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Cladding and Curtain Wall Construction 475
Unitised or panel system of curtain walling
An alternative to the stick system is the unitised or panel system of curtain walling in which
complete panels of an aluminium frame with glazing are fabricated as units ready for hoisting
into position for fixing to the structural frame or to a carrier system. The advantage of
this system is the facility of precision assembly of the components and glazing under cover
in the conditions most favourable for successful glazing, particularly for insulating glass
(IG) units, to provide the most effective protection to edge seals. By virtue of repetitive,
precision assembly, large glazed panels may be fabricated at reasonable cost for fixing to
the structural frame with narrow weather sealed joints between panels, the maximum area
of glass and least exposed area of framing. In effect, this is a system of large, dead light
window frames, ready glazed for fixing as a glazed wall system between floors or as a
curtain wall. The members of the frame are designed to show the least area on face necessary
for satisfactory support, bedding and weather sealing of glass, and adequate metal
section and depth to support the weight of glass and wind forces acting on the panel where
it is fixed and restrained at its top and bottom edges to structural floors. The edges of the
frames may be shaped for gasket glazing compressed into rebates by metal strips bolted
from behind or may be shaped so that the long edges of panels interlock and the horizontal
joints overlap over gasket seals.
Large panels may be fixed to a carrier frame, which is fixed to the structural frame. The
carrier frame is designed to support the dead and live loads of the panels, provide a background
for gasket or sealant joints and assist in the alignment of the panels across the face
of the building. Figure 7.52 is an illustration of a glazed curtain wall in which panels of
double glazed units are fixed to a system of aluminium mullions and transoms fixed to the
structural frame at each floor level. The spacer bars at the edges of the double glazed units
are shaped so that the two panes of glass can be secured to the wings of the spacer bar with
adhesive silicone. The silicone acts as a powerful, long-term adhesive.
The glass panels are secured to the carrier frame with dumb-bells that fit to a bar that is
screwed to the mullion, as illustrated in Figure 7.53. The dumb-bells fit into the wings of
the spacer to secure the glazed panel at intervals on all four sides to hold the glass panel
in position. A gasket fixed to the bar provides a backing onto which the silicone sealant
jointing is run between glass panes.
This system of curtain walling is used for the advantage of the flush silicone sealed joints
that provide a flush external face, as illustrated in Photograph 7.3. This system is often used
with coloured glass for the dramatic effect of a large expanse of reflective material. For
reasons of safety, the height of this cladding is limited to about 8 m, unless a system of
mechanical retention of the outer panes is used. Mechanical retention takes the form of
aluminium angles around the edges of each outer pane. This glazing system is used for
small areas of cladding both internally and externally.
Structural glazing sealant
The characteristics of making a powerful bond between glass and metal have been used in
the system known as structural glazing. The advantage of this system is that glass may be
bonded to the face of a metal frame through a narrow edge strip contact of silicone to the
back of glass and the face of a metal frame. The glass is held firmly in place by the silicone,
which will transfer a whole or part of the weight of glass to the frame and the whole of