Monday, January 21, 2013

Have we learned nothing?

The festival season starts early in the south, and Mardi Gras is one of the first events we see set-up for numerous stages and shows.  Outdoor festival stage canopies have proven to present opportunities for accidents because they have large surfaces that can be caught by side winds and uplifting plow winds.  To counter-act the forces developed by gale-force winds, event riggers rely heavily on large ballast weights to tie-off canopy structures in an attempt to anchor the framework to the ground.


These ballast weights work well if they are massive enough for the predicted winds and effective 'sail area' presented by the canopy roof and side walls.  If they are too light, the ballast weights will just be dragged across the ground and bulldoze a path of their own.  Slippery grass, or dirt that turns to mud, just act as a lubricant to free the ballast weights up for sliding in the event of a downpour, so anchoring or pinning the ballast weights to keep them from moving is also part of a good rigging plan.  (The ballast weights shown above were not pinned or anchored to prevent sliding)

Wind is not a steady-state event.  You know that.  It gusts.  It starts and stops.  It changes direction.  It generally whips everything around a bit as it works it mayhem.  For this reason, we need to use secure connections on the guy wires for the ballast weights and stabilizing X's that are used to stiffen the structures against parallelogram movements.  If you don't, a gust of wind can slack the line, unhook the conenction(s), and you loose all anchoring in the blink of an eye.


As you can see above, the anchor ties used in the blocks in the first picture are common racheting tie-downs in the trucking industry, and they only have a open hook at the load securing end.  This may be de rigueur for the transport industry, but it's a no-go for event safety.  If the wind gets to blowin' and this bad boy comes loose, it could rip the flesh right off of someone unlucky enough to get in it's flailing path.

So, it was bad enough to see the above tie-down terminations at almost every corner of the stage, but then I looked further up the anchor line and found this:


Each anchor line had two possible points of failure.  Three stages and a major cable bridge over the street, all in close proximity, four to eight anchor blocks secured in this manner for each one, and two possible failure points per securement.  All of this within a city block of a major open waterway (read: obstruction-free wind-path).  Personally, I don't like those odds.

The City is relying on the Event Producers to Employ Staging Subcontractors that will erect facilities that will be safe for the (possibly highly intoxicated) general public to hang-out around and potentially be leaning-on, climbing on, or even messing with.  Plan for the worst, and you may have a chance of having an incident-free event, but if you leave opportunities for failures in the system like are shown above, be ready to pay the piper (or at least the plaintiffs).

Follow-up to commenter:
If you are going to use ratchet-straps to secure anything (subject to load rating), then utilizing products that have a self-closing clip is highly recommended.  Here is a picture of that type of ratchet and clip:

Deisthttp://www.deist.com/inline-ratchet-tiedown-p-258.html

It is also recommended to have a wire rope safety cable that runs in parallel with this, preferably one that is only slightly longer than the final tensioned length of the ratchet strap.  This can prevent a total release of the system should the ratchet strap fail.  Fabric based straps and slings are subject to weakening due to abrasion, minor cuts, and rot.  Straps exposed to sunlight have shortened lifespans due to the exposure to UV light which weakens the fibers.

2 comments:

  1. Hello Erich. I happened to stumble on your article "have we learned nothing" and had a question for you. I must admit the system of using a ratchet strap to tighten down the anchors of temporary roofing is the most common I have seen through out my career. That being said... I think you have a valid point on the possibility of it coming unhooked should the structure sway for any reason. What would be your suggestion for an alternative method of securing the guy wires? A shackle in the loop of the strap in place of the hook?

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  2. Thank you Erich for your follow up. I guess the reason why I didn't think of this solution is because when I envision tie down ratchet straps for a stage roof, I think of the 2', 10,000 lb straps used on tractor trailers and didn't even think they made straps of that strength with snap hooks. The straps you posted look as though they might be comparable to the ones used in the roof pics you have posted. Most of the ratchet straps with the snap hooks I found tend to be rated lower at 6,000 lb breaking strength. However after a little more extensive searching there are indeed some 10,000 lb straps with snap hooks out there... just harder to find. Assuming the eye of the steel will fit in the snap hooks of the ones I have found, this would indeed be a great solution. Thank you for your input!

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