Safe Rigging Practices
By Rudy Trubitt
This article originally appeared in Live Mix Magazine, an ACT III Publication. The target readers for this piece are working professionals in the event-production industry.
For years a staple of the touring scene, suspending or "rigging" loudspeakers above stages and audiences has become commonplace in smaller venues and installations all over the country. Users and manufacturers alike share a concern for the safety of these systems. Fortunately, those familiar with the do's and don'ts are eager to share their knowledge in the interest of safety for all involved. The information provided here is intended to be food for thought, and not explicit design information. When rigging, always seek the advice of qualified experts.
The most basic rule of safe rigging is to design your rig to support five times the weight of your maximum intended load. While most people have heard of this 5:1 design factor, not everyone understands why it is so important. If a rigging system is designed with a reduced safety factor, say 2:1 or 3:1, it's still two or three times stronger than it needs to be, isn't it? Harry Donovan, a veteran of the tour scene and author warns, "The speakers won't stay up in the air as long--the rigging will eventually will break." Andrew Martin, president of the ATM Group (http://www.atmflyware.com), a manufacturer of loudspeaker flying hardware agrees. "Everybody's always very interested in the right way to do it," says Martin. "At the same time," he continues, "not everyone is ready to build it that way, due to the cost involved. They may not have had accidents using a safety factor of 2:1 or 3:1, but it doesn't mean they won't down the road."
A 5:1 design factor is necessary for a number of reasons. Although load rated hardware is the only kind that should ever be used, it is possible for such a fitting to not meet its published specification. This could be due to wear, a manufacturing defect or other causes. In any case, if the part is only being loaded to 1/5 of its rated capacity, a reduction in its strength is less likely to cause a catastrophic failure.
Another reason for over-design is the dynamic loading put on a rigging system when starting and stopping chain motors. Andrew Martin recently analyzed the problem with a structural engineer, and determined that the shock load adds 85% to the weight, a value higher than previously thought. "A good rule of thumb is to figure you're doubling the load when you start things moving," says Martin. That happens every single time you start a motor or let it drop." Chain motors aren't the only case to consider--earthquakes are a less predictable, but clearly potent source of dynamic loading.
If any doubts remain as to the need for a 5:1 safety factor, reflecting on the risks involved should quickly dispel them. "When you think about rigging," says Donovan, "you're thinking about killing people or not killing them. If you put in just one fitting that doesn't work, or put it in the wrong way, you could kill dozens of people. That fact calls for an entirely different level of concern than plugging a microphone in. If you plug a mic in wrong it's fixable. But most people put in the same level of concern into rigging as they do into other things."
So, what steps are necessary to insure you're working safely? "You have to look at every single element in the system," says Donovan, "and make sure you've got that safety factor for each one. You may have decreased the strength of a part somehow. For instance, pulling an eye bolt at an angle will decrease its strength. You have to figure out what angles you're pulling and what the strength loss is due to the way you're using it, and make sure you still have a 5:1 safety factor."
A basic fact of rigging bears repeating--the effective weight of a load can increase greatly, depending on the angle of the cables suspending it. A simplified example: A 1,000 pound load is held by two cables. If the cables are the same length, hang vertically and are balanced between the load's center of gravity, each will be stressed to 500 pounds. This seems intuitive enough, until the angles of the suspending cables deviate from vertical. Rocky Paulson of Stage Rigging explains. "With bridle legs of equal length hanging at an angle of 30 degrees with a 1,000 pound load, you have 1,000 pounds of force in each leg. If you decrease the angle 15 degrees, it would be roughly 2,000 pounds in each leg, and if you try to make it perfectly horizontal, the load will be an infinite weight," says Paulson. These load angle effects must be figured into the design. "You have to calculate this load [angle], and make sure that even with this increase in force, you still have the 5:1 safety factor," urges Donovan. (These calculations become quite complex when dealing with different leg lengths and angles. Donovan carries a palm-top computer running a 123 spread-sheet to aid in the process.)
"Finally," Donovan continues, "look at the load increase factor from shock loading (an increase ranging from 25% to 75%, as mentioned above) and make sure you still have the 5:1 safety factor. You'll end up with pieces a lot bigger than you thought." Even with a 5:1 safety factor one still can't be too careful. "In most accidents, several things go wrong simultaneously," says Donovan. "The safety factors are so high that doing a single thing wrong usually doesn't cause an accident--usually you have to do two or three things wrong at once."
With safety in mind, let's turn to a familiar issue--getting your loudspeaker array positioned optimally in the room. "[Touring] sound companies want their speakers in the same place every day," says Donovan. "What typically happens is I'll find it's not safe to get that many speakers up in a certain building, or it's not safe to get the height they want. As the bridles get flatter and flatter, you reach a limit as the tension increases. So, between a reasonable bridle angle and the slightly lower beams of a particular building, the speakers are lower than the engineer would like. At that point they'll complain, and the riggers will be under pressure to do things that they think are not safe."
"What happens at that point depends on personalities," Donovan continues. "How dominant are the sound people are and how sure of themselves are the riggers? The trouble is most riggers can't calculate tensions, forces, loads on beams, and what those beams can take. All they can do is estimate--'We did this before and it worked.' But the fact that something's worked before doesn't mean it's got a 5:1 safety factor on it. But because these riggers are guessing and estimating, they are apt to change their minds slightly under a lot of pressure. In some cases, they're doing things that they aren't sure are safe. What's needed is the education so they can do the calculations and tell the sound engineers 'If we get it two feet higher it's going to cause 4,700 lb. tension on this cable rated at 4,000 lb. safe working load, and we can't do that.'" The bottom line? "Don't ask the rigger to do something he doesn't think is safe," concludes Donovan. "It happens frequently, and it's a bad idea because he's liable to do it if you ask him strongly enough."
