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Re-shoring/Shoring - ACI 347

Re-shoring/Shoring - ACI 347

Re-shoring/Shoring - ACI 347

We are currently constructing a 4 floor story building. It has been over 60 days since concrete casting of the second floor slab, The design properties of the heavily reinforced 10" slab is more than safe to carry fresh concrete dead load + dead load of shoring for third floor and construction live loads. We, as the contractor have proven that the third floor slab can be casted only by shoring and no re-shoring required under level 2 slab by calculations and 28 day compressive strength test results of cylinder samples (concrete have achieved more than %100 of design strength in 28days). However, our client is still insisting and ignoring the structural capacity of level 2 slab even after 60 days referring below typical sketch on ACI347. Please share your thoughts and ideas and actions we could take if you had similar experience prior.

RE: Re-shoring/Shoring - ACI 347

I'd say if you have shown via specific design that both deflection and strength are acceptable with a specific shoring arrangement, that this would be acceptable.

Perhaps explaining/demonstrating to your client that you achieve the same intent as ACI is what is required. I suspect that the ACI requirement is geared towards thinner slabs of lesser strength and in fact is only demonstrating the general arrangement which is supposed to be confirmed by specific design checks, which you have done. A specific design check might equally show that you need to reshore 1 or 3 levels vs the 2 levels indicated in the picture from ACI.

RE: Re-shoring/Shoring - ACI 347

Assuming your analysis is right then I agree with your interpretation.

But clients want dumb stuff all the time. If I’ve already proven to them that it’ll work and they still insist, my next step is telling them how much more it’ll cost them to do what they’re insisting. If the still insist, then I charge them the extra money and do it. You tried to save them money, they declined, so now they have to pay money. Only fair.

RE: Re-shoring/Shoring - ACI 347

Not always about money, some clients just like to minimise risk, or are risk adverse.

Its worthwhile having the conversation with them regarding what they see as the issue. Are they approaching it from a different perspective and perhaps seeing something you have not considered. Are they approaching it from an educated perspective or simply insisting on 2 levels of reshoring because the generic picture showed it like this and they actually don't know the first thing about shoring.

Offer to take more cylinders to remove the risk of not achieving 28 day strengths to minimise the risk if they are still nervous. Take more survey levels to show no adverse effects when stripping out the shoring, etc. Ask what would you need to do to satisfy their exact concerns and go from there.

RE: Re-shoring/Shoring - ACI 347

While I agree with your assessment, you have to consider YOUR liability as well. No matter what, if something did not work out you would be responsible. Reshoring is cheap insurance...do it.

RE: Re-shoring/Shoring - ACI 347

1) If your obstinate client is a structural engineer, he may well be concerned about future deflections rather than present strength. Even when slabs possess sufficient strength for shore removal, if they are loaded earlier than anticipated, this can result in unintended cracking which can greatly increase in service deflections. It's the stuff that good lawsuits are made of.

2) Ideally, you and the structural engineer would have agreed to a shoring scheme prior to entering into the contract to do the project. If that is not the case, then you're in a bit of a tough spot in that, as Ron has implied, you probably don't want to own the future deflection performance of the slab and your engineer likely doesn't not see any benefit in being aggressive in that regard.

3) To head off the question before it's asked: yeah, we do design slabs that aggressively for deflections. It's really the only way to stay competitive and win work.

RE: Re-shoring/Shoring - ACI 347

The recommendations of the ACI 347 Simplified Method focus on strength for evaluation of the number of slabs/levels below that much be engaged with reshoring to support the loading above. But this does not explicitly address early age deflection or the delayed stiffness gain (relative to strength gain) in concrete. While some shoring contractors are modeling the structure in parallel with the EOR, the EOR of the structure is truly best suited for evaluating the effects on the structure.

If specifications exist and called for a minimum number of levels of reshoring, you can certainly propose an alternate, but without an explanation of both strength and deflection, you will not likely be granted an alternate.

I have proposed providing no reshoring on various occasions and been successful. I have also provided one level of reshoring where no reshoring was required, but I felt it was best to include, whether due to my thoughts on the structural system or rate of construction, or at times, for maintaining a precedent with my craft labor that felt more logical.

If you can show the in-place strength is adequate AND demonstrate (as noted in 347.2 with reference to ACI 435) that short and long term deflections are not impacted (certainly your slab age helps), then you have a case to discuss with your client or the owner.

But if you can't justify the deflections or if the specifications plainly call for reshoring no matter what, the EOR has every right to hold course and protect his building.

If the lack of reshoring means other trades start work in the strucutre sooner, and the capacity is well over the demand and the project completion would be impacted in a positive light, it may well be the best course of action for all involved to do further investigation - even if it involves paying for additional analysis if the EOR is available to do so.

Additionally, if the capacity to avoid reshoring is dependent on concrete strengths exceeding f'c, I don't know of any engineers who would typically allow the use of additional strength without some additional analysis of the structure.

RE: Re-shoring/Shoring - ACI 347

When working for this rigging outfit in our area, we reshored floors whenever heavy machinery was to be either installed or removed.

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