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Fall Protection

Fall Protection

Fall Protection

(OP)
In this thread, thread190-97369: fall arrest system anchor point , they state the 5000 is an ultimate load. When checking bending with this, we should use the tensile capacity and not the yield?

(Fy=50ksi, Fu=65ksi)

Is that correct?

RC
All that is necessary for the triumph of evil is that good men do nothing.
    Edmund Burke

 

RE: Fall Protection

To save 30% on steel, yes.

RE: Fall Protection

(OP)
haha, thanks.

RC
All that is necessary for the triumph of evil is that good men do nothing.
    Edmund Burke

 

RE: Fall Protection

We use an "engineered" fall arrest system that only puts about 900 lbs into the fall arrest equipment. Tested many times.   OSHA also says that if you design for two times this load - it is OK.

We use a 6' lanyard with shock absorber, full body harness and approved D-ring or hook.

We have many "saves" over the last few years.

RE: Fall Protection

Mike, I'm familiar with the 900# load requirement as well, due to the energy absorber.  However, I would apply this as a live load to the beam, and check it and the supporting connections accordingly, be it ASD or LRFD.  Can you elaborate on your "two times this load" comment?

RE: Fall Protection

(OP)
Nutte,
Is the 900# load the service/stress load case and the 5000# ultimate tensile strength load case?

If I am given a 1000# service and 4000# ultimate for a davit assembly, do I use the 1000# as a live load/impact load and design as normal?

This all seems to be so arbitrary.

RC
All that is necessary for the triumph of evil is that good men do nothing.
    Edmund Burke

 

RE: Fall Protection

If the lanyard limts the maximum arresting force(MAF) to 1800 lbs with a harness (as per OSHA 1926.502(d)(16)(ii)) then you design for the 1800 lbs and apply a FS of two (per 1926.502(d)(15)(i)).  The 5000 lbs is not an 'ultimate' load, but rather a general case for which the ppe is not known.  All of the lanyards I deal with have a MAF of 900 lbs, so I design all my anchor points for 1800 lbs.  Sometimes I add a % of that as a lateral load.  I then add it to the live load as normal.

Sounds like for your case you simply need to design for an 1800 lbs live load. .66Fy works for me.  Or whatever it is in the new code.    

Read the OSHA requiremnts at www.osha.gov.  You'll want standard 1926.502.

RE: Fall Protection

In my mind, the 5000# is an arbitrary load, to be used if the system is not engineered.

If the system is engineered, I use 900# as a service live load and check accordingly.  I'll have to look into the "Factor of Safety=2" thing.  My initial thought is this would be the factor of safety built into the steel specification, and should not be applied to the live load, with that total being applied to the beam.

 

RE: Fall Protection

OK, digging into the OSHA requirements:

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=10758&p_table=STANDARDS

Quote:

1926.502(d)(8)

Horizontal lifelines shall be designed, installed, and used, under the supervision of a qualified person, as part of a complete personal fall arrest system, which maintains a safety factor of at least two.
This doesn't tell me to double the load, then do your design.  I just need a FS of 2 against whatever I'm checking.  AISC uses a FS of 2 against fracture, so that should be it.


Quote:

1926.502(d)(9)

Lanyards and vertical lifelines shall have a minimum breaking strength of 5,000 pounds (22.2 kN).
OK.  This wouldn't affect the 900# applied from the energy absorber.  It just means that regardless of the energy absorber, you need a certain breaking strength for your lanyard and vertical lifeline.


Quote:

1926.502(d)(16)

Personal fall arrest systems, when stopping a fall, shall:

1926.502(d)(16)(i)

limit maximum arresting force on an employee to 900 pounds (4 kN) when used with a body belt;

1926.502(d)(16)(ii)

limit maximum arresting force on an employee to 1,800 pounds (8 kN) when used with a body harness;
Hmm, there's that pesky 1800 pounds.

 

RE: Fall Protection

OSHA says 2:1.  Body belts have not been allowed for years - too dangerous.  Even with a body harness - you should have a rescue plan to get to the victim within 15 minutes - 30 maximum or use some of the newer harnesses which have a drop down foot holder that allows the user to "stand" and take weight off the harness.

We design to 1800 lbs. +/- and have the testing to prove it.  We actaully took 250 lb "dummies" and threw them over the side.  Worked great - fall arrest system that is.  At 1800 lbs - it is my opinion that OSHA put in a 2:1 safety factor - but we design steel to say .66 or .6 which gives you another safety factor.

