two way flat plate with cables 1

[Eldorado]

I have done two way slabs on low-rise buildings with conventional reinforcement (rebar), but I have not done these types of slabs with cables. What pitfalls are the most prevalent for post-tensioned slab construction.

 

[Michel60]

Good afternoon Eldorado,
For 2-way PT (and PT in general) it's important to consider restraining conditions, tendon layout, stressing locations.

- Tendons will "pull" the structure toward locations of higher restraint so imbalance in lateral stiffness and eccentric LFRS placement should be avoided. For example shear walls located at one end of the floor plate will be an anchor and the remainder of the structure will shrink towards it.

- For multi-story construction floor plates will tend to shrink in overall dimension, cumulatively, if columns are positioned plumb around the perimeter. Specifying a slight outward tilt may be necessary.

- Cracking can occur at retaining conditions such at floor along shear walls. For highly stressed slabs it may be necessary to place delay strips in the slab or wall.

- Two way slabs normally have a primary stressing direction where the tendons are banded in the column strips and a secondary direction where the tendons are placed at a uniform spacing. This helps reduce the number of conflicts for the tendon drape.

- Placing tendons at openings is a bit of an art. Sometimes you stop them at each side of the opening (requiring additional reinforcing around the opening) and other times sweep the tendons around the opening, typically if they are smaller.

-Consider the locations for stressing the tendons. One end can be a dead end anchor with no access and the other a live end where you'll be placing the jack and having a place to stand to do the work. You can possibly pull a tendon up to about 200 feet from one end but I wouldn't recommend it if you can have access both ends and pull both ends. Slab edges will need to be plugged and patched at the live ends.

-Sometimes you can be blocked from getting to a live end at the perimeter of a structure. Blockouts/pockets in the interior of the slab can be placed to allow stressing.

This I'm sure is only a beginning to the list. If you have any PT contractors in your neighborhood now is a good time to introduce yourself.

regards,
Michel

 

[rapt]

Eldorado.

Michael has assumed you are from North America and are doing unbonded PT in his 3rd and 5th points. If you are somewhere else in the world and doing bonded PT, the 200' is not p[ossible. The normal limit would be 30 to 35m, so about 100'. Banded/distributed tendon systems are common with unbonded PT and the ACI code has rules that assume you are using them. Other codes do not, so you need to be careful to make sure your design is in accordance with your design code and structural logic and statics.

Michales first 2 answers also apply to RC slabs. The shortening from PT would only be about 20% of the shrinkage shortenming and about 10% of overall shortening. The typical shortening in a building slab from PT would only be 1-2mm at each floor. Well within normal building tolerances.

 

[Eldorado]

Michael60 and rapt, thanks for your pointers. Good overall information to consider. I am working with unbonded cables. I do understand the potential for distress at rigid shear wall and similar elements. Regarding delay strips, how do you incorporate them at a shear wall. If I'm working with a long square building that is, say 300' on edge, how are construction joints handled if they are not an isolating expansion/contraction type joint.

Casting columns out of plum. I'm having trouble with this one. Is it common to do so and if so, how often do they not straighten. I understand we are not talking about a 45 degree tilt.

Regarding your advice to get with a good PT contractor, I have already scheduled a meeting to gleen more of their expertise.

Thanks

 

[Michel60]

Details for delays for shear walls are problematic. Pour closure strips at the tops of the wall to slab interface can be used or a strip in the slab along the wall (with added reinforcing in the joint for both). Obviously you'd like to avoid them. The decision to use delays will be based upon the project conditions and the degree of pre-compression. I don't have a magic number to share when it will be necessary but as the stress moves up above 200-250psi it's worth looking into.

I do disagree with rapt on the need for shrinkage/creep considerations. Likely we've based our assumptions on differing floor plate dimensions and stressing levels. Talk to your contractor about shrinkage across a parking garage size floor plate. It could be up to 1/4"(6mm) to 3/8"(10mm) per floor. So that would entail maybe a 1/8" to 3/16" lean in a outboard column on either side of a floor. So we're not talking about a perceptible lean for construction purposes. But depending on how the initial layout is achieved the loss can accumulate over several levels and be a bigger issue if not considered.

regards,
Michel

 

[rapt]

Michel60

Are you talking about shrinkage or PT shortening? My figures are for PT shortening. Shrinkage happens in all buildings, RC or PT.

For a normal building slab with P/A of 200psi, the PT shortening over a length from edge to restraint of 100' would be about 1/16" or 1.5mm. The shrinkage would be more like 3/8". But the shrinkage happens in all buildings, not just PT. Yes, it must be allowed for but it is not PT specific.

 

[Michel60]

Good afternoon rapt,
Yes I do agree there are two components in the equation with only one being unique to PT. But in my experience the measured PT shrinkage is something greater than the PL/AE value would lead one to believe. Possibly due to localized higher stresses in banded PT, lower than assumed "E" value in the concrete at early age, etc. I understand that the stiffness doesn't rise necessarily as fast as compressive strength at an early age. Local practice has stressing at around 3-days but I've had request to pull as early as 24 hrs after placement.

In any case getting more info from a PT contractor's point of view would be interesting. Eldorado would you mind posting any words of advice from the trade side to the rest of us designers after your meeting?

regards,
Michel

 

[Eldorado]

I'll post what I can after my meeting. I always think it's a great idea to listen to the expertise of the man doing the work. Yes, sometimes they don't realize why we ask them to do it the "hard way", but they often give insight on ways that things can be done more efficiently or more cost affectively. I can't tell you how many times a carpenter has made me feel incompetent by pointing out a great way to put together some wood.

 

[rapt]

Michel,

Yes, if you are stressing that early, concrete modulus will be significantly lower leading to increased PT shortening. Axial Creep will also be much higher with the very early stressing. The concrete must be pretty good if you are able to stress fully at 24 hours (is that from the start of placement or the end).
It also depends on how long after the pour you measure it. An 8" slab will have about 15% of its total shrinkage in 10 days (about the same as the PT shortening again) and 30% in 30 days.

 

[Michel60]

Good morning rapt,
Emphasis was on the "request" part. Early stressing would entail a special recipe concrete mix. I've never actually allowed a 1 day stressing, the statement was more an indication of just how hard the cycle time is sometimes being pushed. Good point on the start vs. finish consideration on the stressing. At 3 days I've haven't had an occasion to differentiate, at 24hrs it could matter.

regards,
Michel