Tree Rigging: Understanding Vector Forces and Bending Moments

[00:00:08]thank you hi it's Daniel Murphy and it's not too often you get to see an anchor Point failure it's a great piece of video I'm going to use it today to talk about Vector forces and bending moment as it relates to tree rigging there's a number of things that could have been done differently here this rigging setup was an accident waiting to happen with that overhead Anchor Point set so high in the tree and the limb used for the overhead Anchor Point had that bend to it and looks like a little twist in the grain these are clear signs of structural weakness the limb ended up fracturing right at this Branch Union here obviously if the block had been set lower in the tree it would have been a lot less likely to cause failure but beyond that let's go over what other changes in the rigging setup the climber could have made to prevent this failure before we do that let's just Define bending moment and Vector forces a moment of force which is often just called a moment is the tendency of a force to twist or rotate an object any Force which is twisting or rotating part of that tree would be considered a moment and its use here it is the root of the word momentum and a bending moment is simply when a force or moment is applied to an element causing the element to bend in this case the element is the tree the force of that rigging line causes that limb to bend we're obviously trying to reduce bending moment on the tree in order to prevent Anchor Point failure another way of looking at it is side loading and rigging we're always trying to manipulate the forces to reduce side loading if we load a tree with the force in line with the axis of the limb the strength of the limb we're going to reduce the bending moment on the tree spending moments also very similar to torque in that both are measured with foot pounds torque would be the force acting to twist an object like a bolt bending moment would be the force causing an arm to bend or in this case the tree to bend the more we can change the direction of that Force the direction of that red arrow to make it in line with the axis of the limb this more resistance that limb will have to failure a force has both magnitude and direction making it a vector quantity so a vector has both magnitude and direction when two separate forces are acting upon an object the resultant Vector shown here in the red arrow will be the sum of the two individual forces so when rigging trees we use redirects also known as multiple overhead anchor points to control the direction of the vectors in this case you can see the direction of the Red Arrows represent the vector forces coming off of those two blocks and they're very much in line with the axis of the limbs in real world rigging scenarios those blocks will line up perfectly with the direction of the vector forces so whether you're using bringing blocks or natural crotch rigging or using a shackle as a redirect using multiple overhead anchor points is going to dramatically increase the trees fracture strength because you're adjusting the direction of the vector forces to put them more in line with the axis of the limbs taking advantage of the woods strength at resistant compression well back to our original video there's only one limb up there so there's no multiple limbs for him to use for redirects how is he going to be able to control the vector forces here to put them more in line with the access of that limb that he's lowering from let's look at the vector Force as it changes throughout the course of this swing here interestingly enough you see the two lowering lines are coming off and that sling that the block is attached to is pointing directly in line with the resulting Vector at this Freeze Frame the piece is just starting to move we've highlighted the lowering line in blue the red arrow is representing the the resultant Vector of force on the tree the Green Line is now representing the axis of the limb and the angle between the red arrow the vector force and the axis of the limb there between that red arrow and the green line is going to significantly affect the bending moment that's put on this tree we're trying to keep that angle as small as possible the larger the angle gets it's the greater the bending moment is going to be next Freeze Frame the piece is almost out of you the angle between the the red arrow and the green line is getting larger and this freeze frame is just prior to failure angle is getting more and more extreme that's over 45 degrees right there so the climber has two potential strategies to reduce the bending moment and he should really use them both in this situation here we are back piece is just starting to move if he had set a redirect on the trunk of the tree at the bend the fall side of the lowering line is going to be kept pretty much in line with the axis of the tree that alone is going to make a big difference in the bending moment just look at the difference between these two vectors especially the further out the P starts traveling the difference between those two vectors becomes more and more significant and the second strategy the climber definitely should be using here so you should be controlling the swing of this piece as this piece starts to swing the top bends which then just continues to accelerate the momentum of the piece in the swing the only way to control that is going to be to use a butt line a second line tied off to the piece to control the swing and it could be set up very easily natural crotch just throwing a line over one of the Nubs above him tying it off letting a ground man hold it controlling the swing so that angle of the vector never gets much past this as that piece gets lowered down slowly a good bit of the weight will be taken up by the second line which is going to reduce the bending moment on the high rigging point the difference between these two angles set by the lowering line creating these two different vectors would most likely have made the difference between that top failing and not the greatest lesson to take away from this video is to understand the importance of Always setting up your rigging systems where the vectors are in line with the axis of the wood and don't just do it when you're rigging heavy pieces get in the habit of throwing all things to your advantage all the time when you're rigging trees so that when you need it when you are rigging heavy pieces it's like second nature let's take a look at a couple other rigging scenarios again this is natural crotch there's a redirect block down in the lower right hand corner of the screen the lowering device is on the back of the bucket truck so it's always good to have that lower block set as a redirect so we're not side loading the top of that tree it's not that much more difficult to set up the rating line through multi-crotch and here on this Oak not only does the multi-crotch help with the vector forces but it also puts the climber in a very good position or the bucket Opera in this case in a very good position to swing those pieces out and away from himself the right side of the tree can go to the right side of that line and the left side of the tree could be rigged out to the left side so one once the line is set the whole tree can be rigged out to either one side or the other there's a shock will talk to that second limb which we're running the lower line through this is keeping both pieces from swinging back into the bucket making it much safer for the operator here the shackle is an extremely versatile and useful tool easy to set up light easy to handle and comes in real Handy for setting up redirects you don't have to worry about the bend ratio being too tight on that small shackle when it's being used for a redirect because the angle of the Rope is still going to stay fairly slight here are two fairly large limbs I recently rigged from this silver maple tree because I was using redirects to control the vector forces I was able to rig from a very small natural crotch in the top of this tree which gave me a good rope angle in order to swing those limbs out the silver maple had a fairly wide canopy and I ran the lowering line from one side to the other this is taking the right side off and then we simply switched sides on the Rope we put a redirect block lower on the right lead to keep from side loading that because the lowering device was attached to the back of the bucket truck again so here it is drawn out showing vectors we're able to get that large heavy Branch to swing quite a ways to clear that cherry tree and you can see the ground man in the lower left hand corner of the picture here both he and the fall side of the lowering line are well out of the way This Is A View From the bucket truck of the natural crotch we were using to rig those large pieces and another for those of you not familiar with silver maple it would be considered very much on the weak side of the spectrum very brittle wood yet with a good rigging system multiple overhead crotch is a nice redirects it's plenty strong to get this limb down safely here I took a natural half wrap around the trunk of this tulip tree before negative rigging that piece of wood there in order to again change the vector force of the Fall side of the lowering line there to make it more in line with the spine of the tree here the lowering line is running around a little nub at the base of the tree in order to keep it in line with the axis of the wood there before going to a lowering device on the bucket truck so thanks very much for watching I hope this helps you improve your rigging systems let me know if it did and come back and take a look at our other videos there's a lot of great videos here about falling rigging and pruning foreign