Still doesn't mean you won't get hurt - cuts, abrasions, concussion - because you will probably hit something on your way down - even at 6'length of lanyard.  But at least you will go home that night..
and clean out your pants....

RE: Fall Protection

True - but the 1800 lb limit is the "max" that the person should "receive" from the harness, not what the harness (hook) might "take" from the person.

So, the test of the 250 lb person dropping 6 foot = x 2 (as safety factor) would tell you want load the hook (the tie-down point and tie-down structure) would need to be designed for.    

I agree, their 5000 lb requirement is only applicable if you do NOT do an engineering (qualified person) review.  

RE: Fall Protection

Actaully I might tend to disagree.  We put the sensor in the lanyard near the hook.  All it ever "saw" was about 900 lbs.

RE: Fall Protection

(OP)
Probel systems would be categorized at engineered systems correct?
But the loads we are getting are 1250# service, 5000# ultimate. Also see comment above about the Davit assembly system.

How would you do use the provided loads?
 

RC
All that is necessary for the triumph of evil is that good men do nothing.
    Edmund Burke

 

RE: Fall Protection

From: MiketheEngineer (Structural) 12 Mar 09 14:08  
Actually I might tend to disagree.  We put the sensor in the lanyard near the hook.  All it ever "saw" was about 900 lbs.

---

Makes sense: 250 lb person dropping 6 ft with a "stretchable" lanyard in a harness would see 3.6 g's real load = the actual stopping force.  (900lbs load)

Double that, and design the restraint point able to handle 1800 lbs.    Probably how the "rules" were written to limit the force (from the restraint on the body) to 1800 lbs.
 

RE: Fall Protection

This is a tricky subject and I am glad to see it discussed here.  OSHA says that the Safety Factor must be at least two, so if you double the expected load, you could justify the use of full yield or full ultimate stress in your design.

I would also like to point out that OSHA requires a 5000lb breaking strength on VERTICAL lifelines.  The required breaking strength on a HORIZONTAL lifeline may be significantly more and as such is not specified by OSHA.

To make matters more confusing, at least to me, is the OSHA requirement that the ANCHORAGE POINT support at least 5,000lb.  This is consistent with the requirement on a VERTICAL lifeline, but not necessarily for a HORIZONTAL one.

 

RE: Fall Protection

I read somewhere the 5000# requirement is for the end ancharge point, not the lanyard and harness. If a horizontal lifeline is used, the lifeline (beam or wire rope) end anchorage points should be capable of resisting the resultant force of 5000#.

RE: Fall Protection

But look up above at the actual OSHA spec, not the "short training handout" answer: that 5000 lbs is a "throwaway" or default-if-nothing-else-is-known-number; You use it only if a 'competent person" has not analyzed the siutation and determined a more accurate number is correct === FOR THAT PARTICULAR COMPANY AND CONDITION.

So: if you need to loop your restraint line around a pipe or a steel column, and that column can withstand 5000 lbs, it is OK - "by the book" - But seriously, how many pipes in the plant are labelled "This piece of steel has been tested and it can hold 5000 vertical load"?   

Only a crane rail.    

RE: Fall Protection

I agree that the 5000# is a fool-proof number, however, if you impose a vertical load of 900# on a horizontal beam, or a wire rope, because the small deflection angle in between the beam and the horizontal plane, the resultant force can be several times of that vertical load - 900#.

I have some reservation on "competent person", other than structural guys, who is competent to tell the strength of the pipes/columns that the lifeline is anchoraged to.

RE: Fall Protection

And the 5000# is per person!

RE: Fall Protection

Correction: The resultant force argument is seemly only valid for horizontal lifeline using wire rope, not beams, or pipes.  

RE: Fall Protection

I agree that the 5000# is a fool-proof number, however, if you impose a vertical load of 900# on a horizontal beam, or a wire rope, because the small deflection angle in between the beam and the horizontal plane, the resultant force can be several times of that vertical load - 900#.

Correction: The resultant force argument is seemly only valid for horizontal lifeline using wire rope, not beams, or pipes.

---

Your second comment is correct:  The "load" (the person at the other end of the lanyard) is falling - by definition, so his lanyard MUST be vertical (or nearly so) then it would have to be wrapped over a rail, or steel, or pipe or something that might turn the vertical load into a side load of nearly equal amount.  

