anchor point for fall arrest device

huangyhg

anchor point for fall arrest device
i wonder if anyone can help with my problem.
we are installing a fall arrest device for maintenance personnel within the plant.
the specs on the device are as follows:
drop forged carabiner - tensile strength 5000lbs.
drop forged harness snap hook - tensile strength 5000lbs.
capacity - one person - 310lbs.
my question is this. why would the literature say that the supporting structure to which the device is attached, be capable of supporting a load of 5000lbs.
the wearer of the harness attached to the device would free fall approx. 3 ft. when the fall is arrested.
does this fall under the same premise that given the rated capacity of a hoisting device, the support structure must be capable of sustaining that load. i can see this in the case of a hoist, but a fall arrest device?
the device is going to be hung mid span from a clevis welded to a member spanning 9'-0", clamped both ends to building joists. now in choosing a member, and keeping the bending stress at around 14.5 ksi, i come up with say a c8 x 11.5 channel.
just seems awfully big for what it has to do. any help greatly appreciated.
thanks in advance.  
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philosophically, you don't want the beam (and/or connecting point) to be the weak point in the safety system.  and while it does seem to be heavily overdesigned,  what standard would you expect if you were the 310 pound guy taking that 3 foot plunge?  and what if the plunge were say, 6 feet instead of 3?
on the other hand, you are talking about a member that could conceivably be a "use once and replace" unit rather than designed for repeated loads.  (assuming, of course, that the plant manager will accept the cost implications and the clamping system can hold the beam when the beam yields i.e. the beam deflects but does not suffer a catastrophic failure.)  this would allow the use of a lighter   
since tensile strength is the force needed for the device to fail why not use a bending stress of 36 ksi (for a36 steel) instead of 14.5 ksi to compute the size of the steel member? then you are making a direct comparison between the failure strength of the device and the failure (yield) strength of the steel   
haggis,
fall arrest devices are under the jurisdiction of osha, check here for requirements:
thanks to all for the replies.
however, a question for slideruleera. shouldn't the stresses for bending in a36 be limited to 0.6*36000 ?
erv,
you provided a great link. i found something that pretty well determines the   
haggis,
glad i was of help. this is what this forum is all about.
erv
haggis - the yield strength (36 ksi for a36 steel) is the value where the steel would be permanently deformed by an applied force.  the tensile strength, where the steel actually fails is quite higher. using 60% (or 67% depending on the member shape) of the yield strength is normally the limit for routine applications.
however your product's information gives the tensile strength for the items (5000 lb). this is the value where they could be expected to fail (break). since you do not have the manufacturer's safe loading on these parts to compare with the steel member's safe loading (60% of 36ksi), i am suggesting that you compare the failure load of harness parts (5000 lb) to one measure of the failure load of the steel   
slideruleera
point taken. i arrived at a much smaller member size (c4 x 5.4) using the 36ksi figure (or close to it) using readily available shapes or shapes we have on hand.
under the circumstances though, the loads that would result in failure of the member would never be experienced and talking with the millwright who would be using the device, i find that he would have no hesitation hooking on to a c4 channel.  this however is beside the point as the ohsa regulations are clear.
it does raise an interesting point though. does the ohsa differentiate between engineered anchor points and convenient anchor points in the existing building structure. in this particular case, the   
not that i disagree with the above discussion, but i just wanted to put out my experience with fall protection.  i spent a few years working out at cape canaveral air force station doing among other things, installing and certifying existing fall protection anchor points on the rocket launch pads and assembly facilites.  some areas i worked in were 250 ft clear height drop inside rocket assembly buildings.  now worker safety is paramount out there, rightly so.  the first 6 months i worked out there, 3 workers were killed in accidents, two relating to falling.   in my time out there, i probably installed about 75 points, certified maybe 100 more, and installed several perimeter cable systems.
the requirements we used were based on osha and some other cfr requirements.  basically, if the fall protection anchor point was on a structural component of the facility, then the component must be within allowable design stresses under the imposed falling load, in addition to service loads expected to be on the structure.  the imposed load for the design was 5000 lb per person.  this is even though the gear either had break away stitching or inertia reels which physically limit the impact force to around 500 lb or less.  but the code requires the 5000 lbs, regardless.  this is in case some jambony hooks up his belt to a 6 ft piece of wire rope, without an inertia reel or other approved device.
there were many cases where a beam brace was intended, but because it was unbraced for a long length, the allowable stresses would be exceed by a fall.  it was not uncommon to be looking at the 50 lb/ft range as a minimum size for any beam span over 20 ft, if unbraced.  
not being familiar with fall protection devices, i have a question.
if the capability of the harness and carabiner is 5000#, is that the ultimate tensile capability of the equipment, or a working limit?  if it is ultimate, should the structure you are tieing into be analyzed on an ultimate and not limit basis?
i realize that it all depends on how throwaway you want the structure to be.  what's important should be that your structure is not the weak link.
looking forward to the discussion.
jetmaker
jetmaker
yes, the 5000lbs is the ultimate strength of the fall arrester components and i agree that the support structure should not be the weak link.
now, you may have noticed that i may be a little conservative on the bolt torquing issue, but i'm not like that on everything.
although i don't want the supporting member to be throw away, by designing it on an ultimate strenght basis, i could have used a much smaller member than the c8 x 11.5 channel.
however, thanks to the link that erv provided, i found the following as mentioned in a previous post:
the anchorage should be rigid, and should not have a deflection greater than .04 inches (1 mm) when a force of 2,250 pounds (10 kn) is applied.
this is an ohsa regulation and i won't argue with this.
in some cases, in the course of my work, i will take into account probability and likelyhood.
first of all, the fall arrest gear will never see a force of 5000lbs, secondly, everytime the guy goes up to change a gearbox, he isn't going to fall so the repetition of stress on the member is negligent.
therefore, theoretically, i could have made the member much smaller without fear of catastrophic failure or having to replace it in the event of a fall situation.
two things govern the decision...the ohsa ruling and the cost. considering we are talking about 9 feet of channel, cost is not a factor so we go with what is mandated and use the bigger member. although, no matter how many we were installing, in this case, cost would have to take a back seat.
to be honest, if it wasn't for the ohsa mandate, i would have used a smaller