Civil Engineering Reference
In-Depth Information
sometimes be used. Lightweight
blockwork similarly finished may
also be used as a non-loadbearing
cladding, for example on timber
framed houses.
Brickwork as a replacement
external skin
Brickwork as a replacement outer
skin finds application mainly in low-
rise construction, although it has
been used in multi-storey
construction where the structure can
be adapted to carry the extra loads
and the brickwork carried on shelf
angles, for example, at each floor
level. Since any concrete supporting
structure has already undergone most
of its shrinkage, the need for
movement joints at each floor level is
less exacting than with new-build;
only thermal movements will be
significant.
Any new brick skin will need to be
tied back to the original structure, for
example by resin-bonded anchors at
the same centres as ties, as if one was
designing new brickwork at that
identical exposure.
Figure 2.21
Corroded examples of strip and wire wall ties
Openings and joints
Openings for windows, doors and
services may create potential
weaknesses in a wall, with narrow
piers between openings, thin
spandrels and lintels often acting as a
focus for any tendency for movement
within a wall (Figure 2.22). Lintels
found during rehabilitation can be of
steel, concrete, stone, timber or
arched brick. Inadequately protected
timber and steel lintels are
particularly vulnerable and may be
found in various stages of
deterioration.
When cavity walls were first
introduced, either a separate lintel or
a brick arch was used for each leaf.
With the increase in popularity of
steel lintels since the 1950s, however,
a number of combined lintels were
introduced which supported the
masonry in both leaves. The most
usual were the common lintel (eg. the
top hat or box lintel with shelf) and
the boot lintel where the main
structural lintel is in the inner leaf but
has a simple metal or concrete boot
unit (or toe) to support the outer leaf
of masonry. Owing to a greater
emphasis on preventing thermal
bridges in the late 1990s, the separate
lintels format is coming back into
popular use.
Double lintels may be a source of
problems. If a cavity tray is installed
above the outer lintel, with weep
Lintel bearings close together
Internal skins
Although often of brick, inner leaves
may be of in situ concrete, clay
hollow blocks, or breeze blocks, and
there has been an increase in the use
of ultra lightweight concrete blocks
for the inner leaves of external walls
since the building regulations
prescribed improved thermal
insulation standards.
Meter box sited inappropriately
leading to high stress at these points
Wall ties
Wall ties may be found made of many
different kinds of materials: wrought
iron, stainless steel, copper, copper
alloy and mild steel coated with zinc,
epoxy, bitumen and zinc with PVC.
The design of the tie itself has also
varied and in the past has included
such variants as cranked bricks, bars
with fish tails, and modern wire
'butterfly' ties (Figure 2.21).
Butterfly ties have been available
since the 1920s and were
standardised from 1945 onwards, but
stainless steel was only introduced in
the 1960s.
Figure 2.22
Positions of high stress caused by
proximity of lintel bearings, and openings
for services
