Stiffness in Timber Floors and Ceilings
Jeff Stott
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Primary and secondary floor structure exposed in the ceiling of the Long Room at Chenies Manor, Buckinghamshire (Photo: Peter Mukherjee, iStock.com) |
The 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 complaints 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, apart from the physical construction survey,
vibrations in the suspended floor need to
be measured to enable a focussed report to
be communicated to the client. Otherwise
it is possible to spend a lot of money trying
to fix a lively floor with little perception of
improvement.
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Recording vibrations from a dynamic excitation device (in this case a drophammer)
to analyse the performance of a timber floor: the data can help assess
deflections, stiffness and dynamic responses |
Steel tensioning to improve the structural performance of over-spanned joists |
ASSESSMENT
Every historic timber structure is unique.
Although it should be possible to predict
a beam’s deflection from its dimensions
based on an assumed modulus of elasticity,
in practice such calculations are unreliable.
The size and location of knots, the quality
and strength of the timber, and the presence
of decay, all impact on its performance. The
strength and performance of the structure
as a whole is also affected by the integrity of
connections, whether primary or secondary
(from beam to wall, and from joist to beam,
for example). A repair previously carried out
on one project rarely suits another. Although
the structure may appear to be similar, the
variables are so great that the probability of an
exact fit is low.
Although obvious, it is worth stressing
that surveying the situation is essential. The
uniqueness of the construction needs to be
understood for the repair to be a complete
design that takes account of the performance
required from the structure, the longevity
required, and the ease of maintenance and
accessibility, as the repair might require future
modification or, indeed, reversal. The design
must also be based on an accurate assessment
of how much fabric needs to be disturbed.
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Dynamic analysis of the Octagonal Gallery at
Mount Stewart House: the top three diagrams
illustrate the mode shapes associated with the first
three natural frequencies of the structure, the bottom
image is a finite element model which represents the
predicted deflection of the structure with a uniformly
distributed loading applied to one half of the gallery. |
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A survey starts with the preparation of
accurate drawings, recording the current
structure, noting any obvious defects. An
assessment of existing records can often
shed further light on past alterations. Some
evaluation of the actual deflection is also
required to give a fuller picture of how the
structure is performing.
The standard method of measuring
deflection involves constructing a stable
scaffold to within 50mm of the underside of
the ceiling and then measuring the deflection
under live loads using a dial gauge fixed
between the top of the scaffold and the
underside of the ceiling. If the scaffold is rigid,
the dial gauge is sufficiently accurate to record
deflections of less than 0.01mm. However, to
achieve the stability required it is best if the
scaffold is built off solid ground, which is not
always possible.
Investigation and analysis techniques are
being developed that can model the dynamic
performance of suspended timber floors in
three dimensions. A dynamic floor appraisal
that records accelerations of a structure
against known excitation forces provides
more information than simple static tests.
To
explain the difference between static stiffness (EI) and dynamic response – accelerations
of the structure – imagine standing still in
the middle of a floor while loads are applied;
you experience static stiffness when the floor
deflects but not necessarily the bounce or
response. It is the combination of EI, mass,
boundary conditions and damping which
creates the dynamic response and thus human
perception of it. Therefore the dynamic analysis
is a more accurate method. Using this approach
data can be collected that can identify the
natural frequencies of the floor as a whole that
account for the uniqueness of its construction.
Mann Williams Consulting Engineers
have carried out a number of tests on historic
floors to develop the technology. Data is
provided using an appropriate dynamic
excitation device at intervals across the floor
surface while the vibrations are recorded from
around the room. Deflections, stiffness and
the dynamic responses of the structure can
be modelled from the data gathered, allowing
the true characteristics of the whole floor
assembly to be analysed.
Not only can this provide a clear picture
of the existing structure and its anomalies, but
it can also be used to explore various ‘what ifs’,
such as the effect of better load management
or the introduction of reinforcement, to
give a better understanding of the potential
of the existing structure. Dynamic testing
and modelling recognises and quantifies
the uniqueness of timber structures and
highlights the failures of simple analyses
based on theoretical magnitudes of deflection.
It can see the structure as a whole, identifying
various areas of damping and areas that can
be exploited to reduce vibration. Essentially
it can give a better idea of whether or not the
vibration of the floor can be improved within a
given budget.
