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Two way slab deflection, ACI 318-08

Two way slab deflection, ACI 318-08

Two way slab deflection, ACI 318-08

(OP)
Dear All,

As per commentary clause "ACI 318-08, R9.5 — Control of deflections" if you satisfy the minimum thickness requirements of clauses 9.5.3.1, 9.5.3.2, and 9.5.3.3 there won't be any need to control deflections!!!

firstly, What sort of deflection it is referring to? immediate, long term?

secondly, I'm concerned about the long-term deflections and cracks! How can you justify not to check long-term deflection for a bay of 7.8m x 6m in a Lobby with 500x800dp beams all around the bay(LL: 5 KPa, SDL: 6 KPa)with just 180 mm slab thickness which satisfies the minimum slab thickness requirements of ACI 318-08, Clause 9.5.3.3 (For slabs with beams spanning between the supports on all sides)?

I'd appreciate to hear your thoughts.
many thanks.

RE: Two way slab deflection, ACI 318-08

With beams all around as you have and a 7" slab and a 20' span, I wouldn't be too concerned with deflection.  Design it as a 2 way slab supported on 2 sides.

Dik

RE: Two way slab deflection, ACI 318-08

For deflections, you may consider calculating the deflection of an elastic plate fixed on all four sides and, maybe, triple this to give you an idea of the long term deflection.

You might consider cambering the slab in the midde by approx 3/4" to 1".  The 1" camber will likely provide a bit of a crown to the finished slab.

Dik

RE: Two way slab deflection, ACI 318-08

SamEtabs,

You will never know unless you do the calculations. Deemed to comply solutions like this in codes tell you nothing about actual performance so you do not know what the deflection will actually be.

Dik,

Multiplying by 6 will give you a better idea, 3 for long term and 2 for cracking.
Cambering does not help if there are worries about effects of deflection of something supported by the floor.

RE: Two way slab deflection, ACI 318-08

Thanks, Rapt... could be, but for a 7" slab with those proportions I had assumed that cracking as well as long term deflection would be minimal.  

In any event deflection should be small...

Dik

RE: Two way slab deflection, ACI 318-08

dik,

You are a braver man than I. It is not a standard lightly loaded slab.

It has 6KPa SDL + 5KPa LL. It may be an end span or even a simply supported span. We do not know. How can you make comments like you have?

Sounds like it is Middle East with a heavy topping and therefore brittle finishes and possibly brickwork .

By my quick calculations, it is cracked and would fail in deflections, both visual and for brittle finishes.

SamEtabs,
If you have a slab where you are worried about the consequences of deflections, ignore deemed to comply solutions and do the calculations properly, allowing correctly for cracking and long term effects. At least you can then get a reasonable idea of what is happening, though it will never be 100% accurate, at least you will be in the ballpark!

 

RE: Two way slab deflection, ACI 318-08

6x is OK...

RE: Two way slab deflection, ACI 318-08

The standard says minimum thickness, not "the thickness shall be...".

Given this I agree with rapt, you need to calculate the deflections with the high loads you have suggested. I wouldn't be surprised if you need a deeper slab.

as for the x6 multiplying of the deflection, lets just hope it isn't post tensioned or similar, because that x6 or x3, will be pissing in the wind.

I fail to understand with all the work that has been done on long term deflections by Ian Gilbert et al, why people are reluctant to do the calculations for long term deflections. They don't take long if you have the right calc's setup.

The span on depths are good for checking and making sure you have a minimum ball park but other than that, they shouldn't be a substitution for full engineering calcs.  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

For two way slabs I always use a multiplier of 4-5 for long term deflections(after including cracking).  How you handle cracking is a crapshoot as there are all sorts of methods out there.  When my calcs match what was measured in the field I tend to think it is more by chance than skill.

I have seen all sorts of problems with slab meeting the 318 limitations.  Typically when higher end/brittle finishes with tight tolerances are required.

RE: Two way slab deflection, ACI 318-08

4-5 has been suggest by some experiments for normally loaded slabs in normal conditions, but once you move outside these boundaries, calculations are a must. I find my calculations match very similar to published results.  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

@RE:
Could you please post a reference for the literature that you have used in your computations for long term deflections? This is certainly an interesting topic. There are many parallel threads on this forum that address this particular issue, one that seems to crop up over and over........just like shear flow!

RE: Two way slab deflection, ACI 318-08

Rowing,

I agree, higher loads require a more detailed approach.  TO me that is in no small part due to the cracking.

What do you mean by published results? Often I am trying to determine why there is a certain amount of deflection existing in a slab.  In that context I have not seen repeatable calculations that consistently predict actual deflections.

RE: Two way slab deflection, ACI 318-08

Slickdeals/dcarr82775 ,

Search for papers by R I Gilbert on this. There are many.

BS8110 Part 2 methodology is reasonable good.

If you want a 'black box" solution (well really not black, probably more Grey than Black!), RAPT software as RE has aluded to previously.

