Strand7 and eccentric prestressed slab 5

[diacivil]

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.

 

[KootK]

@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.

 

[Ingenuity]

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

 

[IDS]

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/

 

[KootK]

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.

 

[BAretired]

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

 

[diacivil]

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.

 

[diacivil]

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

 

[KootK]

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.

 

[BAretired]

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

 

[Ingenuity]

KootK,

Paragraph directly above that you pasted...

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

 

[KootK]

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.

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.

 

[Ingenuity]

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

 

[diacivil]

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

 

[diacivil]

Hi,

Would you check it please?

Cheers

 

[diacivil]

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?

 

[IDS]

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/

 

[diacivil]

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.

 

[IDS]

diacivil - no attachment this time!

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[diacivil]

So sorry. it is here now

 

[Brad805]

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_publications/jp40.pdf

If you want to understand this in greater detail you might want to read the guide below.

http://www.fib-international.org/practitioners-guide-to-finite-element-modelling