Post Tension Transfer girder precompression force and tensile stresses 1

[nampar]

Hi everyone,
I need to design a 60ft span transfer girder supporting a column that sees loads from nine stories above. My girder c/s is 66in wide x 76in deep. My tensile stresses are going beyond 12sqrt(f'c) i.e. in the cracked zone. Also my P/A is around 750psi.

I am using a cracked section. Is this something i should be concerned about?
Also, What is the permissible (industry standard) precompression stress which I should absolutely not exceed?


I am doing this for the first time so even if there are any obvious inputs/comments that shall be really helpful.

 

[rapt]

As long as shortening is not heavily restrained from shortening, I would not be worried about that level of P/A or the fact that it is cracked.

As long as you fully account for

- crack control checks
- deflection check considering cracking, tension stiffening, creep and shrinkage (not by factor method). Deflection limit should be far tighter than the normal beam deflection allowance as the beam is a "settling support" for the structure above.
- compression stress at service
- stage stressing effects

 

[Ingenuity]

Similar to rapt, assuming no significant restraint-to-shortening, I would not be too concerned about these P/A levels.

Given you are in California, I am assuming this will be a UNbonded transfer beam - if so, you need to consider your non-prestressed mild steel reinforcement based upon satisfying: a) your flexural tensile service stresses (crack control), b) your flexural ultimate strength requirements, and also c) robustness/resilience of the beam element with unbonded PT.

Purely for illustrative purposes - not endorsing this design (it is not mine) - here is a photo of one of two transfer girders to FIS Parking Garage at Houston, Tx. [Note the level of P/A]:

 

[rapt]

Ingenuity,

Is it really possible to achieve a Robust Transfer beam with Unbonded PT?

 

[Ingenuity]

Is it really possible to achieve a Robust Transfer beam with Unbonded PT?

Our North American counterparts will argue, YES, with the argument along the lines of 'they have been doing it for decades' without known issues.

Interestingly, is this analogous to UNbonded PT precast segmental bridges sections, where there is ZERO mild steel reinforcement passing through each critical section? Admittedly, the typical 'event' that warrants/requires robustness in a bridge structure is different to a building transfer beam.