Why Things Fall Off Cranes

We talked about crane failures in a previous  video, but you might be surprised to learn that   things can and still go wrong with heavy lifts  even when the crane is perfectly safe and sound.   All cranes use a hook as a connection to the  load, and yet, few things we need to lift have   an attachment that fits nicely over a gigantic  steel hook. “Rigging” is the term used to describe   all the steps we go through to attach a load  to a crane so it can be suspended and moved.   And, like all human endeavors, rigging is prone  to error. Some of the most serious crane failures   in history had nothing to do with the crane  itself but were actually a result of poor   rigging. One of the worst construction accidents  in U.S. history happened in New York in 2008   when a large metal component of a crane was  improperly rigged. The overloaded slings failed,   dropping the collar directly onto its attachment  points to the building under construction,   causing it to detach and collapse. Six workers  and one civilian were killed in the incident,   and many more were seriously injured. There’s  a lot that can go wrong below the hook,   so in this video, we’re going to take a look at a  few of the fundamentals in attaching and securing   a load and some of the hidden hazards that can  pop up if not done properly and carefully. I’m   Grady, and this is Practical Engineering. In  today’s episode, we’re talking about rigging. You’ve probably heard the phrase that if your  only tool is a hammer, every problem starts to   look like a nail. It’s a lighthearted way to  warn about over-reliance on a familiar tool.   But, if you’re a rigger whose job is  to secure loads to cranes for lifting,   you really do have just one tool. You don’t  use a piece of old rope out of your pickup bed.   You don’t use a ratchet strap from  the big box store down the road.   And you definitely never use the crane’s hoist  line to wrap around a load. You have one option:   a sling. Of course, slings come in a wide  variety of sizes and types and materials,   and you also have hardware like hooks and eyes  and shackles and pulleys but, yeah, one main tool.   And here’s why. Slings have a rated capacity. That  rating is a guarantee from the manufacturer. More   importantly, it’s a big responsibility taken off  the shoulders of a rigger to know and trust that   each connection to a crane or hoist can carry the  right amount of load. So what do these tags mean? It’s actually pretty straightforward. The tag  shows how much weight you can put on the sling   using the three basic hitches. If your load has a  hook or a shackle, you can use the vertical hitch:   one eye over the load and one eye over the hook of  the crane or hoist. It’s the most straightforward   configuration for a sling and takes full advantage  of its load capacity. If there is no attachment   point on your load, you might instead use  a basket hitch. In this configuration, the   load is cradled by the sling, and both eyes are on  the hook. One benefit of the basket hitch is that   it doubles the sling’s load capacity since you  have two legs holding instead of just one. But,   it only works if the load is balanced and easy to  control since it’s only cradled from the bottom.   If you need a snug grasp on the load,  you might use the third basic option:   a choker hitch, where the sling passes  through one eye and attaches to the   crane hook on the other. The choke point has  extra stress when used in this configuration,   so the load rating for a choker hitch is less  than that of the vertical or basket hitch. If you’re using a sling to lift something  heavy and don’t see a load rating tag,   just stop. Every sling rated for rigging  has to have a tag, whether it’s a synthetic   sling like this, a wire rope, or a chain.  Even so, the vast majority of rigging failures   happen because a sling was overloaded. You  might be wondering why that’s the case when   the load rating is spelled out right there on  the tag. But those three numbers hide quite a   bit of complexity involved in rigging, and I  have a few examples set up here in my garage   to give you a glimpse into those intricacies. Even  if you aren’t planning to connect a 20-ton beam to   a crawler crane any time soon, this information  applies to lifting just about anything. The first rigging pitfall is center-of-gravity.  Not all loads are evenly distributed or equally   balanced, and that can cause some serious issues  if the rigging doesn’t take it into account.   For example, if you’re using multiple slings to  lift something, your first inclination might be   to simply divide the total weight by the number of  slings to estimate the load each one will carry.   But if the slings aren’t all attached at the same  horizontal distance from the load’s center of   gravity, the total weight won’t be distributed  evenly between them. That may seem obvious,   but many many loads have been dropped because  of misunderstandings with center-of-gravity. For just one example, loads often show  up to a site in a crate where it’s not   quite so easy to see how the weight is  distributed. In a worst-case scenario,   incorrectly estimating the force on each sling may  cause one or more to overload and fail. But even   if the sling doesn’t give out completely, it might  stretch just enough to cause a load to shift.   And if it shifts such that the center of gravity  moves to the wrong side of the attachment points,   there’s a chance it will tip and fall. You  can’t push a rope, after all, so slings   only provide resistance in one direction. So when  lifting a load that isn’t equally balanced between   attachment locations (and especially for the big  lifts that use more than one crane), you have to   calculate the load share between the slings and  make sure each one can handle its portion. The   formula is super simple as long as you know the  center of gravity. And, if there’s a chance a load   could slide if one sling stretches more than the  others, it’s got to be secured before the lift. The next potential rigging pitfall is the  sling angle. Let me show you an example:   Say you have a balanced load, and you  need two slings to attach it to a crane,   but your slings are kind of short. So, when you  get everything hooked up, the connections make a   30-degree angle from the horizontal. Is each sling  carrying half the weight of the load? Would I even   be asking if the answer was yes? In fact, at a  30-degree angle, each sling is subject to double   the force that it would otherwise feel if it were  perfectly vertical from the load. Why is this? Slings can only pull in one direction. For  simplicity, we sometimes divide the force   in the sling into its vertical and horizontal  components. If the sling is perfectly vertical,   it has no horizontal part. But as the angle  of the sling changes, the horizontal component   becomes a greater and greater proportion of  its total load. This may not need to be said,   but we don’t need a horizontal force to lift  something. We need a vertical one. In fact,   the horizontal force isn’t just unnecessary,  but it also has to be canceled out by an equal   and opposite force in the other sling.  