Post Tension Design Rule of Thumb 2

[Aet20]

Dear all professionals,

I recently started to work on PT beams and slab design, particularly with RAPT software. I would like to ask if there is any rule of thumb applicable under EC2 that is good to know for example recommended span-depth ratio and any limiting criteria during design stage. Appreciate if someone could enlighten me as I'm fresh to PT and would like to know more about the idea of designing, it,

 

[rscassar]

TR43 is a good publication. Probably more towards the British Standards. Rules of thumbs that I like to work with include span to depth ratio for different systems (flat plate, flat plate with drop panels, band beams) for different loads and different span configuration (end span v internal span). Always found it good to satisfy deflection and shear requirements with a P/A between 1.4 to 2.0MPa. When this is satisfied many of the other design checks like bending strength and service stresses / crack widths tend to fall into place.

 

[Sheer Force Engineer]

Hi Jaet, here are a couple of links for you that you may find useful..

- This is a beginner primer to PT, its pretty elementary but if you are just starting out, its worth a quick read... Link

- Here are some span-to-depth ratio tables you can use which can give you a ballpark idea of if your design is reasonable... Link


For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[rapt]

Jaet,

I would suggest you read EC2, the RAPT manual and some good texts on PT design before you start.

 

[CURVEB]

Adapt has a lot of good technical notes here:

https://adaptsolutions.wordpress.com/adapt-builder/technical-notes,

especially if you are in ACI territory. Not so many rules of thumb, but a simple explanation of the process used for various aspects of the design.

 

[dik]

acurveb... looked at a couple of the tech papers, and appears to be an excellent source.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[rapt]

Be careful using PTI span/depth ratios. We find they often result in slabs thinner than we would like and the resulting slab is more heavily reinforced and less ductile and has more punching shear problems.

But remember in PT a span/depth ratio is the starting point in the design. Deflection limits are the final controlling condition.

 

[Aet20]

Dear rscassar, Shear Force Engineer, dik, rapt and CURVEB,

Thank you so much on the useful information and resources shared to me! Much appreciated!

On the other hand, would like to check if there is unbalanced loading due to different HCS thickness as well as the span width on both side of the PT beam which will induce a torsional moment to the PT beam. Is there is anyway to consider the torsional moment into the model by using software RAPT? As what I know RAPT is more to a 2D analysis which not allow us to model the torsional moment in it, is it true? Shall I check the torsional moment separately/ manually?


Thank you and happy new year!

 

[rapt]

Yes, you would need to consider torsion separately, but in most cases the torsion would be relatively small and below the minimum torsion levels. Torsion could also come from other effects such as different span lengths, pattern loading etc. But it would depend on your structure and construction method which we know nothing about.

 

[Sheer Force Engineer]

I agree with rapt regarding torsion...

Not sure what code you are designing to but the Australian code allows you to "ignore" incidental torsion on your beam if you have not considered its stiffness in your analysis (such as 2D frame analysis in RAPT, its in Clause 8.2.1.2 AS3600-2018). Providing minimum torsion reinforcement is usually enough to justify this in most cases. However obviously if you have torsion as your primary load path, or you are considering the torsional stiffness in your analysis (such as most 3D analysis software), it needs to be properly checked.

Grey areas in between would need engineering judgement as to whether you need a separate check or not. Like if you have grossly un-balanced spans over a band beam or one bay significantly loaded much higher than the other. In these cases your "incidental" torsion might actually be relatively moderate and something you can't "ignore".


For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[rapt]

Not quite what I said.

In designing the slab direction, RAPT will consider the column stiffness in the frame and result in moment in the columns. This will be the result of Torsion in the member spanning in the transverse direction. The problem with 2D is that when you come to design the beam in the other direction, RAPT knows nothing about this torsion from the transverse direction.

In a flat slab this would normally be ignored.

With a transverse band beam, it should be considered as torsion, but normally the value will be relatively small and give a torsion stress below the minimum code limit (in AS3600 .25Tcr). If it is over this limit, it needs to be designed for as it has been allowed for in the slab design in the transverse direction.

With a transverse beam, it is the same as the band beam case, but the torsion will normally be more significant.

If you want to call it compatibility torsion and ignore it as the code allows you to, the code requires that it be ignored for all calculations (slab in the other direction also so use zero column stiffness) and minimum torsion reinforcement must be supplied. If you do this, I would still check punching shear at columns based on full column stiffness and it will really be there and punching shear is a critical design condition that should be investigated for the loading the column could attract.

 

[Sheer Force Engineer]

Hi rapt, my post was not intended to be a paraphrasing of your previous post or ellaborating upon it.

Merely my own comments and take on things, sorry for any confusion



For the latest insights into the art of Structural Engineering

http://www.sheerforceeng.com

 

[dik]

I did nothing... the other characters 'did the lifting'... but thanks...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[rscassar]

If using finite element program, make sure you interrogate the torsion design settings. One major software package has torsion design set to "None" as the default setting. Default setting should be wood-armer for finite element slab designs.