Construction Joints in Post Tensioned slabs

[strguy11]

How are construction joints handled in a 2-way PT slab construction? If the tendon is continuous through the joint, it seems like the moment capacity of the slab is the same as if the joint weren't there, correct? Why would you use a intermediate stressing anchor vs, not one, but still having the tendon go through? Is the only consideration to provide some mild steel to take the shear forces??

 

[Ingenuity]

Why would you use a intermediate stressing anchor vs, not one, but still having the tendon go through?

Intermediate stressing anchors within a CJ are used when the stressing length of the tendons are such that PT losses (mainly friction) would be excessive and the resulting effective prestress force not economic ($ per kip of PT force).

At a CJ it is typical to use intermittent formed keyways...and I also use mild steel reinforcement too.

The location of your CJ should consider: 1] the tendon profile/drape such that the PT anchors are placed mid-depth of slab, and 2] the flexural/shear actions applicable at the chosen location. Sometime the CJ locations are "selected" by the formwork system used, and sequencing of construction.

If the tendon is continuous through the joint, it seems like the moment capacity of the slab is the same as if the joint weren't there, correct?

The ultimate moment section capacity of a continuous tendon with or without a joint is the same, assuming (for an unbonded tendon) that the effective prestress force is the same. For a tendon with an intermediate stressing anchorage, the moment capacity may differ to that of a continuous tendon, depending on the the anchor hardware used - basically, upon stressing the Pour 2 tendon, the wedges at the CJ of the Pour 1 intermediate anchorage may not be lift-off so differing effective prestress, but it is a small effect.