FOS for Jacking Points on a structure

[dmacdonald]

I'm working on some jacking points for a pumphouse and I had a question about the FOS that should be used. I'm pretty green and haven't dealt with this yet. Basically the structure is going to be taken off the truck with jacks, and lowered onto the foundation.

I was wondering if anybody had any advice on this topic. Are jacking points to be designed similar to crane/lifting components (higher FOS)? Are these loads to be factored as any typical load (lesser FOS)?

I've dug around, asked around, and haven't been able to get a solid answer. If anybody has any info, or specific references where I should look to find this?

Thanks in advance.

 

[ishvaaag]

Since a construction matter, a lower safety factor -that doesn't affect nor the integrity nor the serviceability of what fabricated- can be accepted. Usual safety factors for construction stage are 1.25 to 1.35.

Another thing is that you need to be as accurate as feasible with your solicitations; since dynamical, either a dynamical analyses or impact factors will need to be considered.

 

[csd72]

agree with Ishvaag,

If it is not being used to lift over personnel then the jacking points just need to be designed for code factors. Just remember that it will probably not be the critical case as you only need to allow for the dead weight plus a dynamic factor.

Just make sure that there is a good allowance at the jacking points for abuse and misalignment as they will never get it perfectly right.

The high factor of safety on most construction equipment is due to the fact that this single factor allows for: materials and load factors, dynamic factors, and allowance for wear and tear since they are reused multiple times.

 

[Teguci]

I get really exciteable on jacking projects. I was thinking more like a factor of safety of 2. In addition, I recommend analyzing the support assuming a 3 point pick. 4 or more is indeterminant and requires trust in the jacking company (do you?).

ASME B30.1 looks promising as far as outlining jacking procedures and design.

 

[csd72]

Teguci,

Good point about the 4 points of support though I would think it would depend on the stiffness of the frame. For a flexibly frame it would be more or less equal for a stiff frame it will be exactly as you say.

 

[Teguci]

I outlined a procedure for the jacking company to lift in 1/16" increments thinking the same as you (elastic deflections, load sharing, etc...). As soon as they put the pressure lever up you lose all control.

Very scary

 

[MiketheEngineer]

Typical shoring FS is around 2.2 to 2.5. Scaffolding is 4:1. Red iron is about 1.67:1

Fighter airplanes are about 1.05:1

It all depends on how well you know your loads and the people putting it together. That's why it takes about 10 years to build a new airplane!!

I'd vote for around 2.5:1

 

[cvg]

I would focus more on the risk than the temporary nature or uncertainty of the loads. If a failure might cause costly damage to the equipment or a safety risk to the contractor, than there is high risk and the FOS should be greater. If the risk is low, then use a lower FOS. The loads should always be estimated conservatively.

 

[fancypants]

cvg is correct this is a risk issue. If what ever the object is falls and gets ruined, what is the replacement cost and how much time to fix or replace.

I have designed a lot of jack and skid type jobs. The largest was a 3600 ton bridge.

Three point and four point comments are right on the money. They may use four jacks and pipe two together so you have a four point pick so to speak. Remember that the piped jacks will see the same pressure and thus distribute the load equally. Going this route you will get a flat plane that can pivot

Four point picks have more control but everything is independent. Good experienced crews are required. You need to absolutely specify differential elevations that are acceptable for the design of what ever you are picking. Forced differential elevations put the stress back into the picked object.

Also mind the center of gravity both spatial and in plane. If the vertical cog is high, to much differential lifting can make it go flop.

 

[ishvaaag]

Well, it may sound as if I try to miss the train of safety but really is not; 1.25 after proper allowance of what happens when lifting forfeiting damage may not be as distant of 2.5 on static without much care of such concerns. The point is that, as Mike says, if you know well your solicitations, the usual consensus on safety factor for works use to go as low.

 

[csd72]

ishvaag hit the nail on the head.