Banded Tendons in One-Way Slab

[bertiskorny]

Can you put banded tendons in a one-way slab or is it normally just distributed tendons in one direction for a one-way slab. In a two way slab it is normally distributed tendons in one direction and banded tendons in the perpendicular direction, right?

 

[haynewp]

Banded-distributed is for two-way slabs. Distributed tendons take the load one direction and the bands take it in another.

 

[slickdeals]

It is better to use distributed tendons than banded tendons in a one way slab. You have a more uniform distribution of prestressing force.

 

[pappyirl]

Banded tendons are not used in a 1 way slab. There is no column/middle strip and hence there are no strong or weak bands. There would be tendons running flat perpendicular to the spanning direction in order to provide the minimum prestress required to prevent cracking. Otherwise bonded reinforcement in this direction can be provided.

For two way slabs the tendons can be banded in one or both directions. I typically use banded tendons in both directions. This uses slightly more PT compared to the one way banded direction but provides steel where it is most required i.e. in the column strip and over the columns. It also provides better deflection performance and allows a reduced slab depth - the bands acting as a grillage of sorts.

 

[rapt]

bertiskorny,

Why would you want to? The banded/distributed layout is a simplified layout used for flat slabs (slabs with pure 2way action) to make them easier to construct with unbonded tendons. There is no logic for its use in pone way slabs. As pappyirl says, the better solution even for those two way flat slabs is to use a two way banded layout but it is often harder to install with unbonded tendons.

pappyirl
If you use 2 way banded and distributed, why do you end up with more PT? The same total is used in each direction, except for the minor reduction in drape available because of clashes.
On what basis do you reduce the slab depth in this case? The grossly unconservative assumptions (PTI/ACI) used in the standard banded/distributed flat slab layout using average moment design are hard to beat using proper two way flat slab slab design theory.

 

[haynewp]

pappyirl,

I have never heard of banded in both directions, is that safe? What do you do when there is a point load towards the middle of the slab?

 

[slickdeals]

Banded in both directions is not permitted by ACI 318. Read section 18.12.4, which limits the spacing in one direction to not exceed 5' or 8*slab thickness

 

[hokie66]

I think terminology is the problem, and that neither pappyirl or rapt is saying that you should have no tendons in the middle. I think they mean that it is best to put more in the column strip, which just makes sense.

 

[apsix]

Also I believe that neither pappyirl or rapt are based in the US and therefore are not necessarily limited by ACI requirements.

 

[rapt]

By Banded in both directions, I mean a proper 2 way distribution with distributed tendons in both directions loading bands in both directions.

Basically 50% of the load is carried in each direction by the distributed tendons and these distributed tendons are then supported by banded tendons picking up the distributed tendon load and carrying it to the supports. Thus 100% is carried in each direction. But then, the 50% distributed in each direction can be modified as long as the respective bands are modified accordingly. So the distributed could be split 70/30 as long as the banded is split 30/70, still giving 100 in each direction.

The US one way banded distributed system is one extreme of this where the distributed % is 100/0 and the banded % is 0/100, thus still carrying all of the load in each direction.

100% banded tendons in both directions was proposed and used in Europe in the 70/80's (definitely in the Netherlands but possibly in other countries as well). In this case, the middle strip is reinforced with passive reinforcement. Costs more but easier installation. I was asking my questions of pappyirl to see if this was what he was suggesting.

Concrete slabs work the same way in all countries, simplistic (and sometimes dangerous) Code rules often limit people who know what they are doing from designing a slab in the best way possible. The rigid rule in ACI allowing banded/distributed or 75/25% distributions only is stupidly restrictive to designers who usderstand slab design (not simply how to apply some code rules).