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Rock climbing in the Southeastern USA |
FAQ: Climbing Physics - Climbing Forces - Anchor ForcesIt will be assumed the climber can place solid protection be it chocks, nuts, hexes, tri-cams, or camming devices. It will also be assumed the climber knows to evaluate any bolts or fixed anchors used. What will be explored in this section are the forces generated within an anchor relative to the locations of the anchor points and the angles of force between them.
The diagram shows the loads on each bolt with a 4.5 kN (1000 lb.) force applied. At low angles, 20 degrees or less, the bolts split the load. As the angle between the slings grows, the forces grow rapidly. If the bolts are spaced 2 feet apart, and we build our anchor using 2 foot slings, the angle is about 60 degrees. This raises the force on each bolt to 666 lbs. It should be clear the distance between the anchors is an important factor. Consider the case of building an anchor using either two 2 foot slings, or a single 4 foot sling doubled between the anchors. The angle created by the slings varies from 30 degrees if the anchors are placed 1 foot apart, to about 45 degrees if the anchors are 18 inches apart, and 60 degrees if the anchors are two feet apart. If the anchors are 3 feet apart, the angle approaches 100 degrees.
A good way to determine if your slings are long enough to keep forces small is to compare them to the distance between the anchors. As a rule, the slings should be longer than the distance between the anchors, preferably twice as long. For example if the anchors are 1 foot apart, 2 foot slings are adequate. If the anchors are 2 feet apart, four foot slings would be more appropriate. While 2 good bolts may often be used as a anchor, the situation changes when the anchor is built with gear placed by the climber. Whereas bolts are rated to hold loads starting at 4500 lbs. (20 kN), typical climbing gear is rated for loads roughly half that of bolts. In this case, at least three solid pieces of protection should be placed, more if there is any question of the quality of any single piece of gear, the rock is suspect, or if small gear is used. The same concerns of angles need to be considered with multiple anchors. With gear anchors instead of bolts, it is even more important to pay attention to the loads on each piece related to their location and sling length. Place the anchors close together when possible, and extend with slings or a cordalette as needed to insure the load is minimized and shared equally. Some good recommendations and photographs of equalizing multiple anchors can be found at the Guiding Newsletter - November 2000 - Anchor Equalization Tech Tip page. Rope Systems Analysis (a 13 page DETAILED discussion, complete with physics): http://www.amrg.org/Rope_system_analysis_Attaway.pdf The Physics of Climbing (for the truly mathematically minded, A very technical explanation of how physics applies to rock climbing) http://student.kuleuven.be/~m9916724/physics/physics.htm Loads, Energy & Ropes (the discussion is about caving, but the principles are the same): http://www.bstorage.com/speleo/Pubs/rlenergy/Default.htm
Guiding
Newsletter - November 2000 - Anchor Equalization Tech Tip |
One
of the most common situations climbers encounter are fixed belay
anchors, typically bolts. It is customary to clip into each of the
bolts with a carabiner and sling, then join the slings with a carabiner
at the point of attachment. The climber / belayer hang from a sling
or slings attached to the bolts, attaching to the carabiner at the
midpoint. As the diagram illustrates, this forms a triangle. If
the bolts are at the same height, and the slings are the same length,
the load is shared equally between them.
The
angle formed between the slings affects the force placed on the
individual anchors. As the chart indicates, the greater the angle
between the slings, the greater the load placed on the bolts. At
angles great than 120 degrees, the forces on the bolts can be greater
than the forces placed on the load carabiner. The diagram indicates
we should strive to keep the angle as narrow as possible to reduce
the load on the anchors.