Reshoring - load distribuation between floors 3

[bushmill]

When reshoring a multistory building with many levels of reshores (5+ levels of reshores), how much load would the lower most slab experience from the loads on the top? Using diagram sketch, there 5 levels of reshores and 6 interconnected slabs. When the 7th floor is poured would the bottom most slab (1st floor slab) share the same load as the upper most (6th floor)? If so then the 1st floor slab could easily overloaded by the wet concrete load in addition to, say 50 PSF construction LL on each floor above?

1/6(Constr. LL & 12" of Wet concrete DL)+ 1/5 Constr LL + 1/4 Constr. LL + 1/3 Constr LL + 1/2 Constr LL + 1 * Constr LL = 147.5 PSF

OR is it safe to assume, because the reshores and slab are not perfectly stiff, the upper reshored floors would carry more of the load? I believe they mention this in "ACI-347 Guide to reshoring", that the simplified method overestimates the load on the bottom shores. If so what would the actual distribution be? If you had 10 floors reshored, common sense would say the lower most slab would not experience the same share of the wet concrete dead load as a slab 9 floors above.

 

[SlideRuleEra]

...because the reshores and slab are not perfectly stiff...

...what would the actual distribution be?

Determine installed reshore stiffness and you can make a first-order approximation of the load on each floor.

To carry load, a supported horizontal structural member has to deflect. With a value for reshore stiffness, you can determine deflection of each floor then back figure, based on that deflection, a rough estimate of how much load the floor carries. There are too many other factors for this calculation to be accurate... although the numbers can be made (deceptively) precise.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[MrHershey]

The thing with straying from the 'infinitely stiff' assumption that the simplified method makes for shoring posts is that you need to come up with a better assumption (and be able to prove it). You're going to have to determine your own stiffnesses, not only of the installed shoring but of the slabs as well. That's a fair bit more in depth than is commonly done for reshoring analysis. Like SlideRuleEra notes, you'll get a number that looks convincing. But because of all the assumptions going into it may be no more accurate that the simplified method (and if your assumptions are wrong, may be worse). The simplified method has been used reliably for decades on thousands of projects, I'd have to be pretty sure of myself to go a different route.

As for what it would do to the load distribution, it'll shift more of it to the top floors as you've noted. How much depends on relative stiffnesses between shoring and slabs. Would recommend taking a look at ACI 347.2, which has some discussion on factors affecting load distribution. That guide states that with 'a more compressible shoring/reshoring system, the structural system tends to shift as much as 15% of the slab loads to the uppermost interconnected floors as compared with rigid shores/reshores'.

If that is a major concern, you could consider adding some additional shoring over the top levels to compensate. I would be hesitant to design lower levels for less than what simplified method says you should, however.

 

[EngineerMary]

Hi, Bushmill.

Reshoring is my specialty (I'm a voting member of the ACI 347 committee and have written a continuing ed course as well as a trade magazine article about reshoring topics); I would be happy to lend a little guidance on this question.

You are correct in that the simplified method suggests that all the floors will take the same load. That method has been used reliably for years.

But what happens if your shoring is sitting on a slab that's designed for 100 PSF and the floor below it is designed for 45 PSF? (This is very common on a floor in a multi-use building where lower levels are parking, then there's a transfer floor, then there's residences on top.) Can we say that, once the 100 PSF floor has carried 45 PSF, the rest of the load is going to the floor below it? I would think not.

What I'll generally do is a hand sketch showing the different levels. You'll have an arrow going down for your load and arrows going up for your design capacity.

So let's say that the Level 8 pour weighs 145 PSF (construction live loads and dead loads). And all the floors below are designed for 40 PSF (we won't worry about design live load reductions in this example just to keep it simple). My preliminary calcs would be 145/40 = 4 interconnected slabs (1 level of shoring and 3 levels of reshoring).

But then I think it is reasonable to take that a step further.

Level 7 has a capacity of 40 PSF. So the reshores under L7 are carrying 145 - 40 = 105 PSF.
Level 6 has a capacity of 40 PSF. So the reshores under L6 are carrying 105 - 40 = 65 PSF.
Level 5 has a capacity of 40 PSF. So the reshores under L5 are carrying 65 - 40 = 25 PSF.
Level 4 has a capacity of 40 PSF. So no reshores are required under Level 4.

