Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


Long Term Deflection & Compression Reinforcing

Long Term Deflection & Compression Reinforcing

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 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.

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!

RE: Long Term Deflection & Compression Reinforcing


1. Cannot comment on your first three questions, however, I would use a triangular stress distribution when considering service loads and hence calculating deflections.

2. I believe Prof. Ian Gilbert who chairs the sub-committee is against kcs factors for calculating deflections. I find it to be a very helpful multiple when calculating additional deflections due to creep and shrinkage. When I say I find it a "helpful multiple", I mean using a multiple of 2 gives my spreadsheet deflections very similar answers to what RAPT (software) does.

3. I would never use a kcs of less than 2.0 for slab design because it is not industry practice to reinforce slabs in compression regions hence no help will be provided by the reinforcement to reduce the deflections.

I have attached my spreadsheet.

The process I go through basically follows:

1. Calculate service moments midspan and calculate Ieff based on these service moments.
2. Calculate instantaneous deflections based on short-term loading and Ieff (Normally G+0.7*Q).
3. Calculate additional deflections from creep and shrinkage based on Ieff and kcs=2.0 (2.0*G+2.0*0.4*Q).
4. Sum the two to give a total long-term deflection.

I have only ever used this approach to verify the outputs I have obtained from RAPT (software).


RE: Long Term Deflection & Compression Reinforcing

I believe RAPT is in the UK at the moment on holidays I imagine, having fun sun banking on their beaches on doubt.  So I think you may have to wait a while for him to chime in.

Ian Gilbert says "The use of the deflection multiplier kcs to calculate time-dependent deflections is simple and convenient and, provided the section is initially cracked under short term loads, it sometimes provides a 'ball-park' estimate of final deflection. However, to calculate the shrinkage induced deflection by multiplying the load induced short-term deflection by a long-term deflection multiplier is fundamentally wrong. Shrinkage can cause significant deflection even in unloaded members (where the short-term deflection is zero). The approach ignores the creep and shrinkage characteristics of the concrete, the environment, the age at first loading and so on. At best, it provides a very approximate estimate. At worst, it is not worth the time involved in making the calculation."
(I doubt I can say it any better.)   

From the paper:

I have also heard Ian say you should never use a KCS less than two if you must use the kcs method.

I believe you should treat the compression steel as design compression steel, such that you do you complete analysis as you would in a beam and then  % of yield is % of effectiveness. So you would use the compressive block after analysis with the compression steel.

Now the real question is what about PT slabs??? That one I can't answer I don't know how you would ever get a reasonable answer from the KCS method for PT slabs.

Should KCS be removed, from the standards? YES, it should, we have the ability to better estimate the deflection these days, thus KCS should be put to death, I believe it has only survived this long because of Finite element programs, that needed the KCS to do the deflection calcs in an easy fashion.

As for doing what everyone else is doing, be careful here, I don't use compression steel in slab generally and neither do any of the designers in my office, so you may be the only person using compression steel in slabs. And if you find anyone whom is using the compression steel shoot them down.

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

RE: Long Term Deflection & Compression Reinforcing

Thanks for the responses.

If Kcs = 2.0 then Acs = 0 so I guess it should not be referenced in the slab section. I will go through the paper and try and get my head around it better.

In regards to industry practice, it is common place in my region for entire top and bottom mats of reinforcing to be provided (with additional top reinforcing as required). It is apparently more cost effective to do this (less tying and individual bars). I have specified slabs with staggered/curtailed and discontinuous top mats only to arrive on site with the builder opting to put an entire top mat in (note that I tend to deal more with residential applications with short span, thin slabs). To this end it is commonplace to have reinforcing on the opposite face to the flexural reinforcing (I am too wary to call it compression reinforcing now!).

I have only worked in two offices but in both it was considered appropriate to use compression reinforcing with an upper limit of Asc of about 50-60% of flexural – Kcs of about 1.4. Through another project I was in contact with a senior engineer who insisted that an Asc of up to 100% could be adopted. I have a set of drawings in the office at the moment for another consultancy for a previous stage of a project where the slab thicknesses can only be justified by a Kcs less than 1. Bit of an epidemic really – unless I am just mixing with the wrong people!

