Civil Engineering Reference
In-Depth Information
The bolts are often connected to a cast node (spider fi tting)
that is in turn supported by a sub-frame or cable trusses.
simultaneously, the effective stiffness of the adhesive will tend
to be signifi cantly greater than that of the bolts. It is, therefore,
sensible to assume that all loads will initially be transferred
through the adhesive and the bolts will only come into effect
when the adhesive has deformed signifi cantly which is often at
failure. Nevertheless, bolts can enhance the load-bearing cap-
acity of an adhesive joint by, for example, using an adhesively
bonded friction grip connection as shown in
Figure 21.10(a)
.
It is also possible to use bonding and bolting in series by,
for example, bonding a glass to a steel plate which in turn is
bolted to the steel substructure (
Figure 21.10(b)
). Other novel
methods such as soldered glass joints (
Figure 21.10(c)
) are
emerging, and are the subject of ongoing research.
Adhesive frameless
The adhesives in these connections transfer the principal loads
from glass-to-glass or glass-to-metal. The adhesives used are
generally thermosets (e.g. UV-cured acrylics, two-component
acrylics and two-component epoxies) which are stronger and
stiffer than the aforementioned elastomeric structural silicone.
The principal advantages of adhesive connections over bolted
connections is that they produce lower stress concentrations
and they require little or no preparation, unlike bolted connec-
tion where holes must be drilled into the glass (
Figure 21.9
).
There are, however, uncertainties on the long-term perform-
ance of some of these adhesives and their performance in fi re
tends to be poor. As a result structural adhesive connections are
the subject of ongoing research.
21.2.2 General performance requirements
The performance requirements that inform the design of struc-
tural glass elements often extend beyond the purely structural
considerations and include: natural lighting; thermal perform-
ance; acoustic considerations; structural performance under
normal and exceptional actions; security; durability; and
maintenance.
The non-structural performance requirements are beyond
the scope of this chapter, but it is very important to note that
they will infl uence and in some cases severely constrain the
structural design process (and vice versa). The structural
design of glass elements must therefore be carried out with
frequent consultation of the other design team members such
as the architects, building service engineers, fi re consultants,
acoustic consultants, etc.
Hybrid connections and other novel connections
A new generation of hybrid connections are emerging which
involve a combination of bolting and bonding. It is important to
note that when bolts in clearance holes and adhesives are used
21.2.3 Structural glass design process
The aim of the structural design process is to devise an effi -
cient solution that satisfi es the performance criteria. There are
two important particularities when using glass as a structural
material. Firstly, glass is an inherently brittle material, and the
consequences of failure and the residual post-fracture perform-
ance must be addressed explicitly during the design process.
Secondly, the engineer must have a suitably high degree of
confi dence in their calculations. However, the desktop design
methods and the construction techniques used in glass are still
in their infancy, and it is often necessary to perform prototype
tests to validate the calculations.
The design of glass members and glass structures will gen-
erally require the following steps:
1. Defi ne the performance criteria required for ultimate limit
state (ULS) and serviceability limit state (SLS). The ULS per-
formance requirements should be based on a risk analysis of
the intended use of glass and the consequences of failure. A
simple risk analysis for glass design and the resulting special
performance requirements are shown in
Table 21.1
.
Table 21.1
is based on residential, offi ce and public buildings
for normal use. It is important to note that the consequences of
failure are a function of the type of building. Therefore, the failure
of a secondary structural member in a densely occupied building
Figure 21.9 UV-cured transparent adhesive joint used in specialist
glass furniture application
