Pour Strip 4

[L775]

When should a pour strip be required in a post tensioned concrete slab? (8" thick elevated slab in residential building)
just to give an example;
say the slab is 298' long x 75' wide
or 365' long x 75' wide.
what is the average practical slab length that will not require a pour strip?

Thank you very much for taking the time to respond to this question.


PS: Archived subjects that did not answer this question include thread507-43064

Thank you,

 

[rapt]

L775

Pour strips are useful to allow the axial PT to compress the slab and, if left open long enough (months), to allow some of the shrinkage shortening to occur unrestrained, in situations where there are very stiff vertical elements such as lift cores etc. It is possible they they could be necessary in a 100' long building if there was a stiff core at each end.

In my opinion, any slab as long as the two you have mentioned should have at least one permanent movement joint, not just a pour strip.

The pour strip will only account for the elastic shortening from the PT and the small amount of shrinkage that occurs while the pour strip is open.

The remainder of the shrinkage and any temperature movement, the total of which would be 5 to 10 times the elastic shortening from the PT, will still affect the structure. The shortening from these effects will cause major problems with columns and possibly also the floors for long buildings.

The absolute maximum spacing between permanent movement joints would be about 200' as long as the columns are not stiff with a preferred maximum of about 150'.

 

[JAE]

nice post rapt

 

[sundale]

I also agree w/rapt and JAE. This building should probably have a thermal expansion joint across the long dimension.

Pour strips are a useful and inexpensive way to reduce the elastic shrinkage strains of the concrete upon curing,and in your case, the elastic and inelastic axial shortening strains due to the prestress forces. A 298' or 365' concrete slab dimension with no allowance for delta T thermal movement is a little scary.

 

[L775]

Thanks for your thoughts.
Although I have seen residential buildings up to 360' long with no expanssion joint.
I think it is not practical to have expanssion joints in residential buildings.
But anyway thanks again,

 

[rapt]

L775,

There is nothing impractical about putting expansion joints in residential buildings. It is done all the time by designers who know what they are doing. Unfortunately, there are too many these days who do not. And you want to copy them.

Just because some idiots who do not understand structural action and design have done it and the buildings have not fallen down does not mean it is right.

If someone went over the Niagara Falls in a barrel and survived, would you follow him!! No, because you would analyse the situation and decide your chances are not good.

Analyse this one. The shortening effects due to shrinkage and creep, will
1 Induce extremely large moments and shears in the vertical elements such as columns due to the restraint to shortening. If you are lucky the columns will not shear off and fail.
2 Induce large stresses in the floors. If it is a typical USA designed PT flat plate, everything will be very cracked despite your assumption of uncracked behavior. The ACI code is actually grossly underestimating the stresses in the slab and its cracked/uncracked state even for vertical loading. And the restraint stresses will be far larger and everything will be very cracked. There will be nothing to distribute these cracks evenly and at close spacings through the slab as there is stuff-all bonded reinforcement in the slab. So you will get large cracks, like 1/16 - 1/8" at 10 - 15' centers rather than much smaller cracks.

Joints at 150' - 200' are much preferrable.

Design it, do not just copy someone else.