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T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 5
T W E N T Y S E C O N D E D I T I O N
1
PROFESS IONAL SERV I CES
throughout its length. The timber is oak,
spanning 9m at a depth of 400mm. The phrase
‘dozy beam’ has been used at Chatsworth
to describe some principal beams’ soporific
attempts to do a mighty job. However, these
beams would have had difficulty coping with
dynamic loads from the start, becoming
exhausted over time.
Unfortunately, the cast flitch would
probably have done little to improve users’
perception of floor vibration. There are so few
bolts used to connect the timber to the cast
flitch that load transfer between the materials
would have been negligible, preventing them
from working as a composite whole.
The two examples above show that
liveliness in a historic, suspended timber floor
can be the product of an inherent defect and
may be worsened by interference with the
structure. If a problem can be exacerbated by
interference, this raises the question: can it
be improved by interference? Can we mitigate
vibrations in historic floors that have their
own unique natural frequencies, depending on
their depth, span, and connections? And can
we do enough to satisfy those sensitive human
perceptions of vibration noted above?
THE VYNE, HAMPSHIRE
In the oak gallery at The Vyne in Basingstoke,
floor vibration disturbed timber columns
that supported heavy stone busts. When the
public walked through the gallery the busts
oscillated back and forth into the room as
though nodding. The floor is based on a series
of principal beams carrying secondary floor
joists which in turn support tertiary joists
above, like counter-battens, to which the
floorboards are fixed.
A simple calculation showed that the
principal beams were slightly undersized for
the span, resulting in a live-load deflection
that promoted a low frequency in the floor
vibration. A failure to think laterally would
have resulted in the complete lifting of
the floor finish in an attempt to stiffen the
principals. This would have been difficult in
view of the ornate plaster ceiling below and
the tennoned joints of the floor. However,
because the bust supports sat near the walls
it was apparent that the tertiary joists could
be cut within about 500mm of the walls
thus preventing the maximum oscillation
within the principal beam transferring to the
supporting tertiary joists below the busts.
HOLISTIC SOLUTIONS
The simple solution identified at The Vyne
would have been apparent immediately using
the dynamic testing approach currently being
developed, and it is clear that this technology will
be invaluable for more complex floor structures.
Building structures need to be
considered holistically, not only in terms
of the interaction between structural
components, but also in terms of their use
and the client’s requirements. Practicalities
of construction need to be taken into
account, and the likely benefits of each
solution need to be weighed against their
cost, both financially and in terms of their
impact on the significance of the structure.
Ultimately, the most appropriate
solution can only be identified through
close liaison between the whole
conservation team – client, builder,
quantity surveyor, architect and engineer.
JEFF STOTT
BA Hons CEng MIStructE is a
director of Mann Williams Consulting Civil
and Structural Engineers in Bath, Cardiff and
Belfast
(see page 32)
, and has been working
on historic monuments since 1986.
The Oak Gallery at The Vyne, Basingstoke: the heavy busts which line the gallery oscillated when members of the public walked through it. (Photo: Nadia Mackenzie/
National Trust)