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Strand7 and eccentric prestressed slab
5

Strand7 and eccentric prestressed slab

Strand7 and eccentric prestressed slab

(OP)
There is a proposal suggests that if we used post tensioned slab on expansive soil with zero eccentricity straight tendons, this design can work in both cases of doming and dishing slab in dry and wet condition respectively. The proposal suggest that once the doming and dishing occur, that means we will get some effect of the post tensioned steel. However, I modeled the proposal in Strand7 non-linearly and I got no effect of the post tensioned steel. Once I apply eccentricity, the prestress works perfectly. I am looking to hear from you.

RE: Strand7 and eccentric prestressed slab

Diacivil,

What would be the stresses induced in a concrete section with axial prestress tendons ? If you can answer this, you have answered your question.

If you cannot, give the design to someone who can answer it, because this is the most basic prestress analysis concept and you are not able to even understand a complex analysis software solution to it. Why you need to model it in Strand 7 I cannot understand.

RE: Strand7 and eccentric prestressed slab

(OP)
Hi Rapt

Everyone knows that the this type of prestress results into axial stresses only. The concentric prestress is used to control cracks width on expansive soil.

RE: Strand7 and eccentric prestressed slab

Quote:

Everyone knows that the this type of prestress results into axial stresses only. The concentric prestress is used to control cracks width on expansive soil

So what is the question?

If you tell us in more detail what you did in the analysis, what results you got, and why they are not what you expected, it might help.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)
The results in strand7 are same if there is not prestress at all (in case of eccentric prestress). only if I applied eccentricity, I got what I expected. In opensees, the results was realistic and exactly what I expected.

RE: Strand7 and eccentric prestressed slab

What did you expect, and what did you get in Opensees?

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)
the concentric prestress reduces the deflection

RE: Strand7 and eccentric prestressed slab

Just for a minute imagine someone else had done the analysis, and just given you all the information you have given us so far.

How could you even start to give a useful answer? You wouldn't know what sort of elements had been used, what sort of non-linear analysis, how you had defined the materials properties, how you had modelled the soil, how you modelled the interface between the soil and the slab, how you had applied the loads, etc. All these things make a difference.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

strand7 has a good write up on pt modelling that you may wish to review. would also like to know what you are doing to replicate the soils movement

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Strand7 and eccentric prestressed slab

If strand7 is simply doing analysis on the structure you defined, and if strand 7 simply does an elastic analysis, not allowing for cracking, defining axial prestress will have no effect on the results, as there I no change in the member stiffness.

This is the normal result I would expect from an analysis program unless you requested some form of non-linear analysis or if Strand7 included some form of cracked section check and re-analysis based on the cracked section properties. These are not questions you can ask us. You need to know what Strand7 was supposed to be doing for you.

If you efined the same thing in Microstran, Spacegass, Staad, Robot, SAP2000 etc, I would expect the same as their normal operation is an elastic analysis.

RE: Strand7 and eccentric prestressed slab

Coincidentally the Strand7 web site has just posted a web-note on modelling non-linear behaviour in reinforced concrete by specifying a stress-strain
curve for the concrete.

A much simpler approach is to generate moment-curvature diagrams for the slab (with and without prestress), then use that in conjunction with beam elements.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

If the slab experiences doming or dishing after the tendons have been stressed, some load will be "balanced" by the curvature of the tendons. Theoretically, this should result in less deflection than would occur with an uncracked unstressed slab. With accidental variations in tendon placement and slab thickness, the effect is not predictable and probably not significant.

Edit: The above comment is incorrect and should be disregarded. There is no balancing effect when the tendons are concentric.

BA

RE: Strand7 and eccentric prestressed slab

(OP)
The analysis was nonlinear (nonlinea aterials and nonlinear geometry). However, no effect of the concentric prestress at all.

RE: Strand7 and eccentric prestressed slab

(OP)
I appreciate it if anyone has more info about concentric slabs.

RE: Strand7 and eccentric prestressed slab

diacivil - if you want some useful help please respond to the questions people are asking you.

