huangyhg

pt two way slab
hi, i'm an italian structural engineer.
a question about pt two way slab: is it possible to design reinforcement layout with ordinary reinforcement and unbonded tendons in "column strip" for each direction and just ordinary reinforcement in "middle strip"?
(banded tendons in two way direction just in column strips)
thank you.
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if you are following the aci code, then it is not permitted. you are required to have cables placed no further than 5 ft or 8 * slab thickness. this is in section 18.12.4 of aci 318-02. this forces you to do a banded-distributed pattern.
thank you for reply.
about structural research or technical paper, is it possible to design that layout about unbonded tendons?
i think you could design it that way but as slickdeals said, the aci code apparently doesn't allow it (not sure about the italian code). it would be interesting to know why aci requires the banded approach.
by the way - i was in italy two summers ago. what a great country!

thank you for reply.
there is no italian code about two way slab. imho it is important to ask about it where there is knowledge.
i think it is possible to band tendonds just inside the column strips, because you could provide satisfactory sectional behaviour.
but i need knowledge and experience, so i ask to forum.
a practical reason for bands/uniforms is placement. if bands in both directions there will be a conflict at columns.
i agree that the banded in one direct with unbanded in the other is a practical way of avoiding the congestion you would have with banded in both directions. the section typically are so small, any loss in cable height leads to a big change in vertical component of the cable force.
but i can't think of a structural reason why you couldn't have banded in both directions
history
the banded/distributed system was basically created for unbonded sustems, because the column/middle strip system requires weaving of tendons. this is reasonably easy with bionded pt as the tendons are not normally pre-manufactured full length, they are made up on sode out of 6m lengths of duct. it is very hard to do with full length unbonded tendons. so the usa with unbonded tendons went this way and because of this they cannot do partial prestressed slabs.
as an aside, the aci code is stupid on this as it was written by unbonded people for unbonded industries. also, the pt software available from the us at the time was written to handle only single strips and the developers of those programs had a big influence on the direction the usa pt industry went and on the aci design approach. because of this it limits people from doing bonded patially prestressed slabs and achieving a better result in many cases.
in australia and asia with bonded tendons, column/middle strip arrangements are the norm as is partial prestressing for flat slabs.
lisia
there is no reason why you cannot do it. it was done quite often in netherlands in the 1970/80's (called support strip stresing i think).
as long as you put column strips of tendons in both directions, some top reinfroceemnt over the columns and and check for strength, crack control and cracked deflections properly, it will work properly as a partially prestressed slab for the column strips and a reinforced slab for the middle strips. eurocode does not preclude this approach.

rapt has given a good synopsis of the situation of unbonded vs. bonded/partially prestressed. as a us trained engineer working in australia, i agree that the aci supported method is "stupid", and would encourage us consultants to do something about it.
thank you for reply. i agree with rapt.
is there a link to technical paper about this "partially prestressed" two way slab?

lisia,
the second edition of tr43 by the british concrete society covers it reasonably well, though the explanation in some areas leaves things up to engineering logic and a little reading between the lines as it covers both methods. it is not available online. you would need to purchase a copy.
otherwise, there is nothing different to it than rc flat slab design to get the moments for column and middle strips in each direction and then designing each strip in each direction as a partially prestressed

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