joints in buildings
joints in buildings
(OP)
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
Thanks






RE: joints in buildings
Some of these gaps can get to 6" (150 mm)depending on the height of the structure and seismic event modeled.
However, if you are not in a high seismic area, maybe shrinkage will control.
Mike McCann
MMC Engineering
RE: joints in buildings
RE: joints in buildings
RE: joints in buildings
your suggestion to connect the two "halves" once the initial pre-stress shortening has been allowed to occur is an idea i have been toying with and is the one i will actively pursue now. that way i avoid all the hassle that the joint will bring.
fantastic to feel complete clarity so that I can "get on with it"
RE: joints in buildings
RE: joints in buildings
thanks for your additional comments. The bulding is in Cape Town, South Africa. Not as big a variation in temp as you might have experience with. But I get your point about the total only been made up of a part of the post tensioning effects.
Current thinking here is that we will construct the slabs in 2 parts (near half) but with "slots" in the ends of the slabs to allow later reinforcing bars to be grouted in. For all intents, the break will be treated as a joint as far as masonry panels etc goes. But before the finishes go in (probably about 5 months after the slabs were cast), we will have the contractor grout in reinforcing into the slots to combine the 2 "halves". By then we expect approx 70% of the shrinkage to have taken place already.
The bonded bars across the "joint" will keep the building together seismically
RE: joints in buildings
I will disagree on your numbers on this Hokie66, but the end result is still the same,
About 10% from PT and equal portions of the remainder to shrinkage and temperature. So about 45% of the total is not time dependant.
Wildebond
You will find that the temperature difference does not vary that wildly in different p[arts of the world. If they pour the concrte on a really hot day and it is open to the atmosphere, the temperature difference in Cape Town could still be 30C or more.
RE: joints in buildings
Thanks for your on-going input. With the Atlantic sea a mere 100m away from this project keeping things moderate I'd say a 30degree C range is probably what we're looking at. For a 70m building this implies around 30mm change in length.
It could be that it would be better to spend the money on some extra reinforcing, ignore the joint idea completely and detail all the masonry with goods joints.
We'll get down to calculating the overall change in length of the building to check this out.
Something else that is a pain is that the shrinkage shortening & PT work against the shear wall elements. This too, will require appropriate detailing
RE: joints in buildings
You can use lapped bars across the delayed pour strip rather than having to install tie bars later.
And yes, one of the main reasons for using delayed strips is to lessen the shrinkage effects on the restraining elements. The other is to reduce the direct tension forces, thus reducing slab cracking issues.
rapt,
My numbers came from the memory bank and may not be based on up to date data, but which 45% are you saying is not time dependant? Shrinkage is time dependant, PT creep is time dependant, temperature goes both ways.
RE: joints in buildings
Dik
RE: joints in buildings
The numbers I remember are:
125 ft or less : Stress from one end
125 - 250 ft : Stress from both ends
250' or more : Use a pour strip and stress from both ends.
RE: joints in buildings
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RE: joints in buildings
Dik
RE: joints in buildings
In using a pour strip, you have to stress both parts separately and join with nonprestressed reinforcing. If you leave the two sections unstressed for a period of time, there goes your crack control.
RE: joints in buildings
Dik
RE: joints in buildings
If a joint is a problem (as it usually is) then, yes, reinforce for the restraint due to shrinkage + temperature. It will be a lot of reinforcement. Possibly .6% or more. As I mentioned below, the PT tendons will not help much for this. This is assuming there are stiff concrete cores at both ends of the building.
Even without the expansion joint, I would still provide a pour strip so that the P/A gets into the slab. And stress in both directions from the pour strip, so there is no external stressing and stressing length is only 35m.!
dik,
Stressing takes care of crack control until the concrete cracks. Then, especially unbonded tendons do nothing and bonded tendons do very little as they are too widely spaced. The stress from full restraint of shrinkage if there are cores at each end is equivalent to about 6MPa, so it is cracked and significant reinforcement is needed for crack control. Add another 6 for temperature and you need reinforcement for crack control.
Also, I assume you mean that the pour strip is filled "after stressing", not prior to!
hokie66,
45% that is not time dependant is the temperature one. Shrinkage stress will increase with time and can be reduced by leaving the pour strip open longer. It is also reduced by creep.
The temoerature 45% could happen at any time over the life of the structure and is not affected by pour strips and very little by creep.
RE: joints in buildings
RE: joints in buildings
If you measure the elongation of the strand when you stress it from the opposite end, with the initial overstress in tensioning, you might be surprised in how little the elongation is when stressing the second end.
Dik
RE: joints in buildings
The pour strip is just two construction joints, and deformed bars across the strip can tie the two together.
I know you are talking about unbonded construction, and granted you will have a lot less friction loss in greased strands than in hollow ducts.
RE: joints in buildings
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.
RE: joints in buildings
RE: joints in buildings
RE: joints in buildings
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
RE: joints in buildings
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.
RE: joints in buildings
Dik
RE: joints in buildings
Remmember it is not a general rule, it depends on the locations of stiff elements!
RE: joints in buildings