Monitoring vibration

Colin Jenner <colin<-a t->wallcoll< . >demon< . >co< . >uk> writes

>We are about to have major restructuring work carried out to the
>foundations of our building while the galleries are still open.  We
>are keen to find the most effective method of detecting vibration
>that might carry through the building to the works of art on the
>upper floors.  Has anybody any experience in this area, are there
>acceptable limits of the various modes of vibration and can anybody
>suggest practical ways of limiting the vibration that will occur?

Vibration can cause the expression of cracks in wood that is under
environmental stress. It can, when of low frequency of the sort you
are concerned about, topple artifacts with high centers of gravity
or move artifacts in unpredictable directions. An outside vibration
can resonate within artifacts and occasional cause damage such as
extending cracks in glass and ceramics. Nails in soft back boards
can come loose and screws can occasionally back out of their holes
causing items to fall.

Vibration damage is hard to anticipate because any structural
element in an object's environment may temporarily resonate and
amplify the force of the source. Wooden shelving made of plain
planking without reinforcement can 'trampoline' under artifacts.
Standard steel shelving does the same for higher frequencies of
vibration. Glass shelving has the highest rigidity and will tend not
to be an amplifier of building vibrations. Sometimes the effects may
only happen during the start up and shut down of heavy equipment.

When I consider the effects that an external vibration will have on
an exhibit I think of six concepts: forcing frequency, stiffness,
damping, massiveness, resonance and center of gravity. It is
impossible to apply these concepts intuitively to predict the
response of a complex system- a building and its contents. But
understanding each can help breakdown an existing problem and lead
to a quicker solution.

I recommend a book that has helped me gain some control of the topic
if not the problem:

    Bishop, R. E. D.
    Vibration.
    Cambridge University Press, Cambridge, 1979.

This book is relatively painless; one of the few written for the
Calculus illiterati. While it is dated, being based on 1962
lectures, the principles haven't changed much. Of course there is
also our earthquake literature from which you can extrapolate some
precautions for your situation.

Controlling the problem starts with seeking a vibration or
structural engineer with experience predicting local building
effects from standard excavation and construction methods. It can be
hard to locate such a person who is sensitive to collection-level
fragility. I know I've had no luck finding an engineer who did more
than speak in platitudes unless he was allowed to do budget-breaking
analyses first. In a way I don't blame them but we are still left
with the problem.

The first recommendation they will give is what I'll offer: Isolate
the vibration at its source. Though you may not be too successful,
it's important to bring the building contractors into the equation.
This won't be the first time they've been asked to keep the vibes
down.

Attempts to add quick reinforcement to exhibits can cause more
damage than they prevent. However very thin (e.g 1/32nd inch)
silicone pads provide skid protection when placed under small
artifacts with low centers of gravity. I use the clear silicone GE
RTV 615 or its high end cousin RTV 655. Cast it into thin sheets on
siliconized mylar then cut it into one inch squares or circles that
can be inserted under the artifacts. Its liability is that if not
well-mixed it can transfer silicone oil to the artifact.

Depending on the looseness of your building construction you may
gain damping of your free standing cases by securing their upper
portions to vertical construction elements such as walls or posts.
Free-standing cases can be thought of as car antennas, holding down
two ends is often better than one. Then again the vertical elements
of a building may transmit vibration at a higher frequency adding to
the problem. Hard rubber sheeting should be placed in the path of
any horizontal connection.

I wouldn't introduce hard rubber pads under full cases because of
the liability of the 'lift event' itself. Even removing the contents
prior to lifting the case the quarter inch needed can be more
hazardous than the external risk. To perform well hard rubber pads
should be 'tuned' for the weight of the object they support. Unless
this is done they are more of psychological than practical value.

Monitoring the initial problem and your attempts to reduce it is
essential. I will not recommend the accelerometer driven equipment
used in industry. Rather make up an array of short steel wires of
different lengths and examine them for changes in the degree of
movement of their free ends. Take precautions near artifacts of
course. This is an elaboration of the old vibrating recording
hygrothermograph arm which can be used for time studies in a pinch.

Dennis Piechota
Conservator
Object and Textile Conservation
16 Central Street
Arlington, MA 02174
617-648-3199

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                  Conservation DistList Instance 11:10
                   Distributed: Friday, July 18, 1997
                       Message Id: cdl-11-10-006
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