What is the difference between reverse parabola and parabola profile for tendons in PT slab ? 1

[JDG3718]

For a PT slab with tendon/distributed layout what is the best profile parabola or reverse parabola for each direction ? and what is the difference between these two profiles and their impact on the stresses ?

 

[KootK]

I'm a bit confused by your question. Normally, in a continuous floor plate, you'll need both parabolas and reverse parabolas depending on where you're at in the slab. As we discussed in your other thread, normal parabolas are used where you're looking to "pick up" gravity loads (mid-panel). Reverse parabolas are used where you're looking to deposit those loads (columns, banded tendon groups).

Sometimes folks debate where the reverse curvature over top of columns ought to be parabolic or circular. In my opinion, it doesn't make much difference. Numerically, they produce similar results and, in the field, it can be pretty impractical to rigidly specify one over the other. The reverse curvature takes place over such a short difference that both a parabola and a circular curve may well be able to fit within the same tolerance envelope regardless of which shape is specified. I think that the main thing is to ensure that you have a suitable minimum radius of curvature in the tendon at the reverse curve location. More info here.

A general tip on tendon profiling is that it is often efficient for the tendon profile to match the shape of the member moment diagram as drawn on the tension side. Correspondingly, a uniformly loaded flat plate will usually have parabolic tendon profiling whereas a transfer beam with a big point load dominating demand will often have a harped tendon profile. Although, practically speaking, it's rather difficult to do anything other than straight/parabolic tendons in a thin slab regardless of the nature of the imposed loading. Just not a lot of space in there to get real fancy.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JDG3718]

thank you that cleared things for me !