The membrane roof itself is designed as single panels which span between the arches, varying in length from 5m to 14m. The fabric is fixed to the secondary steelwork, which lifts the line of the fabric above the top chords of the arches and avoids the membrane clashing with the purlins. In the zones between the middle and side halls the membrane roof is pulled down at every second arch position to create low points for the fabric structure. This creates the necessary structural shaping of the fabric in the longitudinal direction as well as the required drainage points for rainwater.
To enhance the natural lighting filtering into the station, linear rooights are planned on top of the arches. These rooights are supported on the secondary steelwork and are designed to allow for natural ventilation and smoke extraction. The investigation of the 14m-wide membrane bays showed a possible membrane deection and conict with the purlins under full live load. To avoid this effect, additional cables have been introduced directly underneath the membrane which will support it under extreme load cases. These so-called 'snow cables' will take forces as soon as the membrane has deected to a certain level. Under normal load cases, these cables are not stressed and only support their own weight.
It was also necessary to introduce snow cables in the pull-down zones to handle the maximum snow-load cases and reinforce the membrane to limit the membrane stresses in this local zone.
Fixing of the membrane to the secondary steelwork is through a typical membrane clamp detail, which is simple and inexpensive.
However, this detail does not allow for the possibility of readjusting the membrane tension following the installation process, so very accurate fabrication of cutting patterns was crucial, together with an installation process which suited this dead-length fit.