Stage rigging and its impact on structural design always present interesting challenges for structural engineers. Going back as far as Leonardo da Vinci and the Court Masques of 15th-century Italy, stage rigging has played an important part in providing the visual effects of stage productions.
Originally, stage rigging was installed and operated by sailors. Men who knew their ropes at sea and were used to working under hazardous conditions high in the air were an ideal choice for employment in theaters. For the most part, modern steel sheaves and wire cables have replaced wooden pulleys (called blocks) and sisal rope. Sandbags are passé, and steel counterweights, similar to those used in traction elevator systems, counterbalance heavy loads. In some instances, theater rigging is motorized, and other systems are computer operated. In spite of those modernizations, the basic principles that underlie the operation of stage-rigging systems have not changed. Operating those systems can still be hazardous.
USING STAGE RIGGING
Stage rigging has two basic uses — to support the stage-lighting system and to support and shift stage scenery. The stage-lighting system consists of a large number of individually dimmed outlets to which various kinds of performance-lighting fixtures can be connected. Large portions of the distribution system for this circuitry are integrated into the stage rigging.
Performance lighting is an art that paints the stage with colored light, so many kinds of portable fixtures are used to achieve particular effects. Multiple specialized fixtures weighing as much as 35 pounds each (or more for some robotic fixtures) are temporarily clamped to the pipe battens in the rigging system. Even modest productions could use 300 or more specialty fixtures, and 60 percent of those could be hung over the stage. The rigging raises and lowers the distribution strips, the multiconductor cables that feed power to them, and the fixtures themselves.
Stage curtains, painted backdrops, and heavier framed scenic pieces are examples of the kinds of scenery raised and lowered by the rigging. Stage scenery tells the audience where the action is taking place. When stage scenery is combined with stage lighting, the overall stage picture establishes the locale, mood, time of day, and weather.
Obviously, if the locale of the play changes, so does the scenery. Sometimes a play has no scene changes, so the action may be confined to just one locale. More often, however, the locale and scenery change frequently. That can involve moving several tons of scenery vertically, in a matter of a few seconds, in absolute silence and total darkness, while performers and stagehands move across the stage below it.
At first that may seem unnecessarily hazardous. But live performances, much like televisions shows and films, have a rhythm and pace that must not be interrupted if they are going to engage and entertain an audience. Of course, the scene shifts are rehearsed, just as the rest of the production is. Nevertheless, split-second timing is essential to avoid injury to personnel and damage to the expensive scenery.
The various kinds of loads imposed on the building structure by these kinds of stage operations cannot be predicted during the design phase. Yet the structure must support them, and the rigging must operate safely and reliably every time it is called upon to move. To understand how this can be achieved, one must first look at how a typical stage is put together.
Fly tower and rigging
The area above the performing area of the stage is called the fly tower, and the part of the building that encloses the stage (the performing area and the wings) is called the stage house. The fly tower houses the stage-rigging system.
A fly tower may be configured in one of two ways: with or without a gridiron. A gridiron is a work floor, located high in the fly tower. It can be used to support the primary, upright rigging. Upright rigging utilizes the gridiron itself to hold the blocks that, in turn, support the live load on the system. Underhung rigging does not stand upright on the gridiron but is suspended from blocks attached below structural steel elements located seven or eight feet above the surface of the gridiron and just below the roof structure. In the design phase, it is possible to predict and control what the maximum loading on this kind of permanent rigging will be.
The gridiron also supports secondary rigging. Secondary rigging may consist of chain hoists or spot blocks that are temporarily installed to support point loads. These point loads might include heavy lighting trusses or any special rigging equipment associated with a particular production. Sometimes secondary rigging is used to actually fly performers, as in the musical Peter Pan. However, when designing the structure, it is not possible to predict or control what these point loads might be or where they will be placed.
The gridiron can also be used to support flexible stage-lighting cables. There are two types of these cables — those that are a part of the rigging and feed the electrical distribution strips and drop boxes, which provide additional flexible electrical connectivity to any of the pipe battens, light ladders, or booms that are used with portable fixtures to light the stage.
In the United States, the most commonly used type of rigging system is a manually operated counterweight system (see Fig. 1). In this system, the scenery or electrical system on the pipe batten is counterbalanced by steel weights. All the lift lines for a particular line set pass through a single pulley (called a head block) that is secured to the headwell. The load on the batten is transferred through the head block to counterbalancing steel weights placed in a frame called an arbor. When the counterweight arbor is raised and lowered, the pipe batten moves, or travels, in the opposite direction.
The arbor is raised and lowered by pulling on the operating line. The operating line passes through a brake and lock assembly. Whereas this device is used to prevent unwanted rigging movement, it is intended to work only when the rigging is in balance. As a result, it usually places no significant load on the building structure.
The loading gallery
When a pipe batten has been lowered to the stage floor in order to attach a load to it, the arbor will be located near the top of the fly tower. While the arbor is in this position, the counterbalancing weights are added to it. The loading gallery provides a place for personnel to carry out the operation. If there are no loads on the pipe battens in the system, the counterweights are stored on the loading gallery floor.
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.
As in any other subcontracting specialty, the installation of the stage rigging will be no better than the contractor's actual work. The rigging steel may be designed and erected as perfectly as is humanly possible, and the construction documents for the rigging itself may be exemplary, but if the installation is not good, all will be for naught.