If you don't specify non-linear material behaviour then the prestress will make no difference, regardless of which program you use. If you do specify different non-linear behaviour for the reinforced and pre-stressed sections then you will get different results.

From what you have told us so far presumably you didn't specify the non-linear material behaviour in Strand7, but you did in Opensees, but without knowing what you actually did we have no way of providing any detailed help.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)
Thanks a lot guys.

I just want to make sure that the concree behaves nonlinearly in Strand7, could you tell me how to do that?

RE: Strand7 and eccentric prestressed slab

Quote:

I just want to make sure that the concree behaves nonlinearly in Strand7, could you tell me how to do that?

It depends.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

The results could still be correct in a non-linear analysis depending on the stress levels.

If the tension stress in the concrete is less than the defined concrete tensile strength, then the section will be uncracked and the results with uniaxial prestress would be the same as for an uncracked reinforced concrete slab.

BARetired, the balancing effect is caused by eccentricity from the centroid of the concrete. As there is no eccentricity of the tendons, there will be no balancing effect, no matter how much slab curling or other effect has occurred.

This whole discussion brings up something I have ranted about previously. Engineers should not use computer software unless they understand both the area of design and what the software is going to do for them and how to use it properly. Software is becoming more and more complex in what it can do, and engineers are using it more and more without understanding what the software is actually doing for them and often without a good working knowledge of the design area in question. In many cases, they are expecting the software to replace the design code and their need to understand it.

RE: Strand7 and eccentric prestressed slab

Quote (rapt)

BARetired, the balancing effect is caused by eccentricity from the centroid of the concrete. As there is no eccentricity of the tendons, there will be no balancing effect, no matter how much slab curling or other effect has occurred.

This should have been immediately obvious to me and upon further consideration, I agree with the above statement.

BA

RE: Strand7 and eccentric prestressed slab

Quote (BAretired)

This should have been immediately obvious to me and upon further consideration, I agree with the above statement.

I don't feel that any retraction was necessary. While the effect that you described may not be significant in this instance (small displacements), there was nothing in your statement that was technically flawed. The phenomenon that you described is in fact the very reason why post tensioned members do not buckle under the axial loads applied by the tendon anchors.

Quote (rapt)

the balancing effect is caused by eccentricity from the centroid of the concrete. As there is no eccentricity of the tendons, there will be no balancing effect, no matter how much slab curling or other effect has occurred.

I disagree with this statement. The balancing effect is not caused by eccentricity between the tendon and the centroid of the concrete. That's just the plain old "prestressing effect" (induced eccentric strain). Load balancing is caused by tendon drape, either installed or induced.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

Quote (diaop)

I appreciate it if anyone has more info about concentric slabs.

I'll leave the software idiosyncrasies to others. Some practical tips:

1) concentric prestressing like this gets you just one thing of any real value: initial compression strain in what will eventually be tensile fibres in the loaded cross sections.

2) #1 implies grater moment capacity, cracking later in the load history and uncracked section stiffness longer in the load history.

3) While #2 will mean less dishing deflection, it's unlikely to meaningfully affect doming deflection. Expansive soils, like frost lenses, expand as they like and are relatively unaffected by SOG stiffness. As such your doming deflection should be unaffected by the post tensioning.

4) Presumably it is top surface cracking caused by doming that is of greatest concern. Because of #3, I would not expect cracking to be greatly improved. The slab will assume the curvature imposed by the expansive soil and cracks will form to facilitate that curvature, just as would be the case in the absence of the post tensioning. Post tensioning will, of course, reduce ordinary temperature and shrinkage cracking.

5) because of points 1-4, I'd be tempted to model the concentric prestressing effect as simply an increase in the modulus of concrete rupture. Depending on the results that you're interested in, and whether or not you have mild reinforcing, it may not even be necessary to perform a non-linear analysis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

Quote (KootK)

I don't feel that any retraction was necessary. While the effect that you described may not be significant in this instance (small displacements), there was nothing in your statement that was technically flawed. The phenomenon that you described is in fact the very reason why post tensioned members do not buckle under the axial loads applied by the tendon anchors.

