Stage Rigging 101
Apr 3, 2010 12:00 PM, Lawrence Graham
Motor-driven rigging systems are still relatively rare in the United States but are much more common in Europe. They are an absolute necessity on cruise ships, because deck space and backstage crew availability are at a premium. Motorized rigging is generally used in the United States to handle unusually heavy loads or where exact repetition of movement is required for long periods of time. Automated shows in theme parks are a good example of the latter use.
There are essentially three kinds of motorized scenery. Drum winches use a winch to take up the cables to raise the load. Drum winches are usually located on one side of the stage and make use of a headwell but no loading gallery.
Counterweight-assisted scenery uses a motor to move the counterweighted line set. In those systems, the operating line is replaced with a cable that is driven by a motorized drum. This system, too, generally needs a headwell and a loading gallery.
Line shaft uses a rotating shaft with multiple drums and is attached to the gridiron or rigging steel to take up and lower the wire cables in the system. This type of motorized rigging imposes no lateral load on the rigging steel and does not need a headwell or loading gallery.
The kinds of loads imposed on the primary rigging can be predicted based on industry standards, a contractually specified maximum, or some other agreed-upon determining factor. For example, the rigging steel in a small theater might be expected to support a maximum load of 60,000 pounds. A larger amateur or educational facility might be called upon to handle 1.5 times that amount, and a commercial operation might handle as much as double or more what is expected in a larger amateur or educational facility.
Sometimes in order to conserve space on the stage floor, a double-purchase counterweight system is used. This system operates like a reversed block and pulley. The counterweight arbor travels half the distance of the pipe batten. The advantage is that the rigging can terminate at a fly floor, part of the way up the fly tower. In this arrangement, the rigging and the operators are out of the way above the stage floor. The disadvantage is the system requires twice the counterweight of a single purchase system and thus imposes twice the load on the headwell.
Manual systems place the operator in a position where the stage is naturally in view. The movement of the rigging can easily be stopped or reversed if a problem occurs. Even on a darkened stage, sufficient ambient light, unseen by the audience, will usually permit a trained operator to see danger. At the very least, it will be possible to feel an obstruction in the operating line and react to it.
The advantage of an automated control system is ease of operation and exact repetition of movement. The disadvantage is the loss of specific human touch and direct control over each moving element. It is possible to incorporate controls that can halt, at a rate of six feet per second, robust equipment used to move a 1,000-pound load at the slightest hint of a problem. However, such a system does not take into account the many minor interferences that are expected to occur with some regularity during an ordinary scene change. A system like that would not be practical because of the realities of a live stage production.
For example, if a soft stage curtain were temporarily pushed out of the way by a stagehand or an actor and into the path of a descending piece of scenery, bringing an entire stage production to an abrupt halt, the resulting automated response simply would not be tolerated in the entertainment industry. However, motorized rigging controls are useful in situations in which they are continuously supervised by trained stage personnel who are intimately familiar with each scene change ballet and personnel who can halt the equipment if something goes awry when performers and technicians weave their way through, around, and under moving scenery in relative darkness.
One of the reasons manual rigging is still the most popular for most applications is its relatively low cost. Motorized systems, depending upon the complexity of control, can cost three to five times as much as manual systems. Although the steel infrastructure probably costs about the same for either type of system, the required electrical infrastructure for a motorized system does add significantly to the expense.
Probably no design professional is more concerned about rigging system failure than structural engineers. Even with the best design, rigging system, and associated structure, failures do occur. Sometimes stage personnel are injured or even killed. Preventing those catastrophes and limiting liability are important considerations.
In light of the inherent hazards associated with stage rigging, it is shocking to discover that there are no building codes that cover rigging installations. It is perhaps even more disturbing to learn that there are no nationally accepted standards for the manufacture of rigging components, nor are the installers of these systems required to be licensed. In addition, national standards for periodic inspection of these systems do not exist.
However, rigging equipment designed and manufactured by reputable companies should not be a source of any problems. These nationally known firms generally employ engineers to design their own components and subject them to rigorous testing. Therefore, an important consideration is to make sure your component supplier is a reputable one.
Next, the structure used to support the rigging components and the anticipated rigging system loads needs to be carefully coordinated and controlled. Insofar as primary rigging is concerned, this is a fairly straightforward thing to do. First, determine what the maximum load per line set should be and multiply that number (A) by the maximum number of line sets that could be installed in the space (B). The number (A × B = C) is then divided by the number of locations where blocks will be installed (D). This same formula (C/D = E) is also used to determine the load on the headwell. The lateral load between the loft blocks and the headblock is calculated, and the reaction between the various components is then determined.
Up to this point, the structural engineer is controlling a relatively simple and clear process. But how does the structural engineer ensure that the rigging does not exceed the load for which the structure was designed? The answer lies in how the rigging system is designed and specified, something over which the structural engineer has little or no control. In general, the architect possesses little or no knowledge of this level of detail.
To solve this problem, a specialist in developing contract documents for rigging systems should be involved as part of the design team to make sure the rigging integrates properly with the structure. An experienced and qualified theater consultant will normally perform this kind of work. That person prepares the drawings and writes the specifications that physically limit the installation to the number of line sets at the calculated load. Just as importantly, this person also works with the structural engineer to ensure the rigging components can be attached to the structure using any manufacturer's standard components in prescribed ways.
A more difficult structural-design challenge arises if the owner intends to install secondary rigging, using spot blocks or chain motors temporarily mounted on the gridiron floor. To determine the kinds of loads that might be placed on the structure, one must think in terms of probabilities. Oddly enough, it is the economics of theatrical production that give the best clues as to what kind of floor loads might be imposed on the gridiron floor. One can anticipate that large professional theaters (the kind that host touring productions and seat large audiences) will have heavy-lifting requirements on the stage and above the auditorium ceiling. Smaller venues, typically with more limited income, usually cannot pay for the larger shows with their heavier and more complex lifting requirements.
A theater consultant can be an important part of the design team. A knowledge of portable theatrical equipment, how and where it is likely to be used, and the kinds of maximum loads it will likely place on the structure become invaluable knowledge.
Good, clear signage is important. Most stage technicians know what their portable equipment weighs. Posting signs stating the lifting capacity of the primary rigging and the point load capacity of the gridiron floor are invaluable steps to promoting safe operations on the stage as well as limiting liability.
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