Thanks for the spreadsheet asixth – I have attached mine (which needs a major rehash) since we are sharing.


RE: Long Term Deflection & Compression Reinforcing

What part of Oz are you in? Adelaide?  

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

RE: Long Term Deflection & Compression Reinforcing

Cairns. What have got against Adelaide?

RE: Long Term Deflection & Compression Reinforcing

Who would go to the UK in winter for a holiday. There is no sun on the beaches (is there such a thing there, beaches or sun as I did not see either), or anywhere. It was freezing, and it was worse in Europe where we were snowed in a couple of times! Fortunately I was busy working all of the time spreading the RAPT gospel in training (mainly) and marketing sessions so I was in heated comfort, except when I had to walk to the train station (about 1000m) in a raging blizzard that grounded all flights out of Copenhagen that night, including mine).

RE the topic, RowingEngineer and Asixth have covered it pretty well.

The comment on the code about compression reinforceemnt having to be in compression was a compromise but was meant to emphasis that just being at the opposite face to the tension reinforcement does not make it effective compression reinforcement. Personally, I think that it has to be at least in the top half of the compression zone to be effective.

RE the compression zone to use, yes the cracked elastic stress zone places the neutral axis much higher than would be the case allowing for the effects of creep and tension stiffening. So it would have an effect at lower depths than the cracked section analysis would show but the effect is not great until it is fairly high in the compression zone anyway, so I would ignore it until is in the cracked section compression zone anyway in terms of kcs.

Even in heavily reinforced RC beams, I have never seen the effect of compression reinforcement be the equivalent of the Kcs factor of .8. Equal top and bottom reinforcement can have an effect from 0 (no effect) to maybe a maximum of 20% in a very heavily reinforced member deeper memebr, never 50% reduction in deflection that a Kcs factor of .8 might suggest.

RAPT actually calculates the strain and stress patterns for the section allowing for shrinkage, creep and tension stiffening (as long as the default tension stiffening method is used, not Bransons guess formula) so it allows for this effect for you and shows the real (relatively) effect of tension face and compression face reinforcement.

From what we have found,
-  compression reinforcement has little effect in lightly loaded memebrs and shallow members (eg slabs). It is more useful in deeper members, rectangular beams without a flange (if you can ever find one) and more heavily reinforced members.
-  extra tension reinforcement is more useful in lightly reinforced memebrs

RE PT members, no code suggests the use of Kcs for PT members. Some software developers use it because it is simple, and they do not understand what they are doing, but it is not correct. ACI and AS3600 only use it for RC members specifically. For instance, if the PT balanced load exactly equals the permanent load, there is no permanent deflection so ther is no creep/shrinkage deflection as kcs * 0 = 0! This is patently stupid as there are still creep and shrinkage effects causing deflection.

R.E. (not you RE) the other designers, yes it is a problem and the code is part of the problem as has been aluded to above. All I know is that people who have done suryeys of slabs after a long period (20 years of service) have all indicated that RAPT's results are in the ballpark of what they are measuring on site, while their code checks have been way out and very unconservative in estimating deflection effects for slabs. The general conclusion is that, unless you account for it properly (not using code simplifications) ignore compression steel in deflection calculations.

RE: Long Term Deflection & Compression Reinforcing

RAPT good to see you survived.

Following on from your above comments, how about compression steel over a column. I would have thought compression steel would be very helpful at this point.

Nothing against Adelaide, just had a few discussion with engineers from that area, and they seem to consider the codes to be books of untruths. Mind you this is only 3 engineer of many so I shouldn't be so harsh.

So are you a JCU man then? I'm an NQ man myself, and attended JCU a while back, I believe Pat Murray is the concrete design lecture these days, Might be worth giving him a call to see what the local NQ standard is, but I can tell you in my office Kcs = 2 for slabs is standard.


