Transfer Plate

[kaldoonh7]

Dear Sir,

I would like to ask how do you design your transfer plate: let's assume I have 2m transfer plate slab:

during the construction, the slab will be cast into two go, first at 0.8m and second at 1.2m.

my questions are:

1) how do you determine the number of tendons, is base on total slab thickness?

2) will I need to consider stress at first cast?

do you have anytext book or manual talk about transfer plate steps?

Regards ,

Kaldoon

 

[KootK]

Transfer plate design is a broad and complex topic, particularly if we're talking PT. As such, my response is bound to be woefully incomplete. Regardless, I'll do my best to help.

how do you determine the number of tendons, is base on total slab thickness?

The number of tendons required will be based on a number of factors and the total slab thickness will definitely be one of them. The level of prestress is usually specified primarily based on serviceability requirements (deflection). That said, flexural capacity, durability (crack control), and code requirements also factor in among other things.

will I need to consider stress at first cast?

You most likely won't be stressing any of your cables until the entire slab has been poured (let me know if that's incorrect). The two stage pour shouldn't have much impact on ultimate flexural strength. It may have an effect on deflection performance however. When the second stage is poured, it will be restrained from shrinking freely by the presence of the first stage pour (already hardened). That will result in some cracking of the concrete that is part of the second stage pour.

With the two pour scheme, you'll definitely need to consider the issue of horizontal shear transfer within the slab. You'll need to assess demand and provide a competent mechanism for shear transfer across the joint such at shear friction etc.

do you have anytext book or manual talk about transfer plate steps?

I know of nothing that deals with transfer plates specifically. Some options for general post-tensioned floor slab design include:

Gilbert
Collins
Khan
Aalami

I've found that, for most transfer plates, I prefer not to use post-tensioning. There are two reasons for this:

1) Generally (hopefully) column offsets above and below the slab are modest. As such, thickness is selected based on shear demand and slab deflection is not critical.

2) Often transfer plate loading and geometery is so irregular that it's difficult to arrive at a sensible tendon layout. Sometimes discrete post-tensioned girders end up making more sense than a full post-tensioned floor plate.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rapt]

You would need to make sure there is sufficient reinforcement in the first stage to control shrinkage cracking. I assume the tendons are within both stages so stressing cannot start until there is sufficient strength in the second stage.

If as Koot suggests the offsets are small between columns above and below then

1 it is really a strut-tie design, not flexural.

2 prestress tendons will not be able to be profiled to top at support and bottom under columns above.

In any case,

1 even if the columns above are at mid span, it is often very difficult to profile the tendons to the bottom under the columns above. Make sure your software checks tendon curvature, several do not!

2 It is important that the tendon layout match the load pattern and moment diagrams. You cannot simply use the ACI average panel layout logic used for normal flat slabs. You cannot use banded/distributed layouts. You need to break the design down to multiple strips in reach panel to match the variation in the moments and loadings.

As Koot said, it can get very complicated to do it properly.

 

[kaldoonh7]

Thanks for the information,

unfortunately, there are no clear design steps for transfer Plate(PT).

 

[rapt]

That is what your engineering training is for!

 

[Trenno]

Push the Architect for greater vertical continuity in the structure before trying to hold up a transfer deck.

Use arguments like... cheaper structure, faster construction etc etc.