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Fall Forces - The Harder you Fall  
Fall Forces - The Harder you Fall
by Duane Raleigh
When you place a bolt you need to make sure it will hold the hardest fall, and will continue to hold falls for years to come. Because of the many variables associated with falls you cannot accurately predict their forces. But based on extensive calculations by Daniel Taupin and Jean Pierre Verdier in the French treatise Amenagement et Equipment d'un Site Naturel d'Escalade, the loads in average "soft" to "hard" falls range from 1300 to 2600 pounds of force.

Toprope falls develop lower impact forces, but can still load the anchors as much as mild leader falls. In live drop tests using a dynamometer to measure peak loads, we generated over 1000 pounds in top-rope falls when there was slack in the system or the wall was so steep the climber swung out when he fell. Because the failure of a top-rope anchor would likely lead to a serious or fatal accident, make sure top-rope anchors are as strong as those you would set for a belay.

Using their calculations and figuring in a safety factor, Taupin and Verdier recommend that belays consist of two bolts, each holding a minimum of 5280 pounds in all directions. They further conclude that first bolts directly above belays or ledges should also hold 5280 pounds, and all other protection bolts must withstand at least 4840 pounds.

The force you put on a bolt depends on many factors including how static or dynamic a belay you have and the distance you fall. But it isn't so much how far you fall, it's how much rope you have out that determines how hard you load an anchor. A short fall on a short length of rope can produce a higher load than a longer fall on a greater length of rope. That is because climbing ropes absorb energy, so the more rope you have out, the more rope you have to cushion the impact.

The relationship of fall distance to rope out is the Fall Factor, which you determine by dividing the length of the fall by the amount of rope between you and your belayer. High Fall Factors equal high impact loads; low Fall Factors equal low impact loads. For example, you haven't been training but go climbing anyway. As anyone who saw you chowing at McDonalds could have predicted, you flame out immediately and fall short of the first bolt only 8.25 feet up the climb, which starts off a small ledge about 30 feet up. Your 176pound carcass drops 16.5 feet and jolts directly onto the belay anchor, producing a Fall Factor of two (neglecting rope stretch, 16.5 feet of air divided by 8.25 feet of rope equals two), the highest possible and the one the UIAA uses to break ropes.

Disgusted with your meager performance and grateful to be alive after nearly wrenching the belay bolts out of the wall, you abuse yourself on a fingerboard all winter and return to the climb in the spring. This time you make it 20 feet past a bolt that is 85 feet up before your home resoles delaminate, sending you screaming 40 feet onto bolt. You take a good whipper, but your Fall Factor is only .38, and you generate a lower force than the one in your shorter fall.

Short falls close to the belay are the harshest because you never have a lot of rope out. Because of this it's vital that the first two or three bolts above the ground or a belay stance are absolutely bomber and spaced relatively closely compared to the anchors on the rest of the route.

You can minimize impact forces by choosing your rope carefully. Climbing ropes absorb energy, but their capacity for assimilation isn't infinite. Every time you fall you destroy part of you rope's ability to dissipate energy, so every fall (assuming all other factor remain equal) yields higher and higher impact loads. Consequently, old ropes give harder catches than new ones.

Your belayer can increase or decrease the impact force, too. If you whip and your belayer reels in slack to keep you from hitting the deck, he then increases the impact force. Conversely, if your belayer waits until he feels your weight coming on the rope and then jumps up, the impact force decreases (use this technique only when the leader isn't in any danger of smacking the ground or a ledge).

The way you load a bolt affects its strength. If you pull on a bolt straight down, as you would with one placed on a vertical wall, you load it in shear, which is typically its strongest position. In a horizontal ceiling you pull out on the bolt, loading it in its weaker tensile mode

When you fall or hang you often pull out and then down on a bolt, or may even load it in both directions simultaneously, which is similar to pulling straight out. Since you never know how you are going to load a bolt, you need to cover all bases by using a bolt that even at its weakest can hold the hardest falls.

Reprinted from Climbing Magazine Oct./Nov. 1992