I'm afraid there was something in my statement which was technically flawed, KootK. A concentric prestress in a prismatic member creates uniform compression at every cross section throughout the length of the member except in the immediate vicinity of the end anchorages. This is true whether the member is straight or curved.

If an unstressed slab is subjected to external forces causing bending moments along the center line, the fiber stress at point x,y will be Mxy/I where Mx is the moment at x and y is the distance above the c.g. Deflection can be calculated based on the magnitude of the moment and the magnitude of I. If the section remains uncracked, I is the uncracked moment of inertia.

If the slab is pre-stressed concentrically, the only difference is that a compressive stress is added to the fiber stress in all locations. Thus the deflection is identical whether it is pre-stressed or not, provided the non pre-stressed slab does not crack.

BA

RE: Strand7 and eccentric prestressed slab

I'm not buying it BA. All of your last post works for me with one exception: you've chosen to ignore the second order load balancing effect that was the subject of your original, retracted comment. Deflection would be slightly less for the post tensioned member because, when deflected, the induced curvature in the tendons would create an upwards balancing load identical in concept to how intentional drape in suspended slabs does.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

Here's some related info supporting the notion that second order balancing load effects are what keep prestressed axial members from buckling under their own prestress.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

(OP)
I totally agreed what you have posted Kootk for column only. However, I am still not sure how to make the concrete to behave non-linearly in Strand7.

RE: Strand7 and eccentric prestressed slab

A post-tensioned slab on ground differs from a column. The ground imparts a tensile force on the slab, which is at the bottom, and thus eccentric.

RE: Strand7 and eccentric prestressed slab

Quote:

I totally agreed what you have posted KootK. However, I am still not sure how to make the concrete to behave non-linearly in Strand7.

Then why don't you provide some information about your analysis, so we can provide some useful advice?

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)
Hi IDS,
I model 8 metres length of simply supported beam with distributed load and prestressed tendons with zero eccentricity. I model the beam as beam element and as plate element.the tendon was stressed to about 85% of the total strength. Concret strength is 32 mpa. Beam dimensions in both cases are 500 mm depth x 300 mm width. Defelction was same in both cases and no effect of the concentric prestress. I have chosen the nonlinear analysis and plastic materials. However, the results fom linear and nonlinear analysis are both same. Please let me knew ifI miss anything.

RE: Strand7 and eccentric prestressed slab

(OP)
The prestress tendon is modelled as truss element with rigid links to the concrete. The axial egfect of the prestress is appeared as expected. However, no effect of the prestress on the moment capacity at all.

Cheears

RE: Strand7 and eccentric prestressed slab

Quote (diacivil (OP))

The prestress tendon is modelled as truss element with rigid links to the concrete. The axial egfect of the prestress is appeared as expected. However, no effect of the prestress on the moment capacity at all.

What do you mean "no effect of the prestress on the moment capacity at all". Without prestress, the member has no moment capacity. With prestress, it has a calculable moment capacity.

If you meant to say that the prestress has no effect on deflections, I would agree with that finding.

BA

RE: Strand7 and eccentric prestressed slab

Quote (op)

However, the results fom linear and nonlinear analysis are both same.

Do all of your models predict concrete cracking?

As was asked above, how are you modelling the soil here.

Quote (op)

The prestress tendon is modelled as truss element with rigid links to the concrete.

Can you sketch this for us? The only longitudinal connection between concrete and tendon should occur at the member ends.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

f'c = 32 MPa
Ec = 4500(f'c)1/2 = 25,450 MPa
L = 8000 mm
I = bd3/12 = 300x5003/12 = 3.12e9 mm4
w = 3.6 N/mm (dead weight of beam only)

Δ = 5wL4/384EI = 2.5 mm

Under the self weight of the beam, I find the deflection to be 2.5 mm with or without prestressing. This assumes the unreinforced beam does not crack under its self weight.

BA

RE: Strand7 and eccentric prestressed slab

(OP)
HI,

thanks all for your responses.

the member has same moment capacity with and without prestress as it can be calculated from wl^2/8.
I modeled the problem as simply supported beam to understand the effect of prestress and then will change to soil support and use winkler model.

