fall protection system 2

[steve1]

I have been asked to review a fall protection system to ensure that it meets the requirements of OSHA regulation 1926.502. The system is fairly simple, a cable stretched between two anchors that workers attach a lifeline to. The OSHA reg states that the anchors shall be capable of supporting 5000 pounds per worker attached and that the fall arrest system shall have sufficient strength to withstand twice the potential impact energy of an employee free falling a distance of 6 feet.

I'm a little unsure how to approach this problem. It's easy enough to determine the capacity of the anchorage and the cable, but how do I analyze the system? Most of the info in my analysis books dealing with cable systems require an initial sag in the cable. Here we have none. Also, how does one go about calculating the initial load in the cable (it has a turnbuckle at each end to take up the slack). Any insights will be appreciated.

 

[Qshake]

I've noticed recently that since the scaffolding incident during a gust of high winds in Chicago last year that SEs are now required to review those kinds of systems.

My first inclination is to direct you to Robert Ratay's Handbook of Temporary Structures in Construction which should be a staple in the library of each structural group. In the reference Ratay has a section on access scaffolding and a section on temporary bracing and guying. The latter may be applicable as you have a cable which has an initial tension applied. Also a book on rigging might be very helpful. There is one on the market but I don't recall the name - it may be the Handbook of Rigging.

 

[evelrod]

I find it a bit odd that after all these years (1896 to present) that safety equiptment (rigging) has suddenly(?)come under scrutiny of SE's outside OSHA. My main concern in the last 20 or so years before retirement was SAFETY. I don't know of ANY engineering texts that spell out what is needed in specifics. There are, however, available from the International Assc. of Bridge, Structural and, Ornamental Irnworkers in D.C. books and pamphlets on safety rigging some of which are available to the public.
Re: Rigging Manual III chapter IX, Rigging Safety

Safety Manual IV chapterIV Wire Rope

These 500+ page manuals are verse and chapter in safety for structural ironworkers in the U.S. and Canada and to a large degree what OSHA uses to establish guidlines.
Hope this helps, but always ere to the 'overkill' side of the equation. You cannot be too safety concious. Good luck.

Rod

 

[amokhta]

While installing a flexible horizontal lifeline is as simple as stringing a line between two anchor points, it is one of the most complex types of fall protection systems when it comes to designing it. Design issues to be considered are:

1. The Load that is applied to the anchor points during a fall incident is a number of times greater than that of the maximum arresting force on the worker's lanyard. The magnification factor depends on geometry and angle of sag. For example for a sag angle of 15 degrees the magnification factor is approximately 2 while for a sag angle of 5 degrees the magnification factor is almost 6.

2. The other factor is the dynamic deflection of the lifeline. The deflection should be small enough to prevent the falling worker from hitting an obstruction or lower level before the fall has come to a complete stop.

3. At all times you have to maintain a minimum safety factor of 2.0

 

[amokhta]

I should add a few more words to my previous response. One should not confuse the 5000 lb OSHA requirement for personal fall arrest systems with end anchor capacity requirements. In fact, after applying the load magnification factor and the minimum safety factor of 2.0, in most cases, the 5000 lb requirement for end anchors would be inadequate for end anchors of most horizontal life lines.

 

[jheidt2543]

The manufacturer of the safety lanyard may be able to provide you with information and insite into the design of their equipment and how it would fit in an overall system.

 

[steve1]

Thanks for some good insights. While researching this issue on the web I came across a book entitled "Introduction to Fall Protection" by J. Ellis. If anyone has a copy of this book please let me know if it's worth purchasing.

 

[steve111]

hi
since we have a few people here interested in roof anchors a pecularity to my area .... we do testing of roof anchors and are continually running into roof anchors with multiple eyelets. the operational drawings show multiple equipment and lifelines tied off to the same anchor albeit different eyelets ... any opinions on this type of arrangement ie tieing equipment and lifelines to the same anchor?

 

[svc430]

Archon Engineering has a software program called "Safety Line" that I think will give you the data you need (rope tension, horizontal and vertical end forces, final sag, etc.). The full program can be purchased for $20 and you can download a demo version for free (demo version is limited to a maximum span of 8 feet). The web site is

http://www.archoneng.com.

In lieu of that, it might help you to know that the initial sag is dependant on the initial horizontal component of the rope tension by the following relationship:

Hx = rope wt. per foot x span^2/(8 x sag)

where Hx is the horizontal tension component (span and sag both in feet).

The forces that develop in the life line due to a center concentrated load (during a fall event) depend on the final stretch of the cable, thus making the system statically indeterminate. Outside of a trial and error solution, the horizontal component of the rope tension can be found by application of the formulas in the sixth edition of Roarkes Formulas for Stress and Strain (page 179). In short, the Roarke formula solves for the rope angle with the horizontal, then uses simple geometry to solve for the horizontal component of the rope tension.

For rope angles less than 12 degrees, Roarkes equation for the rope angle is as follows:

angle (in radians) = (W/EA)^1/3

where W is the applied load.
E = modulus of elasticity (29,000,000 psi for steel).
A = rope cross section area.

From this, the horizontal component of rope tension can be found by dividing the applied load by twice the tangent of the above angle. (Vertical component is simply one half the applied load).

All of the above assumes, naturally, that the end supports are at the same elevation (it gets considerably more complicated if they're not).

Of course, a much easier way around all this (which I highly recommend) is to specify a life line system from a manufacturer of such systems who will give you the forces you need to analyze the systems support structure.

Typically these types of systems will include some sort of shock absorbing device at the end(s), turnbuckles, and rope tension indicating device for installation. For long spans, these manufacturers can also provide special rope hangers for intermediate supports that allow 'pass-thru' provisions when used in conjunction with a special carrier attached to the line to which a lanyard can be connected. This eliminates the need for a worker to unhook his lanyard at each intermediate support.

I think most safety harness manufacturers can provide these systems, but I'm most familiar with Miller. These systems aren't cheap by any means, but you do get all the hardware needed for the installation, much of which will come pre-assembled with the cable. (I'm afraid to think what these guys must pay for insurance).

One thing to keep in mind is the potential for a workers lanyard getting caught on some nearby structure or equipment during a fall event. This could result in the loading from the safety lanyard being applied in the horizontal direction rather than the vertical (fall) direction.