huangyhg

post-tensioned beams parallel to slab
i'm designing a two-story parking garage using post-tensioned beams and slabs. it will be ordinary moment frames with three nominally 62' spans in one direction and 7-9 spans of between 22' and 32' in the other direction. my one-way slabs range in thickness from 7" to 9" and run in the short (22-32') direction.
my question is in regards to those 22'-32' beams running parallel to the slab. during a meeting with a pt supplier, they mentioned that you need to post-tension the beams parallel to the one-way slab to the same p/a value in the slab. i've searched though my mountain of literature and can't find a reference or guide to this situation. my question is this: how much slab do i consider in the design of those beams? will i be accumulating prestressing if the slab is already prestressed to a p/a value and then i add more through the prestresssing of my beam?
tech support at adapt stated that it didn't matter as long as my tendons were anchored at the cg for the tributary cross-section. well, the tributary for my cross section is 31' or 62'! should i just be using the area and cg of the beam itself? i'm worried that i will be introducing a significant moment in the beams if the tendons are anchored higher/lower than they should have been. as these beams aren't carrying a significant gravity load except selfweight (since they're parallel to the one-way slab), i'm concerned that they wouldn't be adequate if a moment did develop.
any help or insight would be appreciated.
"as these beams aren't carrying a significant gravity load except selfweight (since they're parallel to the one-way slab)"
what are the beams there for?
hayneswp,
they are the lateral force resisting system (ordinary moment resistant frames) for the direction perpendicular to the long 62' beams.
you are planning on pouring the slab at the same time as the beams, correct?
yes, as far as i know. not as sure about the stressing schedule though, as the pt supplier will be responsible for that.
i had written up a long reply but i just accidentally deleted it.
anyway, i think you should design the beam as post tensioned (draped strands) with an effective flange width at on side per aci or whatever your code says. i am assuming your beams are only at the ends of the slab and none in the interior?
you can have some extra tendons without any drape running within the effective flange width if you are concerned about a minimum p/a. i think adapt was implying you should anchor the tendons at the centroid of the beam+effective flange width (this is somewhere within the beam). i wouldn't just assume the beam is only taking its own self weight.
there will be some deflection compatibility issues between the beam and adjacent slab as you move away from the beam, but assuming there are no heavy line loads, i don't think it will be a problem.
i don't know the technical answer, but the following is my idea:
provide mild steel to resist a reasonable amount of tributary area and then...
for example, say the slab is designed to have p/a=200. your drawings show enough tendons in the slab to create 200 for the entire slab, including the part of the slab within the aci effective width, which is fictitious for a pt beam anyway. now provide p/a for the rectangular part of the beam. you have approximately 200 everywhere in this case. put the beam tendons at d/2.
dbd
miket14,
mike, dbd is correct regarding the addition of pt tendons in the beams. the addition of the tendons is to ensure that the slab cross section has the correct amount of precompression with the presence of the perimeter beams.
auce98
miket14,
if the beam is there it will be stiffer than the slab and will support some of the slab load. the beam and the attached slab must deflect the sams amount for compatibility so the beam will attract some of the slab load.
in terms of the amount of pt required in the beam, you could put none in there if you want as long as you put enough reinforcement to provide strength to support the loads that the beam will attract and make sure that the slab parallel to the beam has sufficient p/a + reinforcement. the slab for about 6 to 8' on either side of the beamn will be supported by the beams anyway, not by the parallel slab, so if the p/a in this parallel slab is slightly reduced because some of the prestress force leaks into the beam it does not matter as the bending stresses in the slab are much reduced as the slab in this area is actually spanning as a 2way slab.
design the beam as a standard t/l beam with the code flanges (i disagree with dbdavis that the effective flange width is fictious for pt beams, there are too many people designing pt beam/slab systems on the basis that the p/a completely averages over the full width including some stupid illogic put out by some pt software companies) and put the anchors at the centroid of that beam shape. put enough pt in the beam for strength and serviceability checks but it does not have to exactly equal the p/a in the slab.
also, the slab perpendicular to these beams will experience bending stresses and you will need to reinforce the slab in this direction with top reinforcement over the beams and possibly bottom reinforcement as well as you move away from the beams.
thanks for the responses. the approach of designing the beam pt considering an effective width of slab is what i was planning on doing, but limiting the p/a to the rectangular area of the beam, not the effective flanges.
adapt seems to anchor at the cg of the tributary cross-section, not the effective cross section. which is right? rapt, is this what you are talking about with regards to the p/a across the full width?
miket14:
a few bigger picture notes on pt design - is it neccessary to drop the beams in question? the economy of pt design is keeping the slab thickness the same. the additional formwork for drop beams in the direction parallel to the slabs may be costly. also, the spans between 22' and 32' appear to range in thickness from 7" to 9". coming from a contractor background - we'd rather pour one thickness of slab even if it meant increasing the thickness of all spans. simplify. simplify. simplify. i'll now step down from my gc soapbox.
specifically to your question: "what effective flange width should you use?" this is more of an art form gained through experience. i've practiced with some heavily experienced pti

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