But noone can give an exact solution. material properties vary, loading varies etc. For lightly loaded members on the point of cracking, there is a grey area as to whether it is cracked or uncracked and calculations either side of the line will vary significantly, but so will slab performance.

EG in a slab test Gilbert reported in ACI Journals in early 2000's, the initial deflection was about 3mm as the slab was uncracked. After 3 days it was about 9-10mm, because the slab cracked due to shrinkage restraint from the reinforcement. After 273 days deflection was about 29.5mm due to increased cracking and shrinkage and creep (load was constant). ACI code predicted 9.3mm deflction after 273 days! RAPT predicted 30.5mm.

RE: Two way slab deflection, ACI 318-08

slickdeals,
In the thread below two articles are posted discussion deflection, one by RAPT and the other by Doug (IDS), these both explain the theory of the long term deflection.

thread507-299616: Purely Theoretical Concrete Beam Question

I also have attached an article by Ian that has some calcs for long term deflection, which I think would give you what you were hoping for.

Dcarr82775

I use RAPT for for a good upper bound estimate of deflection taking into account long term deflection is RAPT, Note: RAPT doesn't take into account shrinkage restraint due to columns and shear walls, but does take into account shrinkage reo restraint.

The FEA programs I am familiar with are slowly getting to this level, however most still do not take into account reo shrinkage restraint correctly in my opinion.

I will post an article by Ian Gilbert that has long term results for deflection as soon as I can find it. I found it to be a good way of testing the FEA programs about 3 years ago when I was comparing software.

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

RE

That is a good document you uploaded.
I like how he says in the start that measured deflections of identical slabs in the field after 1 year differed by up to 100% in some cases!

I've seen mention of shrinkage restraint due to fixed items (like columns) recently - where can I find out more about this? Does Ian Gilbert talk about it anywhere?


ps I heard NCE has (or had) Inducta Slabs?

RE: Two way slab deflection, ACI 318-08

Demayeng,

Gilbert's work is mainly on cracking, shrinkage and creep effects on cross-sections and members.

The external restraint effects are basic engineering. The slab shortens, the restraints resist shortening, inducing tension stresses and thus earlier cracking and higher steel stresses and more deflection. He has done some work on crack control due to these effects, but their effects on deflections are basically induced tension stresses. You can determine these from analysis.

RE: Two way slab deflection, ACI 318-08

I have red flagged my post above after it was pointed out to me that the attached material is copy righted, thus management will remove the link. below is an open source document that gives similar information, if not as detailed. I will also contact Ian about the above paper.

http://ww2.integer.it/Web_1/database_locale/Fib%20Praga_2011/pdf/030%20596%20065%201%20Gilbert%20SE.pdf

Dcarr82775,
here is a good paper for deflection results.

http://www2.civeng.unsw.edu.au/fileadmin/public/research/publications/uniciv/R-407.pdf

some more good papers from Ian Gilbert, http://www.civeng.unsw.edu.au/staff/ian_gilbert/publications

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

Demayeng
Yes I do know a fair bit about inducta slabs and there other programs.  

The software developer and I have had many discussions about key areas of the software and failure to give appropriate information in there help manuals (I haven't checked in recent time, this comment is based on 2 years ago).

While the FEA model does now include long term calculations beyond that of Kcs, most FEA programs in my mind fair to handle reo shrinkage to a level that would be acceptable to me. This is in part failure of the product to allow you to superimpose your reo layout on the FEA model. This means that reinforcement restraint is never correctly included. I could go on, but lets just say that I use slabs for strip evaluation.

do you want to know anything else in regards of FEA?  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

RE
I still don't get it though. Are you saying that it is something you should calculate on top of the existing deflection formulas? (EC2 or otherwise)

I would have thought that the inclusion of shrinkage restraint caused by the reinforcement steel would cover that, as the mat of steel cannot shrink shorter than the distance between columns - does that make sense?

RE: Two way slab deflection, ACI 318-08

Regarding Inducta Slabs, it is what I use. I find it is very fast to use, so am just trying to reconcile it's use in my mind!

I think Slabs has come a long way in the last 2 years, but I still don't think you can superimpose your reo layout.

RE: Two way slab deflection, ACI 318-08

Demayeng,
if the building is highly restrained without an expansion joint, ie a 50m long building with two heavily reinforced elevator cores at each end of the building. in this case restraint defection would be of concern, especially if the cores didn't crack due to the restraint force.

in regards to the steel situation, it will help with this shrinkage deflection/cracking however you need to make the judgement, for internal slabs the code allows you some judgment on the amount of reo due to restraint, this would need to be considered.

I cover this more in this thread thread507-313219: PT design Ieff=Ig and in the fea discussion thread, this will also help you review inducta slabs. do you know if they have have changed the mxy information in their help files?

