Two-way PT with transfer beam 3

[BulbTheBuilder]

How do you approach or design the two way pt-slab in the sketch below? The slab is PT, but the beam is non-prestressed concrete.
Do you ignore the beam and transfer column and solely design the PT two-slab? Then after you design the beam for the transfer load? In this case I am assuming the beam isn't taking load from the slab but the transfer column only?

But if I do that it means I am not correctly balancing my dead load. However, If I balance all those loads as well, I might have a camber in my slab before the floors above are constructed. What is the best way to approach this? I will use RAM Concept to design this slab, but I would like to have a better understanding before even modelling.

 

[rapt]

The axial prestress in the slab in the beam direction will act as an eccentric axial compression on the beam, deflecting it downwards.

So you are not only not balancing dead load , your prestress in that span is actually countering your balancing loading and deflecting it down!

It is a combined system. Design it as one!

 

[BulbTheBuilder]

Thanks for the response, Rapt. Great response.
In this case, the beam is in the middle of the bay. How will I deal with all the "excessive" precompression? My slab will be taking a max of 300psi, but beam will be taking more. What is the possibility of designing as one system but using equivalent load from the PT to design the beam rather than having it modeled? I am thinking of this to avoid dealing with 6sqrt(f'c) tensile stress limit.

 

[rapt]

What excessive precompression?

Design the beam as a T beam. It supports the slab load on it plus the point load from above. The slab parallel to the beam is supporting nothing at midspan, it is a compression flange for the beam.

It is a transfer beam supporting your whole building. It has a level of importance that is much higher than your desire to make simplistic assumptions to avoid designing it the way it should be designed.

 

[BulbTheBuilder]

I think I am not explaining myself well or I am confusing myself. This is not an actual design; I am learning PT design, so I am trying to figure out things.

I am going to design the system as one, I understand that now. Assuming my beam needs a PT force of 500kips, and my slabs needs only 200kips. How do I deal with the expressive precompression at the end spans of the slab since I have to take the PT tendons all the way to end spans for stressing? Edit:[Can a beam at such location be staged stressed?]

Or the idea is to have tendons for the slab run through the beam, account the eccentric axial compression in the beam and its effect. Yet, the beam itself is non-prestressed. Kindly let me know what you think.

 

[gte447f]

Its been many years, but I designed a lot of these with RAM Concept back in the day. However, the transfer beams I designed were also post-tensioned. Sometimes they were stressed in stages as the building stories above were added. The transfer beams would usually have some banded tendons that were continuous through the beam, but they would also have additional, supplemental tendons that were discrete to just the beam. I would think if the transfer beam did not have additional supplemental tendons, then it would absorb some of the prestress from the slab tendons which would result in a loss or lowering of precompression in the slab in a localized area near the beam.

 

[BulbTheBuilder]

That great information, gte447f. Looking at the image I uploaded, was the supplementary tendons at the CGs of the beam? If that's the case the tendons from the slab thru the beam will be at a different elevation from the supplemental.

 

[gte447f]

BulbTheBuilder, the supplemental tendon anchorages were at the CG of the beam, but the tendons were typically draped or harped to provide resistance to the applied point load from the transfer column. This is just how we typically designed PT transfer beams. There are probably other ways, I suppose.

 

[BulbTheBuilder]

Is it possible to have such layout (see image below)? The slab PT is initially stressed, and the beam is stressed later or staged-stressed. Or the best way is to have straight slab tendons and let that of the beam do all the work?

 

[gte447f]

BulbTheBuilder, yes, like that. You could set the slab tendon drape to balance whatever percentage of dead load is typical for your slab design. This may require more drape than a typical slab span due to the extra depth and dead load at the transfer beam span. Or, don't worry about draping the slab tendons, and go ahead and stress some or all of the beam tendons. Its a balancing act (pardon the pun) that is part of the design of PT transfer beams. Of course, you have to evaluate the stresses, deflections, and required mild steel reinforcing for each stage of loading and/or stressing.

 

[BulbTheBuilder]

Excellent! I have a better understanding of the configuration now. I will try and run some numbers later this evening or during the weekend.