Steel Carabiner Failure to Blame for RI Circus Accident

W/ some structural engineering knowledge... and the numbers tossed above...
1,500 : 10,000 = Safety Factor of 6.667

With 9 performers, who i'm not sure travel with the show and weights are a constant variable, 9*150=1,350. And the structure above which is easily 10' in radius, the actual load had to be much more than 1,500 lbs.

That much mass, not rigidly supported would be subject to torsion. Although I don't have any knowledge of torque, or the mass' distribution in that structure. The inevitable torque created by not stabilizing that structure would be resisted by that cable above, ultimately resulting in a shear force in the cross section of the carabiner. This force easily multiplies the shear force created from the axial loading of the mass.

Lastly the movement of the acrobats, in-sync, would multiply this mass, similar to force exerted by the fall of the climber. In structural engineering, this force is called dynamic force. This force would also multiply the stated mass above.

It is easy to see that if a single carabiner rated for 10,000 lbs were used. The factor of safety for this setup would actually be much smaller than 6:1.

They should have backed it up, simple. But with more rigging above, it may have killed the illusion that the acrobats were 'flying in the air'.

How did I do? Any real engineers want to take a stab at it.

either someone is full of shit on the initial report, or fatigued equipment wasn't tracked and pulled from service.

redundancy won't save you.

Sean McAuley wrote: Huh? Apparently every known bridge and building code is wrong then. Anything non-redundant is usually fracture critical, and taken heavily into consideration during design.

Key structural components of bridge/buildings are rarely REDUNDANT. Please understand the definition of redundant! (look it up if you have to).

For something to be redundant, for example a column, then you should be able to remove that item and have no effect on the structure or its operation.

JSH wrote:Redundancy is a principle of engineering. Often, because many unknowing, trusting lives are at stake. This circus act should have been approached as an engineering problem, regardless of its use of climbing equipment. Redundancy fail.

Lack of redundancy is not the problem here nor is it the best solution.

Tom Sherman wrote:How did I do? Any real engineers want to take a stab at it.

I'm a structural engineer.

What you said makes sense. Though rather than a redundant shackle. The best idea is simply to use a stronger shackle from reputable rigging supplier.

(I agree that 10,000lb is insufficient. Though i doubt i broke at 10,000lb. It was either cross loaded OR of suspect origin.

patto wrote: Key structural components of bridge/buildings are rarely REDUNDANT. Please understand the definition of redundant! (look it up if you have to)

Just...wow

I can mostly speak for bridges, but redundant load paths are required for primary members in all structures. The exception is large substructure elements (columns, piers, etc), which were clarified in the 3rd edition AASHTO LRFD. They need not be redundant due to their relatively bulky size and the cost and complication of making them redundant. The reason 2 longitudinal girder systems are no longer built in the US is they are non-redundant structures. Examples of fracture critical (non-redundant) elements include things like box girders, integral pier caps and often truss elements (gussets, main chords, etc). These elements must be designed to never exceed yield stresses as failure would produce catastrophic failure of the structure. Additionally, factor of safety is rarely used for design. Bridge and building codes use a series of factored load combinations and a reduced allowable stress (LRFD and ASD). However, OSHA does use factor of safety, and their code may be more applicable in this particular incident.

TLDR...I am familiar with the definition of redundancy, and it's use is required by both AASHTO and IBC.

Tom: What you wrote sounds right on, but without knowing the fine details it is hard to make speculations on how it failed. Torsion is not something generally considered for the biners we would use in rock climbing, mostly because the ropes we use are able to stretch and twist without causing huge torsional forces on our gear. With a steep cable under tension, this is not necessarily the case, and a slight twisting can cause huge forces in the gear.

For those without engineering degrees, imagine trying to break a biner that is being held in a vise. You could pull really hard on it (good luck!), smash it (would require lots of really hard blows), or put a crowbar through it and simply twist, which could snap it quite easily. This twisting is one means of failure, and I am sure it is something the investigators in this accident are considering greatly.

Brian, isn't what you describe why climbing carabiners are generally plenty strong for us, except when they aren't?

Since most of our gear is free to rotate and move around in a fall, high torsional forces aren't all that commonly encountered in a fall. So the 'biner is loaded in either it's strong or it's weak axis, but not twisted about much.

But then once in awhile, you nose-hook a carabiner and it CAN be subjected to high torsional forces, and they break like toys.

Yes? No? I'm not an engineer.

Maybe their hair was not placed in the direction of pull from their fall?

and also didn't use vidal sassoon

teece303: Exactly. Nose hooking biners is incredibly dangerous, because they are not designed for those types of stresses. You can even feel some biners flex if you open the gate and pull on the nose. Similarly, biners loaded in three directions, ones loaded when the gate is open, or ones that are twisted really hard can all lead to failures under their maximum.

Luckily for some nose-clippers (but not all), the next draw down might stop you if your top one fails on a sport route before you crater.

Never use a non locking rig when the job calls for a locking rig.

D.Buffum wrote: If there had been a second caribiner, perhaps redundancy would have saved them when the fatigued one failed? .

that's load sharing between elements that can't handle the entire load individually (or a safety factor adjusted load), not redundancy

a redundant system would have just snapped their necks in mid air instead of taking a grounder

Redundancy is being solid at the anchor point- in this case, the steel binder they all tied into.
No one obsessed with redundancy here!

