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Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Allowable Stress for Fall Arrest Beam Related to Unbraced Length

(OP)
I have been asked to certify a group of hoist and fall arrest beams in a plant. I’ve rated hoist beam before and am not having problems with rating them but I am in a dilemma regarding the stress to use for the fall assist beams considering the unbraced lengths of the compression flanges.

The situation involves long fall arrest beams located over railroad tanker cars. The existing beams that we measured (existing drawings were not given to us) are S10x25.4’s, galvanized, constructed about 15 or 20 years ago. These are professionally done beams and I had expected the design to be a slam dunk. The difficulty I have is that these beams have spans ranging from approximately 20 ft to 35 ft with cantilevers as much as 11 ft at their ends. With those spans, the allowable stresses are virtually nothing.

From OSHA regulations I understand that I need to design the beams for a failure load of 5,000 lbs. Also assuming that the steel is either ASTM A36 of ASTM A572 Grade 50 I can assume an ultimate Failure Stress (Fu) of 58 and 65 ksi exists. Further, calculating, my maximum stresses, the maximum is under 30 ksi therefore I have a Factor of Safety of 2.0 or greater.

The problem that I have in my mind is that when you consider the unbraced length using ASME BTH-1-2001 there no allowable stress available for these excessive unbraced lengths.

The bottom line question is: "Is it acceptable to expect the beam to permanently deflect and twist, essentially acting as a rope/tension member, when subject to a fall assist load of 5,000 lbs. (and substantially less too)?"

Any help or guidance would be much appreciated.

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

I would say that it's not acceptable. A flexural capacity calculated ignoring the governing failure mode isn't much of a capacity at all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

I would also agree that it's not acceptable. I don't believe any post-buckling strength should be relied on, though I agree it will be there after having seen monorails buckle laterally from a heavy sideload without collapse. Sounds like for your setup adding bracing (if possible) or compression flange cover plates are the way to go. You might look at a cap channel; I've used them for monorails with large unbraced lengths. The calculations take a bit to go through but it will substantially increase the capacity in lateral-torsional buckling.

Regarding the cantilever, I'd take a look at some of the research on monorails with cantilevers as the lateral-torsional buckling capacity is highly dependent on the backspan after the cantilever. Some good references I've collected that you might find interesting:

Allowable Bending Stresses for Overhanging Monorails by N. STEPHEN TANNER
Lateral-Torsional Buckling of Wide Flange Cantilever Beams by BO DOWSWELL
Design of Crane Runway Beam with Channel Cap by DUANE S. ELLIFRITT and DUNG-MYAU LUE

Also, don't forget to check flange bending for the impact load if your fall arrest system loads up the flange tips and not the web.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

(OP)
Thanks - Oh I'm not saying that this is acceptable which I am agreement that it's not acceptable. - I'm just trying to understand how the previous engineer could justify the use of these beams in this condition. Especially when they are so far over the limits.

The corporate safety officer asked me to look at and certify a bunch of "self build" hangers and beams that were very suspect. He just though these is as and added item expecting a "fly through" in they were engineered with a major plant addition many years ago.

My 1st response for repair will probably be to add a hat channel (there aren't any nearby members to brace off. This will be messy though with the hot dipped galvanization.

I just get concerned with I have to repair something that wasn't done right many many years ago - it really shakes up owners! - We'll see how this all hits the fan!

Again, Thanks, It's nice to know that I'm not stupid!

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Quote (JDWLEW)

I just get concerned with I have to repair something that wasn't done right many many years ago - it really shakes up owners!

I hear 'ya. "Hire me! I'll solve you some problems that you didn't even know you had!" If they care about safety, hopefully they'll value what you're bringing to the table. That said, you need to make damn sure that the engineer down the road can't justifiably second opinion you out of some future work.

I can think of one cheat here. It's hard to say if it would apply to your situation though without more detail. It goes like this:

1) Fall arrest loads often have to be applied in all directions, including laterally.

2) The original designer may have designed the beam for those lateral loads in weak axis bending.

3) If the beam would work for weak axis bending, then it will work for strong axis bending, no matter what the fancy math says.

This is a long shot of course. If the beam worked for weak axis, it's a pretty safe bet that it would also be checking out for strong axis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

I have been down this road before. A couple of key thoughts:

1. The S shapes may have been originally installed to provide a "track" for underhung monorails. The S shape is a bit more favorable to handle local flange bending due to more meat at the web-flange interface, though I agree with others above, the obvious best case is to keep the impact load aligned with the web (if possible, but not always possible).

2. Read the OSHA specification carefully! There is a key word ("OR") that many overlook. 5,000 lbs is not always the design force. In fact, if you are an SE performing this design, it's my opinion that a blind 5,000 lbs should not be the design force. If supporting a single user, it could be less, if supporting multiple users, it could be more.

I've shortened it up, but 1926.502(d)(15) states: "Anchorages....shall be....capable of supporting 5,000 lbs per employee attached OR shall be designed...as part of complete fall arrest system with safety factor of at least two...under the supervision of a qualified person."

The qualified person is YOU! Find out whether the client needs one or multiple people tied off at once. Then you can refer to language just a bit farther ahead under fall arrest systems in 1926.502 to determine your design force (i.e.: the harness limit is 1800 lbs x SF (2.0) = 3600 lbs per person)

Link to OSHA Fall protection provisions (1926.502): Link

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Quote (FoxSE14)

I've shortened it up, but 1926.502(d)(15) states: "Anchorages....shall be....capable of supporting 5,000 lbs per employee attached OR shall be designed...as part of complete fall arrest system with safety factor of at least two...under the supervision of a qualified person."

The qualified person is YOU! Find out whether the client needs one or multiple people tied off at once. Then you can refer to language just a bit farther ahead under fall arrest systems in 1926.502 to determine your design force (i.e.: the harness limit is 1800 lbs x SF (2.0) = 3600 lbs per person)

FoxSE brings up a great point, I've used this provision to design a single user fall arrest system for use with low-force lanyards for a one-time job.

Other thoughts, because the load is applied below the CG of the beam they may have considered the load to help prevent a LTB failure. I dislike doing this but it is worth considering.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Aan S beam 15 to 20 years old will probably be gr 36

WF's went to gr 50 around 2000 to 2005

S beams lagged behind 3 to 5 years - I don't know why

RE: Allowable Stress for Fall Arrest Beam Related to Unbraced Length

Can you look at it as a 'really stiff' cable with adequate anchorage it should be sufficient to resist the loading prescribed? In the event that it is really needed, it will save the person's life and if bent out of shape, it can be replaced. Is there a requirement that it remain elastic and act as a 'beam'?

Dik

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