Civil Engineering Reference
In-Depth Information
workmanship than are single-stage
joints.
Single stage joints, usually mastic
or gasket sealed on the outer face,
where a single mechanism provides
protection against both air and water
penetration, are usual in some types
of curtain wall construction. Face-
sealed joints in window-to-wall
masonry situations should be
designed to be lapped rather than to
butt, wherever possible, since this
will afford some protection to the
seal (Figure 1.71).
Window-to-wall joints are usually
better sealed at the back, and to have
some kind of overlap or mechanical
protection at the front, effectively
creating a two-stage joint. This
applies whether the wall is single or
multi-skin (Figure 1.72).
Each joint design will have a
maximum and a minimum dimension
over which its performance can be
assured. Such ranges will depend
very much on the individual
characteristics of edge profile of the
joined components and the jointing
product material and dimension.
Figure 1.73 shows a typical
Gaussian curve with such limits
indicated. For those cases where
corrections of positioning or other
deviations are not feasible, the
prudent specifier or contractor
should have alternative designs of
joint in mind for those conditions
which lie outside the limits.
Butt
joint
Fixings
Adjustability of fixings, for example
for sheets used in overcladding, is
normally needed in all three planes
(Figure 1.74). It is not possible to
give any universally applicable
recommendations for the amount of
adjustability needed since this will
depend on circumstances such as the
kind of construction, its
characteristic accuracy, and the
amount of control exercised.
Nevertheless it will be possible from
data on accuracy of the original
construction and its movements in
service, to calculate, for example, the
size of slots within brackets and the
size of holes for pegs.
All adjustable fixing mechanisms
must be equipped with provision for
the restoration of positive fixity
following adjustment in order that
accidental slippage under the
maximum design load does not occur.
Probably the most straightforward
method of achieving this is by the
compression of two serrated metal
plates or brackets secured by bolting
through (Figure 1.75).
Lap
joint
Figure 1.71
Face sealed joints should be lapped rather
than butted
Air seal
Special
Normal joint design
Special
Figure 1.72
Two stage joint, with air seal on the inside,
and an overlap on the outside to shed
rainwater
Predicted normal joint range
Predicted total joint range
Width of joint
Figure 1.73
Normal (Gaussian) distribution indicating
upper and lower limits for normal joint
performance
Figure 1.74
Adjustability of fixings is normally needed in
all three planes
