Shoring and Reshoring of multistory buldings 1

[vlad1981]

Good morning, I've been a follower of pretty much every thread regarding shoring/ reshoring. I Hope @EngineerMary is still around.
I got a couple question regarding reshoring.
1. I am new to reshoring, so in my company I am gathering old designs to use somehow as templates. I got the situation, in which I found a couple designs in which no load from the reshores is considered, but when I do consider the loads from the reshores there is a different animal, and the slab "fails". See below
. Design load all floors: DL: 8" Slab (100 psf), LL: 40 psf, additional DL: 10 psf, Factored design capacity: 196 psf.
. Construction loads: DL: 8" Slab (100 psf), LL: 50psf, DL (forms & Shores): 10 psf, DL (reshores) 5 psf, Factored construction loads (during pour): 212 psf.
So, to the left, is my design, considering the weigh of the reshores (5 psf) being added to the slabs 3 & 2, right, is the design without said weight.

As you can see, in the left slab # 2 is overloaded, which suggest another level of reshoring needed, on the right, slab is OK.
which brings me to the second question:
2. As a first approach, I'd use the unfactored loads 160 (Slab + LL + DL (Construction)) / 50 (LL + DL (Design)) = 3.2 interconnected slabs ~ 4. But if I use factored loads, 212 (Construction, factored) / 76 (Design, LL + DL, factored) = 2.78 ~ 3 interconnected slabs. Which approach is correct, and why??
3. Third question, when having tables, or a shoring such that it can not be considered uniformly distributed, SP-4 (14), Formwork for concrete, states in 6.7.3: "... An approximate solution may be obtained by finding max. moments and shears, and determining and equivalent uniform load..." is this, finding moments and shears as it was a two way slab, taking the reshores as supports? or similarly, distributing the load between interconnected slabs, and getting that equivalent uniform load taking columns/ walls as supports?

Thanks all in advance.

 

[Enable]

I'm having a slightly difficult time understanding your assumptions and following what it exactly it is you are doing. If you could walk me through exactly what your loads are floor by floor and how you're assuming they'd be distributed that would be great.

In general reshoring is based on the so-called simplified method (see ACI guide below). Basically it assumes that the shoring/reshoring is infinitely stiff relative to the flexural stiffness of the slabs, loads are distributed uniformly (be careful about this one), and that loaded slabs will take load in proportion to their flexural stiffness (though assumes aging effects are moot on this front).

So if your top slab has 100psf DL, and you apply a 50psf construction load then you sum that number (after multiplying by appropriate LL and DL safety factors) and divide by the LL capacity of the individual slabs beneath. Round to the nearest slab. Note: you need to take in account if the designer has applied a LL reduction which does not always accompany the stated LL.

What I'm getting from your case is we have a total factored load of 1.2*(100) + 1.6*50 = 200 psf to be resisted by floors with a capacity of 1.6*40 = 64 (assuming all the floors are at 28 day strength if not you need to ratio the capacity). You would need 3.13 floors. You would probably round to 3 in this case but if it's your first rodeo go to 4. You use factored loads like anything else you would design.

BTW if you haven't read it please grab yourself a copy of ACI 347 Guide for Shoring/Reshoring of Concrete Multistory buildings. It presents assumptions of the simplified method and is quite detailed.

 

[Enable]

As for your question 3: when you have non-uniform loads you simply calculate the max stresses imposed by that load as you would in original design. If it's less than the design stress you are good to go and if more than the design stress you connect more slabs!

 

[vlad1981]

hey, thanks a lot.
yeah, I did struggle to try and ask as clear as possible, but it seems is a mess.
Heres what's happening, shoring slab # 5, one level shores, two levels reshores.
All slabs are the same, 8" (100 psf), Design loads: LL=40 psf & DL= 10ps, Construction loads: LL= 50 psf, DL = 10 psf.
Reshores weight 5 psf.
When I get to level 5, and there is no ground support, the factored construction load gets equally distributed between slabs 4, 3 & 2. So 212 psf (factored construction load) / 3 = 70.66 ~ 71 psf to each slab. so far, so good.

1. Slab 4 gets on top of its own weight, an additional 71 psf.
2. Slab 3 gets, on top of own weight, 71 psf + (weight of reshores)/ 2 (reshores bearing on 3 distributed between 3 & 2).
3. Slab 2 gets the 71 psf + weight of reshores / 2 (bearing on 3) + weight of reshores (bearing on 2).

For the above, 1S + 2R when pouring slab 5, slab 2 would be overloaded if weight of reshores are considered. But is OK, if not.

