Post-Tensioned Transfer Beam in One-Way Slab Design

[jgwdea]

My main question is whether or not a post-tensioned transfer beam parallel to the direction of slab post-tensioning tendons should be designed as a T-Beam (using slab as flange) or Rectangular beam.

Here is an example to illustrate the question:

Let’s say I have a 180 (north-south direction) foot by 540 (east-west direction) foot elevated slab. The columns are spaced at 20’ centers in the long direction (east-west) and 60’ centers in the short direction (north-south). For a typical PT beam perpendicular to the slab span, I would have a 3-span (60 feet per span in the north-south direction) beam with a 20’ flange width (effective flange width of 106” based on ACI). So these beams are spaced at 20 foot centers typically. Now let’s say the architect wants to eliminate a column, which requires me to add a 40 foot transfer beam running east-west to provide support for the beam line running north-south. It seems to me, I should design this transfer beam as a rectangular beam instead of a T-beam. If I were to design it as a T-Beam, the added flange width would reduce my P/A stress, so I could add quite a bit more PT without overloading precompression. However, it doesn’t seem like I should be able to use slab beyond the width of the transfer beam as flange material since the one-way slab will be post-tensioned parallel to this beam (it will already have its own precompression stress.)

Thanks,
jgwdea

 

[BAretired]

rapt,

In the situation described in this thread, I would omit post-tensioning in the flange of the T-beam. Also, I would consider partially prestressing the beam, using conventional reinforcement to make up the deficit in moment capacity, all with a view to keeping the level of prestress more or less consistent throughout.

BA