Theatre Design & Technology - Winter 1979 - 17

frequently needed to assure the performers' and the public's
safety!
The cables themselves must be in good shape. Always use
two cable clamps (crosby clips), and check the tightness often. Plunk the cables to feel for taughtness. It is important the
tension be as even as possible. If one overtight cable carries
a disproportionate share of the load and snaps, a domino reaction could be set off, with catastrophic results!
Once the platform is hanging in the air, it must be made
stable from lateral motion. This is best accomplished if rigid
members can be braced against a building wall or a column.
These must come into the platform so as to hold the platform
in all directions. Cables can also be used, but remember they
can only resist a pulling force. It is best if the point of attachment to the building can be as much below the point of attachment of the platform as possible.
Pretest the work before allowing actors to use it by weighting it down with something to simulate use, for example.
Safety must be the concern of all working on or around the
project.

Practical Concerns
What of the cost factor in using suspension structures? For
most theatres, financial savings in materials can be had by using wire rope in design. Suspension structures do away with
much of the legging and bracing of most conventional platforms. Using cable supported structures is labor-intensive,
however, as installing the cable properly takes time, and additional time is spent in temporarily shoring up the platform
while the cable is attached. This time expenditure is a draw-

back to some commercial ventures where every hour of labor
is precious, although it may not be a problem to many educational theatres.
Almost any platform can be suspended. In suspending a
heavy structure, a variety of factors must be taken into account: the dead load of the unit (the weight of the unit without
anyone or thing on top of it); its live load (the weight of people
and all movable objects); the dynamic load (the force of moving objects that come in contact with the unit); what type of
cable is to be used (this depends mostly on its anticipated total load-the sum of dead, live, and dynamic loads); where
the cable is to be attached both on and above the unit itself;
how the unit is to be stabilized against vibration and sidesway;
how the cables are to be disguised if they are a visual nuisance; and how the unit is to be put into place so the cables
can be attached.
Determine the total weight of the unit by adding up the
weight of the unit itself (dead load) and the maximum weight of
the people or things on the unit (live load). Then multiply by a
factor of two. This gives the designer a safety margin which
protects against directorial changes of mind regarding the
blocking or minor design changes.
Also consider whether the weight will be evenly distributed
most of the time throughout the unit, or whether there will be a
particular concentration of weight in one spot, and whether
there will be an inordinate amount of jumping up and down on
the unit. Such excessive, repetitive vibration puts extra stress
on a suspended structure and necessitates the safety factor
of (x 3) to be used. An excessive concentration of weight in
one area will cause extra stress there and will necessitate the
installation of extra support (e.g. more cables).

Short Eyes, 1977. Entire "concrete" catwalk constructed of Upson Board, plywood, and 1" x 3", and hung on YB" airplane cable from the grid
40' above. Each cable strand was capable of supporting almost 1 ton. The cables, invisible in the photograph, disappeared behind the catwalk
wall and attached to turnbuckles, which then attached to the catwalk base (built of 2" x 4" and 31." plywood). The turnbuckles enable the strain
to be equalized between the cables.

USITT Winter, 1979

Theatre Design & Technology

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