RE: Strand7 and eccentric prestressed slab

(OP)
distributed load case is only 10 kn/m
combination load case is from distributed load and prestress
note: I applied 1 mm eccentricity to avoid software error (as explained in strand7 manual)

http://s7.postimg.org/xk3e1v9sb/mom.jpg bending moment of combination load case
http://s12.postimg.org/9cfsxhz1p/NEW_3_2.jpg bending moment of distributed load case
http://s4.postimg.org/5m689lx3x/image.jpg deflection of of combination load case
http://s2.postimg.org/3xblnrk89/NEW_3.jpg deflection of distributed load case

RE: Strand7 and eccentric prestressed slab

Quote ((OP))

the member has same moment capacity with and without prestress as it can be calculated from wl^2/8.

That is an incorrect statement. The moment capacity is the ability of the member to resist moment. We don't know what that is because we don't know the details of the tendons but the moment capacity without prestress is zero unless you provide some conventional reinforcement.

What you meant to say, I believe, is that the moment is the same whether or not the member is prestressed. That statement is true only if the unstressed member does not crack. If it cracks, it will collapse unless reinforced. If adequately reinforced, it will deflect more because of the cracking.

Quote ((OP))

distributed load case is only 10 kn/m
combination load case is from distributed load and prestress
note: I applied 1 mm eccentricity to avoid software error (as explained in strand7 manual)

With a distributed load of 10 kn/m, the maximum moment is 10*82/8 = 80 kn-m. If the member is unstressed, an uncracked section would develop a maximum fiber stress of 6.4 MPa or about 928 psi which is far beyond the cracking strength of the concrete.

It is beyond me why a 0 mm eccentricity should produce a software error. That would appear to be a deficiency in the software.

BA

RE: Strand7 and eccentric prestressed slab

What is the area of prestress rebar?

Any limitations to Strand7 in educational mode?

RE: Strand7 and eccentric prestressed slab

(OP)
Dear BAretired
I completely agree with you. but I do not know how to model it correctly.

Dear Brad805
the rebar dia is 10 mm.

and I donot know if there is limitation for student version.

RE: Strand7 and eccentric prestressed slab

@KootK,

We still have an outstanding issue to be addressed in this thread. Concentric post-tensioning tendons in a curved prismatic member produce pure axial compression throughout the curved member. They produce axial shortening but no deflection.

When a slab is tensioned concentrically and then curved by external forces, the effect of the prestress remains the same. It is still concentric no matter what shape the slab takes. It can produce only pure axial stress in any section normal to the curved axis. Deflection produced by external forces does not result in any load balancing by the tendons.

BA

RE: Strand7 and eccentric prestressed slab

What BAretired is stating (curved beam with concentric presterss = only axial compression) was what a professor friend of mine (Dr. M. Tadros - U of Nebraska Omaha) some years ago.
I had a hard time getting my head around it but I think he (and BAretired) is correct.

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RE: Strand7 and eccentric prestressed slab

@BAretired/JAE: are neither of you swayed by the column information that I posted above which, in my opinion, clearly shows the effect that I've been espousing? It's from Bazant for goodness sake. You'd be hard pressed to find a more reputable, living source.

A curved tendon, curved by way of drape or deformation, has components of it's tension oriented vertically. Obviously. If those components don't push upwards against the encasing concrete, then what keeps the cable in vertical equilibrium?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

@BA/JAE: I've had a late breaking revelation and may now agree with your conclusion. Here's how I think that it works:

1) the tendon does push upwards against the concrete to create an uplift force on the member.

2) the horizontal components of the axial forces applied to the ends of the member due to prestressing create P-delta moments on the displaced member that would push the member downwards.

3) #1 and #2 balance exactly.

So there is a balancing load of sorts but it is exhausted countering the P-delta moments that would exacerbate deflection and is therefore unavailable to resist externally applied load.

Another way to see it is that, for the prestress to remain oriented parallel to the axis of the displaced member, something has to "turn" the prestress force. That something is the pseudo-balancing load generated by the tendon curvature.