The "worst case" would be a perfect inertial-less pully with no resistance: Any friction on the lanyard as the lanyard pulls down reduces the "final sideways" load on the lanyard hook.   

Comes back to the [test results case] of a 900 lb load (3+ g's) on a 250 lb person's harness x 2 (factor of safety) = 1800 lb load on the hook.   

RE: Fall Protection

kslee1000 : Your very accurate concern about the increase in tension in a horizontal wire rope being suddenly pulled from below - tension in the wire rope that the lanyard is tied to - is why they also prohibit lanyards from being tied to "life lines" or "handrail lines".   

You have to go back to a real support that can withstand the jerk

RE: Fall Protection

racookpe1978:

The wire rope is used in many industrial settings that have overhead cranes and runways. as long as I can remember, the suppliers claim the rope can sustain 10000# of ultimate load in tension, as well as the end fittings. Don't you think the near vertical pull of 900#/1800# will produce a tension in the rope several times higher than the applied load?

Also, I am not aware that lanyard is prohibit to hook to wire rope through the hooking device that comes with it. Can you point out where I can find info on this "prohibition"? Thanks ahead. (Instead, I am aware that the lanyard shouldn't wrap around a beam, or pipe, without using beam wrap with a D-Ring)  

RE: Fall Protection

Quote:

agree that the 5000# is a fool-proof number, however, if you impose a vertical load of 900# on a horizontal beam, or a wire rope, because the small deflection angle in between the beam and the horizontal plane, the resultant force can be several times of that vertical load - 900#.
Exactly right.  That is why I asked a couple days ago about the stretch of wire rope.  Apply your 900#/1800# load to the center of your line, and with trail and error, determine the elongation and cable tension that gives you the vertical reaction you're looking for.  For the ones I've checked, the cable tension is in the 5000# - 6000# range.  So the anchors and everything else down to the support should be checked for a service live load corresponding to the cable tension.

RE: Fall Protection

There are specific requirements that state that the system as a whole must maintain a FS = 2.0.  For the testing method provided in the appendix that are deemed to meet the OSHA requirements, they state that either the lanyard shall be provided with the system, or the stiffest lanyard available shall be used.  I think this is why we have the 1800#/900# confusion.  

OSHA limits the maximum force on a body from a body harness to 1800#.  In theory, the stiffest lanyard would allow this load.  However, most lanyards in use will limit the impact to 900# because most employees don't like being hit with 1800#.  If the lanyard is not provided with the system, I think you should design for the 1800# as a service load - applying the 2.0 FS to that.  If a 900# lanyard is supplied with the lifeline system, apply the 2.0 FS to 900#.

RE: Fall Protection

And you should ALWAYS use a shock absorbing lanyard!!!  Some well known companies now make a horizontal lifeline with a shock absorbing lanyard built in that limits the horizontal force to less than 2000 lbs for TWO people!!!

Well tested....

RE: Fall Protection

The lanyard and the harness are made per OSHA requirement, and verified by testing. The more critical issues are the strength of lifelines and end anchorages. Again, the suppliers claim the lifelines and the anchorages they furnish are both good for 10,000# = 2x5000# (OSHA). The most critical issue remains to be the structures that the lifelines attached to. Note that the 5000# is usually located 5'-6' above the platform, and the load is close to horizontal. Can your building columns, or pipes, survive during the pull, or prevent fall more than the max. fall distance specified by OSHA (6'). Never been easy to design a good horiz. lifeline system.  

RE: Fall Protection

There are other issues as well... like the workers that did not tie off...

RE: Fall Protection

The way to deal with the load being close to horizontal is to install a predetermined amount of "sag" in the line when it is installed.  Obviously the angle will still be low (about 7 degrees - still close to horizontal), but it will be known.  Place a weight (say 10#) haning from the cable at the center, and tighten the cable until the load is a certain distance below the anchorages (we use length/30).  From here it is possible to determine the initial state with pretty good certainty.  Energy methods or iteration with assumed final deflected shape will get you the deflection (and thus the tension) with the men on the cable.

We will typically have tensions on the order of 7000# (service level) for two men with 900# lanyards.  Design the anchorages for an ultimate tensile load of around 14000#.

RE: Fall Protection

Nutte and enginerding have laid out the end anchorge issue correctly. Beware that analysis on the cable will take some efforts to properly taking into account of initial sagging, and final pull. Also, keep an eye on OSHA specified fall distance from where the worker was standing.

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