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FOS

FOS

(OP)
I have been asked design some lifting beams that will be used to transfer material from a slab on grade and onto a trailer. My client has purchased a machine specifically for this task. The intent is to use a system of tubes spaced 5'-0" o.c to place the material on in the shop. Once a load is complete the equipment will come in and hoist all of these tubes at once and place them on a flatbed. The client wants to pick up a maximum of 50 kips with a trailer that is approximately 48' long.

I know rigging has a FOS of 5:1. I also know Steel has a general FOS of 1.67:1. So, if these were combined together we would end up with a FOS of 8.33:1. I'm not sure if this is appropriate or not (combining the FOS). How should the FOS be handled in this instance? I want to be conservative but not too conservative.

RE: FOS

I would go straight 5:1 ratio of nominal resistance to load. You could add the resistance factors to the resistance side of the equation if you like, to maintain the same reliability across steel, bolts, and welds, but I think adding load factors to get to 1.67 is excessive.

RE: FOS

(OP)
So basically increase my loads by 5x and then don't apply the FOS found in the AISC. It does seem excessive. I wonder if others will think the same.

RE: FOS

Don't forget to mark and proof-test @ 125% per OSHA 1926.251 a(4). Not optional regardless of engineering.

RE: FOS

I generally use the AISC FOS plus an impact factor on the load.....I tend to use an impact factor of 2 for typical cases.....if the lift mechanism is for repeated use/abuse, then, I might increase the impact factor to reflect this....I try never to skimp on the design of lifting mechanisms...

RE: FOS

(OP)
Sail,

AISC FOS + impact will be well below 5:1.

John,

I'm not using it the client is. I will remind him of what is required but that is up to them to mark it up. They should be aware of the requirement as this is something they do on a regular basis however, the machine is new.

RE: FOS

Fair enough. My end-user bias is showing through.

RE: FOS

ASME BTH-1 and ASME B30.20 - The B30.20 being "Below-the-Hook Lifting Devices, a safety standard" and the BTH-1 being "Design of Below-the-Hook Lifting Devices"

Don't try and mix and match FOS between rigging and the beams. Your rigging should all be rated to include the FOS needed (which is usually 5), and per the BTH-1 you get your own fun FOS on the beams.

Sounds like these are going to get a lot of uses - don't forget fatigue.

RE: FOS

(OP)
Fatigue, we are talking 1-3 cycles per day. I have considered it, but I don't think 1-3 cycles per day is a fatigue issue and that is where my consideration stopped.

RE: FOS

the OP might check if that FOS of 5 is for failure/ultimate stress......

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