What a great thread this has turned into.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

...and "Mr. Prestressed Concrete", TY Lin, agrees too...

RE: Strand7 and eccentric prestressed slab

Quote:

Hi IDS,
I model 8 metres length of simply supported beam with distributed load and prestressed tendons with zero eccentricity. I model the beam as beam element and as plate element.the tendon was stressed to about 85% of the total strength. Concret strength is 32 mpa. Beam dimensions in both cases are 500 mm depth x 300 mm width. Defelction was same in both cases and no effect of the concentric prestress. I have chosen the nonlinear analysis and plastic materials. However, the results fom linear and nonlinear analysis are both same. Please let me knew ifI miss anything.

Have a look at the web note I mentioned earlier, if you have access.

If you want to model a material like concrete that has a low tensile strength. set material to elastic, and yield criterion to Max stress, then under Tables, create a stress v strain table, and assign this table to your material.

If you would like to upload your strand7 model I would be happy to have a look (but I might take some time as I am away from home at the moment).

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

I tracked down that TY Lin stuff. Unfortunately:

1) Like the Bazant info that I posted above, it speaks to the "column effect" rather than balancing loads although the topics are surely related.

2) It doesn't have anything to say explicitly about the the presence or absence of a load balancing effect due to the tendency of the tensions wanting to straighten etc.

I submit the sketch below for group review. It illustrates the mechanisms that I mentioned in my previous post.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

My copy of "Prestressed Concrete Structures" by T.Y. Lin, Copyright 1955, Fourth Printing 1959 is probably older than the reference mentioned above. Fig. 12-3-5 in my copy is identical to Fig. 14-5 in the one cited above.

In the discussion on the next page, Lin stated "As an extreme example, even if the member has a reverse curve, Fig. 12-3-5 (b), the application of concentric prestress will not tend to straighten the member."

BA

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)

If you would like to upload your strand7 model I would be happy to have a look (but I might take some time as I am away from home at the moment).

Kindly see the attached file. Now, the tendon is on zero eccentricity without any software errors.

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)

If you want to model a material like concrete that has a low tensile strength. set material to elastic, and yield criterion to Max stress, then under Tables, create a stress v strain table, and assign this table to your material.

Still I can not upload stress strain table

RE: Strand7 and eccentric prestressed slab

Quote (BAretired)

In the discussion on the next page, Lin stated "As an extreme example, even if the member has a reverse curve, Fig. 12-3-5 (b), the application of concentric prestress will not tend to straighten the member."

I wasn't able to find this but I'd very much like to check it out. Can you tell me the title of the section that it's in BA?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

Certainly, KootK. It is in Chapter 12 "Tension and Compression Members". It is Article 12-3 "Column Action due to Prestress", pages 353 and 354. If you wish, I could scan it for you.

BA

RE: Strand7 and eccentric prestressed slab

KootK,

Paragraph directly above that you pasted...



Ref: "DESIGN OF PRESTRESSED CONCRETE STRUCTURES" LIN/BURNS, 3 Edition, 1981.

RE: Strand7 and eccentric prestressed slab

Quote (BA)

Certainly, KootK. It is in Chapter 12 "Tension and Compression Members". It is Article 12-3 "Column Action due to Prestress", pages 353 and 354. If you wish, I could scan it for you.

Gracious of you to offer BA.

Quote (Ingenuity)

Paragraph directly above that you pasted...

Well, don't I feel foolish. Further evidence that middle management is rotting my brain. Of particular interest is the statement about this not being applicable in continuous structures because of secondary effects. I was having a hard time trying to think through the same logic for a multi span member.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Strand7 and eccentric prestressed slab

Sometimes simple (and crude) physical models assist in the understanding:



RE: Strand7 and eccentric prestressed slab

(OP)

Quote (Ingenuity)


Thanks mate. I have done similar model and it works.

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)


Hi,

Would you check it please?

Cheers

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)

If you want to model a material like concrete that has a low tensile strength. set material to elastic, and yield criterion to Max stress, then under Tables, create a stress v strain table, and assign this table to your material.

Could you tell me why you suggested to do so?

