joints in buildings 3

[wildehond]

We are designing a 7 storye building that is approx 70m long. the floor plates will be post-tensioned prestress concrete. to avoid shrinkage and other problems, we have called for a joint in the building. most rule of thumb guides recommend that the joint be 40mm wide, but I can't make out the logic of this. the PT slabs will certainly make the buildings shrink away from one another, so 40mm is more than enough. And during a possible seismic event, a smaller joint would simply mean that the slab edges would knock against each other. defintely causing some damage, but surely not catestrophically? Any thoughts?

Thanks

 

[rapt]

dik,

The idea of the pour strip is to get the P/A into the concrete where there is stiff restraint at the ends. Filling the pour strip before stressing defeats the purpose.

To be effective in significantly reducing shrinkage restraint the pour strip would have to be open for at least 3 months to get rid of 50% of the shrinkage and about 12 months to get rid of 70%, but then tere would be no P/A in the slab to resist the stresses due to the remaining shrinkage, all of the temperature change and any vertical load stresses.

The most effective solution to the shrinkage/temperature dilemna is to only pour concrete when the temperature is low! Then the temperature change is +ve and reverse to the shrinkage, but that is impractical, as is leaving pour strips open for months.

 

[civilperson]

No joint is needed for buildings less than 300 feet maximum dimension if freestanding and correctly detailed. Assume the centerline is fixed and the ends move to accomodate temperature change.

 

[hokie66]

You can't assume the centre is fixed if the ends are fixed. Each building needs to be assessed on its on.

 

[dik]

Dealing with stiff elements restraining the slab is a different issue and has to be addressed separately. My intention of using a middle pour strip is simply to reduce effects of shrinkage on stressing and the effect is minor at best (but I still do it).

For stuff I've done, the strands are normally unbonded and my experience is that if you stress 100' or 200' and from one end or both, the P/A will essentially be the same as will be the effective shrinkage (maybe slightly higher if you stress from both ends <G>). I've never used a bonded system for a floor plate and the friction loss is so small that you will not likely have any problems with stressing it from one end. Spec it as stressing from both ends and check the elongation; I think you will be surprised. I think civildude is correct for relative lengths, again, barring significant restraining elements.

Dik

 

[rapt]

Civilperson,

Have you ever checked the moments and shears in your end columns from shrinkage and temperature for a 300' long concrete building that is shortening towards the centre, as well as the stresses in the slab if the columns are reasonably large? They are enormous, for both RC and PT buildings. 180 200' is the maximum as long as there are not stiff vertical elements, otherwise shorter still.

And as hokie66 says, it shrinks towards your stiffest vertical elements, not the centre. If you have concrete cores towards both ends, that is where it shrinks towards. And the centre is in direct tension!

dik,
Shrinkage does not come into the short term stressing losses calculations, only elastic shortening and it is very small. You are going to get the same shrinkage losses no matter what after the slabs are connected, but most designers of PT in US do not calculate these properly anyway if they use the Zia rules.
You are wasting your time putting in a joint if this is what you think it is for.
This discussion is not about prestress losses, it is about slab shortening and restraint effects.

 

[dik]

Thanks Rapt... next time I'll consider eliminating the joint. I still don't have a problem with stressing from one end.

Dik

 

[rapt]

dik,

Remmember it is not a general rule, it depends on the locations of stiff elements!

 

[dik]

for sure... Dik