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

RE: Long Term Deflection & Compression Reinforcing


Midspan reinforcement seems to have the most effect on deflection, either tension or compression reinforcement depending on the situation as described above. Modifying the reinforcement at the supports does not seem to be nearly as useful.

Are there more than 3 engineers in Adelaide! Actually most of the designers I converse with in Adelaide would never use Kcs of .8, more normally 2 except for deeper members, but then I suppose they are RAPT devotees if I am talking to them so they believe us (and their slabs do not deflect too much!) and try to calculate it properly rather than wasting their time guessing re Ian Gilberts quoted comment.

I did not realise that NQ concrete performed differently to that in other areas of Australia!

RE: Long Term Deflection & Compression Reinforcing

RE - UQ actually, but I just designed the JCU Dental school if that count's for something. Ironically, the engineer you mentioned may have been the one that insisted that 100% Asc could be used (the engineer's company was doing construction supervision of a project and were advocating the removal of a precamber that I had specified on the basis of the compression reinf)..... I trust the wording of this statement is suitable? Let me know if it's not?

We should catch up if you come up this way? I seem to recall you owe me a beer!

RAPT - concrete is the same in NQ but the higher wind speed helps to hold the floors up.

Thanks for all the help – much appreciated as always!

RE: Long Term Deflection & Compression Reinforcing

Interesting about the support steel, I have always lapped my bottom steel over columns hoping to reduce deflection.

as for concrete in NQ, It doesn't shrink as much, due to the high humidity. Roof tops and walls can sweat if too skinny. other than that concrete is concrete, not that any of these are concrete properties just behavior.

I suggested he call Pat so that Pat can tell him that they use Kcs of 2, more because if Ozeng 80 hears it from Pat he may believe it than a guy on the net by the name of rowingengineer.  

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

RE: Long Term Deflection & Compression Reinforcing

Interesting discussion.  I try to control deflection with adequate depth, but do admit to throwing in some "compression" bars from time to time.  RE, I never figured the lapped bottom bars at columns assisted much in deflections, but they do a lot for overall robustness.  Having said that, I can remember several occasions when I used a bunch of compression bars to limit the deflection of cantilevers, tried to compute how much there would be, and had much less than I expected.  Calculating deflections in concrete structures is a very imprecise thing.   

RE: Long Term Deflection & Compression Reinforcing


I agree with Hokie66 that the best reason for lapping bottom steel is for robustness. It does wonders for ductility of connections (punching shear failure capacity is increased immensly) plus helps a lot for high wind/earthquake effects and also in accidental damage situations (eg if a column fails) that some codes now require (BA8110, EC2).

Interesting comment about shrinkage in NQ. Don't they have airconditioning!

In Cantilevers, if they are beams, compression steel will be more effective in them than in a normal beam as the flange is not at the compression face, so the neutral axis depth is normally much deeper!

RE: Long Term Deflection & Compression Reinforcing

I'm always keen for a beer, you should comment on my FAQ. Forget I mentioned Pats name then if he is giving out that advice, He is considered to be the local expert and I would have thought he would have know better, BUT I guess the guy whom suggested it was named Andrew as Pat has been unwell.

I will give Pat a call and chew his ear when i get the chance.  

I think if you add tension and compression reo your transformed modulus would increase, and shrinkage deflection would reduce, but Kcs doesn't tell us this. Yes I agree it dose do a lot more then help with deflections, But i would like to test my theory. But alas I doubt I will ever get the chance, maybe I can swing a uni to do it as a thesis.  

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

RE: Long Term Deflection & Compression Reinforcing

RE, I agree, except that at supports you often have a deeper section for some width (beam, band, column) which already acts to stiffen that area.  Bearing walls would be the exception.  In adding compression steel to help with deflections, I have always thought of it as making the concrete act like steel as much as possible, where the reinforcing forms the flanges.  Deflection of steel members is predictable.  The problem with that analogy is that the webs are not steel.

