Steel is the most suitable material to use to support facades in circumstances where glazing has to be supported over distances of more than about four meters. The strength and stiffness of steel is such that supporting elements are of smaller section and therefore less obtrusive than would be the case with other commonly-used materials such as aluminium.
Tubular and tension elements are used in atrium roofs and vertical facades to form elegant and minimally intrusive support for glazing. Large deflections can be managed by careful detailing of fixings to glazing, such that local bending of the glass is avoided.
Atrium Roofsand Skylights
Atria are common features of office buildings where the layout involves large floor plates. In shopping centers, the external walls tend to have few windows and stores generally face onto an internal "street" which is extensively glazed at roof level. The roof glazing allows natural light to penetrate deep inside the building for the benefit of the users and to reduce the energy-consumption of artificial lighting.
The structural systems used to support these roofs are designed to maximize the amount of light entering the building, using members of the minimum width possible. Tubular members are often used and painted in a light color so as to absorb as little light as possible.
Glazing systems with pressure plates
Skylights consisting of areas of sloped glazing in a single plane and atrium roofs may adopt a system of sloping rafters with orthogonal purlins which are designed to retain the glazing by applying a clamping force. Glass panes can be monolithic or insulated glazing units and in simple skylights are generally clamped on all four edges using a pressure plate which is screwed into the rafter or purlin. Setting blocks are provided on the lower edge of the pane to resist the component of the weight acting in the plane of the glass.
Water management is fundamental to the proper functioning of the skylight glazing system. The slope of the roof has to be sufficient to prevent the deflection of glazing and supporting members from causing ponding and to provide sufficient flow velocity to wash dirt away. Rainwater will inevitably be prevented from draining freely away by purlin pressure plates but gaps should be provided at the junctions between rafter and purlin pressure plates to allow most of the water to run off. Cover caps which clip onto the pressure plate provide an aesthetic external finish to the glazing bars.
Rafters and purlins are designed with drainage channels which interconnect as a secondary drainage path such that any water which passes the gaskets at the edge of a pane of glass is collected and conducted down the roof to discharge at the bottom of a rafter. A secondary potential source of water is condensation on the inside surface of the glass which the supporting elements are also designed to collect. Condensate drainage channels on the purlins discharge into similar channels on rafters and are drained to the exterior at the bottom of the roof. The elements for this type of roof are usually extruded in aluminium because the profiles required to accommodate pockets for gaskets and to form condensate drainage paths are easily extruded.