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Monday, September 19, 2011

Everyone Goes Home Safe: Furthering The Rigging Safety Discussion

I began writing this series of rigging safety articles several months ago motivated by the many dangerous practices I’ve seen over the years. (See part 1 and part 2.)

These disasters-waiting- to-happen lurk in clubs, small shows, medium shows, and sizeable temporary events.

This past summer, we have witnessed staging/rigging tragedies in Canada, Indiana, and Belgium. The common element in each of these truly unfortunate incidents are media reports of extreme weather conditions. There’s a very human tendency to believe these “acts of nature” are unavoidable, and in some respects, it’s true.

But those who promote outdoor shows, and those who stage them, have an implicit responsibility to both closely monitor worst-case weather patterns and to have decisive, immediate, and safe plans of action to do everything possible to keep everyone safe.

Further, the reality of a sudden storm – versus a slow onset – has become all too well-known in recent weeks, but regardless, the stage and all associated structures must be designed and built to withstand the worst that nature has been known to deal out in the past, plus a margin of headroom. Must be. Anything less is a gamble; a misguided dice roll that risks other people’s lives and well being.

A suggestion: if an event is held in an area known for weather conditions that cannot be mitigated by strength-of-build, such as violent storm squalls or tornados capable of leveling permanent construction, then further precautions must be undertaken. For example, spotters could be deployed a mile or two out, watching the skies and able to radio in warnings in time to get everyone off the stage and away from the towers.

At the least, a competent individual should be assigned to monitor all available weather reports. He /she must have the authority, and the clarity of purpose, to stop the show at the first sign of trouble. A false alarm or two is far better than waiting until it’s too late.

In addition, fall zones should be established around the stage, and any support towers and cordoned off, so that in the event of structural failure, no one will be in the danger zone when the structure collapses.

Safety guy wires (wires that run from ground anchors to the top of the stage structure) can greatly help to stabilize a structure when it’s subjected to wind load - and even seismic loading - though they also produce a vector force of their own pulling outwards and downwards: the structure therefore must be strong enough to handle the added load of the safety wires.

Safety wires represent a lot of extra work and expense - but nowhere near the cost of a single human life. It’s also possible to add extra tension to the stabilizing wires in the rear so that if the structure does fail, it will topple rearwards, away from audience members, rather than forwards and towards them.

Further Steps
Load cells - small devices that provide tensile information electronically to a remote readout - are relatively inexpensive these days. They can be built into the stabilizing wires and into other key load points of a stage, or a sound tower or lighting tower. The increasing force from a freshening storm can be monitored electronically from a safe distance.

A rapid-deployment retractable roof system can be readily designed to remove the wind load on the huge fabric surface in seconds, in the event of a storm that sneaks up suddenly. Using the same mechanical arrangement as a ball-bearing sail track on a sailboat, and a few electric cable-spools, a roof could be retracted in seconds.

Sure, the equipment on stage is going to get wet. That’s O.K. if lives are saved. (And anyway, that’s what tarps are for.) There’s likely to be plenty of time to deploy tarpaulins, but only after the threat of the roof collapsing has been reduced by removing the roof “sail” area.

The bottom line is that a wide range of solutions need to be considered, adopted, and implemented.

The alternative, which I fear we’re close to, is the cancellation of all future outdoor events until safety guidelines are drafted by a government agency, and a method of policing the guidelines is put in place. That could take years.

So more than ever, I believe that we desperately need to establish a safety charter that governs all 50 of the United States.

If we don’t, it will be done for us and we probably won’t like the results.

Fault Conditions
Now let’s pick up where we left off last month: bridling and reeving. This is more than incidental knowledge because these two practices, while often misunderstood, are two of the most likely culprits for inadvertently setting up potential fault conditions.

Bridling doesn’t refer to horses, as you may have guessed. It refers to angular loads. When a load is suspended by two or more suspension points that are not directly in line with the mass of the load, the resultant rig is said to be bridled.

As you can see in Figure 1, the vector force of a bridle can create loads within the rigging parts that far exceed the actual load of the object itself. This does not mean that the 100-pound object used in these four examples will ever weigh more than 100 pounds. Of course not.

Figure 1: The vector force of a bridle can create loads within the rigging parts that far exceed the actual load of the object itself. (click to enlarge)

But the load on the rigging parts can greatly exceed 100 pounds due to the vector loading of those parts (which might be wire rope, Spansets, slings, chain, or other materials).

Because this is counterintuitive, I like to show the effect in real time by using three 30-pound fish scales - which I do when teaching classes in person. I’ll actually rig up a 10-pound load with a sliding spreader bar. I put one fish scale on the top-most hang point – which is always going to read 10 pounds – and then put the other two scales on each of the two bridle parts of the rigging.

Then, as the angles of the bridle are adjusted, everyone can see how the load on each of the bridle parts varies. It’s particularly dramatic when the angles are acute in either compression or expansion, and the fish scales quickly max out at 30 pounds - then suddenly the light bulb goes off!

As the bridle angle approaches 90 degrees (full horizontal) the loading on each bridle part mathematically approaches infinite. In real life that doesn’t happen because of material elasticity and other factors. But it’s worth knowing that the true force in a bridle part can exceed the actual mass of the suspended object by a very large factor.

Greater Forces
It’s equally important to understand that an acute bridle angle places a considerable compression (or expansion) force on the object of mass, as a function of the angle of the bridle.

While this may be no problem for an industrial spreader beam that came at “no extra charge” with your recent 90-ton crane purchase, it often is a considerable problem when you suspend a loudspeaker, or loudspeaker array, using an acute bridle arrangement.