 

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

I will get into the FEA discussion thread today

in regards to Mxy in Slabs, the help file says:

"Include Mxy in Design: According to the Elastic Plate Theory, at each point on a plate there are three moments: Mx, My and Mxy. In SLABS, the twisting moment Mxy can be included in the design of the slab reinforcement by checking the box labelled 'Include Mxy in Design'. Strictly speaking, Mxy should be included in the design of the steel as it is one of the internal actions in a plate. However, AS 3600 does not mention Mxy explicitly, and in general design practices it is ignored.  We recommend that when considering the overall design of a concrete slab, the twisting moment Mxy is a secondary effect and can generally be ignored. However, the twisting moment should be considered for over-loaded thin slabs, where the thickness is less than 120mm. It is ultimately up to the software user to decide if Mxy is to be included in the design of the reinforcement."


Additionally, they have told me that for slabs greater than 200mm thick, Mxy makes no difference. I have not checked this yet.
 

RE: Two way slab deflection, ACI 318-08

Just reading through this thread, and although I respect the work of Gilbert and others in trying to more accurately calculate deflections, it will always be a very imprecise activity.  The preface to his paper acknowledges that there are many variables beyond the control of the designer which contribute substantially to deflection.

With the type of heavy restraint RE mentions in his latest post, I believe the design approach should be to eliminate this restraint insofar as possible rather than trying to design for it.

RE: Two way slab deflection, ACI 318-08

demayeng,
That is were I left off with them, the statement is wrong, the advice is wrong, and it really gets me hot under the collar.

here are the previous threads were in depth discussion has taken place. the letter attached by me that was written by rapt is really good in the cracked torsion stiffness thread.

thread507-275796: cracked torsional slab stiffness

thread744-266307: Mxy Moments in concrete floor design

Hokie,
The example was theoretical never to be a particle situation hopefully, yes the restraint should be minimized rather than the slab try to be designed for it. regarding all these unknowns, this is why you should assume Ieff=Ig, if you do you really should take into account all these actions that are very hard to take into account.  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

RE,
I think you missed a "not" there.

RE: Two way slab deflection, ACI 318-08

Hokie,
I sure did

rewrite:
Regarding all these unknowns, this is why you should NOT assume Ieff=Ig. And if you do decide to take Ieff=Ig then you really should take into account all these actions. Knowing that these are very hard to take into account.
 

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

Demayeng,

AS RE has said, you cannot ignore Mxy (RAM Concept has the same problem!!). AS3600 does say it must be included as AS3600 says that design has to satisfy equilibrium, and to satisfy Equilibrium, you must include Mxy (this is the official response from the AS3600 committee) in the design.
It does not specifically refer to Mxy because it would then have to refer explicitly to every aspect of the forces on a structure and if it left one out accidently, then someone would leave it out of design, so it is a general requirement of the code that equilibrium be satisfied, and engineers are supposed to know what that means. Unfortunately some engineers spend their days trying to find their way aroiund code wording rather than complying with structural engineering logic. Statics rules in design, that is all the code needs to say!

I hope that the judge will accept their comments in court when you are sued for a resulting problem if you ever have one! I know 10 experts who will shoot them down in flames and there are a lot more out there who will agree! They are being very irresponsible with their comments.

Hokie,

Yes, it is relatively imprecise, but the aim is to be in the ballpark with our results, rather than not having a clue of what the possible defelctions may be, especially in deflection sensitive situations. Then the designer should investigate a range of possibilities with varying material properties etc. It is better to know that your deflections wioll be 20mm +- 20% than have a comment saying deflections are acceptable without knowing values when you are putting something expensive and deflection sensitive on the slab.

RE: Two way slab deflection, ACI 318-08

I think it is actually OK as they give you the capacity to include it if you want to.  All they are saying is that for many situations it doesn't make much difference to results. I just checked this for a simple slab at 200mm thick and seemed to agree.

The only statement out of their manual that doesn't sound correct is:
"However, AS 3600 does not mention Mxy explicitly, and in general design practices it is ignored"
I can see that this doesn't seem to have any relevance as plate theory is not discussed in AS3600
 

RE: Two way slab deflection, ACI 318-08

demayeng,
does it slow down the compute time? I would say no. why would someone want to ignore it if it doesn't slow your computer. what reason is there for giving this advice? all it does is show a lack of understanding of the works of fea and other methods of design.

Don't be afraid to ask the guys at inducta question about there software,if you can get an in depth understanding of the workings in the post processor, you will be able to adjust the workings such that the results are correct, may take a bit of extra time to model but at least you can defend you design to anyone.  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

Thanks for the links I will definitely look at them.  This topic interests me greatly, I just don't get a lot of chances to work with 2 way slabs at work.
 

RE: Two way slab deflection, ACI 318-08

Dcarr82775,
no problems, don't be afraid to discuss further any items of interest. Gilbert has published a book or two on deflections, might be worth reviewing as well.  

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

RE: Two way slab deflection, ACI 318-08

specifically this one : GILBERT, R.I. and RANZI, G. (2010), "Time-dependent Behaviour of Concrete Structures", in press, Taylor & Francis, London (A1).

http://www.nceng.com.au/
"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected" Petroski 1992

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