**It's also important to note that this changes if your reshores extend to grade. That's a horse of a different color.

Depending on the type of structure and the clear heights of the reshores, I will often remove reshores from under Level 5 in this case. (A good example would be a parking garage. A lot of times we have 2 full floors of reshores and then leave reshores under the girders only for that 3rd level.)

We have successfully designed many millions of square feet of reshoring using this method, but it's important to be aware of your load paths if you are going to do this.

I hope this helps.

Mary

 

[EngineerMary]

There is an old article from Concrete International floating around called "Simplied Design of Reshoring" or something like that.

 

[Shu Jiang PEng SE]

@EngineerMary

I agree with you. the vertical stiffness of the floors and the reshore structure should be analyzed. The stiffness of the reshore structure should be larger than those of the floors.

 

[Ingenuity]

ACI has a FREE spreadsheet called AutoReShore V1.1 available here: Link

Description:

This series of Excel spreadsheet programs allows the user to perform the shoring and reshoring analysis of multi-story concrete buildings during construction. The programs provide both numerical results and an illustration of the step-by-step construction process. The programs are based on the typical assumptions used in ACI SP-4 Formwork for Concrete, 8th ed., and in ACI 347.2R-05 Guide for Shoring/Reshoring of Concrete Multistory Buildings.

- All previously cast slabs are identical and have equal stiffness;

- Ground level or other grade base support is rigid;

- Shores and reshores are spaced closely enough to treat their reactions as a distributed load;

- Shores and reshores are infinitely stiff relative to the slabs; and

- Reshores are installed snug tight without initially carrying any load.

I have no idea how good/bad it is - I stumbled across it some time back - so use at your own peril!

 

[bushmill]

Thanks all for the replies.

If anyone has an example calculation or article on stiffness analysis for reshoring that would be helpful. So far I have not been able to find any example calcs. I'm thinking the stiffness of reshores could simply calculated using k=A*E/H but determining slab stiffness will not be so simple. And then how it affects load distribution.

@Maryengineer thanks for the help.

I also may have a follow up question or two since you are specialist in this topic!

 

[Ingenuity]

There is an old article from Concrete International floating around called "Simplied Design of Reshoring" or something like that.

Maybe "Practical Design of Reshoring" by Randy Bordner, Concrete International, Volume 24, Issue 10, 2002: Link

 

[EngineerMary]

Ingenuity; Yes, that is the article.

Bushmill; I don't think that calculating the stiffness of the reshores themselves is a very practical field analysis, mainly because your loads have a lot of indeterminancy in them and the reshores are not equally loaded. 347 suggests designing for a live load of 50 PSF, but that's allowed to be reduced by ASCE Loads on Building During Construction. However, even you took the maximum live load reduction and used 25 PSF, I think you are still high sometimes. There hasn't been a ton of work done on the question, but I have a couple articles where the actual live load on a reshore was measured and came out to be about 16-17 PSF. You also have to account for the fact that your floors are distributing load around (especially on a post-tensioned floor) so some reshores might have 17 PSF of live load and some may see significantly less. Then you have the added issue of reshore clips. If you are using reshore clips, it would be difficult to assume that they are all installed in exactly the same manner with the exact same snugness. It would be extremely difficult and time consuming to figure out how much your clip will compress and then find the compression in the reshore itself. Therefore, I think it's reasonable under field conditions to assume the reshores are infinitely stiff as the Simplified Method in 347 suggests.

My previous post has a worked example that demonstrates how to account for slab stiffness (remember to account for your load paths and PT-Transfer). Chapter 6 of SP-4 also has a worked example. I will email you a couple of articles that I have. I hope that helps.

*I do not recommend designing reshores with a 17 PSF live load. I don't do it, 347 doesn't recommend it, and I don't believe ASCE allows it.

I love reshoring and can talk about it all day so ask away!

 

[MrHershey]

I'm thinking the stiffness of reshores could simply calculated using k=A*E/H but determining slab stiffness will not be so simple. And then how it affects load distribution.

The stiffness of your reshoring depends on a lot more than the axial stiffness of the shores themselves. As Mary has noted, calculating the stiffness of a shoring system really isn't very practical. Assuming shores are infinitely rigid in comparison to the floors has a long history of reliable use and really should be the default assumption unless you have a really, really good reason to do something else.