Many suspension frames are simply not designed or built to be loaded at sharp angles, and especially with far greater forces than the actual weight they were intended to support.

Not all frame designers and manufacturers are adept at imagining the full gamut of strange (and inappropriate) rigging arrangements that their frames may become subjected to.

If a loudspeaker uses simple eyebolts as a means of attachment (this is very common in small systems), even a shallow bridle angle will subject the eyebolts to forces they were not designed to accommodate.

Eyebolts – even the good ones – are intended primarily for a straight line-of-force load that aligns with their threads. (Figure 2) They should never be loaded laterally.

Figure 2: An example showing the de-rating of an eyebolt that has a Safe Working Load of 1,000 pounds in a straight line of force. A direction of pull (in-line) of 45 degrees reduces it to 30 percent of the rated working load, and at 90 degrees, it is 25 percent of the rated working load. (click to enlarge)

Though they can tolerate loading in the direction of their eye, as the angle decreases from a straight line of force, their load rating in turn decreases dramatically.

Other devices such as rigging frames, bumpers, etc, can exhibit similar limitations as eyebolts, but here the issues and properties are product specific.

One product may be able to tolerate an acute bridle, while another may not. It depends on the design geometry of the device being rigged, so there is no simple or single answer.

That said, the additional stress that’s put on the wire rope, the shackles, the Spansets and the other parts of the “hang” will all be magnified by using an acute bridle – even if the rigging frame or bumper itself can handle the load. Best idea: always use shallow bridles!

Figure 3: Rule of Life - do not bridle past a 30-degree angle. (click to enlarge)

Rule of Life (this is like a “Rule of Thumb” but more so): do not bridle past a 30-degree angle (0 degrees being a straight line pull). See Figure 3. Doing so will place a significant compression force on the loudspeaker (or other object) that you’re intending to suspend.

Situations may arise in which you must bridle tighter than 30 degrees. In such case, the shackles should be oriented towards the line-of-force so that they pivot easily towards the load point. If there are eyebolts in the rig, they will need to become expensive swivel eyes, rather than inexpensive static eyebolts.

With sharp bridle angles, considerable care must be exercised to insure that no parts are overloaded.

Stress Effects Of Reeving
Reeving is when a Spanset, nylon strap, wire rope, or other suspension part that’s already fastened to an attachment point is then passed through a shackle, an eyebolt, a handle, or some other fitting, and then is fastened to a third attachment point.

It’s common to see reeving in all manner of rigs ranging from small club systems to large PA hangs.

For reasons unknown, people just like to do this. I’ve overheard talk in which the reckless reever believes it’s actually safer to do so. I did it once or twice myself, before I understood the stress effects.

What happens with reeving is a lot like a 90-degree bridle. The force on the reeved part, as well as the shackles and the other fittings associated with the reeve, is magnified many times over to the point where it can damage otherwise perfectly constructed materials and weldments in a fly frame. The compression force on the object, be it a loudspeaker, a rigging frame, or other, will be many times that of the object’s actual weight.

Additionally, the load can rotate or shift within the reeve, increasing the stress yet again when the load shifts – very possibly leading to sudden failure. Therefore don’t do it!

The only arguable reason might be to “use up” an excessive length of a Spanset or wire rope sling, in order to shorten the distance to the upper attachment point(s) when headroom is limited. Sorry, that’s not an acceptable reason. Obtain the proper length of wire rope slings or Spansets. Again, do not reeve!

Now, just so I don’t get letters from irate construction workers, reeving does have a place in the construction industry. If you reeve the attachment fittings on a 100-ton water valve, you can use your crew to shift the valve within the cradle created by the reeve to align it with the 80-inch pipe you’re fitting it to.

But please note that when this takes place, it does not occur over people’s heads. And it usually doesn’t last for more than a few moments of work in a restricted-access construction zone with trained personnel. Neither of the aforementioned conditions typically prevail in an entertainment technology rigging setting.

Figure 4: Note the differences between a shackle (left) and a pairing ring (center). A circular ring is shown at right. (click to enlarge)

Straight & Lateral
A frequently asked question: do pairing rings always come in pairs (like earrings)? No. You can buy them one at a time. In the previous installment, I harped on the fact that shackles are not pairing rings (Figure 4); in other words you should not use a shackle to join two bridle parts together, unless the bridle angle is very shallow and the shackle is rated for limited “pairing” use.

So instead, you’ll want to carry a selection of pairing rings in your rigging kit. A pairing ring is intended to take lateral forces in addition to a straight line force. Like anything else, it too has its limitations. You can’t pull it in all directions at once; if you need to do that, you’ll want a Weldless Circular Ring, such as the one shown above, which is also easy to obtain from any good industrial supplier.

And when using either a pairing ring or a circular ring, simply attach the shackles to the ring. In both cases, you’ve avoided placing lateral loads on the shackles themselves. Now it’s all good.

While we’ve conquered many things once deemed impossible but now takenf or granted – such as air transport, wireless communication, modern PA systems of amazing quality, and much, much more - let not our quest for pure technology overshadow our intrinsic responsibility to our community.

Be it a rock concert or a political rally, we need to be able to say, “Everyone gets to go home safely.”

Ken DeLoria is the founder and former owner of Apogee Sound, a manufacturer of loudspeakers and many associated rigging accessories. For more than 30 years, he has been a sound engineer, a hands-on rigger, and a safety supervisor at numerous events and permanent installations.

(See part 1 and part 2.)