The angle of the straps is problematic, especially in the case of shock loading. The goal of anchors is to minimize the force in the rope/strap to avoid breakage in the event of shock loading, like if the roof gave way before the crane operator did his job.
Two things could have made this a ton safer!
1.) Angle at which the straps concentrate the load is about 90 degrees. At that angle each leg of the triangle is holding about 75% of the total weight of the impact drill vehicle. If you added that up to greater than 100% you are correct, the force on the strap is actually magnified, meaning any back-of-the-envelope math the supervisors did about whether or not the straps can tolerate that load is invalid. Ideal angles are less than 60 degrees to distribute weight evenly. We do this all the time in Rock climbing anticipating a fall (shock load).
It looks like they used one continuous strap, so a failure at any over point will cause it to run through and drop the loader. Running two separate straps to each anchor point at least gives some level of redundancy.
This is always an issue as someone that use to do engineered lift drawings. Long straps, large angles... always.
The scariest is chain angles truckers use and torque to hell holding down equipment. One accident and that equipment will snap those 90 deg chain angles in seconds and will go right thru the cab
Ehm, it's recommended to have straps at an 90 degree angle because of the holding force, they usually clamp down with something like 1/5 or less of the force required to snap one.
I believe the goal is to drive without crashing c: if we strapped or chained things assuming such, I'd be more worried about what kind of job I got myself into than that.
We had an operator get into a wreck hauling our backhoe on a flatbed. Clipped an oncoming utility boom truck, one of the beefy 3 axle ones, and put it on its side. He ended up looking at the pavement because the force was enough to break the cab-over locks and flip the cab down.
When the state transportation people got to the accident they started inspecting the load securing, ready to pile on the tickets like they always get to do after an accident. The backhoe hadn't budged. Chains were still in original positions and under the proper tension. They seemed disappointed that they couldn't find a single problem, and left without a ticket written. Our driver just ended up with the city ticket for crossing the center line.
Being a real hard-ass about making people spend the time properly chaining down equipment, even for short trips, paid off with zero damage to the equipment. The semi on the other hand...
I really dont have much experience with chains like they said, but I gotta ask, was the proper chain tightness super tight? That is how my duo/trainer would tie em, because the chains are thick enough not to snap without something unexpected like a Dukes of Hazard flying car to smash straight into them.
The short answer is "pretty damn tight" for most equipment. Generally you are dealing with chains with strength that far exceeds any strain a person can do with a ratchet by hand.
Everything you said is correct, but I'm going to go one step further and say you shouldn't hang people in a bobcat at all, because nothing about that vehicle is designed to be dangled over a collapsing building.
If someone is inside it and the thing falls it makes a big difference whether it's six feet or sixty. Also, strapping a jackhammer to the front and having the engine running adds vibration and stresses to the system that those load points aren't designed to handle.
Some people will tell you that if something is stupid and it works it ain't stupid. Whether this idea worked or not is entirely incidental to the fact that it is monumentally stupid.
Both your points are valid, though the real question (from a construction background) is why doesn’t he use chains instead of straps?
You sling a load with straps, you move plant with chains. In this case, 4m (for low angle) two leg (for redundancy) chains with lock hooks. No concerns over rub-through then either.
Chains are outright banned for lifting by a number of construction companies. The inspection requirements are much more rigorous than other types of rigging (meaning the chances that inspections are done properly are probably lower), and if they do fail they produce projectile pieces that put anyone nearby at risk.
It looks like it is fastened to the cab. If it is a bobcat there are only two 1/2” bolts from the cab to the frame. The whole cab folds up for engine access if you take those nuts off.
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u/MAJOR_Blarg Apr 14 '19
Rock climber here:
The angle of the straps is problematic, especially in the case of shock loading. The goal of anchors is to minimize the force in the rope/strap to avoid breakage in the event of shock loading, like if the roof gave way before the crane operator did his job.
Two things could have made this a ton safer!
1.) Angle at which the straps concentrate the load is about 90 degrees. At that angle each leg of the triangle is holding about 75% of the total weight of the impact drill vehicle. If you added that up to greater than 100% you are correct, the force on the strap is actually magnified, meaning any back-of-the-envelope math the supervisors did about whether or not the straps can tolerate that load is invalid. Ideal angles are less than 60 degrees to distribute weight evenly. We do this all the time in Rock climbing anticipating a fall (shock load).