Civil Engineering Reference
In-Depth Information
which will bear on the temporary
support must be checked; a simplified
method for assessing load is given in
BRE Good Building Guide 10
(93)
.
More detailed advice on loading and
method of support will be needed if
the existing construction is in poor
condition
(94)
.
Inspection
Although a rare occurrence (apart from
boundary walls) there have been some
collapses of masonry structures caused by
the wind. Several of these failures have been
investigated by BRE and the results have led
to recommendations on appraising the
stability of large masonry structures. Ways
of appraising the stability of existing walls
and techniques for strengthening them, and
the design of new structures to ensure
stability are discussed in Safety of large
masonry walls
(95)
.
All external walls should be inspected in
detail and positions and directions of cracks
noted; if more than a hairline, crack widths
should be recorded and long term
monitoring considered. If visual evidence of
distress is found then a full plumbline survey
should be undertaken. The possibility that
the walls might have been originally built
'out of true' should not be discounted
(Figure 2.19).
Solid brick walls should be inspected to
determine any potential overstressed
narrow piers such as the walling above each
opening, internally and externally. Lintels
may need to be examined to establish
material, shape, size, condition and
bearing width.
The position of the DPC relative to ground
level all round the external perimeter of the
building should be noted, together with the
effect of abutments. Obvious signs of rising
damp, for example the presence of salting
and tidemarks, should be noted.
The external elevations should be
examined for cracking and any features
likely to affect the flow of rainwater down the
building face such as the protection provided
by the overhangs, leaks from rainwater
goods or unsound renderings.
Any loadbearing door or window frames
need to be identified, and checked that fire
resistance is adequate. It may be useful to
compare the extent of 'unprotected areas'
with current regulations to see whether
modifications are needed or justified.
Internal faces of external walls should be
inspected for evidence of dampness such as
mould growth.
The problems to look for in the main parts
of walls are:
◊
narrow piers or returns overstressed
◊
defective bearings of long lintels
◊
spalling, cracking, oversailing, bulging or
leaning
◊
dampness and algal growth
◊
inadequate or blocked air bricks
In situ assessmentof masonry
walls
It is often of value to be able to
measure the performance of existing
walls, especially in cases having (or
possibly having) legal consequences.
Examples are where there are signs of
deterioration, cracking etc; where a
wall has partly or wholly collapsed;
where the loading or usage of the
walls may be changed; or where the
imposed loading has changed due to
external influences (eg an adjoining
building has been removed).
A number of techniques have been
developed for in situ assessment of
masonry. The bond wrench was
developed in the USA as a control test
for the bonding of bricks to mortar in
new work. This has been further
developed by BRE into a site
procedure which enables the bond of
bricks to mortar to be assessed in
built brick or blockwork, and is
described in BRE Digest 360. In
some cases it is also very useful to
know the level of stress in a wall.
This can sometimes point to
overstressing or foundation
movement, or give useful insight into
movement related stressing. The test
technique is based on the flat jack
method and is described in BRE
Digest 409.
Normally where mortar problems
occur (eg of poor durability), they
have been investigated by chemical
analysis and the results may be
compared with the specification
which is normally by content of
cement etc. The trend, however, is
towards performance specification of
mortars and strength then becomes
the key parameter. A simple in situ
test for mortar strength is described in
BRE Digest 421
(29)
.
Figure 2.19
A bulging outer 'leaf' in a solid wall. Is this
sulfate attack, or simply the parting of
two wythes?
◊
pointing erosion
◊
slipping or spreading of brick arches
◊
sulfate attack
lime, silica or iron staining
◊
◊
lime popping
The problems to look for in respect of DPCs
are:
◊
missing, damaged or deteriorated
◊
bridged by rendering
◊
pointed over
below external ground level
◊
◊
liquids injected into joints, not bricks
◊
plaster contaminated with hygroscopic
salts
