Theatre Design & Technology - Winter 1979 - 16

Figure 1

Figure 2

reclion of a cable without rubbing action impeding the cable
travel. Thimbles serve the function of protecting the cable
when tight loops are made. Crosby clips, first grip clips, (and
others), clamp around two pieces of the same size cable and
make a loop or splice. Turnbuckles, or any similar threaded
device, are used to adjust the length of a piece of rigging. All
of these can be seen in any supplier catalog.
It has been the experience of the writers, that the rigging
used most in theatre has been Ya" diameter galvanized aircraft cable, and Ya" crosby clips and turnbuckles. Those are
available in most hardware stores. The allowable load of new
Ya" aircraft cable is 1100 pounds. This is far less than the
breaking strength of 2100 Ibs. Metal is a very elastic material,
and the prevention of permanently bent or stretched members
is the actual criteria for establishing maximum safe loads. The
point at which the metal yields is called the elastic limit. In
general, flexible cable loads must be no greater than half the
published breaking strength to assure that yielding of metal
will not occur. Each type of cable is different, however, and
the theatre practitioner would be well advised to research
thoroughly each actual application.

pairs or groups of cables work against each other, or the mechanics and geometry tend to be self aligning. The bridge in
Figure 1 is cable stayed. Imagine a load placed on the bridge.
The load wants to deflect the cable down at the point of contact. For this to happen, the cable to either side of the load
must move up. The bottom cable cannot push up in the
middle, but holds down to either side of the load.
The bridge in Figure 2 is a simple stabilized structure. The
cable still carries the vertical load, but the rigidity is the result
of the strong triangulated deck structure below it. Both of
these principles can be applied to the problem of hanging a
platform from a theatre fly grid.

Theatre Applications of Hanging a
Platform
To start with, this guide will be presented in an intuitive
framework, rather than a rigorous engineering one. This is a
valid way to design structures. However, the more daring or
unfamiliar a particular scheme is to the designer, the greater
the risk. For a designer wishing to build something with which
he is unfamiliar, it is recommended that the advice of an engineer be sought. (With a good sketch, this can often be done
by mail.) In general, a cable-supported work is more flexible
or "bouncy" than post and beam platform construction.
A good place to start when a specific platform is to be hung
from a fly grid is to view the platform as if it were to be supported on legs. Determine where the legs must be located so
that adequate support is provided such that the platform will
not break apart. These are the same locations at which cables
must be attached. If this criteria is met, there is reasonable assurance that the cables are spaced closely enough so that the
framing will not be overloaded. If a platform is made up of several lids bolted together, legs need not be placed on all edges
of all the lids. The same is true of the cables. But the cables
must be wrapped around or bolted to the on-edge lid framing
just as a leg would not be placed in the middle of the deck
board material.
Next consider the cables as they attach to fly grids. When
working from a well-designed, modern theater grid there is
usually no problem of overloading the steel members. The grid
is usually designed for many times the load the platform will
carry. When hanging from pipes or light bridges, this is much
less true, especially if they were added later. In all cases, caution must be used in applying loads--a competent engineer is

General Principles of Cable Structures
Cables are pure tension members. A cable will resist loads
at its ends trying to pull it apart. Other loads from the sides, or
compression at the ends will either slacken the cable and render it useless, or realign it so that it will be loaded in tension.
The fact that the cable is flexible and realigns itself is the secret to its high strength.
Cables must always be used in conjunction with columns,
beams or a land mass, in compression. A tent would not stand
without posts loaded at their ends in compression. The fact
that cables work in unison with compression members adds a
characteristic to the structure that is crucial-that of overall
stability. An example of an unstable structure would be a tent
column held in a vertical position by only two cables. Wind
from the proper direction will blow the column over. The third
cable must be added to stabilize the system. Another unstable
system would be a rectangular platform hung from a theatre
fly grid with a vertical cable at each of the four corners. Although the platform will not fall, it has no stability in a lateral
direction. Any actor's movements will excite the platform and
swing it uncontrollably.
The methods to stabilize a structure can be classified into
two groups. Cable-stayed structures are designed so that

Aircraft Strand and Cord
Zinc Coated
1 x 19 Wire Strand
Rope
diameter
(inches)

1/8
3/16
1/4
5/16

Breaking
Strength
(Ibs.)

2100
4700
8200
12,500

7 x 7 Aircraft Cord

7 x 19 Aircraft Cord

Safe
Strength
(Ibs.)

Breaking
Strength
(Ibs.)

Safe
Strength
(Ibs.)

Breaking
Strength
(Ibs.)

Safe
Strength
(Ibs.)

1050
2350
4100
6250

1700
3700
5200
9200

850
1850
2600
4600

2000
4200
7000
9800

1000
2100
3500
4900

"Safe strength" can be interpreted as "allowable strength."
It has been established as 50 0 0 of the breaking strength to assure that yielding does not occur.
14

Theatre Design & Technology

USITT /Winler, 1979

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1979