Despite their popularity, bolted connections are not the ideal type of connection for glass. Firstly, glass is a brittle material, which is why local stress concentrations around the bolt cannot be reduced through stress redistribution. This makes bolted connections relatively inefficient from a structural point of view. Secondly, the surface flaws in glass caused by the drilling of holes and the distortions of the tempering stresses around holes (cf. Section 3.6) mean that bolted connections are inducing the highest stress concentrations in the weakest possible area of the glass panel.
Research carried out by Overend [260] investigates the strength of steel-to-glass adhesive joints that eliminate the need to drill through the glass. A circular 60 mm diameter adhesive area was used and three different adhesive were tested. The best performing adhesive was an acrylic based adhesive and achieved an average load bearing capacity of 85 kN. A series of equivalent 60 mm diameter through bolt connections were also tested for comparison, these archived an average capacity of 29 kN. These tests showed that with the correct surface preparation and adhesive selection it is possible to provide an adhesive connection that would improve the short term strength of bolted connections by close to 300 percent.
A further development in this area is the combination of a glued connection and a pretensioned bolted connection (Figure 7.26). This connection consists of two stainless steel adhesive discs, which are glued exactly opposite of each other on either side of a fully tempered glass plate. The discs are connected with each other by means of a stud bolt that fits tightly in the discs but goes through a clearance hole in the glass thereby bearing onto the steel discs but not the glass. The discs have an annular area for the adhesive that is clear of the relatively weak hole edges in the glass. The stud bolt is pretensioned after the adhesive has completely set. This reduces the deleterious effects caused by the
peel stresses. The hole in the glass is large enough to cope with the tolerances of the glass manufacturer and to allow the bolt to bend. The adhesive has the function of an intermediate material and is being used for the transfer of the shear force between the steel discs and the glass surface and consists of a thin layer (0.1 mm) that guarantees a stiff and creep resistant joint. The pre-stressing of this connection is only possible if the discs are glued to monolithic glass. When laminated glass is required, the discs should be connected to only one of the glass layers, normally the one which is protected from weathering and vandalism.
This connection has been tested at the Faculty of Civil Engineering, Delft University of Technology. For the tests a 19 mm fully tempered middle layer glass and stainless steel discs with a diameter of 120 mm, a thickness of 15 mm and a bolt diameter M24 has been used. The specimen failed by local overload of the glass cross-section just below the stainless steel discs. The adhesive joint survived in all the tests. The average strength of the connections was 230 kN. The stud bolt through the connection showed a visible plastic deformation before the glass failed. Such a behaviour might be used in practice as an early warning mechanism in case of overloading.
Figure 7.26:
8
Special Topics
8.1 Design assisted by testing
This text has been compiled in collaboration with the following experts: Benjamin BEER, Dr. Iris MANIATIS, Prof. Dr. Geralt SIEBERT
8.1.1 Introduction
Despite advances in the field of computational analysis, the design of complex glass structures can not be based solely on numerical simulation. The reasons why full scale prototype testing remains an integral part of the design process of innovative glass structures, as well as the main issues that should be considered when testing glass elements, were discussed in Section 6.4.1.
Computational modelling, typically finite element models verified by rules of thumb, are required to predict the structural behaviour with an acceptable level of accuracy. The results from these calculations are often the basis for the first test prototype or specimen. Geometrical imperfections as well as tolerances should be taken into account to achieve a realistic test setup. A comparison between test results and the corresponding predicted values given by the model should be carried out. If major discrepancies are found, both the test setup and the model should be checked.
The fracture strength of heat treated glass is the sum of the absolute value of the residual (compressive) surface stress and the inherent glass strength (see Section 3.3.2). Only the latter is influenced by subcritical crack growth and depends, therefore, on time and environmental conditions. The residual stress is constant. Consequently, results from experiments with heat treated glass (HSG or FTG) in ambient conditions depend significantly less on time and environmental conditions than the results from tests on annealed glass.
General guidelines for design assisted by testing are given in the annex of EN 1990:2002[133]. The engineer must, however, bear in mind that this standard has not been specifically written for glass structures. Detailed reviews of the countless national standards, regional standards, building regulations and recommendations for project
specific glass testing is beyond the scope of this document. For any project, the testing
procedure has to be chosen to suit the project specific needs as well as of the requirements defined by building owners, insurers and authorities.Nevertheless, in order to provide the reader with a general idea, a few examples are discussed in the following.