It is an unfortunate fact that many, but not all, vendors of theatrical supplies and services are willing to sell rigging equipment and even install it with little or no knowledge of its interaction with the structure. All of the rigging failures I have seen have resulted from operator error. However, damage to the structure has always resulted because the rigging interfaced poorly with the structural elements themselves.
The answer to this conundrum is to write specifications that limit fabricators and installers to those who can demonstrate a satisfactory performance history. Specifications should also require the submission of shop drawings, provide for independent inspection(s) of the installation, and call for as-built drawings once the installation has been completed.
So how does one prevent accidents? The first rule is to limit access. Particularly in the case of schools, access to the gridiron and the rigging should be controlled. One of the best ways is to use an enclosed stairway with a locked door. Another method is to key the brake and lock assemblies in the rigging system so that the rigging system cannot be operated without being accessed by a key.
The second rule is mandated training. No one should operate stage-rigging systems until he or she has been thoroughly trained in how to operate them. Contract documents should include mandated training, and the owner should be advised to have an ongoing training program for new hires and a refresher course for existing operators on a regular basis.
Routine inspections are needed. Before any production opens, the rigging should be thoroughly inspected by a knowledgeable stage technician. It should also be inspected yearly by an outside inspector, and a written report on its condition should be submitted to the owner.
Jean Rosenthal, the Broadway lighting designer, once said, “The theater is a dangerous place. Until you recognize that fact, you are not likely to produce much excitement in it.” She was right, and much of that danger is inherent in the operations that involve the stage rigging. However, careful attention as to how the structure is designed and how the rigging is installed can greatly reduce the chance of failure or accident.
Lawrence Graham, A.S.T.C., is a senior consultant at CDAI with more than 40 years experience in theater design, consulting, and project management. He has a master's degree in theater arts from the University of Tulsa. Graham is a member and past officer of the U.S. Institute for Theatre Technology.
Professional engineers encounter other types of rigging in theaters, ballrooms, arenas, general-purpose auditoriums, and churches. The following are three examples of audio and video rigging issues.
Loudspeakers and speaker clusters are often encountered in assembly facilities. For the structural engineer, obtaining an estimate at the beginning of the structural-design process of the probable load is an important consideration. In addition, designing access to the speaker clusters becomes vital when installing, aiming, and maintaining the speakers. If not well designed, these areas are often not easily accessed once the auditorium's fixed seating has been installed. In these instances, catwalks and work platforms become a necessity, requiring additional expense.
Installing the speakers themselves involves the use of various kinds of rigging components. Collectively, these components are usually referred to as a harness. Each harness interfaces with the structure at a minimum of three hard points and perhaps more. In larger venues, most speakers are assembled into clusters that must be carefully aimed in order to provide the appropriate coverage.
Most loudspeaker manufacturers provide connection points in the speaker cabinets where the harness can be safely attached. Obviously, attaching the harness to other parts of the cabinets is not a good idea — it could void any warranty and may be hazardous.
The least desirable kind of harness, from a safety point of view, is a harness fabricated by an installer in the field. This sort of work, no matter now carefully undertaken, presents many opportunities for human error. Harnesses made by specialty rigging companies provide a much better solution. The best possible harnesses are those made by the speaker manufacturers themselves. Factory fabricated to interface correctly with the manufacturer's speakers, they offer the additional advantage of being able to aim individual speakers within a cluster.
Projection screens, particularly large ones, can impose loads on the structure serious enough to warrant careful evaluation. Compared with speaker installations, installing projection screens — even very large ones — is relatively simple.
Projection screens can be grouped into three categories: framed screens that stand on the floor, framed screens that hang from the structure, and roll-up screens that hang from the structure.
Screens that stand on the floor impose no serious load and can be handled with normal floor loading. Hanging screens, whether they roll up or not, come with built-in attachment points designed by the screen manufacturer. The interface between the screen and the structure is usually a cable or chain. The location of the screen, the number of attachment points, and the estimated live load should be estimated early in the design process.
The actual installation of the screen should be undertaken by a qualified rigger, and the type of chain or cable — and the method for securing it — should be carefully specified as a part of the design documents. Video projectors in larger venues are often quite large and must be located to provide an essentially straight shot at the stage so that the image will not keystone or be distorted by the projection angle.
Usually, that means providing a projection room in an appropriate location or having the projector drop from the ceiling to the appropriate projection location. In the latter case, a projector lift is incorporated into the building design.
Projectors require regular maintenance, so the point of maintenance becomes an important issue. In large venues, it is often wise to provide a catwalk to the lift and configure the lift so the projector can be serviced at that level. In smaller installations, the lift can be configured to be lowered to the floor for access to the projector. In either case, early planning as a part of the structural design is essential.
Rigging of systems for lighting, loudspeakers, video products, trusses, and scenery in stage production applications is inherently dangerous and should not be undertaken without a full and complete understanding of the principles involved. This article is an overview of some of the aspects in the subject of rigging but should not be construed as a designer's guide. Before undertaking any installation in which rigging is required, it is imperative that a mechanical engineer with the proper permits for the locale and any licensing required be consulted so that you are not subject to liability and the consequences of improperly installing these systems. S&VC, Primedia, and the author of this article assume no liability as to the statements expressed or implied within this article.