Factors of Safety

[oyvey]

Hello All,

I have been tasked with doing structural analyses of overhead lifting slings. They are for lifting ~10,000 Lbs in a factory setting.

I've been "told" that the requirement is SF = 5.0 for the entire assembly: I-beams, lugs, bolts, bearings, wires, welds... every little detail. However, in all my web searches I can only find sf=5 for wires, ropes, etc. For example MIL-S-8512D specifies designing the main structure for SF=3 against yield allowables. I found another document (from NASA) specifying main structure using the lesser of sf=3 against yield or sf=5 against ultimate, plus sf=5 for wires, etc. others yet have different sf's per failure mode (compression, tension, shear).

Can anyone please point me to an authoritative source (preferably readily available in the public domain) that specifies sf's for structural steel used in overhead lifting?

Thanks very much,
oyvey

 

[bridgebuster]

The ultimate capacity of a sling is determined by the manufacturer. If your boss wants FS = 5, and your pick = 10,000 #, you need to buy a sling with 50K capacity; or am I missing something?

 

[BAretired]

OSHA regulations:

1926.753(e)(2)

Components of the multiple lift rigging assembly shall be specifically designed and assembled with a maximum capacity for total assembly and for each individual attachment point. This capacity, certified by the manufacturer or a qualified rigger, shall be based on the manufacturer's specifications with a 5 to 1 safety factor for all components.

BA

 

[oyvey]

Yep. A miss. 1) it was a peer, not a boss. 2) if i had an mfr rating I wouldn't need to analyze it, would I? This is an in-house manufactured sling.

 

[oyvey]

BAretired - your response was helpful and is much appreciated. The OSHA spec makes no mention of plastic bending. Seems to me it should be permitted with such a high sf, but once again, I've been "told" it is not permitted in this application. The structure has a bolt bending problem. Do you know of any specs/regs that address this?

Thx

 

[racookpe1978]

Don't go multiplying things too far:
If the intended load is 10,000;
and the sling is rated for 10,000;
then you are allowed to pick up the 10,000 load. You do not need to go look for a 50,000 sling.

On the other hand, you SHOULD have a reasonable "lift" ratio: The sling is rated for 10,000 (and based on a design factor of safety of 5x, was designed to withstand 50,000 load.) But YOU should have 13,000 or 15,000 rated sling in case the LOAD (that you have assumed is 10,000) is actually heavier or has shifted or moved or whatever. Only use the last 0.00001 of a lifting device if you have checked everything you "expect" to lift and know absolutely with 100% certainty that conditions are perfect and will remain perfect for the entire lift with no changes. NEVER go into that 5x "safety factor" to make assumptions about the lift, use ONLY the rated nameplate capacity.

Then conduct a safety review during your pre-lift brief and double-check everything.

Now, you're going into this backwards:
So your rated load will be 10,000.
Your design load will be 50,000.
EVERYTHING on the sling (or bar or device) needs to pass 50,000 without failure.

 

[rb1957]

FWIW ... if you're loading the bolt with 50,000 lbs, i'd let you use plastic bending; with a check that 10,000 lbs is still elastic (which i'd expect). but maybe your company's practices are different (more conservative). is this already made, or just designed ? pins are not where i'd look to save weight ... over-size them if at all possible.

question, is this going to be used very infrequently or daily/hourly ? if frequent use is expected, then there might be a fatigue question ... this is another reason for the high SF (to "eliminate" fatigue)

Quando Omni Flunkus Moritati

 

[BAretired]

BAretired - your response was helpful and is much appreciated. The OSHA spec makes no mention of plastic bending. Seems to me it should be permitted with such a high sf, but once again, I've been "told" it is not permitted in this application. The structure has a bolt bending problem. Do you know of any specs/regs that address this?

I don't know of any such spec/reg and I doubt that one exists because bolts are not intended to be used in bending.

NASA's table is interesting...a factor of 3 based on yield and 5 based on ultimate. To me, that seems like a rational stance and I'm not quite sure why OSHA insists on a safety factor of 5 to 1 for all components of a lifting device. Perhaps OSHA has a different interpretation of safety factor than I do, but failure is deemed to occur when a member becomes fully plastic and safety factor is the load causing failure divided by the service load.

I do not necessarily agree with OSHA on all matters, but their regulations must be satisfied by law.

BA

 

[dhengr]

oyvey:
You say... “I have been tasked with doing structural analyses of overhead lifting slings.” You don’t usually design lifting slings, you buy them, because you don’t have the know-how and equipment to make them or test them. Racookpe... is right, wire rope, a sling, a chain or rigging fittings and hardware such as shackles which are rated for 10k, can break/fail at 50k. The 5:1 FS is to ultimate strength or failure. I think BA’s OSHA finding 1926.753 pertains primarily (only) to Steel Erection on a Construction Site, and more particularly to the wire rope, slings and fittings used in that process. This 5:1 FS is probably appropriate because these particular pieces of equipment are abused, handled and stored roughly, and used over and over in many different configurations. OSHA is very convoluted and difficult to understand, or follow from issue to issue or btwn. various subjects or areas covered. But, BA’s 5:1 in 1926.753(e)(2) is in keeping with the above. OSHA 1910, General Industry, is probably the more appropriate Standard for in-plant usage. It’s no less difficult to interpret and follow, but it does refer to ASME and ANSI stds. for cranes and lifting devices, and these do talk about a FS of 3:1 to yield strength. I would talk with your local OSHA office with any questions. They’re the ones who make the final interpretation in your area. Most of them are generally helpful when you approach them beforehand, with intelligent and reasonable questions, rather than trying to fight with them after the fact.

If in fact, you are going to design the entire lifting system for in-plant use and for lifting particular items, that’s another matter. Take a look at ASME BTH-1-2008 “Below-the-Hook Lifting Devices.” It’s pretty much the bible in this design area. Along with good stress analysis, good structural design and detailing, good clean welding and welding details, etc. Keep it as clean and simple as is possible, follow a simple, clean load path from the load to the hook, avoid stress concentrations.

I’m not suggesting that there can’t be any yielding, but don’t start talking about plastic bending or plastic hinges, because you likely have big problems in usability before you do any serious plastic bending. You should try your darnedest to design pin bending out of your system.

Show us a sketch with some dimensions, loads, CG’s, etc. of what you are trying to do; the info you need to start your design.

 

[TXStructural]

Why is there a bolt in bending?

OSHA does have a rule or two for general industry:

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9834

Specifically (since I don't know how your device is constructed, this is a typical example):
29 CFR 1910.184(e)(4)
Proof testing. The employer shall ensure that before use, each new, repaired, or reconditioned alloy steel chain sling, including all welded components in the sling assembly, shall be proof tested by the sling manufacturer or equivalent entity, in accordance with paragraph 5.2 of the American Society of Testing and Materials Specification A391-65, which is incorporated by reference as specified in Sec. 1910.6 (ANSI G61.1-1968). The employer shall retain a certificate of the proof test and shall make it available for examination.