Fall Forces and Fall factor
0 views
Skip to first unread message
benjamin desouza
May 9, 2005, 4:04:36 AM5/9/05
to npte...@googlegroups.com
Fall Forces - The Harder you Fall
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
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
0 new messages