Two way slab deflection, ACI 318-08 2

[SamETABS]

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.

 

[rowingengineer]

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 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

 

[demayeng]

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.

 

[hokie66]

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.

 

[rowingengineer]

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

thread744-266307

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

 

[hokie66]

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

 

[rowingengineer]

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

 

[rapt]

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.

 

[demayeng]

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

 

[rowingengineer]

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

 

[dcarr82775]

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.

 

[rowingengineer]

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

 

[rowingengineer]

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