PT Slab design - something called "Drop bands or Slab Bands" 5

[DEL2000]

A supervisor went to a seminar on PT Design. In the seminar, they defined something called a "Slab Band" or a "Drop Band" as a slab thickening used primarily to get more drape on your banded tendons, but was not to be designed as a beam (ie, no beam shear steel). Limitations were placed on the width and thickness of the band (Band Width > 3* Band Thickness, and Band Thickness <= 2*slab thickness), presumably to make a very wide and very shallow section that would still behave as part of the 2 way slab, and not become stiff enough that it collected too much load where it would behave like a beam or edge supported slab.

I was curious if anyone has heard or used something like this. With the exception of this seminar, I haven't found a reference for Slab bands in any PT texts. Note that I have seen projects from other engineers that have come across my desk (in both PT and Conventional RC) that had slab bands, but these have always had shear steel in the band. The seminar notes made it very clear that shear steel was not required or intended for these very wide and shallow bands.

Thanks for any information you can provide.

 

[haynewp]

In Bijan's "Design Fundamentals of PT Concrete Floors" there is some discussion of slab bands and why they "usually do not require stirrups" but still uses some mild steel and stirrups for crack mitigation. It states the stirrup requirement from force attraction in bands is offset significantly by the upward component of prestress force.

You should be able to check for yourself whether they really need the stirrups for shear or not based on ACI, this is what I would do.

 

[dik]

RJC of Vancouver has used slab bands for > 20 years... not sure if for PT work... but should work as well... the depth is greater than a couple of inches and is of the same magnitude as the slab thickness... the added stiffness attracts neg moment and is better accommodated by the greater depth...

Dik

 

[rapt]

DEL2000,

This is a very common structural system in Australia and Asia for PT slabs. It is very quick to build (one way), very economical, reduces materials if the sizing is done correctly and performs very well in service if detailed correctly.

It is basically a continuous drop panel in one direction (but the band width is less than a drop panel width, normally .2 to .25 of the cross span length). The thickness is often 2 to 3 times the slab thickness (not limited to 2 times) and depends on relative span lengths in the 2 directions. Normally it is stiff enough to make the action very close to one way action and they are normally designed this way (flat slabs in USA are designed as one way slabs anyway using the banded/distributed system). Some designers trying to get a little extra capacity out of the system use the small amount of 2 way action but this complicates the reinforcing.

The slab direction (distributed) is designed as a one way slab with continuous drop panels (the transverse bands) at the supports and the tendons are draped to the face of the bands except at edge columns. This thickening at the supports allows the slabs to be thinner than would normally be used for the slab span.

The band direction is normally designed as a T-beam for the full load in that direction with the width of the flange limited as for a beam. The full width is not used (though some try to get away with it by ignoring statics and good engineering principles).
Beam Shear is checked as for any design but the minimum rules are based on Vc, not .5Vc (this is in accordance with ACI and AS3600 for wide beams). Because of this, shear reinforceemnt is normally only needed near the columns. Often a nominal cage is still provided full length simply to support the reinforcement in the band and the transverse reinforcement and tendons. Punching shear is also checked as the load path into the column could give a punching failure also. Edge bands should always have minimum torsion reinforcement.
The slab between the bands also needs minimum shrinkage and temperature reinfrocement independent of the tendons and reinforcement in the bands.

They have not been used extensively in USA because of the standard beam systems used in parking garages there (would be much better off with band beams) and the desire to reduce formwork costs by using flat plate slabs in other structures.

 

[hokie66]

The reason that slab bands often do not require stirrups is that the minimum shear reinforcement provisions do not apply for slabs and wide, flat beams. This is true for both posttensioned and conventionally reinforced. But the bands still must be checked for both beam shear and punching shear. Think of slab bands as drop panels connected.

 

[hokie66]

Ignore my comments, rapt has answered your question much more comprehensively, and I agree with him completely. One area that I find needs a bit more attention is the shrinkage reinforcement in the slabs between the bands. Restraint cracking often occurs transverse to the bands in the thinner slab section due to restraint of the heavy bands. For PT, use distribution tendon or two, for reinforced, I like to use a bit more than min temperature & shrinkage reinforcement.

 

[Taro]

This is also a VERY common system in the northwest region of the U.S., usually for office buildings.

Bijan Aalami (I think this is who haynewp was thinking of) has published several articles, PTI tech notes, etc. on the subject. He refers to the continuous dropped bands as "wide shallow beams".

 

[DEL2000]

Thanks for all the insight. I may have misrepresented the seminar notes . . . the seminar was an ASCE seminar in Vegas, at least one of the speakers was Bijan Aalami (from the seminar cover sheet, who I didn't know about personally), they did define the slab band as I described it above, but the official text in the seminar's notes said this about slab bands:

"Slab band is treated as part of a two way system. One way shear design provisions meant for beams do not apply to slab bands."

So, with this definition, it would seem to match with what people are talking about above regarding wide shallow beams and ACI shear requirements. Thanks for the reference on that PT Book and all of the great information.

 

[hokie66]

I think the seminar notes misspoke about the shear requirements. The reason the shear requirement is not the same for beams is that the band is treated as a slab because of its width to depth ratio, but not because two-way provisions are different than one-way.

 

[rapt]

Hokie66 is correct about the cracking problem in the slabs perpendicular to the bands.

I prefer to use normal reinforcement for this rather than tendons, especially for roof slabs that experience a combination of elastic bending stresses and differential shrinkage problems as well as temperature differential stresses. The differential shrinkage stresses are fairly high in these floors and when combined with the other stress could easily result in stresses greater than 500psi at the maximum negative and positive moment locations.
0ne or 2 tendons in a 20-25' width of slab providing 100psi P/A do not offer any crack control once the tensile strength of the concrete is exceeded while bonded reinforcement does. A layer of top reinforcement over the whole slab works best though most PT companies perefer to go for the distribution tendons.

RE Aalami's design methods, he is probably proposing that the full slab width be used as a flange for the band direction. This is completely wrong and should not be done.

Also, one way shear must be checked for all two way slabs. The only difference is the different limit on the shear stress at which minimum reinforcement is required.

 

[hokie66]

Rapt,
By your use of psi, you are talking to the American audience. If the distribution tendons are grouted as we do in Australia, do you still recommend deformed bars for restraing crack control?

I agree roof slabs without a membrane need a lot more reinforcement, whether bars or strands.

 

[rapt]

hokie66,

I only use psi on this site because the major part of the audience have not evolved to more logical units as we have and those that have normally understand the conversions.

My comments on minimum reinforcement in this case apply to bonded PT also. Bonded tendons at 2m (7') centres do not provide control of crack width once the concrete has cracked. It may stop cracking occuring until the stress is higher (by the P/A) but once it cracks, there is no crack control. For this reason I would prefer bonded reinforcement at 200-300mm (8-12") centres. I would insist on this in an exposed slab.