Safety Factors - outrigger platform design 4

[RHT2020]

Hi, I am designing a temporary outrigger platform for a Multi - story building. I am wondering what is safety factor I should use for the platform floor and outrigger beam design? It is lightly loaded. S.F of 2 is good enough? Any codes and standards I should refer to? I don’t think outrigger platform should be designed for SF of 5 as outlined in ASME except lifting lugs. please advise.

 

[JStephen]

If this is something that would be considered a scaffold, and is in the US, then see the OSHA rules pertaining to scaffolding.
I'm not aware of an ASME standard that covers something like this, so I don't know where the ASME reference would come in.

 

[Lomarandil]

He may be referring to ASME BTH-1 if the platform is lifted into place by a crane.

Can you provide a picture or link to the sort of platform that you mean?

----
just call me Lo.

 

[RHT2020]

It will be like "PERI RCS MP materiel platform" to move materials and formworks to high rise building.

 

[JStephen]

If it's suspended by a crane and used to lift other stuff, it'd fall under ASME BTH, etc.
If it's supported off the building while the crane is detached, I'd say it falls under the scaffolding provisions of OSHA.
And if it performs both functions, it would need to comply with both.
And also needs to be coordinated with the building design if supported off the building.

 

[DTGT2002]

I'd say JStephen is right on. The outrigger platform itself when locked into the structure is a form of scaffolding as outlined in OSHA 1926 and would be expected to maintain a 4:1 FOS against failure.

When rigged for flying, the anchor components engaged for lifting would be looked at under the ASME and likely be at 5:1. This can get into debate as it is the load being supported, and typically one would never permit the outrigger to carry load while attached to the crane. Our approach has been that the anchor points themselves are considered as part of the rigging and their attachment to the overall equipment would be at the higher FOS.

I am assuming this is in the U.S.

In recent years, we have gotten quite a bit of pushback regarding the hold down loads applying uplift at the end of the outrigger beams, with the general rationale that they have not designed for a concentrated load pushing up at the slab. In the end, the load is generally less than the weight of a reasonable area of the slab dead load and sufficient capacity can be provided by the bottom cover and bars, but it is still something that comes up.

We also get great concerns at the props at the edge of slab. For our details, these loads are much lower than the typical formwork shoring loads, but it always seems that folks get more alarmed by two 3000 lb props at the edge than a few hundred 10,000 lb props across the floor footprint.

 

[Ceinostuv]

Use ASCE 37 to determine loading. Apply the ASCE 7-10 load combinations, pay special attention to D+L combos. If you're still uncomfortable, add an additional FOS for your own sake. I like to use the typical D+L combos plus an additional FOS of 2 when I'm using things like sand bags, water barrels, or deadmen to resist overturning; otherwise I stick to the ASCE combos.

Make sure your guardrail spacing and heights comply with OSHA.

As you can see in the PERI diagram, they like to install posts on their walers at the interior of the slab to resist overturning; if you plan on using this, make sure you check the slab appropriately or VERY CLEARLY state it is outside your scope on your drawings. Use the correct, factual concrete strength at the time of installation (if you install at 7-days, use 7-day strength, not 28). If you like to sleep well at night, follow through and make sure someone looks at the slab.

Judgement-In-Training

 

[dhengr]

Ceinostuv:
It seems to me that I’ve seen someplace in the codes and stds. that using any easily moveable materials (your, “using things like sand bags, water barrels, or deadmen to resist overturning”), and I’ll add things like bundles of lumber, stack of conc. blk., etc. is not a good idea, is not allowed. This is because some turkey will decide that those conc. blks. would be better stored over there, not here in the middle of the floor space; or the lumber can be used to start the conc. forming on the next floor, etc. I’ll grant you that your items are somewhat less likely to be mistaken for framing supplies or mason’s materials. The semi-fixed adjustable posts are a much better solution for the needed extra support. Otherwise, all the above posts are really good advice.

 

[Ceinostuv]

Dhengr, I wouldn't be surprised if there is something out there, if you ever come across it again I'd love to find out where. The tough thing for me with temporary structures is figuring out which codes apply.. but if there is a code out there prohibiting it, well I'd love to keep that as ammo for when I'm talking to clients. Counterweights are specifically allowed and their requirements codified by OSHA's Safety Standards for Scaffolds Used in the Construction Industry (SS. 1926.450) with the only exception applied to a mason's platform, because of the high variability of weight that may accompany a mason. This section does not allow the use of construction materials as counterweights (like bundles of lumber, stack of conc. blk., etc), to prevent the turkeys from getting confused.

Using moveable deadweight during construction is, at least in my experience, surprisingly common even on largescale commercial projects I've seen. This is doubly true when construction is left completely up to the contractor's means and methods.

While the semi-fixed adjustable posts may seem to be the better solution, consider what happens if there is an accidental bump to whatever is providing the resisting force. A bump to a prop that's not positively attached at the top or a bump to it's adjusting nut could send the platform tumbling. A bump to a deadweight item that has been positively attached to the platform properly may just drag the platform a bit, and even if you bump the platform towards the edge, at least you haven't lost all capacity (especially with the integrated safety factors of ASCE 7's load combinations) and will hopefully give the field guys enough time to evacuate. Maybe a better way to explain my understanding is that a prop is much more sensitive to movement than a counterweight, and that scares me.

EDIT: When I used props to resist overturning, I usually added an extra prop or two for some redundancy in case the event described above were to occur.

Judgement-In-Training

 

[jjl317]

One regulation that limits the material used for counterweights is the suspended scaffold section of OSHA 1926.

"1926.451(d)(3)(ii)- Counterweights shall be made of non-flowable material. Sand, gravel and similar materials that can be easily dislocated shall not be used as counterweights."

 

[Ceinostuv]

JJ: That put me in my place , my original post has been edited.

What's done is done, those structures were topped out years ago and I had no incidents and no issues with OSHA inspections or building officials; no harm, no foul I guess. I don't necessarily agree with the code on this one; but, I'm out of the scaffolding and formwork business anyways. Probably for good reason, apparently.

Judgement-In-Training

 

[jjl317]

I don't necessarily agree with the code on this one either - while I completely understand the potential concerns with flowable material, I would prefer that the regulations let me decide as a professional engineer whether or not I can safely address these concerns.

Though on the flip side, it could be a benefit to be able to point to this when dealing with argumentative contractor.

 

[dhengr]

OSHA is probably where I saw this issue first. I certainly don’t agree with a lot of what I read, and try to understand the rationale for, in OSHA. But, we should also remember that they are writing and promulgating for a lot of people, generally more disagreeable, less knowledgeable and less deep thinking (what are the negative possibilities of this design or action) than most engineers. Unfortunately, we then end up caught up in the middle sometimes because we don’t know some obscure section or para., or the thinking behind that particular item. I haven’t had many dealings with OSHA for quite a while, but my understanding is that they have been told, from above, to be more helpful (more teacher, less sheriff) in our understanding what they want and mean, and less confrontational, when we come to them for advice and help early in the process, and in a less confrontational way ourselves. The old…, ‘getting more bees with honey than vinegar thing,’ I guess. It probably pays to get to know someone at the local OSHA office who you can talk with and is generally more agreeable, easier for you to talk with.