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Thread: Question About Anchor

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    Question Question About Anchor

    Hello Guys,

    I've been a firefighter for almost ten years, but i must say i'm a complete newbie in rope rescue. I live in a small town, with no high buildings, mountains or big slopes, so rope rescue interventions are not usual.

    But in any case i like to stay updated and learn about this techniques, so i'm taking a vertical operations course, and talking about anchors and anchoring systems, one guy asked about the use of our emergency trucks as anchor points, for example for rappelling..

    We know we can use the wheels for setup anchors, creating negative anchors (tie low?), but the question was:

    What about changing the direction of the anchor system, to create a positive anchor, using the whole structure of the truck, like in the picture attached.. Is it possible?

    Name:  AnchorTruck.jpg
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    I mean, setting up the anchor in one side of the truck and then redirect the rope by the same side, going up, and then crossing by the roof to the other side...

    The answer was: "it might be....."

    What do you guys think...? is it possible, ??

    Maybe adding some pads in the contact points... i don't know..

    Thanks in advance..

    Mark

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    Have done something similar once or twice...should work...just pad where padding in needed.

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    Thanks for the feedback..

    Mark

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    I would be wary of this as you are essentially anchoring to the top of the vehicle, and providing a fairly long lever arm to roll the truck over. I know we get away with things a lot because our forces rarely approach "worst case" magnitudes, but it is something we need to consider.

    How much force would it take to roll a piece of apparatus over? Would a 6000lb pull at the top edge do it? Because worst case that's the kind of force you could be experiencing...

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    Quote Originally Posted by snoboy View Post
    I would be wary of this as you are essentially anchoring to the top of the vehicle, and providing a fairly long lever arm to roll the truck over. I know we get away with things a lot because our forces rarely approach "worst case" magnitudes, but it is something we need to consider.

    How much force would it take to roll a piece of apparatus over? Would a 6000lb pull at the top edge do it? Because worst case that's the kind of force you could be experiencing...
    How are you able to put fire truck tipping force on a rope and not break the rescuer on the other end of it first?

    I think tipping over a fire truck with a rope system is not something I would consider a realistic danger at all. Worst case scenario is usually (right or wrong) in the BCCTR BCDT. One meter fall on 3m or rope... That doesn't even come close to the arbitrary 6000lb number.

    If the rope is a lever arm, where's the fulcrum? I would say it is at the top of the truck. Which makes it a a Class 1 lever hugely in favor of the fire truck.
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    I went too quickly through my in the head calculations...The theoretical Maximum arrest force (with a 2kN load) discussed in the BCDTM is 15 kN, which is 3400 lbf, not 4500lbf. I used 20kN forgetting that was breaking strength not MAF...

    I rounded up to 6000lbf as the fire service usually works with a larger "rescue load" of 2.7kN - aproximately 1/3 more. That should have been 4500lbf as a final answer. Still a pretty big load.

    So you see, 6000 was just wrong, not arbitrary. Thanks for making me rethink.

    The rope is not the lever, the truck itself is the lever. The fulcrum is at the wheels on the side of the vehicle facing the load. Class 2 lever as I see it.

    I'm not saying it's possible, but I am saying it's something that needs careful consideration, not just a "Oh it should be fine, we did it once," response...

    Anybody have a firetruck and a 3 Ton hoist??? Can you go try something for me?
    Last edited by snoboy; 01-20-2014 at 10:44 AM.

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    Quote Originally Posted by snoboy View Post
    I went too quickly through my in the head calculations...The theoretical Maximum arrest force (with a 2kN load) discussed in the BCDTM is 15 kN, which is 3400 lbf, not 4500lbf. I used 20kN forgetting that was breaking strength not MAF...

    I rounded up to 6000lbf as the fire service usually works with a larger "rescue load" of 2.7kN - aproximately 1/3 more. That should have been 4500lbf as a final answer. Still a pretty big load.

    So you see, 6000 was just wrong, not arbitrary. Thanks for making me rethink.

    The rope is not the lever, the truck itself is the lever. The fulcrum is at the wheels on the side of the vehicle facing the load. Class 2 lever as I see it.

    I'm not saying it's possible, but I am saying it's something that needs careful consideration, not just a "Oh it should be fine, we did it once," response...

