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.

 

[rapt]

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.

 

[diacivil]

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.

 

[IDS]

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/

 

[diacivil]

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.

 

[IDS]

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

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[diacivil]

the concentric prestress reduces the deflection

 

[IDS]

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/

 

[rowingengineer]

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."

 

[rapt]

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.

 

[IDS]

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/

 

[BAretired]

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

 

[diacivil]

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

 

[diacivil]

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

 

[IDS]

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/

 

[diacivil]

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?

 

[IDS]

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/

 

[rapt]

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.

 

[BAretired]

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

 

[KootK]

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.

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.