In all instances, rigging safety must be the overriding concern.
The practice and definition of rigging takes many forms. These range from hanging some sound deadening murals on the rear wall to suspending thousands of pounds of loudspeakers, lighting equipment, video screens and the like over living people's heads. We'll include the use of suspension towers, whether small crank-up Genie types, or large scaffolding structures in this initial treatise.
From a creative standpoint, suspension of loudspeakers, lighting, screens, and properties, can open up sightlines while putting the loudspeakers in an optimal position to cover the space. The same is true for lighting; illumination coming from the wrong direction does little good and can even harm a scene. Unless it's an overflow room, video screens must be positioned where they support the action on stage, not detract from it. And finally “stage properties” which include set pieces, scenic backdrops, and other suspended or freestanding physical elements, can make all the difference in the quality and perception of a special show—or perhaps only the normal “look” in the sanctuary during a regular service, which is important in the impression that the church conveys. This creative side of stage design has opened up tremendously since rigging became commonplace in houses of worship.
The elephant in the room
But in all cases, safety MUST be the overriding concern. The hero of the day is not he who found an unused beam that looked like it could withstand some strange lateral load to get that screen properly situated. The heroine is she who said, “No, that is not safe. The vector force on that beam might be enough to torque it out if its cradle and cause it to fail. We need to have it studied professionally.” And while that might be the cause of a few snickers from those in the know, who used that same beam for every year's gala Easter show, my admiration is with the lone voice who spoke up and said, “No.”
Our industry is getting more and more accomplished at designing and hanging huge set pieces, tons of lighting and sound (sometimes literally), expansive video, large and heavy backdrops, and set pieces of all kinds (think huge Christmas trees with vocalists in them). But as an industry we have experienced more stage, roof, and truss collapses during the past five years than at any time previously. What father or mother wants to deny their child visiting a church's special production that's been promoted for weeks, only to get a call that she isn't coming home? The loss of life is a terrible, but preventable tragedy. And that's the key word: preventable.
Two hang points are never twice as strong as a single one, unless their relative angle is 0 degrees. Doubling up can put dangerous lateral and torsional stresses on the suspension points they are bridled to, as well as the chain, wire rope, and line.
Most rigging accidents can be avoided by simply adopting good practices, but some are more complicated. The complexity of forces on ceiling beams and other building fixtures is not something to be judged casually, or worse, just guessed at.
To give you an idea of what this means in real time: When a load is suspended vertically, with gravity being the only force, the function is simple.
The sum total of the load is the same as the weight of the object and its accessories being suspended. But not so fast! When you need to bridle between two suspension points (and bridling is the proper terminology) to derive a hang position that is somewhere between the two points, you are not—repeat NOT—increasing the strength of the suspension system. You will be categorically decreasing it as well as adding lateral forces to the two (or more) hang points that you are bridling from. This seems counterintuitive, but is mathematically correct. Vector forces come into play whenever there is more than 0 degrees of angle between two rigging parts suspending the object.
Another way of stating this is that two hang points are never twice as strong as a single one, unless their relative angle is 0 degrees. Doubling up can put dangerous lateral and torsional stresses on the suspension points they are bridled to, as well as on the chain, wire rope, line, or other suspension cable being used for suspension.
Mind your vector loading
Let's look at this mathematically for a moment. If, like some folks working in churches, you've given up math for ecclesiastical pursuits or the love of mixing music for the Lord, it's still a good idea to have a working knowledge of vector loading. We promise to keep it simple. Vector analysis is one of the things that keeps airplanes in the sky, and hopefully your understanding of it will help to keep your loudspeakers, lighting instruments, and video screens in the air safely.
As you can see in Figure #1, supporting the 1,000-pound load in a plumb line results in 1,000 pounds of weight on the suspension points, plus the negligible additional weight of the shackles, wire rope, and other rigging fittings. Equally clear is that when bridling between two points, the 1,000-pound load does not change in weight—but the load on the two points can be greater or even double than the original 1,000-pound weight.
The greater the bridle angle, the greater the load with the ultimate being 180 degrees, which mathematically results in an infinite load on each of the suspension cables. Backyard zip-liners beware. Don't stress the zip lines too tightly, and use flexible trees that will take up some of the shock as the weight load reaches the center of the line, which is the heaviest point of loading.
The important takeaway here is although basic physics tells us that an object cannot weigh more than it weighs when subject to a known gravitational force, the manner in which it is suspended can cause far more stress on the suspension parts than is often realized. I once had to buy three fish scales to prove to a computer specialist that this is true: a 50-pound weight could exert as much as 150 pounds of force on not one, but both of two suspension lines. Moreover, the angle of force may cause an otherwise healthy beam to move laterally, which could damage its anchorage to other ceiling support members. Under extreme conditions, especially in aging buildings where wooden beams may have become rotten, or when concrete and steel construction materials might have fatigued, the torsional forces could set up a domino effect that might bring the whole building down. Add some unexpected snow pack on the roof, perhaps some seismic activity or high winds, and the possibility for complete structural failure in an older building, or a sub-standard newer one, becomes quite possible.
When more than two bridle points are utilized, and/or if the bridles are unequal in length, the math becomes much more complex. At that point it's probably time to call in a professional mechanical engineer or rigging consultant. That said, if the loads are very small, like in the 40-pound range, and the beams are huge, there's no point in overdoing it ... provided that you are using the proper cable, rigging fittings, and attachment methods. But anything much heavier, or when beams are aging or already stressed with lighting or other heavy fixtures, then a professional engineer's services are warranted. Who knows, maybe he or she might even become a member of the church.
As a final comment about rigging safety, many people take it for granted that a freestanding structure, such as speaker towers, stage platforms, ramps, and other load-bearing objects are as solid as rock. But even rocks can collapse or be moved from seismic activity, mudslides, and hurricane-force winds. Seismic activity should always be considered, even if you live and work in a zone that's considered relatively safe. During an earthquake, suspended loudspeakers and other equipment may have an advantage over freestanding gear. The suspension cables may allow the equipment to swing without causing cable failure, but the freestanding stacks and staging have nowhere to go but down. For this reason guy wires to stabilize them and a “fall zone” marked with yellow cautionary tape should be established to keep spectators and crew away from possibly being crushed. We'll cover this topic in more depth in a future article, along with a lot more information about how to use rigging and suspension to achieve the creative look and performance that is desired, while maintaining a professional and concerned attitude towards safety.