INTERVENTION AND MITIGATION
Solutions to excessive deflection under
dynamic load fall under two categories: load
management and structural improvements.
The former includes measures to reduce
the loading or better distribute it across
a wider area, or to bypass defective
elements altogether. The latter includes
repairs and, if necessary, alterations to improve the performance of the structure.
However, the ability to carry out repairs
to principal beams and floor or ceiling
joists is almost invariably confined by the
historic fabric that surrounds them.
Repairs should be honest and disturb the
surrounding fabric as little as possible. Ideally
they should be limited to within the depth
of the floor zones. It is sometimes possible to
supplement the timber with steel splints fixed
to the sides of the joists to improve shear and
bending strength and stiffness. On a beam
supporting secondary joists, however, this
may be impossible without removing all of the
original tenons from the floor joists, causing
substantial damage to its historic significance.
Repairs may be needed that improve the
bearing while maintaining honesty, protecting
the original fabric as much as possible
and allowing for a degree of reversibility
(a complex subject in its own right).
The following cases further illustrate
some of these options.
MOUNT STEWART, NORTHERN IRELAND
Service notches can severely disturb the
performance of suspended timber floors both
in terms of strength and serviceability. While
most plumbers and electricians are aware of
this now, in the past there was an arrogant
disregard for structure when new services
were installed within floors. An example of
the problems caused by notching is the drop
in the floor of the Octagonal Gallery above the
central hall at Mount Stewart, an 18th-century
mansion house owned by the National Trust.
This drop was severe enough to bring the
floor’s safety into question, not just because
of the dramatic distortion but because of the
structure’s response to footfall.
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The Octagonal Gallery at Mount Stewart, Northern Ireland after partial opening up
revealed the extent of service notching (Photo: David McClimond/National Trust) |
The original structural design of the gallery was perfectly sound but the service
notches had caused the marked drop in the gallery floor shown here. |
For the gallery to work unsupported on
the balustrade side, there had to be load-sharing
between the joists, fanning out from
the wall, and a central ring beam acting
in a complicated display of compression
and tension. The central ring beam is not a
perfect circle but is elliptical or a stretched
octagon and would naturally have difficulty
maintaining the structure in place along the
near straight sections that relate to the major
axis. The joists need to function as cantilevers
with a particular stiffness to ensure these long straight sections are not overloaded and
the connections between joist and ring beam
need to have a high degree of fixity. If the
joists deflect too far, the stresses in the inner
beam can change to the point of inversion;
this in turn brings the effect of the balustrade
into question. Notches in the joists could be
catastrophic but notches there are – or were –
causing increased deflections and locally high
stresses in the joists.
The structural concept of the original
design was correct: it was the interference
that was damaging and this has now been
corrected by filling the notches and installing
straps across them to ensure the tension
is catered for and by improving the fixity
between joists and ring beam. In addition,
the gallery has been jacked into position,
connections improved and timber repairs
completed, leaving the gallery structure
nearer to horizontal with an 80 per cent
improvement.
Simply by reinstating the loss, the repair
successfully addressed the two structural
concerns particular to the structure:
degradation and serviceability/vibration.
CHATSWORTH HOUSE, DERBYSHIRE
An early solution to the problem of vibration
in suspended timber floors was the use of cast
iron flitches in the staterooms at Chatsworth.
A flitch had been inserted between split
timber beams (the beams appear to have been
cleaved rather than sawn, possibly indicating
the work was carried out in-situ). The flitch
is a cast inverted ‘T’ with a slot formed in its
base. This slot ends with an anchor block at
each bearing against which a rod is tensioned.
The use of cast iron probably dates the repair
to around the mid-19th century. There is no
evidence so far that would link the repair to
Joseph Paxton, designer of the 1851 Crystal
Palace, but as he was head gardener at
Chatsworth from the 1820s and was designing
the Chatsworth greenhouses in 1832, it is hard
not to make the link.
The evidence that this early flitch
design was used as an attempt to mitigate
the liveliness of the floor is clear. It was
not installed because of decayed bearings
or because the beams were overstressed;
even now the timber is in good condition 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
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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) |
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.
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The Building Conservation Directory, 2015
Author
JEFF STOTT BA(Hons) CEng MIStructE is a
director of Mann Williams Consulting Civil
and Structural Engineers in Bath, Cardiff and
Belfast, and has been working
on historic monuments since 1986.
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