I employ SRENE in my thinking whenever possible at natural anchors - the circus act is a controlled environment unlike nature. The riggers are fools for not taking advantage of that fact.

patto wrote: Key structural components of bridge/buildings are rarely REDUNDANT. Please understand the definition of redundant! (look it up if you have to). For something to be redundant, for example a column, then you should be able to remove that item and have no effect on the structure or its operation. Lack of redundancy is not the problem here nor is it the best solution. I'm a structural engineer. What you said makes sense. Though rather than a redundant shackle. The best idea is simply to use a stronger shackle from reputable rigging supplier. (I agree that 10,000lb is insufficient. Though i doubt i broke at 10,000lb. It was either cross loaded OR of suspect origin.

what's your SE number and what states are you licensed in? first of all, that isn't the only definition of redundant in structural design. there are different levels of redundancy requirements for different levels of risk outcomes. your example of removing a column either demonstrates that you don't know what you are talking about, or that you are trying to manipulate an example to somehow prove your point. removing a column in a structure, whether it be a building or a vehicle, and when possible with bridges, is expected to not result in collapse of the structure or to result in injury or loss of life. it is acceptable that there may be some or all loss of function of the structure, depending on the case.

an example of where this comes into play is structural design in seismic areas (like california). the structure is designed with enough redundancy (whether it be additional members, connection designs that redistribute loads to other members more effectively, etc) to ensure that catastrophic failure doesn't occur. in california, the structural design code openly admits that preservation of the structure is not the priority - preservation of life is the priority.

slim wrote: what's your SE number and what states are you licensed in? first of all, that isn't the only definition of redundant in structural design. there are different levels of redundancy requirements for different levels of risk outcomes.

Sounds like you are the one trying to alter the meaning of words. I'm sure you can use the dictionary as competently as I can. Naturally context is everything.

There is a big difference between saying:
"slim, you have a redundant amount of skills to offer this company" and "slim, you are redundant to this company".

slim wrote:your example of removing a column either demonstrates that you don't know what you are talking about, or that you are trying to manipulate an example to somehow prove your point. removing a column in a structure, whether it be a building or a vehicle, and when possible with bridges, is expected to not result in collapse of the structure or to result in injury or loss of life. it is acceptable that there may be some or all loss of function of the structure, depending on the case.

Expected? Sure there are certainly some structures which have redundant columns, but for most structures if you remove a structural column then localized collapse is likely or at the very least problems of servicability. By definition the column is not redundant. A simply supported slab doesn't enjoy being turned into a cantilevered slab with the removal of a column. A portal frame likewise doesn't continue standing happily if one of the vertical members is removed.

slim wrote:an example of where this comes into play is structural design in seismic areas (like california). the structure is designed with enough redundancy (whether it be additional members, connection designs that redistribute loads to other members more effectively, etc) to ensure that catastrophic failure doesn't occur.

Saying something is designed with "enough redundancy" is different to saying something is redundant.

slim wrote:in california, the structural design code openly admits that preservation of the structure is not the priority - preservation of life is the priority.

Great. Not sure how this is relevant.

you're completely missing my point. your definition of redundancy (ie removing a column with no effect on the structure) is not the definition of redundancy requirements as used in structural engineering. the reference to the CA requirements explain the defition of redundancy requirements (ie in extreme cases loss of function of the structure is acceptable, but catastrophic failure isn't).

so, just answer the question - are you a LICENSED STRUCTURAL ENGINEER, or are you a PE who tinkers with some structural stuff, or an EIT that works at a structural firm?

David Peterson wrote:Not sure how reliable the source of this picture is...

Has anything come out about the equipment used?

why would they use that piece of crap and not a shackle

I don't think I've seen it mentioned yet, but the simplest reason for the carabiner to fail under the load that it did, would be that the gate was not properly closed.

I have had plenty of sticky biners myself and have seen plenty of auto lockers not locked before. This seems to be the simplest reason, given the biner breaking in three along the major axis.

Buff Johnson wrote: Has anything come out about the equipment used? why would they use that piece of crap and not a shackle

http://tinyurl.com/mufybjx

Still says "a carabiner", as far as I know, they haven't confirmed it was a locker or not, but the visual would suggest so. Makes me wonder if the two rings in the bottom of the image were loaded in semi-opposite directions.

Internet news and social media is probably not the best place to get my info from.

David Peterson wrote: tinyurl.com/mufybjx Still says "a carabiner", as far as I know, they haven't confirmed it was a locker or not, but the visual would suggest so.

Even though they don't offer pictures of the failed gear, the video from the incident command follow-up provides enough information. What they have pictured (in that follow up incident reporting) is a carabiner and not that piece of crap.

Fatigue is still an issue (it is probably the issue), and in which the circus realizes it must go through and replace gear. I assume they have no idea on history, so they probably are replacing everything.

From what I see, the equipment was adequately rated, and it was not improper equipment, or improper equipment use.

Gear replacement plans are important for hardware even if the gear doesn't take a big hit.

Buff Johnson wrote: Even though they don't offer pictures of the failed gear, the video from the incident command follow-up provides enough information. What they have pictured (in that follow up incident reporting) is a carabiner and not that piece of crap. Fatigue is still an issue (it is probably the issue), and in which the circus realizes it must go through and replace gear. I assume they have no idea on history, so they probably are replacing everything. From what I see, the equipment was adequately rated, and it was not improper equipment, or improper equipment use. Gear replacement plans are important for hardware even if the gear doesn't take a big hit.

How often should you replace a carabiner?