Behind this inter-personal dynamic looms the issue of liability. Andrew Martin explains. "If I'm a sound company and I pay a rigger to hang my points, I'm responsible for what that person does. If I say 'I need my points here and here and I want you to go from that beam to that beam in order to get them there,' [you're assuming additional liability by specifying how they should do their job]. If you tell them to get the points however they can, the liability issue is somewhat clouded--it has to be decided in court. But you can bet that in any situation, you are at least 50% responsible." Talk of responsibility quickly leads to thoughts of insurance. "If you give any kind of instruction and it's not your profession," Martin continues, "you're in big trouble and no one is going to insure you on that. For a contractor to get liability insurance is not a big deal--it's just a general liability policy. But, the insurance company does need to be informed that you are hanging things above people's heads so that they can make a notation on the policy. Most of the insurance writers will overlook that and call it a general liability policy, but if it comes to a claim, they'll try to find every loophole they can. In order to protect yourself," Martin adds, "you want to make sure [the policy] specifically notes that you are rigging things above populated areas."
Another fact not to be overlooked is the building your hanging the equipment in. Most structures are designed to support the extra loads which result from wind, rain and snow, but architects of smaller structures may not have considered the load an average-sized sound system can present to their building's roof. Compared to rigging's 5:1 design factor, building codes typically require far smaller design factors--well under 2:1. "Buildings are not designed to waste the builder's money," says Martin. "They design them as cheaply as possible for the purpose. If a wall and girder system's purpose is to hold up a ceiling, then it will be designed to use the minimal parts and be the cheapest for the builder, which means they are not going to design for any tensional stress (as created by flown equipment) at all. In big venues, it is considered, but in smaller facilities, they don't consider it." Where any doubt exists, consultation with a structural engineer is necessary to insure safety.
Even if your sound company hires experienced riggers to set your points, there's still the design of your loudspeaker grid to consider. Those choosing to build their own should follow the same 5:1 design factors. "There are dozens of designs out," says Donovan. "Some of them are extremely flexible and adjustable, and some have no adjustment to them. Some companies have done a very thorough engineering job on their grids, while some have never been tested and are obviously put together by some apprentice in the metal shop. I wish we could convince all sound companies to get their stuff engineered and tested--it's the ones who aren't making that much money who think they can't afford to do it. These companies are also usually the ones with the least training for their people, so they're the most dangerous."
Testing should also be approached with care. Martin described a situation where testing itself caused a hazardous situation. A sound company had designed a loudspeaker grid with only a 2:1 safety factor. To test it, they loaded to 150%. It held, and out the door it went. If the system had a 5:1 design factor, 150% of the grid's safe working load would have represented 30% of the system's theoretical limit. But, with only a 2:1 design factor their test load was actually 75% of the weight required to cause catastrophic failure. Add to that an extra 30% shock load when hoisting the rig and the maximum load could have been reached. Stress like that causes permanent damage to the system. "There's a point in any alloy called the yield point," explains Martin, "where the material gets flexed so far that it doesn't return to it's original shape. You've changed the molecular structure of that part, and weakened it. It's very bad to test something until it starts to bend and then say 'this is strong, use it.' But a lot of people will do that."
"Every piece of hardware has to be load rated by the manufacturer, [and be rated] for holding loads over people," says Donovan. "They should probably also be proof tested at twice the safe working load so you know there's nothing wrong with it. Some manufacturers proof test every single piece, some don't test any, some test a few samples from each batch. Standard proof test is 2/5ths the yield point," he adds. "You shouldn't test it to anywhere near it's yield point, but twice the safe working load [assuming a 5:1 design factor] should be fine."
It's worth noting that metal parts will reach their yield point before breaking. The additional force required to reach break point varies, but is often in the 20%-30% range. All rigging components should be rated for a maximum load, but weather that value represents the yield or breaking point is often omitted by the manufacturer. Finally, remember that testing won't guarantee the part won't fail someday. Unless a part is tested to its destruction, the test will only prove the part is capable of supporting the weight you've tested it to, and since that point should not be over 2/5ths of its maximum, you can't be sure how it will perform under extraordinary conditions while in use.
Finally, safety requires maintenance, which means keeping a watchful eye on all your equipment. "If any equipment shows any kind of abuse or wear, don't take chances--throw it away," says Stan Miller of Stage Manufacturing. "A $5 fitting or a $20 SpanSet or $50 Aeroquip part is cheap. Throw it away." Miller takes the extra precaution of destroying potentially damaged parts before disposal to prevent their re-use. Hoist motors also need looking after from time to time. Wally Blount of hoist manufacturer CM explains. "One thing we stress is proper maintenance of the hoists. These are mechanical devices and they require periodic maintenance and inspection. If people don't do that, it could ultimately lead to an accident." To help avoid such situations, the company offers periodic maintenance classes across the country.
So, what's a conscientious would-be rigger to do? Those looking to further their education in safe rigging practices should check in with the US Institute for Theater Technology. They oversee several independently run seminars on rigging and other aspects of performance facility safety.
For further information, please see ESTA BSR E1.4-3 - 201x Entertainment Technology—Manually Operated Hoist Rigging Systems.
Special acknowledgement to Harry Donovan (1943-2009), who gave generously of his time for this article. He also wrote two books on rigging, Entertainment Rigging and Arena Rigging: A practical guide for riggers, designers and managers. His author page on Amazon is https://www.amazon.com/Harry-Donovan/e/B001K8XDSC/ref=dp_byline_cont_pop_book_1