10.05 Bolted glass facades
Open full view...Categories: Structural Glazings
Introduction
Bolted glass connections are described in Section 10.04. This section describes the ways these connections may be used to create glass facades. In some cases the glass pane is only required to transmit its own weight and wind load to a supporting structure and for these applications bolts bearing against holes in the glass are normally used. In other cases glass panes can be connected together to form structural assemblies in which the glass can carry the dead and wind load of the whole façade. Patch plate connections are normally used for these applications as they can accommodate higher loads due to the greater area over which the stresses are distributed.
Glass panes supported by a rigid structure
Bolted connections may be used to fix glass to the edges of concrete floors, to mullions spanning between floors or to structural steelwork, commonly fabricated from tubular steel, image.
Glass panes supported by tension structures
These systems take various forms including
- Glass supported by vertically spanning trusses with lateral restraint to the trusses provided by horizontal wires, image,
- Glass supported by bowstring trusses, image,
- Dead load carried by suspension wires or rods with tubular steel horizontal wind truss, image,
- Dead load carried by suspension wires with prestressed cable truss carrying wind loads, image,
Suspended glass assembly
Suspended glass assemblies were first constructed using patch plate connections. Dead load of the glass was carried to the top of the façade through the glass façade panels and wind load restraint was provided by glass fins attached to the façade. Current practice is to use the glass mullions to carry both dead and wind load using patch plates to connect the mullion sections together and bearing bolts to attach the façade panels to the mullions, image.
The suspended glass facades of the Serres at La Villette in Paris used bearing bolt fixings to connect the panes forming the suspended façade with cable trusses carrying the wind load. However suspension wires are usually used to carry the dead load when using bearing bolt fixings.
- Distribution of loads and stresses
- Redundancy
- Tolerances
As with any other form of façade construction, the loads on the façade must be transmitted to the supporting structure and then to the foundations. With bolted glass facades there are no established design codes and the designer must either prove his design by testing or refer to the guidance of the fixing system manufacturer. Section 10.01 discusses the strength of glass and Section 10.04 describes the main features of different types of connection.
All structural design should ensure that failure of one element does not lead to disproportionate failure. This is particularly important when using brittle materials like glass which can fail suddenly without warning. Where glass is supported as discrete panes this is not a problem but where glass is used structurally it is necessary to provide alternative load paths, image.
Bolted glass facades differ from traditional frame supported glass in two respects. Firstly the edges of the glass panes are exposed so that variations in pane size are more apparent. Secondly deviations in hole size and position may lead to additional stresses in the glass unless there is a means of adjustment provided. Permitted deviations on the glass and any supporting steelwork will normally be smaller than for other forms of construction.
- A clear distinction should be made between provision for tolerances and provision for movement to ensure that the allowance for movement is not used to overcome deviations during erection leading to failure in service when movements are restricted.
- Tolerances are discussed in more detail in Section 03.07 and Section 03.08.
- Movements
- Sealing
When glass is supported in a frame, in plane movements of the glass and supporting structure can be accommodated by movement of the glass within the frame rebate resulting in relative movement of adjacent panes of glass. Where bolted glass connections are used relative movement between panes may not be possible and movements have to be accommodated at the edge of the glazed area. The movements to be accommodated will therefore be larger as they relate to a larger area of glass.
Deflection of the glass causes rotation at the fixing points which leads to bending stresses in the glass unless the fixings are designed to permit rotation as described in Section 10.04.
Section 01.09 and Section 01.10 discuss movements in more detail.
Weathertightness of bolted glass facades is dependent on the formation of sealed joints between the edges of the glass panes. There is no scope for secondary seals or drainage as in the case of framed glazing.
The design and construction of sealant joints is described in Section 01.07 and Section 01.08.