Post-tension Beam Seismic Design

[MikeT14]

I'm currently designing a multi-span (4 bays at 62') PT beam and am trying to reconsile the seismic design between RAM Concrete and ADAPT-PT. I'm trying to decide/calculate the amount of PT to utilize in the lateral force resisting system. The PTI Manual makes a reference to using , 0% or 25% to 65% for pre-stressing, but I'm unable to find a "hard-and-fast" reference as to the amount of PT to use in the ADAPT post-processor calculations.

Any insight would be helpful.

Mike

 

[AUCE98]

Mike,

As I am sure you are aware, the PT will not help at the beam/column joint in a situation where there is moment reversal. If the load combonation including the lateral forces creates a positive bending moment the contribution due to the PT would not be effective. However, I would say that it would be conservative on the other hand to not use some portion of the PT forces when a moment reversal does not occur.

Our office typically uses 50% of the PT in lateral case when using the post-processor in ADAPT-PT.

 

[MikeT14]

Thanks AUCE98, that's just the recommedation I was looking for.

Anyone else have any opinions on the amount of post-tensioning to use in ADAPT post-processor?

 

[rapt]

If it is a PT beam and the beam is connected fully to the column why can't you use the full PT.

The only time you should need to use a reduced amount of PT is if it is positioned well away from the column, say in a slab. Then, only the tendons within the normal column strip (I know you do not use column strips in USA for PT slabs but the width can still be calculated) width should be used at the maximum. Possibly only half of this amount would really contribute. This gives a nominal range (depending on longitudinal and transverse span lengths) between .5 and .25.

 

[AUCE98]

Rapt,

The PT tendons are typically draped in the beams/girders over the column supports such that it mitigates the negitive moment more effectivly. In a lateral load condition there is the possibility that a load reversal may occur at the joint causing a positive moment at the joint. In this situation the PT would positioned in the compression zone of the beam, therefore not aiding to the tension face of the beam. The reason you would still use some portion of the effective prestress is due to the P/A (axial compression)component of the PT force. I personally have never used a PT slab to transfer lateral moments and consider a portion of the slab as the effective slab width to transfer such loads. I would contend in that situation you would have the same possible occurance of moment reversal as well.

Regards,

AUCE98

 

[MikeT14]

Good point Rapt.

After speaking to the support people at ADAPT, they suggested that as long as 100% of my PT was going through the column and that I was in a low seismic zone, there was no practical reason to not utilize all of the PT for lateral resistance. The post-processor's utilized by ADAPT allows for a reduction in order to increase ductility by adding additional mild steel at the support and at midspan, or as Rapt suggested, when a portion of the PT does not go through the column support. For my case, using anything over 45-50% of the PT meant adding perhaps one additional bar at midspan or lengthing the bars already in the design due to the gravity loading (even with load reversal). As it stands, using 100% meant that no additional mild reinforcement was necessary.

ADAPT also said that there is no practical rule for this and its a discussion they have frequently with users as there is no reference available suggesting how to handle it.

Thanks,

Mike

 

[rapt]

MikeT14

Auce98: I understand how PT design works. The question was not specifically about reversal moment conditions. My reply was general in any caseas explained below.

In a reversal moment condition, if you do a strain compatibility analysis to determine the section capacity (as you should), the tendons would be near the compression face and would have a very small effective depth. They would provide a very small capacity but the section would not be ductile as is necessary for earthquake members. This all comes out in the calculations so there is no need to ignore some of the tendons except in the cases as noted in the earlier response.

I would always place bonded reinforcement at the tension face in this case, even if calculations showed that the tendons at the compression face were adequate for strength (which I very much doubt). A check on ductility would quickly show that the section was not adequate under this load condition without bonded reinforcement at the tension face to improve ductility and to provide more strength as a bonus.

In lieu of doing the proper strain compatibility calculation for reversal cases, I would ignore the PT completely in the calculation. Remember that for unbonded strands the ultimate force formula for the tendons for the design of the section does not apply for reversal cases where the tendons are near the compression face (even if the code neglects to tell you this). The maximum force you could use would be the working force but this should be reduced also as the tendons are in the compression zone.

 

[Taro]

MikeT14, what code are you using? The IBC says that prestressing in flexural members resisting seismic forces:
- shall be the lesser of 700 psi or f'c/6
- shall not provide more than 1/4 of the strength
- have anchors located at the exterior face of the joint and be tested for a minimum of 50 cycles of loading

 

[MikeT14]

IBC 2003 and ACI 318-02 for this project. I am unaware of the IBC requirement for prestressing. Does it apply to Post-Tensioning Members too? Where is it in the code?

 

[Taro]

I am not in the office right now, so I can't quote you the exact section. But look in the IBC concrete chapter for the exceptions/amendments to ACI 318.

 

[MikeT14]

Taro,

I found your reference, it's in Chapter 21. However, as my structure is in a "region of low seismic risk" (OKC, OK) I'm not sure the provisions of chapter 21 would apply. According to the commentary, the provisions of Ch. 1-18 and 22 provide adequate toughness for such structures.

I'd appreciate opinions on whether Ch. 21 applies to my structure.