unreinforced concrete slab 1

[morganjoe]

Can anyone tell me the procedures, assumptons, and formulas to use when checking a thick unreiforced concrete slab. I am thinking that if the slab is thick enough you just need to check the shear values because the tension steel will not develope. Is there a ratio of length to depth, or other guidlines on when this assumtion is correct?

Any help would be greatly appreciated.

Thanks

 

[UcfSE]

That assumption is not correct.

 

[jike]

ACI 318 has a section on the design of plain (unreinforced) concrete. I think it is Chapter 22.

 

[morganjoe]

UcfSE, I have a thick bottom slab to a cofferdam in which there will be uplift forces. I can brace the end of the slab on opposing sides so I will end up with a one-way slab with a bending moment. I have seen it designed where a very thick slab is used and they checked the bending stress verse a 6*sqrtf`c for what I assume is the tension in the concrete. I have never designed a slab that was not reinforced and will notfeel comforatable unless I can back it up with some type of references or acceptable design parameters. I see one of your replies to a different post as "How thick is your footing? You may not need reinforcing steel for moment if the footing is thick enough, so you wouldn't have to develop the yield strength of the bar." My situation may be different in that I have a simply supported slab that will have a bending moment at the midspan and not a column at the middle of the slab.

Thanks for any input.

 

[UcfSE]

Not developing tension steel is not reason to only check shear. You still have tension on the section, you just have to resist it with concrete.

 

[steve1]

My understanding is that the allowbale bending stress in tension for unreinforced concrete is equal to the "modulus of rupture" which indeed is equal to 6 x sqrt f'c (at least it was in the older ACI codes).

On a different note you say you have an existing slab with uplift. That puts the tension face on top which should be readily inspectable. Pay careful attention to any visible cracks espicially any running perpindicular to your span direction.

 

[jike]

Double check me on this, but I believe that 6 x sqrt of f'c is ultimate stress not an allowable.

 

[DRC1]

If you are desiging a tremie seal for a cofferdam, this typically is not designed for bending. This is a lot trickier in the details than it might first look and you should someone who has done one look at your final design. Ratey's Handook of Temporary Construction has a good chapter. If it is not secured by anchors, the bouyant weight of the concrete should be 125% of the uplift force. I have seen tremies 7 feet thick and some I have heard being 15 to 20 feet. It does act as a lower brace once the dam is dewatered, and the forces are generally taken in compression and are far less than the allowable. If you use anchors it is better to have more low capacity than a few high capacity to avoid bending in the slab. Typically Tremmie concrete for cofferdams is unreinforced and should not be counted on for high strength.

 

[morganjoe]

I have designed numerous tremie seal slabs for cofferdams and they have always been unreinforced and the weight of the slab alone resisted the uplift. This typically results in very thick slabs. This particular cofferdam will be designed with a reinforced tremie seal slab that will be braced on opposing sides. I have seen a design that used bonding of the concrete to the sheet piles to help resist the uplift and a check for tension in the unreinforced concrete for the bending stress in the tremie seal slab. I was wondering if anyone else has experience designing a slab that way. I do not like that type of design. I will only design a slab that resists uplift by the weight of the slab alone, or I will switch to a reinforced braced slab. I ignore tension in the slab, and friction and bonding between the piles and the concrete. Just wanting to see if maybe I am being too conservative.

Thanks all

 

[DRC1]

I have seen some designs that have used the bond, but I don't like it either. Between when you pour the tremie and you actually have the dam dewatered and completly braced, you have a lot of funky things going on with the stess and deflections. Exactly how much bond you do get is debatable. I have used anchors to stabilize a slab rather than using mass alone. I have also lft sumps and dewatered from the bottom of the slab to eliminate uplift. Note this was for a smaller dam with soil that did not have high permiability. How does the braced slab work? I don't think I have seen that one before.

 

[Qshake]

Please remember for you to develop the bending stresses in the middle of the cofferdam seal, you need to ensure you've developed the necessary boundary conditions. Whatever those conditions are.

If you don't develop any support restraints you're slab will move like a rigid body and will not have any stresses at mid section.

And depending on the depth of the seal it could be reasoned out that moment is transferred to the sheetpile/soil or in a conserative fashion, the edges are simply supported.

You'll need a steel/concrete bond stress that is reliable for your calcs.

Lastly, with the depth of the seal (as is the case for most seals)being significantly deep relative to it plan dimensions you may not develop bending stresses at all. At this depth it may act like a shear slab.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[rholder98]

I don't know anything about cofferdams, but Section 22 has the modulus of rupture as 5*sqrt(f'c). Section 9 has it as 7.5*sqrt(f'c), but for structural plain concrete, Sec. 22 governs.

 

[mitchelon]

Just FYI, the real value of fr is much greater than the allowable value in ACI. Around 9-11*sqrt(f'c). Most codes are conservative and use 6-7.5. 12 is allowed in some cases in prestressed concrete design.

 

[miecz]

rholder98-

Where in Section 22 is the modulus of rupture given?

 

[Lion06]

jmeic-
the modulus of rupture is on page 345 (ACI 318-05) equation 22-2. It says Mn=5(f'c^0.5)*Sm. Therefore, modulus of rupture is 5f'c^0.5.

All-
Am I missing something here. I would think that in order to get a slab to work unreinforced, it would have to be relatively thick. Once you start making it that thick, the DL is increasing, causing you to increase the thickness more. It seems like a never-ending cycle. I have never designed an unreinforced slab, but it seems kind of scary to me.
Possibly if the spans are very short and the joists are reinforced I would feel better.

 

[UcfSE]

StructuralEIT,

It seems to me that the weight of the slab increases linearly with depth, while strength increases with the square of the depth. It should be self evident from here.

 

[dirtsqueezer]

Have you ever thought about fibermesh? I don't have a reference for the values, but this might help your unreinforced concrete woes a bit. I believe you can get steel or polyfiber.

 

[Lion06]

I just made a quick formula relating the required slab thickness to the slab span for a 4ksi mix (for nothing more than supporting its own weight) and got h>(L^2)/303.4 for phi=0.9. Using phi=0.65, as I assume you would have to for unreinforced concrete (someone please confirm or correct this), I get h>(L^2)/219. The less conservative of these two means an 4.5" slab for a 3' span and only increases from there. The more conservative means a 6" slab for a 3' span, and this is just to support its own weight?
Can someone please help me to see the value in this type of system?

 

[miecz]

The way I read Section 22.2.2, a plain concrete slab is not allowed to resist uplift (due to bouyancy) by flexure. It may only resist the uplift by dead weight of the concrete, or by arching action that keeps the concrete in compression. As such, the flexural strength, modulus of rupture, etc. are irrelevant.

 

[UcfSE]

According to 9.3.5 [φ] is 0.55 for structural plain concrete.

Plain concrete is good, for a common case, to show that a residential footing, or any sufficiently narrow or deep strip footing does not need cross bars, i.e. bars perpindicular to the longitudinal axis of the footing. As the OP teaches us, it has uses elsewhere too.