Long Term Deflection & Compression Reinforcing 1

[OzEng80]

I recall a side comment by RAPT on a thread (that I haven’t been able to find) regarding the appropriate use of compressive reinforcing for reducing long term creep and shrinkage deflections. As AS3600, CL 8.5.3.2 now clearly states that compression steel needs to be located in the compression zone (who would have thought!) – I wanted to clarify the application of Asc in relation to slabs.

I had a quick look at Ku values for typical slabs that I have designed (deflection controlled) and Ku seems to generally be in the order of 0.1 (treating as singly reinforced). Adopting this value of Ku =0.1 and assuming 25mm cover and d = D – 25 – 10, the minimum slab thickness to just scrape any reinforcing into the compressive zone is D = 300mm (kud = 26.5mm). As this is based on a singly reinforced section – accounting for this ‘compressive reinforcing’ would actually lower Ku further (or does it – if it is not in compression?!). I have read that under sustained loading (ie appropriate to the consideration of long term deflections) the neutral axis lowers further as internal stresses are redistributed into the compression reinforcing – I have not found anything to quantify this however.

So…
How do you determine the long term neutral axis depth and is this appropriate for the assessment of ‘compressive reinforcing’.

How far into the compression zone does reinforcing have to be to be effective - Should the reinforcing be located within the ‘compressive stress block’?

Is there a limit to how much compression reinforcing can be effective (right up to 100%)?

Should the Kcs reference be removed from the slab chapter (is it really on appropriate to beams)?

What is industry practice? Every engineer I have encountered (including myself) has used 'compressive' reinforcing to reduce deflections in slabs much thinner than 300mm… Since the industry dictates predominantly slabs with complete top and bottom mats of reinforcing (for ease of construction), I am loathe to end up specifying thicker slabs than everyone else.

I understand that the formulas are significant simplifications of very complicated theory and are ‘deemed to comply’ – I am not disputing their appropriateness, I just want to know to apply them correctly.

Thanks in advance!

 

[asixth]

I had way too many beers last night.

 

[asixth]

Now that I have had time to rest my hangover. My statement where I mentioned the basic creep strain in Cairns has increased from 850E-6 to 1000E-6 had nothing to do with the local concrete batches but more the wording of the 2009 code. Clause 6.1.7.1 of the 2001 codes states that the basic shrinkage strain is to be taken as 850E-6 while Clause 3.1.7.2 of the 2009 code says that the final drying shrinkage strain shall be taken as 1000E-6 outside of Brisbane, Sydney and Melbourne.

Running an example, the final shrinkage strain for a slab with a thickness of 200mm (th=200mm) in a tropical or near costal environment is 440E-6 by the 2001 model.

Whereas the same example to the 2009 model gives a final shrinkage strain of 500E-6, consisting of 454E-6 due to drying shrinkage and 46E-6 of autogenous shrinkage.

I have attached my excel macros for both the 2001 and 2009 code to the end of the pdf. They can be copied into a module.

I also noticed that basic and advanced workshops on the new concrete code are coming up in the next few months which may cover these topics.

http://cementandconcrete.com.au/

 

[IDS]

asixth - if the strain is unrealistic for your local concrete the code does allow you to use a shrinkage based on either meaurements of similar concrete or lab tests.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rapt]

Doug,

As you know I agree with everything you are saying. Item 4 is very important in the kcs debate. Once a section is cracked, the tension reinforcement is a lot further from the N/A than the compression reinforcement and for lower reinforcement ratios it is a lot further away again, so its effect on shrinkage warping is a lot more than the reduction effects from the compression reinforcement. So the kcs assumption of exact balancing of effects is patently stupid.

R.E. Shrinkage in the new code, it is recognised around Australia that as soon as you go outside the major capital cities, concrete shrinkage increases significantly. There was talk at one stage of making it generally 1200, with reduced values in Sydney, Brisbane and Melbourne, but they decided on 1000 with the reductions to 800 and 900 in those cities! But as mentioned, anyone can use figures based on testing of local concrete.

R.E. Beer, yes it is good, even the old NQ draught used to be drinkable!

R.E Jeffs comments - he uses RAPT for a lot of his deflection calculations, and does not believe in kcs as far as I know! RE AAEM method, that only covers creep. I hope Emil has included shrinkage in there somewhere.

R.E. Mxy, I will have a go at posting it today.

R.E. deflections on a 220 slab spanning 8200, you would probably not notice it walking on the slab, but I would think you would be up to 60-70mm! The problem buildings in Melbourne were not noticed until someone actually did a survey on the floors. The occupants never complained.

