Theatre Design & Technology - Winter 1979 - 18

When the live and dead loads are evently distributed,
placement of the cables becomes relatively simple. The platform can be suspended at even points along its perimeter; the
number of cables multiplied by their allowable load should exceed the combined live and dead loads multi pled by the safety
factor. A sufficient number of cables should be provided so
that no points of the unit are less supported than the others. In
placing cables for support of a platform, the structure of the
platform being suspended itself, as well as the distribution of
the live load, must also be taken into account. Conventional
platforming with 2 x 4 framing needs support at least every
8'0". If the platform is being framed with another material, different rules apply. Of course, the type of cable to be used depends on the strength needed. Most uses require cable at
least Ya" to 'I." thick.
Once the cable is selected and the points at which it is to
be attached to the unit are chosen, one must determine how
to attach it to the platform. In most cases the cable is best attached to a suitably heavy eye bolt run through both the platform surface and its framing. The cable is run through the eye
and then cable clamped back on itself (always use two cable
clamps and a thimble for safety). In some instances the surface eye bolt may be dispensed with and the cable run directly
through a hole drilled through the depth of the framing, and
then clamped to an eye bolt hidden within the framing. This
latter method allows a clearer visual look to the audience, but
requires a bit more skill in making an effective connection.
Up above, the cable is best wrapped around and securely
clamped to an I-beam or heavy pipe which is itself bolted or
tied to a very strong and stable object such as the grid over
the fly loft. Screw eyes placed into wooden or plaster ceilings
are to be avoided, as it's possible for these screw eyes to
gradually work themselves out, with potentially disastrous results.
As mentioned before, it is standard practice to include
within each line a turnbuckle, which is a device for lessening
or increasing tension in the cable. It's quite important that
each cable be taking a truly equal share of the load. If any
lines are slack, another line may be taking more than its fair
share. Turnbuckles allow some lines to be loosened and other
lines tightened, and this allows the weight to be more or less
evenly distributed.
A big problem for the technical director is getting the platform into position in the first place. It's usually best to attach
the cables up above first, and let them hang freely while the
platform is being put in place.
Platforms are best positioned in sections. Each section can
be temporarily shored up with 2 x 4 legs and 1 x 3 bracing,
moved into place, the cables attached and secured, and then
the shoring removed. Individual sections of the platform can
then be bolted together to form the total unit. The same
method, in reverse, can be used when it is time to strike the
set.
This article does not purport to be a "how to guide" in the
use of cables, but merely to be a discussion of some of the
basic possibilities and problems of suspension structures in
the theatre. Suspension structures usually involve heavy objects and some very real dangers to life and limb, and the
services of a qualified engineering consultant are usually required. Most designers are well advised to find a structural engineer or consultant who will spend a few hours of time checking their plans for safety and accuracy. Complete planning is
always important, but never more so than in working with suspended structures.
When care is taken, suspended structures open up a whole
new realm to the designer in theatre. No longer are the apparent restrictions of gravity a limitation. Suspended sets can
soar with the designer's imagination; and, like the suspension
bridge over water, can sometimes solve the previously unsolvable design problem.D

The
Light-Link
Lighting
Control

System
by Charles H. Cox, III

Light-Link is the name of a project that has reinvestigated
the distributed dimmer concept in terms of recent advances,
primarily in electronics technology, that have been made over
the last few years.
Investigations of distributed dimmer systems have been
conducted in the past. The earliest systems known to the author are those designed under the direction of Ned Lustig' in
the late 1950's. For his systems the dimmer control information was conveyed in an analog fashion using a separate control wire from the control console to each dimmer. This system
has undergone a succession of improvements since its introduction and is still in limited production.
Dirk Epperson,2 in a graduate thesis done at Yale several
years ago, also investigated distributed dimming. In his system the control information, which was in a digital format, was
multiplexed onto a single channel. Using a method generally
referred to as power line carriers, this composite signal was
sent over the same AC power lines from which the dimmer received its power. Each one of the dimmers was equipped with
a receiver to detect the composite control signal and to select
the control information appropriate for that dimmer.
From these earlier systems it is clear that the construction
of a distributed dimmer system requires attention to the
means by which the control information is conveyed to the remote dimmer, and to the design of the remote dimmer itself.
The control signal, for example, must be made more immune
to interference if it is to be run throughout the theatre as opposed to traversing from control console to dimmer rack. The
importance of dimmer specifications, such as its size, weight,
and cooling, increase considerably in significance when the
transition from dimmer rack to light pipe is made.
Recent advances in large scale integrated (LSI) circuits has
made it feasible to consider an integrated circuit (IC) dimmer,
i.e., one in which all the functions required to support the control of the SCR (or whatever other control device is used) are
performed by a single integrated circuit. The benefits of an IC
dimmer would be similar to those demonstrated by the pocket
calculator; the result would be a compact, reliable and low
cost (approximately $25 in OEM costs) dimmer. Other ad© Charles H. Cox. III, 1979

Theatre Design & Technology

USITT IWinler, 1979

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