PT Tendon Layout - Square Grids 1

[Luceid]

Hi all, I'm looking at a schematic flat plate with square 9m x 9m column grid across the entire plan. Normally the shorter/longer spans of the column grid can help to inform which direction should be banded vs distributed using ACI 318, however in this case the square column grid opens up the options. Are there any other reasons to send the banded/distributed tendons in a specific direction for a case like this, or should I just flip a coin?

Simplified floor plate below (movement joints/pour strips can be laid out as desired)

 

[OldDawgNewTricks]

If the column grid spacing is equal in both directions, then the directions of the banded/distributed tendons don't make much difference. But in real life, it is rarely the case that we get such convenient uniformly spaced gridlines. It might start out that way during conceptual design, but it gets messy quickly when they actually start laying out the spaces in the building.

As you mentioned, you probably need at least one expansion joint to accommodate temperature changes and some pour strips for tendon stressing.

 

[dik]

I'd be looking at something like:

for a start...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Luceid]

Yep, totally with you dik. Got a fair bit of leeway on the layout for once which is nice

 

[rapt]

That still leaves a 400' and 450' lengths with no joints.

My maximum has always been 50-60m,so 150 to 200', and even then you need to check the shortening effects on the column connections for the columns closer to the edge

 

[hokie66]

In addition to the shrinkage/PT shortening effects which rapt is talking about, consider the quantity of concrete placement and area of finishing which your contractor is prepared for.

 

[dik]

rapt... have you tried using poly covered strand for that length and stressing from both ends? You might be surprised to see how little stressing losses there are... assuming it's unbonded.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[rapt]

dik,

I was not talking about prestress losses.

I was talking about the forces and moments induced in the frame, and especially the outer columns, by slab shortening due to axial prestress, shrinkage, creep and temperature changes.

 

[dik]

Thanks rapt...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Luceid]

Yes rapt, totally agree. There will be more joints than that. Thanks all!

 

[RobertHale]

If it were me, I would place the uniform tendons so they are crossing the construction joint. That way, you do not have to splay out a banded tendon group at the joint. For this L-shape, you can't get away from some banded groups passing through a joint, but you can minimize it.

 

[Luceid]

Great point, thanks Robert

 

[hokie66]

Nothing should pass that joint. It should be a movement joint, not just a construction joint.

 

[Luceid]

I believe Robert's intention (at least my interpretation) was that the distributed tendons would be perpendicular to the movement joint in orientation, not that they would be crossing the joint in any way. I'm definitely planning on having nothing crossing the joints. Good clarification though hokie, thanks.