10.02 Bonding

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Categories: Structural Glazings

Introduction
Due to its brittle nature the use of glass in bolted assemblies is limited by the concentrated stresses around the fixings. Using adhesives to connect glass elements allows the loads to be distributed over larger areas than when using bolted connections reducing the resulting stresses. Bonded glass is currently used in the following ways:
 

  • Laminated glass. Laminated glass is formed from two or more leaves of glass bonded together with a sheet interlayer, usually polyvinylbutyral (pvb), or resin, usually acrylic or polyester. The properties of the laminate depend on the type(s) of glass and the interlayer as described in Section 10.01.
  • Structural silicone glazing. In structural silicone glazing a structural grade silicone is used to attach glass panes to the supporting frame as described in Section 10.03.
  • Attaching wind restraint fins to glass facades. To provide a high degree of visual access to buildings, for example for shop fronts and entrances, glass facades may be formed by a single row of glass panes supported along their top and bottom edges.  Where this method of support does not give sufficient resistance to wind loads, additional wind restraint may be provided, while still keeping the transparent fa├žade, by using glass fins. Traditionally this has been achieved using low modulus silicone sealants which allow loads to be transferred by tension and compression, image.  BS 6262 gives limited guidance on the use of bonded glass fins for this purpose particularly for determining glass thickness but refers to adhesive manufacturers for further information on adhesive properties.



Recent developments
The sealants traditionally used for bonding fins to glass facades allow slip due to shear at the interface between the glass elements under bending stresses so that the joint does not develop composite action, image, resulting in a loss of structural efficiency.

Recent research (Pye and Ledbetter) has shown that it is possible to bond glass using modified epoxy resin adhesives, which have a high shear modulus and hence allow composite action of the assembly. Laboratory tests on T beams formed from two glass plates with an adhesive joint at the web/flange interface achieved almost complete composite action.

Although the ability to create bonded glass beams has been demonstrated under laboratory conditions, use in real structures will require consideration of long-term behaviour. In particular the durability of the adhesive bond and the effects of repeated and long-term loads on the adhesive bond. Good workmanship is also essential to ensure that a good bond is achieved. This must be accompanied by a full understanding of factors which affect the overall performance of the assembly. For example it may be thought desirable to etch the glass to improve the bond strength however whilst this may be the case it will also weaken the glass and is not recommended.

A more detailed discussion of the use of bonded structural glass is given in 'Structural use of glass in buildings' published by the Institution of Structural engineers.