Basic Two Way PT Slab Design

[eit23]

Hello,

I am a new EIT and do not have much experience with two way PT slab design. When designing two way slabs using the banded and distributed tendon layout, does the banded tendons between columns act as a "pin" support for the design of the tendons in the distributed direction? Or do you still consider the columns support the distributed direction and provide extra stiffness? It is my understanding that when designing using the banded/distributed tendon layout the two slab acts as almost a one way slab with beams, therefore leading me to believe the uniform direction should be designed using "pin" supports for the beams. However, when designing with the EFM method for two way slabs, both directions are considered to be supported by the columns. In the end, I just wanted to know is it common practice to design the distributed tendon direction with "pin" supports or the stiffness of columns. Thanks

 

[BAretired]

Here is one example. Use your favorite search engine to find more on the internet.

BA

 

[eit23]

thanks, but I am not sure if that answers my question about the supports.

 

[KBVT]

I'm not sure what you mean by "pin" support, but the analysis method is almost identical to mild reinforced slabs without the middle strip/ column differentiation. The "column strip" is providing support for your perpindicular frames.

The slab is still functioning as a two-way slab, you are just distributing your banded tendon force over the entire strip width while the uniform tendon is already being distributed with the placement of the tendons.

 

[ash060]

Do not treat the bands as pin supports, if you are doing Equal. Frame method use strips in both directions one will be banded the other distributed.

Check out PTI for more information

 

[rapt]

eit23,

The column stiffness is normally allowed for in both directions. For this type of design, If you want good crack control, do not believe the ACI allowable stress rules! Deflection results are problematic also.

I love it when people try to justify calling this a "2way" design. The logic of "distributing your banded tendon force over the entire strip width" as suggested by KBVT above completely defies statics and concrete slab design logic.

I agree with your comment in the opriginal question that the slab is almost a 1way system. Yes, the failure mechanism is basically as a 1way slab, especially if no bottom reinforcement is added (as is allowed in some cases by ACI318). This works ok for ultimate failure conditioon as you are basically getting a yield line result without doing a yield line design. However, the same logic cannot be applied for crack control and deflection design. Hence my comments above about crack control.

Interestingly, the results of an FEM analysis cannot be used for a yield line design as is being done in this type of case by some FEM design software packages.

 

[BAretired]

There is no particular reason to consider the banded tendons between columns to act as a "pin" support for the orthogonal tendons. It is usual to consider continuity in both directions just as you would for a normally reinforced slab.

On the other hand, if you choose to consider the banded tendons as "pin" connections, your design will be satisfactory too. There really is a considerable degree of latitude in the assumptions made for two way slab design, prestressed or otherwise. We do not know precisely how two way slabs behave and any reasonable assumption is acceptable as long as statics are satisfied.

Concrete is not an elastic material. It will crack as required to suit the assumptions you make in your design. Many different designs are admissible provided that statics are satisfied.



BA

 

[rapt]

BAretired,

"Concrete is not an elastic material. It will crack as required to suit the assumptions you make in your design. Many different designs are admissible provided that statics are satisfied."

Agreed, for ultimate strength. Unfortunately, for service conditions, both crack control and deflections, the assumptions you make can have severe effects on these 2 areas. You can let the concrete crack in an unrestrained manner at ultimate strength and the slab will still stand up, but you need to place sufficient reinforcement in any area where a crack will occur to ensure adequate crack control, and if you do not you will get some wide cracks and significantly increased deflections. These are controlled by the elastic stress pattern, not the final reinforcement layout.
The best solution is always to reinforce to the elastic stress/moment distruibution, then cracking and deflection effects can be logically estimated and controlled using the same reinforcement pattern as is used for ultimate capacity.

 

[rowingengineer]

eit23,
While the advice given above is good and more than helps you with you slab design. I would like to just add a little comment as to why you should model columns in your slab designs all of the time.

PUNCHING SHEAR, This is important for flat slabs, and modeling the columns at the correct stiffness level is important to ensure this brittle failure mode doesn't become the failure mode. Failure to take into account eh moment transfer from the slab to the column in this calculations can lead to gross errors.


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