Does anyone have test result information regarding the load forces applied to an anchor when the rope goes over say an edge of say a parapet wall while lowering a 1 person load? Also what would be the load on the parapet wall? Any angles that have been tested and proven would be appriciated.
GOD Bless FDNY and ALL of the Lost Brother's and thier families.
FTM, PTB, RFB
As far as I know, the forces applied to the anchors would be dependent on the angle between the two points. The parapet wall would not be a factor. Also, the load on the wall would be the result of the load on the end of the rope. This would probably be somewhat equal to the full weight of the load.
From the different trig classes I have taken in college (several years ago), I know the force applied to the anchor would be decided by your angles. The greater the angle between the two anchors, the greater the force applied to each one. This is why we keep our angle below 120 degrees. Ideally, the angle should be less than 90 degrees between the anchors.
The parapet wall would only be a factor based on the strength of the construction of the wall. If the wall fails during the load, then the system is compromised. Otherwise, figure the wall is holding the weight of the load. An edge roller would be used over the wall to reduce abrasion on the ropes, but it shouldn't change the load.
Forces & Angles
Loads change wherever elements in the system join together and/or change direction (like your edge).
Every time your main load bearing rope(s) contact any surface between the anchor and the load there will be a shift of part of the total load away from the anchor. Running the load over a high friction edge could theoretically halve the anchor load (a "perfect" edge would allow the anchor to see the full load). Anchor strength could then be a secondary concern to rope failure (due to very small contact points). Rollers and similar items will reduce the friction and even out the load to some degree.
Look in the books by CMC, On Rope, Tim Setnika's book on rescue and other sources.
If I remember right, forces on directionals are calculated as being = 2 x cos(angle/2). When you look at two legs of a simple anchor, generally speaking, force increases with angle to the point where each anchor (assuming two legs) sees 2x the main load.
It is very useful to test this for yourself - a visual demonstration is always good. Using simple spring scales you can see what the forces are at various angles. Remember these concepts hold true for STATIC systems. Once you introduce motion (up /down, whatever), the system becomes dynamic and the math gets very complex, very quickly. Try it on your simple models - you see spikes and other interesting things - VERY instructional.
I would encourage you to forget about memorizing angles, etc. and become comfortable with the underlying physics. You will never be stumped that way.