RE: Strand7 and eccentric prestressed slab

Quote:


Could you tell me why you suggested to do so?

Because if you want to model a material with non-linear behaviour and different tension and compression behaviour, that's what you need to do.

I haven't had time to look at your model properly, but from a quick look it seems that you haven't assigned any stress-strain table for the plates.

You need to read the manual or look at the on-line help to see how to assign non-linear behaviour.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)

that you haven't assigned any stress-strain table for the plates
Sorry mate, I gave you the wrong file. the last one is attached. please have a look.

RE: Strand7 and eccentric prestressed slab

I think you should go back and read one of rapt's earlier comments about understanding the software. IDS is being very kind. Frankly, I would not help you after your last question because it shows to me you need to do more research or reading. NLinear analysis of concrete involves cracking. You must define when that occurs so the program can alter the section stiffness as it steps thru the loads. Maybe I am off base since I have not used Strand7, but this is a very basic (and important) step in the software we use.

Non-Linear analysis of concrete can be useful, but you need to keep in mind some of the comments from this report. http://www.civ.utoronto.ca/vector/journal_publicat...

If you want to understand this in greater detail you might want to read the guide below.
http://www.fib-international.org/practitioners-gui...

RE: Strand7 and eccentric prestressed slab

(OP)
So you agree the proposal is not going to work?

RE: Strand7 and eccentric prestressed slab

Diacivil, I am not sure what proposal you mean, but I can't see that Brad805 suggested that anything would not work, if done properly.

One problem with your latest model is that you have used von mises stress criterion for yielding, which is not appropriate for a brittle material like concrete.

There are probably other problems. I will look tomorrow.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

I have been out of contact so have missed the discussion.

Tendon curvature/change in angle causes bending /curvature effects.

Concrete centroid curvature/change in angle when subject to axial load causes bending/curvature effects.

When you combine the 2 of them with a tendon that follows the concrete centroid exactly, the 2 sets of effects, one internal and one external exactly balance each other and you end up with axial compression.

Forget about balanced loads, it is just a nice term. It is all about moments. A parabolic profiled tendon resulting in a balanced load of w over a length L simply represents the moments based on wl2/8 in a much nicer fashion. If the curvature was random, it would be much harder to represent it as a balanced load and much easier as moments.

RE: Strand7 and eccentric prestressed slab

Augmenting what rapt said above, the attached sketch shows a member with three crank points. If the member is concentrically prestressed from A to E, the blue arrows represent the axial compression in each member. The tension in the tendons is equal and opposite to the axial compression and is not shown. If there are no losses due to friction or other source, the axial compression will be equal in each leg.

The resultant of the two forces acting at each of points B, C or D is the bursting force where the tendons would break out of the concrete if not anchored securely.

If there are friction losses at each of the crank points and stressing is done from point A, axial compression will be greatest in leg A-B. Compression in leg B-C will be diminished by the loss at point B and the compression in each of the remaining legs will be reduced further. Even when there are prestress losses, except for axial shortening, the cranked member retains its shape if concentrically stressed throughout each part.

A curve can be approximated by a series of short straight lines so the same argument holds for a curved member.

BA

RE: Strand7 and eccentric prestressed slab

2
I have now looked at the Strand7 analysis and made a number of changes:

1. I have changed the 3 rows of plate elements to 2, so I can connect the truss members representing the tendon directly to the plate nodes, rather than using an "attachment" link.

2. I have deleted the vertical rigid links because they weren't doing anything. You can always add them back in if they have a purpose in a later stage of the analysis.

3. I have changed the node restraints to remove all moment restraint, and remove the restraint in the Y direction except for the central node at each end. The other restrained nodes are restrained in the Z direction at both ends and in the X direction at one end only.

4. On the Property-Material tab the plate Material has been changed to elastic, and the Yield Criterion to Max Stress.

5. On the stress-strain table I have adjusted the stresses in the tensile region so that: a) the initial elastic modulus is the same in tension and compression; b) the maximum tensile stress is close to zero, c) the stress-strain slope after cracking has a very small positive slope.

