10.04 Bolted connections

Categories: Structural Glazings

Introduction
Traditionally glass has been supported around the perimeter in a frame. More recently various methods have been developed to support glass with bolted connections. These methods of fixing allow the creation of large areas of glazing uninterrupted by framing. Bolted connections may simply be used as an alternative method of fixing glass panes to a supporting structure however they may also be used to connect glass panes together allowing loads to be transferred from pane to pane forming a structural glass assembly. In structural glass assemblies the glass may carry the dead load of the façade with wind load carried by secondary steelwork alternatively glass fins may be used allowing the glass to carry both dead and wind load. Methods of façade construction using bolted glass are described in Section 10.05.

Bolted connections must be capable of carrying in plane loads in the glass (normally dead load) and loads perpendicular to the plane (normally wind loads). Loads perpendicular to the plane are carried by contact between the glass surface and the fixing. Two techniques are used to carry in plane loads as follows:

• Friction - Patch plates are used to clamp the glass and transfer in plane loads by friction on the surfaces of the glass.
• Bearing - Bolts passing through the glass transfer in plane loads by bearing of the bolt against the sides of the hole in the glass.

The glass will deflect in response to wind loads causing the glass to try to rotate about the fixings. Either the fixings must be designed to permit rotation or the glass must be strong enough to resist the resulting bending stresses.

Toughened glass is normally used to accommodate the stresses induced by the concentrated loads at the connections. Toughened glass cannot be cut or drilled so all cutting and drilling must be carried out on annealed glass before toughening. Holes are cut using diamond drills and must have a diameter at least equal to the thickness to allow toughening to be carried out. Toughened glass should be heat soaked to reduce the risk of nickel sulfide failure.
 

Patch Plates
Patch plate connections use stainless steel plates either side of the glass which grip the glass as a result of the clamping effect of a bolt passing through the assembly, image. Direct contact between the metal plates and the glass is prevented by fibre gaskets. Contact between the bolt and glass is prevented by using oversize holes in the glass; a nylon bush is normally used as an additional precaution.

This system is unsuitable for use with laminated glass as creep of the interlayer will reduce the clamping force and hence the friction between the glass and the patch plates. It is also unsuitable for use with insulating glass units as they would not be able to withstand the clamping force. Annealed glass may be suitable in some locations but toughened glass is normally used due to its greater strength and safer failure mode.

The fixing method does not allow movement between glass panes and the system must therefore be designed to accommodate movement at the edge of the glazed area. The fixings do not allow rotation of the glass but the area of the fixing is greater than for other bolted connections reducing the resulting stresses.

The in-plane load capacity of a connection is dependent on the clamping force and the coefficient of friction. The clamping force is dependent on the torque in the bolts, which must be checked during erection. The coefficient of friction is typically about 0.2 but should be checked with the supplier when using proprietary fittings. The supplier should also be able to provide design guidance for the system including maximum spans for different glass thicknesses. For bespoke designs, tests are likely to be required to prove the design strength.

Patch plates may be used in a range of shapes and sizes and can be used in the form of flat plates for joining glass panes in the same plane and as angles for attaching fins to a façade. Patch plates were developed before hole-bearing bolted connections and their use now tends to be limited to applications where their ability to carry greater in plane loads is required such as suspended glass assemblies.
 

Hole-bearing bolted connections
A number of bolted connections have been developed which work by transfer of load from the glass to the bolt by bearing of the bolt against the sides of the hole in the glass. Due to the high contact stresses, toughened glass is necessary and direct contact between the glass and the bolt is avoided by using bushes which are normally made of polyamide. The use of countersunk fittings allows an almost perfectly flat surface to be produced.
 

• Planar system. The planar system developed by Pilkington is shown in image. The glass is connected to a bracket with sufficient flexibility to allow rotation of the glass. As the centre of rotation is offset from the plane of the glass, rotation will be accompanied by in plane movement which must be accommodated by the fixing. This may be achieved by the use of an oversize fixing hole in the supporting bracket. The system may be used with single glazing or double glazing units in which case the load bearing fixing is to the outer pane.
• Articulated bolt. An articulated bolt system, image, was developed by Rice Francis Ritchie for use in the Serres at La Villette in Paris. In this system the spherical seating in the plane of the glass allows rotation without inducing in plane movement.
• Resilient inserts. In this method a larger hole is made in the glass and filled with resilient material. The bolted connection is then attached to the resilient material. Stresses in the glass are reduced by using a relatively large hole in the glass and the flexibility of the insert which allows rotation to take place.

Stress analysis of bolted connections, image, may be carried out using finite element methods, image, however it will normally be necessary to confirm the predicted performance by testing. When using proprietary fixings the manufacturer will normally be able to provide design guidance for his fixings based on previous testing. If testing is used to prove the performance of a new design a sufficiently large number of tests must be undertaken to allow for the inherent variability of the materials, particularly the glass. Testing a single panel will be of little value.