Fall Protection

[PEinVA]

In this thread, thread190-97369 , 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

 

[civilperson]

To save 30% on steel, yes.

 

[PEinVA]

haha, thanks.

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

 

[MiketheEngineer]

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.

 

[nutte]

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?

 

[PEinVA]

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

 

[JJEL]

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

http://www.osha.gov.

You'll want standard 1926.502.

 

[nutte]

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.

 

[nutte]

OK, digging into the OSHA requirements:

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

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.

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.

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.

 

[MiketheEngineer]

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....

 

[racookpe1978]

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.

 

[MiketheEngineer]

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

 

[PEinVA]

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

 

[racookpe1978]

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.

 

[rdeltoro]

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.

 

[kslee1000]

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#.

 

[racookpe1978]

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.

 

[kslee1000]

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.

 

[IFRs]

And the 5000# is per person!

 

[kslee1000]

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