Post Tension I girder failing concrete stress compression

[SatoshiNakamoto09]

A one span simple beam I-girder post tension with parabolic cable layout only fails concrete stress at top in midspan during transfer stage calculations,
May I ask how do you go about this failure in concrete stress only problem? Aside from decreasin the jacking force at trance, also since selfweight is already fixed.

How about adding non-prestressing bars at the top, then if so should we use working stress method just to solve this concrete stress failure during transfer stage calculations?

 

[bugbus]

If you're over the concrete stress limit at transfer, it's only going to get worse in service right?

Deeper girder and/or stronger concrete are probably the two easiest ways to do it.

 

[Luceid]

Does "transfer stage" mean during stressing?

I don't think the top stress failing in compression makes much sense during stressing at all - seems like something is way off or maybe I don't know something about I-girders. If that's the case - bugbus is right, service loads will only introduce more compression into the top flange which will just make the problem worse.

If this is more normal than I'm reading - I would expect that your top flange may be failing in tension during stressing - a symptom of overbalancing. Not sure what the real issue is here though.

 

[BAretired]

If it is failing in tension at top fibers in midspan, you can add either top strands or top rebar as required.

 

[Just Some Nerd]

the wording "failing concrete stress" makes it seem like a compression failure at the top, which would only get worse once adding load, but I'm guessing it is just tension in the top as you suggest BA

My personal preference raising the low-point of the parabola slightly first before adding top reinforcement if reasonable to do so while keeping the other aspects of design working, but top reinforcement will do the job.

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Why yes, I do in fact have no idea what I'm talking about

 

[BridgeNerdGuy]

Transfer typically refers to the stage immediately after stressing but before time dependent losses can occur. At this point you can add steel to the top of your section, minimize the eccentricity of your strands, reduce strand quantity, or change sections to increase the denominators recalling the basic P/A+-M/S.

AASHTO has a higher limit for tension at this stage if reinforcing steel is provided in the tensile zone, I’d think similar logic applies in whatever code you’re governed by.

Controlling tension at the top of the section at this stage is a classic issue in prestressed concrete design, especially if you are using a shallow section and stuffing a lot of PS force into it.

 

[KootK]

I'll assume that we're talking about tension failures in the concrete under prestress. In the world of precast concrete it is relative common to address that by:

1) Adding reinforcement. This, conventionally, is done via ULS design methods although I certain understand the impetus to use something like working stress if crack control is very important.

2) Adding straight prestress to the top of the beam as BAretired mentioned. You'd tend to think that this would be counter productive in the sense that it would work in opposition to your primary prestressing. Not so.

 

[Lomarandil]

All good advice above.

Please also leave a little extra margin in your top flange stress capacity for constructability concerns (like flange lateral stresses due to overhang brackets or rotation during picking due to sweep/curvature).