 

[OzEng80]

In defence of Cairns and it’s aggregates, I contacted a local concrete manufacturer today who confirmed that the final drying basic strain in accordance with AS1012.13 for their typical mixes to be in the order of 550-700 micro strain. So, is it appropriate to use these values for design purposes or is something more formal (than an email) required? Test results for the specific batch of concrete used on a project would be a little late in the design process…

RE – with inducta you recommend using AAEM and EC2 methods (which I am not familiar with) for deflections over the AS3600 approach. Given that these methods appear to give less conservative results (I will check closer tomorrow) how can you justify being outside the Australian code? I ask because I want a copy of your stamp/disclaimer!

Thanks again.

 

[IDS]

OzEng80 - I don't think that EC 2 is in general less conservative than AS 3600, and in some aspects it will give greater deflections than AS 3600. In particular:

- It is more realistic (i.e. greter deflections) for lightly reinforced sections with moments just above the cracking moment.

- It handles shrinkage curvature better.

- Total long term shrinkage and creep values are very similar.

I'll do some comparisons and post the results when I have time.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rowingengineer]

9.3.2 Slab deflection by refined calculation
The calculation of the deflection of a slab by refined calculation shall make allowance for
the following:
(a) Two-way action.
(b) Cracking and tension stiffening.
(c) Shrinkage and creep properties of the concrete.
(d) Expected load history.
(e) Deflection of formwork or settlement of props during construction, particularly when the slab formwork is supported off suspended floors below.

The two methods I have mentioned take into account most of these requirements for refined calculations; I do however make modifications to all software to ensure the part (e) is included and restraint effects. The AAME is written by Ian Gilbert.

So no disclaimer, no need.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

RAPT,
Missed your comment about aircon, While we have the same aircon systems, plastic cracking is easier to control.


Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rowingengineer]

Ozeng80,
How about this for a disclaimer:

"The theoretical evaluation of the long term deflections (regardless of the method) may be very unreliable in practice as there are many factors that can influence the final results. In some cases these factors may be impossible to control and quantify (such as weather conditions, construction tolerance, material consistency and development in new construction products).

Controlled laboratory experiments may have correlated closely to more advanced theories, but the fact remains that the deflection prediction of concrete structures is very unreliable, which diminishes the practical value of the more accurate theories. This is especially applicable for a simplified beam approach, which requires subjective judgment and may provide misleading results. The 2D plate theory (used in SLABS) is a much more superior analytical tool and provides a much more accurate prediction of the instantaneous deflections. Considering the unreliability of long term concrete behaviour, it is suggested that users consider the deflection results for all methods available in SLABS 2009 to determine if the design is within acceptable deflection limits."

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[IDS]

Controlled laboratory experiments may have correlated closely to more advanced theories, but the fact remains that the deflection prediction of concrete structures is very unreliable, which diminishes the practical value of the more accurate theories.

I don't agree with that. An analysis that includes all the applicable factors will provide a reasonably accurate estimate of upper bound deflections. The actual deflection may be significantly less (especially in the short to medium term), but it is unlikely to be significantly more (if the analysis is done properly).





Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rowingengineer]

Agreed Doug, this disclaimer is a direct copy from the software Ozeng80 and I are discussing, it was pasted in tongue and cheek.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rapt]

RE,

For aircon, I was making the point that humidity in FNQ might be 99.99% in the open air but not inside airconditioned buildings, where it is the same as in Tasmania (approximately) or even lower as they do not have to turn on the aircon most days in summer, only the heaters in winter.

RE your quoted disclaimer from that other software vendor, looks like a marketing blurb rather than a technical comment to me! The same manual tells you to ignore Mxy moments in design, because it gives you cheaper buildings and makes his software comparable to other software that also ignore Mxy moments and cheaper than when using RAPT as RAPT includes the effects being 2D software.

If he included Mxy in his designs you might get more reliable answers! Also, if he understood the lack of accuracy that FEM software has in dealing with cracked concrete slabs reinforced orthogonally, he might not make such "accuracy" comments.

Maybe that is where my marketing falls down, I cannot write BS like that (or more correcxtly, refuse to)!

R.E. Cairns Concrete Shrinkage
AS3600-2009 has the following clause on Shrinkage values

"3.1.7.1 Calculation of design shrinkage strain
The design shrinkage strain of concrete (?cs) shall be determined—
(a) from measurements on similar local concrete;
(b) by tests after eight weeks of drying modified for long-term value, in accordance with AS 1012.13; or
(c) by calculation in accordance with Clause 3.1.7.2."

So as long as your supplier can guarantee that the concrete he is supplying will conform to the results from his tests in accordance with 3.1.7.1(b) and to AS1012.13 at 8 weeks and modified appropriately, then you can use those values. That has always been the case in AS3600 (previously it was clause 6.1.7.1(b)(ii).

