Theatre Design & Technology - Summer 1986 - 34

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equally small load transfer. It is quite
likely that in this case the engineering
analysis would show that the increased
loads remain well within the safety factors of the components and only minimal equipment size increases (if any)
can provide the required single-failureproof design with adequate safety. The
tilting of the canopy is also prevented.
The above examples represent only
a few basic considerations for singlefailure-proof design systems. In each
case many other system components
have to be evaluated.
I f the ny set on Fig. 2 was motorized, for example, and often used, the
wire ropes would be subject to fatigue.
Also, their connections may represent
weak links. The failure of an outside
line would certainly be more critical
than that of an inside line because the
end of the pipe would become a cantilever, subject to high bending stresses.
It would yield and bend down but in
this process the bending movement is
reduced. Whether it will bend or break
can be calculated and should be designed not to break. In addition, the
head block sheave diameters should be
selected to provide long cable life.
While this type of analysis is not
commonly done for simple pipe battens, it illustrates the events to be considered in design. A similar but much
more complex analysis would be necessary and must be done for the stage lift
and canopy structures shown on Fig. 4
and Fig. 6.
While the single-failure-proof de-

Theatre Design ,I< Tl'rhnolog)

sign principles are not known to many
theatre designers, these requirements
are strictly enforced by the codes for
elevators and other hoisting equipment
outside the theatres. Windo\\'-washing
scaffolds, for example, must be singlefailure-proof. This is typically accomplished by using a second brake directly
applied to the cable drum. In case the
motor brake or dri\'e gears fail the second brake is automatically actuated by
sensors detecting overs peed .
In this case, hO\\'ever, only a single
cable drum is used. This drum is considered to be failure proof because it is
designed to work at very low stresses.
The gears, brakes, and the drum bearings may, however, wear down and fail.
Therefore, they have to be provided
with backups. The drum, for example,
has to be mounted so that it would continue to support the load if a bearing
fails.
Should a theatregoer be as safe, enjoying a performance, as a window
washer, working high above a city on a
scaffold, suspended from the top of a
building?
The unsuspecting theatregoer would
not believe that the two situations are
even comparable. But the fact is that all
hoists lifting the scaffolds have safety
brakes, whereas in most theatres the
light bridges, orchestra shells, and
acoustical canopies, weighing tons,
hang over the audiences and often rely
on one gear, one coupling, or one shaft
for their support. There is no difference in safety, whether one stands on
top of this equipment or below it.
As a blessing in disguise, some of
these facilities are not in use anymore
because they are too cumbersome to
operate. Others have been permanently
tied to the building structures because
they were considered to be dangerous.
But many are still there, built with minimal engineering, held together by a
single linkage without an independent
safety backup.
In view of theatre design practice as
it nists today, and the competitive nature of the business, safety cannot be
achieved without the proper codes,
standards, and quality control. The
field, however, is a complex one, because it involves a wide variety of equipment and installations. Therefore, a
thorough detailed code (equivalent to
A SI / ASME A 17.1 elevator code and
variou codes for power operated scaf-

SITT SlIllllller

1l)~6

folding) would fill thousands of page
"ith fine print. To prepare it withou
outside funding would be an almos
prohibitive task for the relati\'ely sma
theatre equipment community.
On the other hand, a "cookbook"
type code, if not thorough, would b
misleading and even dangerous becaus
it would encourage simplistic solution
for some very sophisticated problems
Such a code would also be restrictiv
and limit the designer without address
ing the real problems.
It \\'ould be rather simple, howeveJ
to add provisions to the existing build
ing codes that would make the single
failure- proof design mandatory for sys
tems that affect the safety of an audi
ence. The same should apply for som
heavy stage installations, such as lift~
orchestra shells, and light bridges. Th
designs, based on such provisions
would greatly increase theatre safet
for minimal cost and are, indeed, Ion
overdue.
Making the single failure proof de
sign requirement (with related analysi~
documentation and change reviews
part of the building code for critic,"
equipment, would not be restrictive t
the competent designer and would, i
fact, be advantageous to all partie
because:
* The public and the theatre opel
atar would be better protected.
* Safety provisions for installation
would be clearly defined by the respon
sible engineer in the single-failure
analysis-a document necessary for ac
ceptance of the design. Such documen
tation, showing reduced risk, would b
beneficial for the insurance carriers an
may reduce the premiums, which hav
soared because of the many accidents
* The architect's and theatre consul
tant's liabilities, regarding the equir:
ment designed by others, would b
reduced.
* The manufacturer's liabilit
would be reduced because of the im
proved safety of the equipment. Sinc
all manufacturers would have to bid fe
a single-failure-proof system, the com
petitive positions of qualified manufac
turers would remain unchanged. Inne
vative design could improve the com
petitive positions of some because ther
are many ways to achieve the single fail
ure proo f design.
L

Olaf Soot is President of Olaf Soot As
sociates P.c.

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