fall protection loading

[Tdog67]

Fellers,

I can't remember the dyanmics of this problem? If I have a 500 pound weight falling 6' starting at zero velocity what is the final static loading compared to? this is for a fall protection design system

thanks

 

[dik]

Many jurisdictions have their own loading requirements.

Dik

 

[Tdog67]

One allowable loading is the actual load with an impact factor. I'm hoping to do this in lieu of designing for a 5000 pound load for each person

 

[zerosum]

I think the loading would be very dependent on the arresting distance. That's why the 5000 lbs (probably obtained emperically) is the easy way out.

 

[Teguci]

If you are talking about OSHA fall protection, your maximum restraining force on the person is limited to 1,800 lbs for a harness and 900 lbs for a hip belt.

As for the anchorage, we are allowed to use an engineered design with a safety factor of 2. So, in lieu of the 5,000 lbs, you can engineer a system for 2 x 1,800 lbs for a harness = 3,600 lbs

You, the engineer are required to determine your own jurisdictional requirments and do your own investigation (do not rely on my info alone).

Reference that may help - "Fall Protection" - J. Nigel Ellis (and a lot of initials)

 

[UcfSE]

5000 lbs if you're under OSHA'a juridiction.

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

 

[kslee1000]

Tdog67:

The increase of the design load is not direct multiple. I couldn't cite the source at this moment, but it goes like for 1 person with free fall limited to 6', the vertical load is 1800#...then for 6 persons, the load is 3000+ pounds. The killer is the horizontal load effect.

 

[Teguci]

kslee
I believe you are referring to a horizontal lifeline system. The diminishing effect of additional persons falling is due to an assumption that the loading any one person will not occur at exactly the same time as the next.

The loading does not usually happen at the same time because of:
Horizontal Location along the lifeline
Height of the person
Length of the lanyards being used
etc...

Based on limited testing, the diminishing return on multiple persons on a horizontal lifeline winds up being +20% for each additional person on the system.

This is a contentious issue and lifeline engineering companies disagree on what to do regarding this issue (my Boss and I had an disagreement on this one)

 

[csd72]

Teguci,

Your calculation above ignores the fundamentals of how cables work. The fall load of 1800lb will result in a large horizontal reaction from the cable as well as the reaction in opposition of the applied force.

Look up the formula in roarks for calculating the horizontal reactions from cables and you will understand what I mean. You will actually find that the loads on a horizontal lifeline calculated by first principles will be much more than 5000lb.

 

[oneintheeye]

get the manufacturer of the system to specify for you

 

[Teguci]

csd72

So quick to criticize without much consideration - not very kind.

Initial response is replying to TDog67 who doesn't mention anything about a horizontal lifeline.

Second response is replying to kslee100 who seems to be looking at a horizontal lifeline. The diminishing return does not apply to fixed vertical anchorages.

In either case, the load that needs to be designed for under an engineered system is the maximum restraining force with a safety factor of 2. It is just that the chance of this dynamic load occuring at exactly the same time on a horizontal line is very slim, hence the diminishing return.

Before criticizing them, I would assume that any engineer who can draw a free body diagram to understand that the vertical force on a horizontal line will not equal the horizontal reaction and give them the benefit of the doubt.

 

[Tdog67]

Fellers,

here is the situation. We will add a monorail to the underside of long span roof trusses in an airplane hanger. Two workers will need to be supported for working on the top side of a corporate jet. The worst case loading on the trusses would be for both workers falling in the same area and one truss could potentially have a large load placed on the bottom chord. What load for both workers would I have to add to the trusses for analysis?

thanks

 

[Teguci]

Using self retracting lanyards and OSHA approved harnesses you could use 3,600 lbs x 2 = 7,200 lbs at the bottom of the roof truss chord.

If that doesn't work, you can start specifying tighter restrictions. For instance, shock absorbing lanyards can limit your arresting force down to about 900 lbs which means you can design your bottom chord for 3,600 lbs with tighter specifications. (Numbers are for a 300 lb person max)

Definitely want to tie into node of the truss if possible. Otherwise you may need to reinforce the chord. We've done that as well.

 

[MiketheEngineer]

Per OSHA - you must use a full body harness and Teguci has it nailed from my opinion.

Check with some manufacturers

Web Devices
Miller
etc., etc.

Google "fall protection" or "body harness" Unfortunately - the last one will probably bring up a bunch of sex sites.

 

[csd72]

Apologies Taguci,

But I would much rather you have a wounded ego than for someone to design these fall arrest systems improperly (not really aimed at you but at any reader of this blog that may be new to this type of thing).

 

[oneintheeye]

i'm sorry but this is a safety critical item and can depend on elasticity of the wire, absorbers used etc/ cant believe you are attempting to determine yourself, get a proper installer on board and get loadings. from experience they are not always what you'd imagine.

 

[kslee1000]

Please review OSHA 29CFR Part 1926.502(d).