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Re: Fall Protection System - Loading

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>From: RajSTC(--nospam--at)aol.com

> "Lifelines shall be secured above the point of operation to an anchorage
> or structural member capable of supporting a minimum dead weight of
> 5,400 pounds".  Design based on this requirement resulted in lot of
> structural steel, which seems to me to be an overkill.  The project was on

> From: "Armijo, John J (PWCSD 273)" <ArmijoJJ(--nospam--at)PWCSD.NAVY.MIL>

> Look closely, the 5400lb requirement may be the caternary reaction at the
> supporting steel member.

Raj is dealing with an anchorage directly overhead of the worker. No
catenary involved. The static & impact loads will be MUCH higher with
horizontal tie lines that restrain by axial tension - basic statics.

OSHA 1926.104(b)
"... anchorage or structural member capable of supporting a minimum dead
weight of 5400 pounds."

but

OSHA 1926.502(d)
"Anchorages used for attachment of personal fall arrest equipment shall
be ... capable of supporting at least 5000 pounds per employee ..."
"... safety factor of at least two ..."
"limit maximum arresting force on an employee to 1800 pounds when used
with a body harness"

and

OSHA 1926.502(e)
"Connecting assemblies shall have a minimum tensile strength of 5000
pounds"
"Dee-rings and snap hooks shall be proof-tested to a minimum tensile
load of 3600 pounds without cracking, breaking or taking permanent
deformation."

There is also a mention of 310 lbs max for employee + tools for the
above.

> From: "Ritter, Mike" <mritter(--nospam--at)lgt.lg.com>

> use tie-offs.  We didn't have anything in the structure capable of
> supporting the loads prescribed in OSHA.  Instead, the contractor
> eventually used railing or one of the other options.  The bottom line was
> that he saw the engineer as being unable to do something that common sense
> says should be easy.

Why would the railing be any better than any other option? Be realistic.
The ANCHORAGE must resist the 5000 or 5400 pounds but that is a rapid
impact load. If there is any mass to the load path, this momentary tug
will not have significant effects at any further point of the structure.
So, I apply this to stanchions and lugs but not much else. After that,
it's 310 pounds per employee ....

> concerning cables...we tried using a long cable running parallel with the
> edge of the building.  The workers were going to tie-off to it.  Bad idea.
> Just a small lateral force on a cable produced enormous end reactions.
> Try it sometime.  It got into an iterative process of 1) how much does the

> it elongate.  It got silly after a while.  It appears a lot of sag would
> be required in the initial cable design just to make it feasible.

Yup. You need a loose cable and/or closely spaced supports to make that
work without huge supports. This, unfortunately, is NOT an issue of
single impact although the impact loads become immense at the support.

> From: "Ritter, Mike" <mritter(--nospam--at)lgt.lg.com>

> I forgot to say -- We have installed proprietary fall arrest systems that
> used cables.  However, those fall under the "engineered systems" of OSHA
> and probably don't impart the full OSHA loading on the structural
> elements.  They have springs to absorb energy and other special devices to
> allow an elongation in the cable system without damage.  The system we
> used also limited the falling distance to about 1 or 2 feet.  This also
> keeps the forces lower than OSHA.

These issues are all covered in OSHA.

> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>

> saw video tape of a wrecker operator tipping an overturned (on its side)
> car back on its wheels just by securing the tow cable to a frame member,
> taking the slack out of the cable and giving the cable a sideways tug
> with one arm. The car rolled upright just slick as glass. 

This sounds practical. I'll have to file it away to use some day. Maybe
win a bar bet ...

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>