    Anybody have a firetruck and a 3 Ton hoist??? Can you go try something for me?
    I still think that worrying about a fire truck tipping over at 6000lbs is arbitrary, no matter how you add up the forces. If, as you say, it is not possible, why would you even consider a firetruck flipping over a realistic thing to worry about?

    I do think that analyzing every part of your system is necessary though. So maybe even inserting the BCDT into this is too extreme. You're obviously going to have more than 3m of rope in service and it's coming from a high point; both of which turn things in our favor quite a bit.

    As for the lever argument, for the sake of academics, I'll still probably go with a Class 1. Even with the tires as a fulcrum. If the firetruck is the load, the tires the fulcrum, then the resultant force of the rope changing direction at the top of the box is somewhere on the opposite side of the fulcrum than the load.

    But to be clear, I think the possibility of flipping the firetruck is not something I would waste one second worrying about on scene.
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    Let's be honest here, if you are worried about "tipping" a piece of apparatus with this extremely basic evolution you have no business performing and type of technical rescue. Evaluate the system, consider your loads and critical points, but this shouldn't even be a discussion here. Spend your time on more important often overlooked things like edge pro, critical angles, over engineering, and over redundant systems.
    Rockbreaker likes this.
    John D. Calamia, BS, NREMTP, FP-C
    Firefighter/Flight Paramedic
    Broomall, PA

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    Since I'm the one who just assumed that using a rescue in such a manner is OK, I've attached a calculation in an attempt to redeem myself a little bit.

    I assumed somewhat realistic numbers for a heavy rescue unit. They're not spot on, but are good enough for illustration.

    The result indicates that the "stabilizing moment" due to the weight of the vehicle is over three times the "tipping moment" due to a worst case dynamic loading. Or, the "dynamic tipping safety factor" is greater than 3.

    That is, the vehicle is capable of withstanding a tipping moment that's over three times larger than the maximum possible worst case dynamic tipping moment we can apply to it. That's pretty good.

    Regardless, I made an assumption that I shouldn't have without closer examination.
    Attached Images Attached Images  
    Last edited by servantleader; 01-21-2014 at 04:19 PM.

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    Quote Originally Posted by Rescue 2 Training View Post
    IIf, as you say, it is not possible, why would you even consider a firetruck flipping over a realistic thing to worry about?
    That's actually not what I said. My point is more that a realistic analysis of a system is important, and that real hard numbers arrived at from theory or practical testing are a lot more valuable than, "Should be good to go."

    I do think that analyzing every part of your system is necessary though. So maybe even inserting the BCDT into this is too extreme. You're obviously going to have more than 3m of rope in service and it's coming from a high point; both of which turn things in our favor quite a bit.
    Agreed. Until someone decides to use a nice long, static sling instead of the rope to extend the anchor. Worst Case Scenario and all...

    I never meant to imply that I thought it was a likely scenario. I was simply pointing out a need to think it through. I also think that the Fire Apparatus is a "best case scenario" and would be realistically concerned if someone extrapolated that it would be OK to do this with 3/4 Ton pickup or the like.

    The whole idea that it worked once so it must be safe is a big red flag to me and I was asking questions to provoke conversation which I have achieved. Analysis like servantleader's response is the kind of answer we should be striving to base our practice on. If we use my revised assumption of 4500 lbf then his analysis looks even better!

    As an aside, this would be totally against our SOP for S&R in BC. But we generally don't have 40,000 lb firetrucks to tie off to, so there is that...

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    Quote Originally Posted by snoboy View Post

    I'm not saying it's possible, but I am saying it's something that needs careful consideration, not just a "Oh it should be fine, we did it once," response...
    If, as you say, it is not possible, why would you even consider a firetruck flipping over a realistic thing to worry about?

    Quote Originally Posted by snoboy View Post

    That's actually not what I said. My point is more that a realistic analysis of a system is important, and that real hard numbers arrived at from theory or practical testing are a lot more valuable than, "Should be good to go."
    Okay.......
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    Thanks a lot guys, this is a Great feedback!!!!!!!!!!!

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    Quote Originally Posted by Markmol424 View Post
    Thanks a lot guys, this is a Great feedback!!!!!!!!!!!
    Did your head explode?

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