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PT design Ieff=Ig

PT design Ieff=Ig

PT design Ieff=Ig

(OP)
following from thread "Post Tensioning as a Deferred Submittal" a discussion about Ieff=Ig for PT has been sparked, trying not to take over this thread I have started a new thread.

Lion06 had mention that Ieff=Ig is allowed by the ACI code, i believe this is in reference to ultimate design, and not serviceable design as Asixth was describing.

In most flat slab PT designs in Australia the design is for PT bonded combined with reo, thus it is likely that the slab is going to crack under loading at some stage. so a Ieff=Ig assumption is not correct and needs to be reviewed for each loading case. However I limit my maximum Ief to 0.7Ig.

The reason we do this is for restrained shrinkage and other similar effects.  

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: PT design Ieff=Ig

(OP)
extract from previous thread:

Lion 06
"I don't think there's anything dodgy about using Ieff/Ig = 1.0 for slabs.  ACI explicitly allows you to do this.  I do this for PT design and every engineer I know that does PT (and precast, prestressed) does this as well.  

Is there any literature out there that says that you should be using something other than Ig for slabs?"

Rapt:
"Lion06,

Just because ACI allows something does not mean it is correct. ACI and Bransons formula both grossly underestimate deflections in lightly reinforced RC members. This has been shown extensively in tests. But ACI still allows it.

Part of the reason is Bransons formula for tension stiffening which is completely incorrect at the point of cracking, saying that there is no increase in deflection when the first crack occurs (Ms = Mcr). The other problem is that a lot of stresses that are created in a memeber are ignored by most designers eg shrinkage restraint by internal reinforcement (in a normal member this will be between .5 and 2MPa, depending on how heavily reinforced it is). So some sensible codes have allowed for this in an attempt to get results from simplified code deflection calculation methods to match real world deflections. There was a paper several years ago in an ACI journal by Gilbert that gave an example of this. Actual deflection of the slab after 9 months was 29.5mm, while ACI predicted about 9.3mm (interestingly RAPT got 30.1!).

In PT design, shrinkage restraint by the reinforcement is much less as the percentage of steel is reduced. But it still occurs.
But the other thing that is ignored in most deflection calculations is tension stress induced by external restraints to shrinkage and temperature change shortening. This just about always exists and in many structures is very significant. It needs to be allowed for. One way, without calculating it, is to make a blanket assumption that Ieff is limited to a percentage of Igross. Another way is to add a tension stress into all sections in the member, or alternatively, reduce the tensile strength of the concrete by an equivalent amount.

The most stupid thing to do is to say ACI does not mention it so I do not have to worry about it. ACI does mention it where it says shringage and temperature effects must be considered in calculating deflections. Most designers simply ignore this as either "too hard to do" or as "I dont know how so lets forget it" and put their heads into the sand."

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: PT design Ieff=Ig

(OP)
I use both the maximum 0.7 Ig and also the volume change design out of the PCI. I use the volume change to adjust the tension strength. I did like the idea of a tension force, due to it being non repeatable for all problems.  

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: PT design Ieff=Ig

0.9 Ig for stressed slabs. Push it to the limit.

RE: PT design Ieff=Ig

Quote:

I don't think there's anything dodgy about using Ieff/Ig = 1.0 for slabs.  ACI explicitly allows you to do this.  I do this for PT design and every engineer I know that does PT (and precast, prestressed) does this as well.

What section of ACI states this?

RE: PT design Ieff=Ig

ACI requires that prestressed two-way slabs be designed as Class U with ft < 6 (fc')^0.5.

Table R18.3.3 states that gross section properties to be used for deflection calculations per 9.5.4.1

RE: PT design Ieff=Ig

Slickdeals,

And clause 18.3.3 changes the class U limit to .5 for two-way slabs instead of .62 because an average moment is being used. This is still not adequate as the stress at the support in the "column strip" zone will still be much higher and this area will be cracked.

Ok, but in calculating ft, you must take into account restraint to shrinkage and temperature shortening.

Most designers ignore restraint stresses except in heavily restrained cases. And even in those case, they only consider it for crack control, not deflections! It needs to be considered for deflections also if you are going to assume full Ig.

RE: PT design Ieff=Ig

I'm skiing for the weekend, so I'll respond when I get back, but I'm almost 100% positive that ACI 318 allows use of full Ig for deflections for class U slabs, which a two way PT slab is, by virtue of limiting the tensile stress to 6(sqrt f'c).

RE: PT design Ieff=Ig

Just to ask the question.  From a practical standpoint, where do you see a real benefit in PT over a mild slab?  You're using less mild steel, but you're not really gaining anything with respect to deflections.  This is not a big deal on interior spans, but what about the exterior slab edges?  I try to keep the deflections at the slab edges to 3/8" to respect a 3/4" caulk joint with 50% compressibility.  

