Restrained prestressed slabs.
Restrained prestressed slabs.
(OP)
I have seen many structures where large prestressed slabs are restrained by walls or other rigid objects (e.g. basement pile walls, stiff columns, stair walls, lift cores etc). This is obviously not theoreticallly a very good thing, nor so great in practice, but yet I've seen it enough times in my limited career to know it must happen all the time and that many designers and contractors mustn't give it as much thought as they probably should. When I've seen it typically the slabs or support structures form their own "movement joints".
Aside from unwanted cracks what are the real world consequences? How is strength affected, how is deflection affected? What cases have you seen? What was done in response to such issues?
Aside from unwanted cracks what are the real world consequences? How is strength affected, how is deflection affected? What cases have you seen? What was done in response to such issues?






RE: Restrained prestressed slabs.
A big issue could be that the prestress doesn't get into the slab and that it ends up in the walls. The could result in higher deflections because you don't get to balance out as much of the load as you were hoping.
RE: Restrained prestressed slabs.
Other than unsightly cracking, I mostly worry about the impact on the vertical elements. You can inadvertently impose an unintended shear demand on your walls that rivals the original design loads.
I've worked on a few buildings that were large in plan and had shafts at the far ends. The contractors poured half of the floor plate a few stories ahead of the other half. The strategy is similar to delay strips in mildly reinforced concrete. It doesn't solve all your issues though.
You may find this document useful: Link
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Restrained prestressed slabs.
RE: Restrained prestressed slabs.
You still have the same cables, the same force in those cables, and the same drape. Therefore, the load balancing is the same.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Restrained prestressed slabs.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Restrained prestressed slabs.
RE: Restrained prestressed slabs.
Asixths solution is helpful in that it gets the prestress compression into the slab. Unfortunately, that is far less than the tension stresses that could be induced by restraint to shrinkage or temperature change shortening restraint. So while it will help to reduce the effects of cracking it will not stop the cracks and you will normally end up with some very wide cracks, as prestress tendons only provide good crack control until the concrete cracks. After it cracks, unbonded tendons are basically useless for crack control and bonded tendons are too far apart to offer good crack control (max 300 centres required compared to 1m - 1.5m spacing). The only real solution is to add extra untensioned reinforcement in both faces at a maximum of 300 and preferable at 200mm centres or less.
The best solution is to check the cracking under combined bending and direct tension loading and determine crack widths.
Even the AS3600 rule for restrained slabs should require the maximum spacing of bonded steel of 300mm, even where sufficient prestress has been added to solve it with prestress alone according to the formulae (hopefully this limitation will find its way into the next version of the code).
RE: Restrained prestressed slabs.
RE: Restrained prestressed slabs.
What I meant in my post is that a slab with restraint will need more precompression compared to a slab without significant restraint. Load balancing is a function of the force and drape. If I had a slab with minimal restraint and balancing 90%, addition of restraining walls will reduce my load balancing as the force is sucked up by the walls and not delivered into the slabs.
RE: Restrained prestressed slabs.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Restrained prestressed slabs.
The axial pre-compression into the slab (the P/A) will get "sucked up by the walls" and reduce the P/A into the CONCRETE slab, BUT the FORCE on the PT tendon will not change (well it will due to losses, but let's neglect for this discussion) so the balanced loads are NOT effected by restraint. Think of it by isolating the tendon from the concrete - the prestressing force from the tendon imparts 'equivalent loads' into the concrete, both axial (P/A) and transverse (due to tendon curvature). The 'transverse load' (in your case the 90%) of the tendon/s within the span (the 'meat' that does the load balancing) is only dependent on the TENDON force and the ANGLE change (i.e. drape). The prestressing tendon FORCE does not change, so the magnitude of the load balancing does not change due to restraint. BUT, the restraint actions of slab/wall/columns etc will reduce the P/A that the slab.
RE: Restrained prestressed slabs.
True.
RE: Restrained prestressed slabs.