Post Tensioned Mat foundation design 2

[PSR_1]

I am trying to gather as much information as I can inorder to enhance my understanding on the design of post tensioned mat foundation which am facing. I only managed to find some practical approaches on a book by dirk bondy. Since I haven't practiced it am feeling ddiscomfort and I really appreciate it if you guys help me with do's, don't s and practical tips.

Thanks in advance!

 

[Ingenuity]

The attached VSL technical report entitled Post-Tensioned Foundations is a bit dated, but still relevant.

 

[hokie66]

Have you done a prestressed mat foundation, Ingenuity? It seems a strange concept to me, but I haven't thought it through.

 

[rapt]

Hokie66,

I looked at doing them back in my PT company days.

If you anyalyse as a rigid raft, it all looks logical. Just an upside down loading and tendon profiling exercise compared to a transfer slab.

Once you start doing FEM analysis modelling the soil below as a "compressible" support material, things go pearshaped.

You tend to get an overall dishing effect, with the pressure around the edge area of the slab distributing out into the soil outside the slab width giving lower pressure in that area compared to the centre of the slab. So there is a lot more deflection at the centre compared to the edge. So instead of getting a pattern of moments similar to an upsidedown transfer slab which you would with a rigid raft assumption with +Ve moment peaks at the supported column locations and -ve span moments between, you tended to get an overall +ve moment pattern with the soil around the edge acting as a sort of support to the whole slab. Then the normal moment pattern for the loads was superimposed onto that giving peaks and troughs, but the overall moment was +ve.

In the end they did them as RC as there was no logic to draped tendons basically spanning the full slab width and length.

This analysis was done for sand subgrade (Gold Coast beachside) but I would assume something similar for clay.

 

[PSR_1]

rapt, are u suggesting post tensioned raft doesn't make much difference?

 

[rapt]

efr,

You would have to look at the BM pattern on the raft.

The times I looked at it many years ago, it was basically a big simply supported slab, supported around the edges. There was no benefit to using prestressing. Plus it depends on your site layout. You would normally require multistrand tendons so would need a clear working area around the slab about 1.5m outside the slab edge. Depending on the site that may not be available.

But it would depend on the soil type and the slab dimensions and site dimensions.

 

[PSR_1]

How about stiffening the raft with beams and designing the beams as PT and the Slab as RC to help get better moment distribution?

 

[PSR_1]

Rapt, one more thing, using unbounded PT for mat foundation, is it a good practice? Since corrosion related to water movement in soil is going to be an issue,.

 

[rapt]

efr,

Missed the one on the 13th. If you made it stiff enough! Deep Beams in the ground are a pain for a start. But why the effort to make it PT. Is there any benefit to PT in it over RC?

PT normally helps where you want deflection control (not this case) and where you have too much normal reinforcement (presumably you have lots of depth available so reinforcement quantity is not a problem). You also have the possible PT problems of over excavation, possible need for stage stressing etc.

Is it worth it in this case?

 

[rapt]

I never think unbonded is good practice so I wouldn't ask me that question.

With unbonded, you lose 20-30% capacity because you do not have bond. So you need more PT or reinforcement to make up for it. Plus you have the corrosion problems. If it is very heavily loaded, you might need multistrand tendons. I do not like that idea with unbonded.

Is there any benefit of using it? No!

I do not know enough about what you are doing to say any more.