Issue 38 Summer web 23 - Flipbook - Page 46
both nibs and underlying plaster was required, to improve
the plaster’s material condition and its structural stability,
in conjunction with mechanical suspension. Suspension
of the plaster without consolidation of the friable plaster
would not be possible as it would be too weak and
powdery to hold any support. Extensive off-site tests on
samples of failed plaster nibs suggest the use of an aqueous
colloidal silica as an ideal consolidant. Subsequently
on-site treatment progressed and, once dry, core samples
and results of limited pull tests showed a satisfactory
increase in strength.
scientifically conclusive trials would have meant testing
the panels to the point of destruction which was not a
viable option. As such, to fully mitigate any risk of failure,
reopen the building and obtain insurance cover, the team
had to alleviate the residual risk of any potential future
failure.
The team explored a variety of methods to alleviate this
risk in conjunction with the Conservation Officer and
Historic England, such as angled supports, structural wire,
and a variety of netting types. Visually a fine net of a
similar colour to the panels behind was deemed most
appropriate and barely visible from floor level. However,
there was no test evidence to prove that such a net would
be capable of taking the 70kg load of an existing plaster
panel and there were too many variables to prove the
integrity of the netting system through engineering
calculations. The team built a test rig to fully replicate
conditions on site which comprised a 1.2m x 2.1m
rectangular frame of 90x45mm timbers of the same
strength value assumed for non-decayed existing timbers.
The rig was angled at the same pitch as the roof and a
netting system identical to that which the design team
intended to install was attached. A representative 70kg
panel was dropped onto the rig a total of 5 times, each
repetition carefully observed and recorded by the structural engineers. On every repetition the netting prevented
the panel from falling and therefore the netting system
was deemed suitable to be installed to alleviate any risk
of potential panel failure to the public below.
This method of stabilisation was combined with the
installation of a series of pads, suspended from above as
not to add additional weight to the panels. Non-functioning laths were carefully removed, and small holes carefully
drilled to allow the consolidant and plaster to penetrate
the panels below. The area was cleaned, and the surface
treated, then pads formed from a build-up of Quadaxial
fabric, rib lath and a plaster of Paris mix were applied
before being mechanically fixed by a stainless-steel tie
attached to a stainless-steel band spanning across the
joists [Fig 2].
The conservation works were deemed to be a success and
risk of panel failure had been reduced, however, there was
no way of definitively proving that the works carried out
could support the panels. The methods used to conserve
the ceiling panels were highly innovative and had
never been used before, so no test data existed with the
exception of the limited trials carried out on site. Any
Above, figure two application of supporting pad - Photograph by Damian Griffiths
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