RE: Long Term Deflection & Compression Reinforcing

A few comments:

1. Nice spreadsheet OzEng. I am going through it and seeing if there is anything I can steal for my spreadsheet.
2. I'm keen for a beer.
3. How is the job market in Cairns and Adelaide. I'm ready to up and leave Brisbane.
4. The basic shrinkage strain for Cairns recently increased from 850E-6 to 1000E-6. What I am led to believe is that local aggregates can greatly affect properties of the concrete such as shrinkage and elastic modulus. So concrete's aren't concrete's.
5. The kcs approach doesn't account for the creep and shrinkage characteristics of the concrete, the environment, the age at first loading etc. But does anyone go into RAPT (software) and change the relative humidity from 50% or the time of loading from 10 days? I am unfamiliar with Inducta or the percentage of the Australian market that is using Inducta at the moment.
6. I have come across two buildings lately constructed in the 1980's that in my opinion would not satisfy today's standards or design methodologies. One of the buildings is our office building (designed by others) which has very noticeable end span deflections. I haven't surveyed it but I would expect the deflections to be in the order of 40-50mm for an 8400 span. Tomorrow afternoon, I may survey the slab to measure the deflections, and compare to software (and my spreadsheet) to see how accurate the calculations are. See the attachment, span between Grid 4 and 5 on Grid C, 8400 span, 250mm thick slab. Of course, this will all depend on how many beers I have.

RE: Long Term Deflection & Compression Reinforcing


1. Help yourself.
2. Me too
3. Can't speak for Adelaide but Cairns is still in a hole. Apparently one of the worst unemployment rates in Australia. I thought things were starting to pick up a bit – but today we had a SE with 5 years experience apply for a drafting role... Not a good sign.
4. I had no idea. Where did you get that info?
5. We have inducta (and RAPT) which contains humidity and hypothetical thicknesses etc. as well as the ability to analyse to other standards. I haven't played enough with it yet, but the Australian method of analysis results in significantly larger deflections then the other codes. The software still relies on you to enter your own load combinations (and therefore interpret Kcs).
6. At least you have drop panels. I am working on a new classroom block which is basically a carbon copy of 3 previous stages since 94. The classrooms have 8200 clear spans 220 thick with a short tapering (to 170mm) balcony cantilever. Classing this as an 'end span' (which is pushing it) and using a Kcs = 0.8 (Asc was less than Ast even if it was effective) results in a 40-50mm deflection. I went out there today expecting to have to duck my head into the classrooms or to roll an ankle as I negotiated the steeply sloping floors – but alas, there was no discernible deflection. Given that two of the blocks are now 15 years old would make it pretty hard to justify the 25% increase in slab thickness (though the architect hasn't noticed – yet).


RE: Long Term Deflection & Compression Reinforcing

Some comments from the heavier end of structural engineering:

It is standard practice in non-building structures to reinforce every face, in fact it's a requirement of the Bridge Code (AS 5100), but I agree with earlier comments that for thin slabs the "compression" reinforcement is likely to be at about the level of the neutral axis, and will make little or no difference to deflections.

In recent years I have been involved in a number of projects where deflections were much greater than predicted in statically determinate structures, where prediction of deflections would appear to be simple.  The main factors that I have found to be neglected (or underestimated) in a simplistic analysis are:

1) Tension stiffening is much less than predicted by the "Branson" equation for lightly reinforced members with maximum moments just over the cracking moment.

2) The reduction in cracking moment due to shrinkage stresses has a huge effect on deflections, and must be properly taken into account.

3) Differential thermal stresses may also result in a significant reduction in cracking moment, especially if the tension face is in full sun.   Even in the UK!

4) After cracking the section is asymmetric, and shrinkage will have a significant effect on deflections, even if the section is symmetrically reinforced.

5) It is important to take account of temporary load sitiuations during construction or due to live loads, if these are higher than the long term loads.  Cracked sections stay cracked!

Taking account of these factors I have found a good match between measured and predicted deflections, with the deflections being about three times greater than predicted by a simplistic analysis, complying with code requirements.  