6. I have added rigid links connecting the central nodes to the edge nodes at each end. If you don't do that you get problems with transverse tensile stresses when the pretension is applied, because we aren't modelling the confinement reinforcement.

Note that although concrete has a tensile strength of a few MPa, after cracking it has zero strength, so it should be modelled as close to zero maximum tensile strength, rather than the plastic 2 MPa you had.

After making those changes the deflections look reasonable: 0.76 mm without prestress and 0.26 mm with prestress. Also the stress plots show that the section is all in compression with prestress, and cracked with tension on the bottom without prestress.

Four node plate elements are not a good model for beam bending, so I have also run the analysis with 8 node plate elements (NEW3.st7) which you will see gives a much smoother stress plot. Deflections in this case are 0.83 mm and 0.27 mm.

Finally I would repeat that for this application using beam elements with a moment-curvature table is not only much easier to set up, it is likely to give better results, and will certainly converge much quicker, because modelling reinforced concrete as a composite material with plate and truss elements is fraught with difficulties (see links posted by Brad805, which are really worth reading).

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

(OP)

Quote (IDS)


Dear IDS,

I do not know how to thank you for your special efforts and time. I really appreciate your knowledge, effort and time.

I will see the models and get back to you.

Thanks a lot.

RE: Strand7 and eccentric prestressed slab

I do believe tracking him down thru his website https://newtonexcelbach.wordpress.com/, find a place to send him a carton is required. Doug's response is well above and beyond the call of duty.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Strand7 and eccentric prestressed slab

I only do these things when there is something I find interesting. In this case it has brought to light several things that we take for granted, but are not at all obvious.

diacivil - it looks like you took your stress-strain diagram from one of the Strand7 web-notes (which has the same problem with the initial elastic modulus in tension being much higher than in compression). If you do have access to the web notes have a look at:
ST7-1.20.40.10 Modelling Nonlinear Concrete with Nonlinear Elastic Material
which I think covers the subject better than the other one.

I have had a look at using a beam element to represent the concrete, rather than plates. This makes it easier to set up the model, and if you set the material to elastic it gives the same results as before. If you use elastic-plastic properties it depends on the order of loading. In the attached file New5a.st7 the load is applied with prestress (small deflection), then removed (deflection returns to zero), then load without prestress (large deflection), then removed (only part of deflection recovered), then load + prestress again (large deflection).

Which all makes sense.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

What no screenshots for the herd? Just joking of course. You are a heck of a good contributor like several other in the thread.

RE: Strand7 and eccentric prestressed slab

I think a screen-shot might be a good idea.

Also I should expand on the "all makes sense" comment. The results using elastic-plastic properties make sense for an elastic-plastic material, but reinforced concrete is non-linear elastic if the steel doesn't yield. When you remove the load the cracks close up and the beam returns to zero deflection (ignoring creep and shrinkage).

In the attached screen shot the contours show concrete stresses for the load without prestress, and the graph shows curvature along the beam for each load case:

1. Normal load + prestress: Concrete is uncracked and curvature profile is parabolic
2. Load removed: Curvature returns to zero
3. Normal load without prestress: Curvature initially follows the same line as Stage 1, then after concrete cracking, increases rapidly, then again more rapidly as the moment increases. At the moment I am not sure why we are getting this two-stage non-linearity.
4: Load removed: Only the elastic part of the deflection is recovered - this is not how concrete behaves (ignoring creep).
5: Normal Load + prestress: Curvature follows almost the same line as without prestress, because the non-linear deflections of Stage 3 are treated as being locked in. Again, this is not how concrete behaves.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Strand7 and eccentric prestressed slab

The bi-linear profile of the curvature line after cracking was due to the small number of elements in the beams. Subdividing each beam into 8 I get the attached results, which shows a smooth curvature in the section curvature line.

The deflection (without prestress) increases to 0.81 mm, but note that the model ignores many factors that will have a much more significant effect on the long term stresses and deflections:

1) Creep
2) Shrinkage
3) Tension stiffening
4) Loss of tension stiffening
5) Differential temperature effects
6) Differential shrinkage effects

And we haven't even started on soil-structure interaction effects.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

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