Civil Engineering Reference
In-Depth Information
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thin sheet claddings may relieve
rapid stress development by
bowing (oil-canning)
All joints, whether sealed or open,
should be designed to prevent rain
and snow reaching the inside of the
building.
Winds tend to drive run-off
sideways across the façades of
buildings, and consequently the water
load on vertical joints is not
necessarily any lower than that on
horizontal joints (Figure 3.61a).
Vertical ribbing of the surface will to
some extent divert sideways flow
(Figure 3.61b), but this cannot be
quantified into simple design rules.
For coefficients of linear thermal
and moisture expansion etc see
Chapter 1.2.
Thermal movement may be one of
the principal actions; the fixings and
joints between panels must allow in-
plane expansion or contraction to
occur freely, otherwise excessive
unintended forces can develop. In
thicker materials, especially if highly
insulating on the central or interior
laminate, bowing movements can
occur due to the differential
movement between outer and inner
layers (Figure 3.60) and very high
and destructive shear stresses may
develop in poorly designed systems.
It is important that movements are
allowed for in the fixings of fibre
cement sheets, and in no
circumstances should tightly butted
joints be permitted.
Case study
Rainwater leakage in infilling spandrel panels
The BRE Advisory Service was asked to carry
out an investigation of damp penetration
problems in a concrete framed seven storey
building located near the coast. Exposed
concrete beams and columns were clad with
mosaics. The seaward-facing southern
elevation had been infilled from first to sixth
floor with PVC-U framed windows and
insulated spandrel panel units. During the
conversion to an hotel the original infill
framing between floors and columns was
replaced with new glazing and spandrel
units, but original metal sub-frames were left
in place. Head and jamb sub-frames had
been covered with PVC-U trims, but the
original aluminium sills were retained as
functional components at the base of the
new panels.
Problems with water penetration were
evident soon after refurbishment, showing as
damage to wall plaster and paint, mainly
concentrated at column positions, and at
outside edges and corners of the partition
walls.
Externally, the infill panels appeared to be
well sealed to the surrounding structure,
albeit with cellular PVC-U trims and profiles
covering the existing metal sub-framing. There
were only a very few places where
deficiencies seen in the silicone sealant
provided possible leakage points and some of
these had been caused by previous
investigation work by others.
Removal and dismantling of a complete
infill frame confirmed the following points:
crudely drilled drain holes had been
provided in the spandrel panel transoms
which drained the front part of the
transoms ineffectively. The rear sections
of the transoms could hold water which
would be pumped through past the inner
gasket seals. The inner gasket seals of
the spandrels were not very carefully
installed at the lower corners and a
screw fixing from the windowboard
penetrated right through into the top
gasket, disturbing its seal
●
rainwater penetrating round the opening
windows collected on the windowboards
and ran through the unsealed joints
between windowboards and plastered
reveals. It then entered the cavity behind
the old metal subframes and wetted
plaster, partition walls etc and also built
up at the floor edges and ran into the
rooms below
●
Weathertightness, dampness and
condensation
All cladding should resist rain and
snow penetration to the inside of the
building, and the cladding itself not
be damaged by rain or snow. In order
to satisfy this requirement the sheet
cladding may be either:
●
routes for rain penetration directly
through the spandrel panel gaskets into
the cavity and blockwork backup wall
were identified
●
sealing of the panels to the concrete
structure appeared to have been
executed quite well and rain penetration
through edge seals was only thought to
make a minor contribution to leakage
●
a lap-jointed moisture resisting
outer layer (eg tiles, see
Chapter 9.1)
●
backed by a moisture resisting
inner layer
●
backed by a clear cavity across
which rain will not transfer
To achieve a weathertight wall, replacement
of the spandrel infill panels was likely to be a
preferable option to modification and repair
of the existing construction.
The face laminate expands
more than the rear laminate
Run-off driven
sideways on
smooth
surfaces
Run-off
thrown clear
of the
surface
Run-off
diver ted down
by ribs
Figure 3.60
Bowing of a panel caused by
differential thermal movements
in layers bonded together
a
b
c
300-600 mm
Figure 3.61
Runoff of rainwater on vertical surfaces