More details are in a paper I presented at the recent Concrete Conference in Sydney:

Finally it should be emphasised that in many situations the best we can do is come up with an upper bound prediction of deflections.  In real structures deflections vary widely, even in nominally identical members.  Actual deflections will be much less than a predicted upper bound if for any  reason a section that has been predicted to crack remains uncracked.  The best we can do is to consider the consequences of the upper bound deflections occurring, and if these are unacceptable take measures to reduce them.   

Doug Jenkins
Interactive Design Services

RE: Long Term Deflection & Compression Reinforcing

I think you know what PT3D is, maybe you should ask Jeff what he thinks of this statement taken from there home page (maybe it is a typo) But i knew at least one PT company had stated that they were using Kcs for deflections.

"Evaluating of the deflections using Ieff

Based on the adopted reinforcement, PT3D will evaluate Ieff in each point, in both orthogonal directions and will calculate the short-term deflections based on the reduced section properties.  The long term deflections are evaluate using Kcs factor, and increased load factors."


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

RE: Long Term Deflection & Compression Reinforcing

Thankyou Doug for your input, nice paper.

Following on two points, I would like to discuss shrinkage and thermal stresses; these are all related to restraint, either by reinforcement or supports.  The reinforcement is an easy restraint to account for as there are papers dealing with this issue, however there are very few references dealing with support restraint forces, and associated deflections. The only method I know of that attempts to account for support restraint forces is the PCA method. Then I use a bit of bluff and mirrors and convert this from forces in the member, to a reduction in flexural strength.

This is one area that the AS codes do not treat at all well, with one clause that is a bit puzzling giving extra reo based crack control and if is restrained or not, with a partially restrained clause giving you judgement choice.  But no discussion about restraint associated deflection.

1.    I noticed Asixth you have the ACI method in your spreadsheet, you may wish to down load Alex's metric version, for the other parts (shear & torision).
2.    Beers is good
3.    Job markets all over the place are slow, but if you're good you will find a position. I am/have changed jobs twice in the last 8 months, no problems. Once forced by conflict with my employer and then got an offer I couldn't refuse. In Brisbane/Townsville at the moment a few companies looking for Good people.
4.    Where did you get that info asixth? Is it just cairns or Townsville too?
5.    I have access to both RAPT, Inducta and Strand7. I change all the inputs in everything be it  RAPT, Inducta or Strand7. From a  marketing point of very RAPT hasn't spent much time in the NQ, so Inducta is more established, but is developed by a software guru not a structural engineer as RAPT.  Inducta have just made available the AAEM and EC2 methods for deflection, I would recommend any engineer use these over the Kcs method. Inducta also has a note in there manual and will give out advice that Mxy can be ignored for slabs due to what the standards have told them. This is incorrect and RAPT will post a message soon showing the standards stance on this issue.
6.    With older building you have to remember that often the construction pace was slower thus suspended slabs were on forms for longer and the concrete agg was better generally. Also with deflections, I am less conservative in my modelling then with strength design. Make sure you don't be to conservative int eh deflection modelling, with regards to column stiffness  ect.

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

RE: Long Term Deflection & Compression Reinforcing

I had way too many beers last night.

RE: Long Term Deflection & Compression Reinforcing

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 of the 2001 codes states that the basic shrinkage strain is to be taken as 850E-6 while Clause 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.


RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing


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.

RE: Long Term Deflection & Compression Reinforcing

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.

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing


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

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing


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

" Calculation of design shrinkage strain
The design shrinkage strain of concrete (εcs) 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"

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 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

RE: Long Term Deflection & Compression Reinforcing

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

" Calculation of design shrinkage strainThe design shrinkage strain of concrete (εcs) 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"

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 calculations. The only shrinkages that can be used in 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.

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing


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 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 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.

RE: Long Term Deflection & Compression Reinforcing

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

RE: Long Term Deflection & Compression Reinforcing

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.

RE: Long Term Deflection & Compression Reinforcing


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.

RE: Long Term Deflection & Compression Reinforcing


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!

RE: Long Term Deflection & Compression Reinforcing

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close