So, the shallower the angle of the sling,   the harder you’ll have to pull on it to  get enough vertical force to lift the load. Watch the spring scale as I change the angle of  the slings holding this steel bar. It’s a little   hard to read on camera, but it’s still clear  what’s happening. When the slings are vertical,   each one holds half the weight (in this case,  about 800 grams each). But as I bring the tops   of the slings toward the center (like they  would be if attached to a single crane hook),   you can see the plunger of the scale descending.  When the tops of the slings touch, the scale reads   about 1200 grams, an increase of about 50 percent.  I’m sure you can imagine what would happen if you   incorrectly divided the weight of the load by two  and assumed that to be the force in the slings.   You’d be underestimating by quite a lot.  So, when using slings that aren’t vertical,   you have to apply a reduction  in capacity based on the angle.   Again, the formula is simple, but you  have to know how and when to use it. Shallow horizontal angles aren’t just an issue  with sling tension, though. Those horizontal   components of force that I mentioned have another  disadvantage related to the third and final   rigging pitfall I want to discuss: abrasion. As  I said, slings can be made froma few materials,   including chain and wire rope, but one of the  most common materials is woven synthetic fibers   like nylon and polyester. These synthetic slings  have a lot of advantages. They’re lightweight   and easy to move around. They don’t create sparks  that can be dangerous in industrial environments.   And, they’re soft, so they won’t scrape  or damage whatever they’re connected to.   But, they have disadvantages too - mainly that  synthetic slings are more susceptible to abrasion. Those horizontal forces I mentioned earlier  don’t just increase the sling tension beyond the   weight of the load. They can also cause a sling to  slide. Obviously, that’s an issue if they slide so   far that the load falls. But even if they don’t,  the friction with the load can lead to abrasion   and even failure of the sling. Synthetic materials  are much easier to cut than wire rope or chain,   so they have to be protected from sharp  edges, corners, and burrs. Synthetic fibers   can also melt. You might not think that a  little sliding would generate much heat,   but consider that friction is a function of  the contact pressure between the two surfaces.   These slings are pretty small compared to the  weight they carry, meaning the pressure they   exert can be enormous. Watch what happens on the  thermal camera when I pull down on this loop as   it slides along the pipe. Even a small amount  of sliding under so much pressure can create   enough heat to melt the fibers. One way to avoid  the possibility of sliding is to use a spreader   bar - a device that helps distribute the singular  lifting force of the hook among attachment points   that can be further apart. This kind of device  lets you reduce the angle of your slings,   giving them more capacity and reducing the  possibility of them sliding and abrading. I’ve been referring to “you” a lot in  this video, putting you in the shoes   of a rigger learning the ropes. But I just want  to clarify that this video is not for training.   If this is your first exposure to the topic, I  hope you’ll agree that you’re not ready. Rigging   is a vital but dangerous job, so if you’re  going to be involved in any heavy lifts,   there is a lot more to learn than my examples  in this video. Finally, if you enjoyed this one,   check out the companion video about crane  failures and what can go wrong above the hook. Hey, the fact that you stuck around to the  end of this video means that you’re pretty   thoughtful about the kind of content you  spend your time enjoying. In other words,   you probably prefer learning new things  about the world more than run-of-the-mill   television programming. 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That’s why this bundle is such   an awesome deal. CuriosityStream is taking  26% off an annual plan - that makes it only   $15 for an entire year of access to thousands of  thought-provoking documentaries on CuriosityStream   AND everything on Nebula as well. You can watch  awesome big-budget content on CuriosityStream.   I love nature documentaries, and this special  called Wild Galapagos narrated by Mike Rowe   is one of my favorites. Or, watch Tom Scott  pit other YouTubers against each other in his   Nebula-exclusive gameshow, Money. It’s a great  way to support my channel and a whole host of   your other favorite educational creators. Plus  it’s just a great deal. Do us both a favor and   click that link in the description. Thank you  for watching, and let me know what you think!

Channel: Practical Engineering

Views: 917,451

Keywords: rigging, rigging failures, overloaded, sling, hook, eye, shackle, pulley, crane, hoist, rating capacity, vertical hitch, basket hitch, choker hitch, center-of-gravity, sling angle, abrasion, synthetic sling, engineer, practical engineering, Grady

Id: swk3IjxzZB4

Length: 12min 21sec (741 seconds)

Published: Tue Sep 07 2021

Reddit Comments

[u/FireWireBestWire]

I love Grady's videos - he boils down some complicated things in easy to understand bites, and his demos that he builds are fun to watch.

[u/medic6560]

Nice video, thanks for posting