Long term deflection on prestressed block and beam slab

I think the prestress force creates an upward moment that cancels the long term deflections on dead loads

Click to expand...

If anything it may remove a percentage of the instantaneous dead load deflection, typically in the neighborhood of 80%. Long term creep, etc is still in play so primary impact to the long term deflection is that the starting point is smaller but you still have long term deflections.

the prestress only avoids the cracking on the bottom on the slab so long term deflections are the same as in reinforced concrete, but calculated using the gross inertia.

Click to expand...

only if the design was done to an uncracked stress level, though a lot of my reading seems to indicate even at the stress limits currently posed by ACI for uncracked stress level the concrete still gets beyond the rupture stress and so you still end up getting some cracked sections throughout.


My Personal Open Source Structural Applications:

Open Source Structural GitHub Group:

So the long term deflections would be the creep factor (lets say 2) times 20% the dead load deflections? Or you're saying the prestress only affects the instantaneous deflection, and the long term deflections should be the creep factor times the 100% of the dead load deflections.

Couple of points

1 it was take out some of the instantaneous DL deflection. How much will depend on the design. You cannot just assume a percentage.

2 Re total long term deflections, it depends on what deflection you are looking at and even which surface you are looking at depending on hot the top surface was levelled (flat or constant thickness) when pouring.

3 Re incremental deflection, this is an amount of movement, not a deflection. The prestress uplift will not affect this. The crack control from the prestress may, depending on the design.

4 Long term deflections for prestressed members cannot be calculated by multiplying the short term deflection by a factor. That only applies to RC members and is very approximate for those as well But it does not apply to PT members.
You have to do a full cracking, tension stiffening, creep and shrinkage analysis.