The rules of thumb that I've learned (from my mentor and several PT suppliers, including at least two who sit on ACI committee 423) are that the slab thickness for a two way PT slab is the thickness is span/40 (to 45).  This is significantly more lenient than for a mild slab.

Additionally, the same people have recommended using Ieff=Ig.  One thing that I've learned is that it's not good to be viewed as an engineer who is overly conservative merely for conservatism's sake.  I've been burned on that before and it didn't feel too good (and the reason for being conservative was cometely justified based on the schedule of packages).  

I'm all about learning, and I admittedly don't know all there is about PT, but to reject out-of-hand something that ACI explicitly allows seems foolish, IMHO.

So my logical next question is this - if you assume Ieff<Ig then that implies at least moderate cracking (beyond shrinkage cracking, I get that concrete cracks).  How does that get reconciled with the requirement to have the tensile stress in the concrete be less than 6(sqrt f'c)?   

RE: PT design Ieff=Ig

Will those same people call you too conservative when you have got to defend the engineering design for a floor slab which deflects too far to be serviceable? They didn't for me.

RE: PT design Ieff=Ig

(OP)
Lion06,
your confusing me I must be getting old, with max Ieff=.7Ig your deflections will still be less than a mild steel slab of the same thickness. Yes for two way flat slabs the difference will be smaller . The main advantage for PT from my perspective is in the speed on construction. However I am a fan of the one way slab and band beam approach or drop panel slabs.  

As for slab edges I am a fan of the edge beam, reduces deflections in this area and gives you some help with regards to punching shear.

as for tensile stress in the concrete be less than 6(sqrt f'c), one is a theoretical requirement and the other is practical knowledge.

I don't believe 0.7Ig is conservative, I very rarely have Pt designers come back to me and request thinner slabs. Mind you the guys in my local PT supplier have been burned a few times on promising deflections requirements and not delivering.

I like to have all my eggs in a basket, such that if I am ever taken to task by another engineer I can justify most things, funny thing is that rarely when you go to court do they care about about the 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: PT design Ieff=Ig

Lion06,

You need to look in detail at the effects of different things on design.

PT will reduce cracking and curvature which reduces deflection.

If you are checking deflections on a flat slab/two-way slab on columns, calculating it based on the full panel width as per the ACI PT flat slab design method is useless. Column and middle strip zone deflections are completely different for PT flat slabs just as they are for RC flat slabs. The amount of load carried by each zone is completely different, the stresses and amount of cracking in the different zones is completely different, but ACI says to use an average, so that sklabs that are really cracked around the columns show as being uncracked according to ACI.

If the "experts" you are refering to are supporters of the ACI PT flat slab rules, then I would not be listening to them. The rules are completely wrong and very misleading. You cannot defy equilibrium and statics.

We are not rejecting it out of hand. I have been rejecting it for over 30 years because it is wrong. If you think a PT flat slab works the way ACI says, you need to go back and study plate behaviour.

You are not including all of the stresses in the slab in your calculation of the applied stress. So you cannot compare it to the tensile strength of the concrete. If you do include all stresses, then yes you can. This includes the distribution of stress around different areas of the slab, internal restraint to shrinkage shortening from the reinforcement, external restraint to shrinkage and temperature shortening due to columns, walls, beams etc. If all of these are calculated and included, then es you can use the tensile strength of the concrete. But all you are probably calculating is the vertical load stresses. And you are probably even using an averaged gestimate of the prestress losses rather than calculating them properly.
If you include all of the effects then there is no problem, but you do not. That is why so many PT buildings in USA have major cracking problems (though PTI will never admit it) and also deflection problems.

RE: PT design Ieff=Ig

The only thing not included in my PT analysis is restraint from mild steel and temperature.  Any external restraint is captured by the FE analysis and the inclusion of the columns and shearwalls above and below.

Regarding the internal restraint to shrinkage from mild steel - I believe this is extremely small and essentially negligible.  If you look at a 7.5" thick slab of 5000 psi concrete with #4 bars @ 18" o.c. bottom and assume the top bars over the column match the #4 bars @ 18", then, on a per foot basis, the EA of the concrete is 362750k and EA of the steel is 7714K.  Therefore 98% of the precompression is being taken by the concrete.  

Thermal volume changes are small if it's in a conditioned space and the temperature differential is small.

This really just leaves creep and shrinkage.  I'm just not convinced that they play that big of a role.

I'm by no means an expert in PT, but just telling me that ACI is wrong and I need to revisit plate theory really does nothing to sway me.  Not that you need to sway me, but it's not an effective way to make an argument.

Also


Rowing-
I agree that Ieff=0.7Ig results in deflections that are less than a mild slab, but not by enough to justify the bump the span/20 to a span/40 for slab thickness.  

