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Post Tensioned Slab Ramps

Post Tensioned Slab Ramps

(OP)
I am designing a Post-Tensioned concrete parking structure (flat plate) with ramps between levels. I have obtained a few publications from PTI about the topic but my main question doesn't seem to be answered in them. That question is: do I induce crazy large moments in my slab from the transition of post-tensioning strands from the horizontal portion of the slab to the ramped portion of the slab? To me it would make sense that there is a large moment created here, but I am confused that I haven't seen this addressed in the literature. Should I create closure pours at the ramp start and stop locations?

I am aware that this thread exists: http://www.eng-tips.com/viewthread.cfm?qid=277880 however it doesn't seem to answer the question.

Thanks for any help,
JJ

John-Jozef Proczka (Structural EI)

RE: Post Tensioned Slab Ramps

You should find a good book on prestress theory! It will be covered there. I would not call PTI a good source of high level technical information on PT design!

Forces induced in the concrete by the prestress are dependant on the angle change of tendon from the centroid of the concrete. As your concrete centroid is (probably) changing slope at the same location as the tendon, there is theoretically no change in angle between the tendon and the concrete centroid. This is not 100% correct as there will be a radius of curvature. Basically there is a force induced by the change in tendon slope and an exactly compensating force induced by the change in slope of the concrete centroid, so they cancel.

You do need to check however if the change in angle of the tendon is near the surface and inducing forces towards that surface and in danger of popping a chunk of concrete out.

RE: Post Tensioned Slab Ramps

(OP)
Thanks for the answer Rapt,

I agree PTI alone is not the only source that should be used for design. It won't be the sole source, especially not the book I was mentioning.

I am viewing the ramp start and stop in my mind as essentially a giant P-big delta force. I think that type of moment induced does exist, and is not solely dependent on the change of the tendon centroid w.r.t. concrete centroid.

I am wondering if anyone else has designed with post-tensioned ramped structures and if they took this into account.

John-Jozef Proczka (Structural EI)

RE: Post Tensioned Slab Ramps

I sincerely hope you are wrong, or all of the design theory over the last 80 years from Freyssinet, Guyon, Leonhardt, (and possibly even me as I have specialised in it and lectured in it) since the mid 70's) etc will have to be rewritten and many of the structures we have designed over that period will either be in danger of collapse or have already collapsed without our knowledge!

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