Civil Engineering Reference
In-Depth Information
However, whereas most will have
stood the test of time and will not
warrant remedial work, some walls
will be found in a precarious
condition. Very careful judgement is
necessary to determine into which
category each wall falls. Collapse of
walls during rehabilitation does
happen.
Walls are propped, stiffened and
restrained by floors and roofs; also by
buttresses in the form of returns,
internal walls and chimney breasts;
and even added features such as
external buttresses and piers.
Changes to any of these supporting
elements may weaken a wall. Walls
are also weakened by openings for
doors and windows.
The current building regulations
and codes require that building
components providing lateral
restraint (eg floors) must be tied or
strapped to walls to give resistance to
wind suction forces and to outward
bowing due to cyclic movement. Ties
are often absent in Edwardian and
earlier structures (straps at the roof
verge may be absent in quite modern
buildings) but may be restored to a
wall by providing through ties at floor
level, either to the opposite wall or to
a suitable anchor incorporated within
the floor or roof structure. These ties
often have decorative pattresses
threaded onto the tie (Figure 2.11).
Further information about remedial
work is available in BRE Good
Building Guide 29
(87)
.
The bonding of bricks also
determines the strength of a wall.
Some housing was built of two
leaves tied together with occasional
brick headers and laid in a weak
lime mortar. Under certain
conditions, for example where the
through headers fracture, the wythes
can move apart, with potentially
disastrous results. This can be dealt
with easily by installing remedial
shear ties as described later in
Chapter 2.2.
Openings in walls - windows,
doors and services apertures - create
potential weaknesses in the form of
narrow piers, thin spandrels and
lintels. Any tendency for movement
in a wall is likely to focus at these
openings. In the past it was common
to provide relieving or discharging
arches over windows, the theory
being that the load coming onto the
lintel was reduced, though the
corbelling effect of brickwork
arguably had much the same effect.
Narrow piers of brickwork
between openings may be
overstressed or offer inadequate
bearing length for the lintels around
the opening. Reliance on the wall
plate, rather than a lintel, to span top
floor windows is a common fault.
Proprietary composite concrete
gutter/lintels over top floor
windows may be structurally
unsound over long spans. Window
and door frames may be the only
support to the wall above.
Figure 2.11
Pattress threaded on the end of a remedial
lateral restraint tie. It is still functioning,
despite considerable deterioration of the
masonry
Figure 2.12
This well-buttressed one brick thick wall to an outhouse has been destroyed over the
comparatively short space of four years by a combination of thrust from the monopitch
roof, which still stands, and vegetation. It is now in a very dangerous condition
