Post Tensioned Podium Slabs - Bottom Mat and Balanced Load 6

[RFreund]

Two questions regarding post tensioned podium slabs:

1) Do you use a bottom mat of reinforcement. It seems that they are fairly common, help distribute loading, and help minimize/distribute restraint cracks. However, if "challenged" do you think they are 'necessary'.

2) You have most of your slab balanced for something around 100% of the slab self weight, but you have a few pesky spans that are longer or more heavily loaded. These will require a large amount of precompression and drape which will end up well exceeding 100% of the slab self weight... Is there anything additional or different you will do to avoid any issues during stressing. For example - You likely need bottom bars over the column (or top bars mid span) to avoid exceeding stress limits at transfer. Anything additional along these lines? Maybe more column dowels?

Thanks!

 

[hokie66]

Maybe if you switched over to bonded PT, it would be more palatable to builders. Partially prestressed, bonded PT is the norm in Australia, and builders here are not doing it just to be different.

 

[KootK]

...and builders here are not doing it just to be different.

I suspect that they are doing it because the engineering community in their locale doesn't really give them the ability to use unbonded systems.

Maybe if you switched over to bonded PT, it would be more palatable to builders.

That's actually a pretty good idea. It's the insurers that need to be persuaded. The builders that are comfortable with PT in general love unbonded. Same fundamental concept though: bonded = more convincing durabilty.

 

[KootK]

This is where I suppose my question really exists - If you end up being overbalanced (and possible still within allowalbe transfer stress), how 'bad' is that?

Truly, I do not know. And I want to be forthcoming about that. As I said, I've never overbalanced as a deliberate design strategy. Nor do I know of anyone who has. I also have never seen any kind of detailed discussion of what might happen if one did overbalance. Who knows though, maybe my reluctance to try this has been nothing more than me failing to think outside of the box. Consider what follows to be nothing more than me trying to create a detailed discussion of what might be problematic about an unbalanced design.

1) Reinforcing complexity. To limit cracking well, I would think that you would have to detail your bottom steel in much the same way that one details their top steel. Bottom mats where you have top mats and that kind of thing. I could see builders responding unfavorably to that. That said, you can just show your builders what this looks like and let them be the judge of its desirability.

2) Deflection. Utilizing the reinforcing intended to deal with the overbalance means cracking the concrete. And by definition that means that, during the initial stages of loading, the slab will deflect in accordance with it's cracked moment of inertia. As such, you've somewhat undone what I consider to be the major benefit of using PT in the first place: kick ass deflection control. Theoretically, I think that you're back to I_gross once you've closed the cracks. That said, I kind of question whether or not a crack truly closes up in this sense in the real world. Lastly, if there's a software package capable of dealing with this aspect of things well, I'm not aware of it.

 

[Celt83]

I think it's important to remember that balancing is not really a design consideration but more of a rule of thumb practice, it's a means to relate the quantity of PT relative the loading condition being balanced. I tend to think of balancing as the percent of tensile stress of the loading I'm trying to balance being negated by the PT. So at any cross section whatever and wherever the tensile stress is under the loading condition the relative reduction of that tensile stress by the PT is the "balance" on the cross-section.

Assuming all stress requirements are satisfied the implication of "over balancing" to the extent that it usually happens is likely more a serviceability concern for humping of the floor. From a mechanics perspective "over balancing" results in a reversal of curvature from that of the applied loading at each section that is over balanced.

 

[RFreund]

Thanks again for the responses.

@Rapt - I appreciate that design summary. I think it is well said and you're right, it does make a lot of sense.

For some reason Unbonded P/T in the Midwest US is a dominating force right now. Not really sure what is driving it. We've had to redesign a couple mild slabs with fairly short spans to P/T even though we only saved 1/2" of concrete.

 

[rapt]

US building PT went the unbonded route back in the 1960's. It is hard to change once the whole system has gone that way. It was basically a factory manufacturing operation delivering pre-assembled tendons to site where the site works were done by others. Bonded PT is the opposite and all works need to be done by the PT company. In recent years the US has realised the deficiencies of the system and have at least started to use trained operatives on site rather than untrained labourers for very specialised work.

Australia went bonded at the same time. We looked at unbonded in the late 1970's because it was being pushed so hard by the PTI but after a few months review decided to stay with bonded because of the design advantages and the low level of the unbonded technology at the time.