As per you explanation, its basically, only Construction load Factored/ Design LL factored, 212/ 64 = 3.3 ~ 4 interconnected slabs? No imposed or misc. dead loads considered?

 

[jayrod12]

I likely would be considering slab 3 and 2 carrying a full weight of reshores each as opposed to distributing 3/2 of the total reshore weight to slab 2. Does that make it work.

 

[Enable]

If the weight of the reshoring is what causes you to go to another floor I would be very much inclined to say that you would be fine not going to another floor because:

A) the actual construction live load is never going to be 50psf on the top floor which can be used to offset the reshoring weight

B) the simplified method over estimates the pass-through force from the top floor. In compressible shoring arrangements it's been shown that as much as 15% of the loads get shifted to the upper slab. If you're using aluminum props it'll be less than 15% but more than 0%. You can use this to offset the reshoring weight on the 2nd/3rd floors.

I would generally add the formwork/falsework weight which you have done.

 

[vlad1981]

Thanks a lot, Enable, you've truly saved my day.
A) is more than enough to justify the difference.
B) is absolutely smart, being my overload around 1.87% .
I wish there was more literature and not just "experience", everyone I know just don't use the reshore weight, nobody been able to explain why. I'd say, using it, and if said overload is less than 1-5% it could be considered as part of that "more than 0 % but less than 15%" shifted to the uppermost slab?

As for your question 3: when you have non-uniform loads you simply calculate the max stresses imposed by that load as you would in original design. If it's less than the design stress you are good to go and if more than the design stress you connect more slabs!

Do you mean in here, to calculate stress as in Load/ contact area and compare it to compressive strength of concrete?

 

[Enable]

I wish there was more literature and not just "experience", everyone I know just don't use the reshore weight, nobody been able to explain why. I'd say, using it, and if said overload is less than 1-5% it could be considered as part of that "more than 0 % but less than 15%" shifted to the uppermost slab?

We have to take care when fashioning rules of thumb like this. For example, if slab cycles are particularly quick (say under 3 days) or if the upper slab is not of the same construction as the lower slabs (maybe thinner or weaker concrete), I may be hesitant to rely on this without thinking about it more deeply. That said, for typical arrangements with similar slab types/strengths across levels and not super early upper slabs I'd probably say this would be true.

We do a lot of silly things in the construction of buildings that almost no one wants to commit themselves to in writing (at least not as an engineer). Hence the "experience" component, ha! By the way, a heuristic I would rely on is what is typical for your market. If it's typical for contractors to connect 3 slabs for a garden variety pour or if it's typical for them to connect 4 or 5 or whatever it is in your area, I would take a long hard look at that. You don't even need to be in a design office to see what they do. Just drive around your downtown and see how many floors are reshored on the high-rise projects. It'll give you a sense of what the industry norm is for your area.

Do you mean in here, to calculate stress as in Load/ contact area and compare it to compressive strength of concrete?

Unfortunately it's not that simple. You need to calculate (or at least bound) all relevant induced stresses not just bearing. For example, lets say you have a new slab you're pouring and at one area based on frame setup or whatever you only have a few point loads coming down (maybe you have some spreader beams that concentrate loads on particular feet). Lets also say that the floor directly below is a 1-way slab between cantilevered beams. Well, even if the shoring/applied load spread over the entire support area below would be less than the design UDL, you cannot safely use that since your load is coming down in the form of discrete points. That means you need to think about what your point loads do to the slab (flexure, shear, and bearing), and what they do to the cantilevered beams (flexure, shear, torsion). If your load exceeds the design criteria of any of those members you need to do something about it.

The benefit of the simplified method is we don't need to think about any of that because the UDL design loads are provided. But for unique spots with point-loads or any non-udl load you need to in effect back-calculate the design resistance of the members for the way that you are imposing the loads.

I Hope @EngineerMary is still around.

There's an ACI-U on-demand course entitled: Reshoring and Early-Age Concrete Behavior that I highly recommend to you. It's $40 and it is definitely money well spent. The lectures (recorded seminar presentations) are pretty interesting and provide good info. Also, an engineer named Mary Bordner provides a presentation on shoring pre-stressed elements, and while I cannot be sure it is the same Mary that once frequented these boards, I'd give it a pretty decent chance they are one and the same. How many Mary shoring engineers can there possibly be?!

As a fun aside: from her presentation it's clear that she has an affinity for wood shoring. I do not understand it one bit and if I ever met her I'd love to ask her about that. Here we use aluminum or steel props all day long. I cannot think of too many advantages of wood (for our locale where aluminum props are abundant) but certainly many disadvantages.