PT Slab - Stressing Blockout

[CURVEB]

Hi all - I am looking for some guidance on the correct way to design stressing blockouts for PT slabs. We have a situation where our tendons cannot be stressed outside of the slab edge due to an adjacent structure. We have typical details showing the general reinforcement arrangement, but I need to get a better understanding of what are the typical design considerations. Because the tendon is not anchored directly over the support, I can imagine that the vertical component of the PT needs to be resolved in bending of the slab, thus an appropriate amount of mild-reinforcement should be provided.

There is also a provision in the ACI318-11 (18.13.5.5) that indicates bonded reinforcement of at least 0.35Ppu needs to be provided when the anchor is located away from the end of the member.

Any thoughts on whether it is appropriate to consider either or both of these requirements? Anything else?

See attached for a quick sketch.

 

[BAretired]

Why don't you stress from the other end and put a dead end anchor next to the existing building?



BA

 

[CURVEB]

We would, but there are constraints at both ends so we have to stress out of the slab either way.

 

[BAretired]

Can you stress from the middle and overlap the tendons?

BA

 

[Retrograde]

You do not blockout the whole depth of the slab. You leave a recess in the top of the slab. This is referred to as a "pan" and is a very common detail. [edit to fix typo]

 

[hokie66]

You could have picked some neater photos, Retrograde.

 

[Ingenuity]

Retrograde: 'pans' are used in bonded PT markets, but seldom (never?) in unbonded PT slabs. Given the OP is in Colorado (and therefore unbonded PT) then the contractor is going to use a full-depth stressing blockout/recess, with multiple monostrand anchorages.

CURVEB: VSL has an old technical guide entitled "Detailing for Post-Tensioning' which has a brief discussion on the subject on page 19 and 20: Link

PTI has some very basic/generic info on stressing blockouts, but no design/tech info as far as I am aware.

I am not sure how many monostrand anchorages you have to blockout and if they are uniform or banded tendons, but you do need to account for the transfer of the full tributary factored load/reaction all the way to your end support, so you need to develop mild steel rebar beyond the anchorage face of the blockout, and carry it back to the vertical end support, AND you need to consider the axial component on the prestress and the tensile forces it will impart on slab where there is no PT. And then consider trimmer bars around the blockout, and also bent-out rebar for the pour-back.

 

[Retrograde]

'pans' are used in bonded PT markets, but seldom (never?) in unbonded PT slabs

I'll admit I have zero experience with unbonded PT. Not sure why you wouldn't use pans though - seems to be an easier detail to achieve on site. The VSL technical manual shows a pan.

 

[Ingenuity]

Pans do not physically exist in the North American market from a supply perspective, and stressing blockouts are seldom used. Unbonded monostrand anchors are not like the AU 'monostrand anchors' where 4 or 5 strands are anchored in a common casting. US monostrand anchors are single-strand anchorages of 5"D x 2.25"W. PT in the US is part of the rebar/ironworkers trade/scope - and all edge-forms, blockouts, etc are part of the concrete/formwork trade - never do the trades cross over!

The VSL tech guide was largely based upon bonded PT, except some brief sections on unbonded monostrand anchors.

 

[CURVEB]

All - thanks for the feedback. Ingenuity - that is the guidance I am looking for.

 

[Tomfh]

No pans in America?! Mind blown!

 

[rapt]

If there is no tension from restraint of slab shortening, you need reinforcement to provide a tension force of about 2 * V / phi developed at the face of the support.

If there is tension restraint, add that to 2 * V.

V would be the shear at the end support.

This is based on the shear requirements.