post tension slab design in ETABS vs Ram Concept

[Moam93]

Greetings,

I usually use Ram concept for post tension slab design . However , I tried to make a verification model by creating same model in ETABS and check results.
Tendon forces are calculated the same in both Ram and Etabs . I made same identical design strip layout in Ram and Etabs . but when I check top stresses in Etabs , they are much higher than Ram Concept . I am not sure why that is happening ... any one can advise where this big difference comes from ?

Regards

 

[BAretired]

How do they compare with your hand calculations? Or sanity check?

BA

 

[johnie134]

etabs cannot handle prestressed slabs, as far as I remember. Correct if wrong.

 

[BAretired]

Etabs claims that it can:

https://www.youtube.com/watch?v=_OVwCTDuxuw

BA

 

[Agent666]

Johnie134, etabs can handle post tensioned slabs. Noting the distinction between prestressed and post tensioned here.

Moam93, are there differences in the way any degree of cracking is handled in terms of affecting the stiffness as this could be impacting on distribution of moments and stresses (though tendon force being the same suggests the same design actions).

Are the analysis meshes identical? I'm not sure how etabs or concept calculate and apply stiffness reduction factors with a PT slab, whether it an iterative thing, or taking some user defined reduction. That may be responsible for some differences.

 

[Moam93]

@agent666
I didn't set any cracknig modifiers to Ram or Etabs . It can be modified ,but I let them all equal to 1 as default .
what I noticed is deformation due to dead load a bit different . defelction at mid span is 35 mm at Ram concept ,but 26 mm in Etabs . and of course negative moment at support is high in Etabs than Ram . I am guessing it might be because of rigid zone around column ? I didn't include automatic rigid zone in Etabs . In ram , there is no option for that , I guess it is already included.
regarding analysis meshes , they are not identical I use both automatic mesh in each program . I cannot control it . but obviously Ram makes more mesh elements than Etabs .

 

[Moam93]

@BAtired , to do it manually . I need to do equalient frame method to get Moment at column strip and I am afraid it won't be close to finite element method analysis especially the slab I am studying is not a typical one for using equalivent frame method .

 

[SethGuthrie]

Is ETABS giving an averaged, effective stress for a design strip like Ram Concept, or some sort of point stress, contour? If it's a point stress, then it's bound to be higher. There are also load combinations to compare, and the balanced loading of the post-tensioning. You might look at DL alone, or even better live load alone without the PT or any self-weight to see if it's just a matter of the slab mesh, stiffness.

 

[Moam93]

@Seth
I am designing both ETABS and Ram for design strip and checking stresses on Service load .
I checked deflection and moment due to dead load only . There is a difference. I believe it is due to analysis mesh and how design strips average forces and then design the slab in both forces ... For example In ram concept if a design strip just doesnot include one tendon which is just outside its width , it will reduce capacity of section . I can send the model if you or any structural engineer would like to review it.

 

[SethGuthrie]

You started off asking about stresses, then jumped to capacity, be aware those are quite different in Ram Concept. Since we calculate strip stresses based on nodal forces a tendon just outside of the strip will affect the nodal forces and hence the stress for the strip, though it will not be considered in the strength design capacity. Regardless if the models diverge when comparing deflections under gravity loads only then the post-tensioning has nothing to do with it. I could look at your Concept model for gross errors, but it sounds like you are already experienced and I would not have any ability to run the ETABS model so it would not help much.