structure and composed of arcs made with curved 120 x 80 Ts on which glazing supports are fixed; 1.50 m grids
• Glazing: single laminated and cold formed glazing (some marked with double silk screen printing), composed of two hardened or tempered HST glass panels (6+6) with a low emissivity layer on face 4 (thermal insulation) and an insert composed of a film (solar protection) sandwiched between 2PVB layers.