Civil Engineering Reference
In-Depth Information
Weathertightness
Single leaves of limited thickness can
be expected to leak rainwater through
to the cavity in conditions of driving
rain unless protected externally with
a rain resistant cladding. This water
must be prevented from reaching the
frame by suitable detailing of
copings, DPCs, cavity trays and
flashings. Defects characteristic of
masonry on steel or concrete frame
walls include rain penetration and
draughts through defects in the
original cladding, particularly in
conditions of high exposure. Poor
drainage will also take its toll (Figure
3.10).
Case study
Fall of stone cladding
Some stone slabs fell from the south
elevation of a building, injuring a number of
people. The building had been erected
around 20 years earlier, and comprised a
reinforced concrete frame with windows and
stone facings.
The damaged elevation above second floor
level was divided into 12 bays by stone clad
columns. Between the columns were windows
with the spandrel panels beneath each window
formed in stone. The stone slabs that fell
came from what was essentially a string
course at second floor level one stone slab
deep which ran the whole length of the
elevation. They fell from a position about one
third of the length of the elevation from one
corner of the building.
The slabs were natural stone about
890 mm x 470 mm and about 38 mm thick
(Figure 3.9). They were fixed to the building
with the long side horizontal. The top edge of
bolts appeared to be loose and not to have
been tightened to fully expand the fixings,
and were easily removed from the sockets.
The angle was in good condition and there
was no sign of significant corrosion.
There were a number of fractured stone
pieces, from fragments to quite sizeable
pieces. All the pieces together would
probably make up three or four full slabs. In
addition, some of the slabs were virtually
undamaged, but a number of these
apparently had been removed by the fire
brigade which first attended the scene. The
blind over a shop front had been extended at
the time the slabs fell and had probably
absorbed some of the impact.
Measurements on the debris showed
some variation in the thickness of the slab
and in the dimensions of the slot. The full
depth of the slot as cut, that is to say the
maximum possible bearing, was 15-20 mm,
whereas the actual bearing was in many
cases less that this, down to 6 mm. Also
some slots had indications that they had
been damaged prior to fixing.
There was a hard mortar in the thin
vertical joints between stones. There was
no evidence of a soft joint in the whole
length of the elevation in a vertical position,
but there was some indication that a soft
joint had been used at the head of the slabs,
consisting of sheet polystyrene and a
mastic pointing.
The fall had resulted from thermal
movement in the long run of stonework,
causing it to bow and lose its bearing. The
thermal coefficient of the stone was of the
order of 3-4 x 10
-6
/ °C. A temperature rise
of as little as 18 °C would produce
expansion in the 54 m length of the order of
8 mm. If completely restrained at the ends
this could produce a very substantial bow of
the order of 280 mm. Obviously the lateral
movement had not been of this magnitude
but there was every reason to suppose that
repeated movement of the stone had
caused some of the slabs to lose their
bearing on the supporting angle.
Figure 3.10
Evidence of poor drainage in a brick clad
reinforced concrete frame building
Figure 3.9
A piece of the fallen stone cladding
the stone was restrained by copper cramps
inserted into two holes drilled into the top
edge of each slab. These cramps were still in
place on the building. Support for the slabs
was provided by short (75 mm) angles (which
appeared to be brass) the horizontal leg of
which located in two slots cut in the back face
of the slab near the lower edge and bedded in
cement mortar. The angle was secured to the
concrete backing by an expanding 6 mm
proprietary fixing bolt. Each angle served the
adjacent edges of two slabs, and some of the
