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PROFESS IONAL SERV I CES
STIFFNESS INTIMBER
FLOORS AND CEILINGS
JEFF STOTT
T
HE REDUCTION
in the performance
of suspended timber floors in historic
buildings, whether perceived or real,
continues to absorb many hours of structural
analysis. Sometimes the drivers are the
obvious ones, like failed principal beams that
disturb plaster finishes, sometimes they are
less obvious, like the tinkling of a chandelier;
either way, the problem exists.
The safe approach is to follow the repair
principles of the Society for the Protection
of Ancient Buildings, especially in terms
of minimum intervention. However, for
this to be successful, it is first necessary
to understand all the relevant facts and
influences, including human perception.
Problems with suspended timber floors
usually relate to one of two structural design
considerations – strength and stiffness
(serviceability). The solutions for reduction
in strength due to various external influences
are normally clear but can be difficult to
execute (see Robin Russell’s ‘Structural
Timber Repairs’ in
The Building Conservation
Directory 2013
). On the other hand, the
solutions for correcting ‘lively’ floors, which
are the focus of this article, are generally
more complex and client expectations are
often difficult to satisfy.
CAUSES AND EFFECTS
All suspended timber floors deflect to some
degree with changes in dead load and the
more complicated live loading. In modern
designs, the movement is barely noticeable to
a person walking across the floor. However, in
some cases, especially older floors, there may
be a discernible bounce. In the worst cases
the vibration can cause cracks in fine historic
plasterwork on the ceiling below, threatening
its survival. Inadequate stiffness can also make
a timber structure susceptible to vibration
from less direct sources such as traffic, live
music performances and machinery, so plaster
ceilings with no floor above may also be liable
to similar issues.
Before any intervention can be
contemplated, it is essential to fully
understand the nature of the structure, its
condition and the cause of the problem.
Defects may be inherent – undersized primary
beams or joists for example – or the result of
changes which have occurred over time, such
as holes and notches cut for services, or due to
the effects of decay. Changes in loading may
also have occurred, caused for example by a
change in the use of the floor above that may
in turn require the addition of new equipment.
Partitions added on to a floor could alter
load paths, transferring new loads onto the
structure, while the removal of partitions
below may have increased the spans.
It is also important to understand
the likely consequences of the condition,
including not only the physical effects on
historic fabric such as plasterwork, but also
the perceived effects and expectations of the
client. According to Annex B of ISO 2631-pt2
2003, which gives guidance on human
response to building vibrations:
Human response to vibration in
buildings is very complex. In many
circumstances the degree of annoyance
and complaint cannot be explained
directly by the magnitude of monitored
vibration alone. ...The basic human
response to vibrations in buildings is
adverse comment.
This suggests that human sensitivity to
vibrations in structures is subjective and
therefore difficult to satisfy. It appears that,
Primary and secondary floor structure exposed in the ceiling of the Long Room at Chenies Manor, Buckinghamshire (Photo: Peter Mukherjee, iStock.com)