 

RE: PT design Ieff=Ig

(OP)
Lion06,
you are losing me again, why the span/20? are we talking two way slabs still?

here is a previous discussion in regards to the full width assumption were rapt expands on his reasons for calling the ACI and PTI design assumption incorrect. thread588-202462: Post-tensioned slab applicabilty

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: PT design Ieff=Ig

Lion06,

What FE analysis are you using to get all of the shortening effects?

If the time at pout is 35C (very possible depening on the location) and airconditioning  is set to 20C, there is a 15C degree temperature difference that needs to be allowed for.

I have made this arguement several times on this site (thanks for ther reference RE) and am not inclined to go through it all again. Though any engineer who understands flat slabs should be able to work it out for himself, which is what I was trying to encourage you to do!

RE: PT design Ieff=Ig

I'm using RAM Concept.

RE: PT design Ieff=Ig

Lion06,

Unless it has changed recently, RAM Concept only considers shortening due to prestress. It does not consider shortening restraint effects due to shrinkage etc. Shrinkage/creep effects are only considered in the prestress loss calculations and in long term deflection calculations if you select the top tier deflection design method. Restraint to shrinkage effects etc are NOT included!

RE: PT design Ieff=Ig

Why are the restraints not included.  If you have the shearwalls modeled and the boundary conditions in and it's doing a true FE analysis, how is that restraint not being accounted for?

Also, why does it matter if the actual prestress force accounts shrinkage and creep with the loss calculations or if the program physically takes it into account?  What is the difference?

RE: PT design Ieff=Ig

Lion06,

Have you ever done in depth (complete) design calculations by hand? You obviously do not understand computer software and FEA.

A computer program does what the programmer tells it to do. FEA can be programmed to do what the programmers wants it to do. If the programmers have included a shrinkage model/loading into the program and added it to the other load effects, it's effects will be included. If they have not, it will not be included.

RAM Concept does not include a shrinkage loading into the analysis so there is no allowance for shrinkage restraint in the design. They do give the option of allowing for restraint to axial shortening from the prestress force if you want to include it.

They have allowed for a reduction in the prestress force due to shrinkage, but this is not an FEA analysis, it is a pre-processor calculation of the prestress force which is then used for analysis.

RE: PT design Ieff=Ig

(OP)
Lion06,
There are a number of different elements to restraint design, these are based on a number of different aspects and has not as yet been fully tested. I doubt even if you program was using a shrinkage design value in the loading that the results would be any more correct. truth is that restraint shrinkage is occurring while creep, reo restraint are progressing.

This makes it very hard to define and give an appropriate number during analysis. The designer needs to be aware of the situation and include a appropriate adjustment to account for this within his design. This can be conducted in many different manners.

I once wrote an article that I hoped to have published, but alas that never happened. I have posted a copy of this article under another post as I will be seeking discussion of this article for all perspectives. I hope you will take the time to review this article, it discusses FE modeling and associated interesting aspects. I also hope you will provide comment on the article.   

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: PT design Ieff=Ig

I have not gone through the calcs by hand.  I do wish I had the time, but, to be honest, I'm backchecking one software with another.

As for the program taking shrinkage and creep into account........ I'm not saying the model does that.  The point I was trying to make is what's the difference whether I let the program do it and I use the actual jacking force for the cables or if I neglect the creep and shrinkage in the model, but use a final effect force that is reduced for creep and shrinkage?  I'm not being a smartass, I'm honestly asking the question, because I'm not seeing the difference. Why is this any different than the friction and seating losses.  You don't model the friction of the tendons or the actual wedges getting seated, but it still works out ok, presumably because you account for this with the prestress force used.  What's the difference?

As for the FE analysis accounting for the restraint from walls - how is this not accounted for with the analysis.  The walls (with their correct stiffness) is modeled and connected to the slab as it will be constructed.  The walls are preventing the slab from shrinking.  I believe this to be true, because I've done some trial runs of identical strips with and without walls at the ends and there is a difference in P/A, so that leads me to the conclusion that the analysis is capturing the restraint offered by the walls.

RE: PT design Ieff=Ig

Lion06,

1   How do you know which one is correct, or if either is correct?

2   We are talkin about 2 different things.
-  You are talking about the effects of shrinkage and creep on the prestressing force. Yes, creep and shrinkage reduce the prestress force and traditionally this is calculated before the analysis and included as an effective prestress force. WE are not disagreeing with this.
-  In determining the stresses in a concrete member, vertical loads and prestress will cause stresses that we account for. Most designers ignor other stress in the member that are less obvious and harder to calculate. The biggest of these is restraint to shortening from shrinkage and temperature change. These are the stresses we are saying need to be included in the calculations if you want to use Fct as the tensile strength of the concrete and Ieff = Ig in deflection calculations.

