huangyhg

pt effects on shoring
in a two way pt floor the distributed tendons are usually stressed (100%) first. this would create a line of uplift at midspan and a line of downward force along the column lines. this increase in force is not accounted for in the shoring designs of most contractors. although the loading is only temporary (until the banded tendons are stressed) is it something i should be concerned about? has anyone considered or heard of contractors considering this subject? thank you.
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i've never heard of this being considered. if you cut a free body, the load applied hasn't changed.

i am unclear as to how the applied load hasnt changed. the horizontal force in the tendon is creating upward and downward forces in the slab, basically redistributing the slab weight towards the tendon high points.
so? if you take bandline with it's tributary, it'll still the same load is it was prior to stressing.
i am concerned about the loads on the shores. if the shores are placed at every 4 feet they are designed to take a 4' x 4' tributary weight of the slab. the shores and slab below are designed to distribute that weight out. if only the distributed tendons are stressed it will decrease the shore loads in the middle of the span and increase the shore loads near the column line. the shores and slab below near the column line were designed to distribute a 4' x 4' weight of slab, but are now seeing a larger force due to the redistribution of slab weight...
there's a flaw in your understanding:
the stressed tendons do not induce load... they precompress parts of the slab to increase the flexural capacity. concrete still weighs 150pcf.
you might be confused if you're using the load balance method and draw a "reverse loading" diagram on a slab strip. that's just something to make the math work.
think about it... if you have a simple-span pt beam, stress the tendons to balance 100% of the dead load, then remove the supports, would the beam float?
the concrete still has the same dead load.
jkstruct is right. you only induce a lateral load from prestressing, not a vertical load.
i think what he is asking is if the entire slab is supported on shoring and the distributed tendons are stressed, part of the slab between the banded region is no longer fully supported by the shores since the prestressing is now taking some of the weight. this would therefore place an additional downward vertical reaction on the banded areas where the shores are still firmly in place below, therefore these shores are now receiving more load than what they were before.

yes there will be an increase in force in the shoring under transverse beam lines, but i've never heard of this being accounted for.
(analogous to a precast bridge girder with camber, only supported at the ends since the center has lifted off the ground)
swivel, jkstruct, and slickdeals are incorrect. the prestressing does change the loading distribution on the props. the props should be designed with adequate overload capacity to account for more load being taken along the column lines before the slab carries the load itself.
has anyone ever seen this taken into account in shoring shops or discussions with the shoring engineer? i see where he's going with it, but i still don't think it really matters.

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