Civil Engineering Reference
In-Depth Information
accidental-damage loadings to a
domestic house were simulated by
removing the masonry between lower
window openings when a
load was being applied at both first
floor and upstairs ceiling level. The
masonry was removed both from the
front wall which supported the loaded
floor and from the rear wall where the
floor spanned parallel to it. In neither
case was there any collapse due to the
damage and there was evidence of a
redistribution in the support to the
load
(97)
.
Main performance
requirements and defects
Strength and stability
The materials used in cavity walls of
buildings up to three storeys will
rarely be overstressed unless in
narrow piers, but if constructed in
accordance with Approved
Document A1/2
(20)
, the lowest storey
has to be built of 7 N/mm
2
units,
whereas 2.8 N/mm
2
units are allowed
for the upper two storeys. Piers of
brickwork between
openings may be overstressed or offer
inadequate bearing length for two
lintels, or possibly three if an opening
in an internal wall also abuts.
Loadbearing masonry can be built to
any height but above three storeys it
has to be a calculated design to BS
5628-3
(19)
.
Some form of structural linking (eg
wall ties) between cavity wall leaves
will have been included in most
designs of cavity walls so that both
leaves to some extent share loads, but
their frequency and distribution will
differ between
walls. The current prescribed
frequency is 2.5 ties/m
2
for most
walls, and 5 ties/m
2
for walls thinner
than 90 mm. Some metal types are
liable to corrode. There is also some
evidence that wall ties may be
deficient, more especially in cavities
which exceed 75 mm in width, and
in narrower cavities at openings and
unbonded edges.
Cavity walls are restrained by
floors and roofs and propped by
buttresses in the form of returns,
internal walls and chimney breasts.
Changes to any of these supporting
elements may weaken a wall. Walls
are also weakened by doors and
windows and other openings.
Lintels may be absent over top
floor windows and reliance placed on
the wall plate to support the
construction above. Proprietary
concrete gutters/lintels over top floor
windows may be structurally
inadequate over large spans.
Figure 2.23
The brickwork over this meter box has
collapsed because there was no lintel or
other support
Wind pressures and suctions
Although damage to masonry walls is
sometimes encountered after very
strong winds, and collapses are not
unknown, well built walls are
remarkably strong
(98)
.
In practice, most collapses of both
leaves of cavity walls are a result of
wind suction, since inward movement
is resisted by floors and buttress
walls. In the 1987 and 1990 gales,
however, large numbers of houses
lost their outer leaves due to suction
forces on walls with corroded ties.
Dimensional stability, deflections
etc
Minor cracking of walls caused by
wall settlement or expansion, or by
differential movement between the
leaves is common. Major cracking of
both leaves usually indicates
differential settlement due to ground
conditions, but some cracking may
also be due to settlement of bearings.
Calcium silicate bricks and some
blocks can shrink appreciably. A
summary of the main points in
relation to cracking is included here,
and further information may be found
in
Cracking in buildings
(99)
and BRE
Digest 360
(100)
.
For coefficients of linear thermal
and moisture expansion etc, see
Chapter 1.2.
Regular horizontal cracks indicate
expansion caused by advanced wall
tie corrosion. A 'belly' in a wall
might indicate lack of restraint, lack
of buttressing, lack of wall ties or
wall tie failure. A leaning wall may
indicate roof thrust, or inadequate
thickness or restraint.
Long lengths of masonry walling
Figure 2.24
A poorly adhered sealant in a masonry wall
movement joint. There are other joint
defects too
holes above, it is too high to fully
protect the lintel and window head.
On the other hand, if it is installed to
terminate below the outer lintel,
weeps are not possible. Battening
out the tray below the lintel is a
possible solution.
Accommodation of services
One of the most common faults found
on site inspections of dwellings has
been absence of lintels over service
entry points, and even brickwork
above meter boxes can suffer from
cracking if there is no adequate lintel
(Figure 2.23).
Accidental damage
Masonry can be remarkably robust in
resisting collapse after impact
damage. In a BRE experiment,