3   It is not accounted for in the analysis because the shrinkage is not modelled as an effect that produces stress and strains in the concrete in RAM Concept and other design programs.
I told you that they allowed for the shortening effect on the prestress force. And that is what you are seeing. Its effect has been modelled in a simplistic way that is giving a result which is not really correct, but it has been modelled. Construction sequence needs to be included in the modelling to get more realistic answers but cannot be done currently.
But you are not seeing any stresses induced in the concrete by restraint to shrinkage and temperature change, because neither effect has been modelled in the program.

You cannot look at results from computer programs whose internal workings you have no idea about or understanding of and make the assumptions that you are. That is not engineering. You should not even be using software if you have no design experience in the area. Complicated software is for use by experienced designers. And you are not one in this area by your own admission!


 

RE: PT design Ieff=Ig

I tend to agree with rapt.  We should all stop using software written by someone else, as none of us understands the internal workings.

In other words, back off, rapt.  Lion06 is not one of the irresponsible ones.

RE: PT design Ieff=Ig

Hokie66,

You raise a philosophical arguement that I have been debating with myself (worrysome) and others for some time. Is design software the cause of declining engineering standards? Is software a black box because
-  it is written by someone other than the user,
-  or is it because the user has not investigated what it does properly and so does not understand it
-  or is it because the user has no experience in the field and expects the software to be 100% correct no matter what input is supplied and to replace his lack of understanding of the design area?

I suggested in one of my responses that RAPT is a black box and received a private reply below
"also I disagree with rapt being a black box or grey, your help manual, (while the manual is slowly getting out of date) has the theory for what a program is doing this is better than most programs. I don't use black box solutions."

When you start thinking about it, if software is a black box, text books and design codes are also. Incorrect interpretation of or illogical data into those will also suffer from the garbage in - garbage out problem. I have had a lot of experience with people doing this in design codes when they have questioned RAPT's results compared to their hand calculation from code formulae. Some of the interpretations of code rules and manipulation of data to get the desired result is amazing.

So should everyone go back to developing their own design theories because text books and codes are open to interpretation?

After all of my debate on this (with myself and others), the conclusion is that
-  engineers are responsible for understanding the design area they are involved in. They should not use design tools of any sort until they understand the theory and practice in that area, and then they should not use the tools productively until they understand the tool. The theoretical education of the designer/user is not the responsibility of the software company.

-  Software developers should explain what their software does and put in the effort to try to make the software as useful to the designer as possible in applying the rules logically and correctly, and market it ethically not in a way purely to boost sales!

And all of this gets down to education of designers, both at university and in practice. To me, that is Lion06's problem as he is not being given time to to what he should do (by his own admission). Is that my fault! Is it his bosses fault? Is it the industry's fault?

This forum can help people with questions on design and interpretation of design logic. It cannot teach designers a complete area of design.

RE: PT design Ieff=Ig

rapt,
I have debated the same philosophical questions myself over the years.  The first programs I used were self written, stored either on punch cards for running on an IBM360 (while in university), or on paper tape for a little HP box.  As analysis software became available commercially, then design programs, then all sorts of combinations like we have today, it has become more and more difficult to interrogate and verify software.  Thus, it has for many of us become a matter of developing confidence, and that is what Lion06 is trying to do.

Your contributions to the site are valued by many here, but in reality, and with no disrespect, you are somewhat of a "one trick pony" when compared to many of the practicing engineers trying to deal with a wide variety of materials, methods, code requirements, and structural forms.  My snarky comment was just because I thought you were tending toward being disdainful of genuine questions, and that is not going to help anyone.   

RE: PT design Ieff=Ig

(OP)
There is an obvious problem with the computer programs becoming too complex, the reason for engineers not spending the time to review these to an in-depth level is due to many factors, costs, time, experience, understanding, and programs documentation.

What we do need to encourage is for engineers to spend the time reviewing the inner workings of a particular software. weather it be a finite element program like microstran for steel design or concrete design. each time these turn up at our desk we should ask those around use what errors they have found and to what depth they have reviewed. We should also be keen to do some calcs by hand to review the effectiveness of the software, but in saying this I don't think you need to design a complete building by hand and then compare every aspect. I think you need to review it in pieces, choosing simple concepts to review, ie torsion or high shear ect. once this is completed I think your understanding of the program will ensure that you don't allow garbage out.

But getting back on topic,

in regards to lions question "As for the FE analysis accounting for the restraint from walls - how is this not accounted for with the analysis.  The walls (with their correct stiffness) is modeled and connected to the slab as it will be constructed.  The walls are preventing the slab from shrinking."

we need to establish the programs basis, I don't know ram concept for PT design as well as I would like to for the purposes of this conversation. does the program allow for a shrinkage theory in the manual? if so how does this theory work? what type of model does it use? plate or shell.  

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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