 

[rapt]

Following on from my last message regarding shrinkage values to use,

"3.1.7.1 Calculation of design shrinkage strainThe design shrinkage strain of concrete (?cs) shall be determined—
(a) from measurements on similar local concrete;
(b) by tests after eight weeks of drying modified for long-term value, in accordance with AS 1012.13; or
(c) by calculation in accordance with Clause 3.1.7.2."

The wording in this clause has changed from the 2001 code. It no longer says you can use the 8 week test values as the basic shrinkage strain in 3.1.7.2 calculations. The only shrinkages that can be used in 3.1.7.2 are the values defined there, 800, 900 or 1000 depending on location.

I am trying to get clarification on what the options are as options a and b above are not explained very well. I will get back when I have more information.

 

[rowingengineer]

Rapt,
I was only pointing out that this is true only once hte air-con is turned on.

you mean this Mxy - "Mxy represents the twist, that is, the rate of change of slope in the x-direction as one moves in the y-direction or vise versa. The twisting moment results in shear stress parallel to the plate surface except near the ends." Somehow the idea of apportioning loads to the moment Mx, My and Mxy in FEA and equivalent frame ect being different is lost on some people.

As for the disclaimer, it is blurb alright, a right lot of BS. I believe Doug's approach is the correct approach.





Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[rapt]

OzEng80,

Following on from my earlier post on local shrinkage tests, I have just spent an hour discussing it with Ian Gilbert.

A summary of our discussions follows,

- 56 day tests on shrinkage test prisms do NOT give a true estimate of the Basic Shrinkage Strain used in AS3600 clause 3.1.7.2. Earlier versions of AS3600 that suggested this were incorrect. That is why it has been changed. So the tests you local supplier is basing his numbers on are not correct, or at least the results are not anyway as a Basic Shrinkage Strain!

- We are still formalising an approach for people who want to test their own local concrete to determine values by test, but the essence of the procedure to follow is
- Do 2 - 3 month shrinkage tests on concrete test specimens of a certain size, under constant humidity and zero stress (so that there is no load induced stress, no restraint to shortening or creep possible), so that the only strain on the concrete is shrinkage.
- Based on the Theoretical Thickness of the specimens, time of tests and humidity, calculate from the AS3600 clause 3.1.7.2 factors for k1 and k4, the total expected shrinkage over 30 years (basically extrapolate the answer from the shrinkage time curves and humidity effects).
- Theoretically you need to allow for autogenous shrinkage that has occurred in the test specimen, but autogenous shrinkage is very small for lower strength concretes normally used in floors.

So it is all a lot more complicated than some simple 56 day shrinkage prism tests, unfortunately! We hope to egt something more formal describing this in future when a commentary to the code (in some form) is released.

 

[hokie66]

Must be a slow day. Concrete shrinks. How much? Anybody's guess.

 

[OzEng80]

Hokie – seems to apply to concrete deflection as well…

Rapt - Bugger. I was surprised that I got a response out of the local manufacturers as it was - It will be an age before they will be able to provide something that meets those guidelines!
So is AS1012 going to be updated? For that matter what about AS2870 (and the CCA paving manuals etc) that have joint spacing and reinforcing determined from shrinkage (I’m assuming 850 MS)? It was my understanding that these weren’t entirely empirical based.

Thanks again.

 

[rapt]

Hokie66,

But to try to optimize our designs we like to get our guesses with reasonable agreement with what is actually happening.

Using something like kcs factor and Bransons formula from ACI code could leave you expecting 10mm deflections and getting 50mm. This sort of inaccuracy does no go well with glass curtain walls and movable partitions, marble floors etc.

I know you do not think deflectioon calculations are worth the electrons wasted calculating them. That may be a hangover from you US heritage. We like to think different and have a reasonable reputation with RAPT of at least getting our answers for deflections within the right ballpark.

To do this, we need reasonable estimates of shrinkage properties of concrete, and if someone thinks they have better concrete than the code is assuming then they should be able to estimate its effect on deflections.

 

[rapt]

OzEng80,

Yes, I was surprised when I noticed the difference in wording and had it explained to me too.

I once asked a concrete supplier years ago for shrinkage test results for the concrete used in a slab to try to explain some restraint cracks, and was handed a sheet of results that was 10 years old!

Ian understands the problem know and hopefully something will be done to explain it all properly.

RE getting other codes updated, no one knows what is happening with new codes now. Standards still has not sorted out how it is going to fund/produce any new code! I would not think anyone has thought about the effects of all of this on other codes! maybe they will have to satrt paying som,eone to put the effort in to do it all properly, rather then relying on freebys provided by us suckers do do it for them!

 

[rowingengineer]

There is a draft code out for AS2870 at the moment, haven't made it past the first page yet.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it