What does a stated working stress in PS Tendons imply? 1

[Future_SE]

I am looking at a PS girder bridge from the 50's and trying to perform an analysis on the beams to ensure it can handle an increased load due to a deck modification. The as-builts indicate that 250 ksi high-strength, stress-relieved strands are to be used and state a working stress of 140 ksi per strand.

My questions is very simple: since the stated working stress is only 56% of the ultimate stress, does this mean that the working stress is the stress in the strands after initial PS losses during transfer/release? Or is this the initial stress the strands are to experience prior to any prestress losses?

 

[BridgeSmith]

For Working Stress Design (WSD), I think working stress would be the maximum stress allowed in the strands under the applied nominal (unfactored, expected) loads. When analyzing for additional loading applied after it's been in service, I would say it should be the stress calculated after losses.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Ingenuity]

I think it would be be the effective prestress, P[sub]eff[/sub], that was used in design, after ALL losses (short and long term).

I encounter this in old (1960's) unbonded PT buildings where the drawing notes often state:

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Although your bridge girder is pretensioned, the nomenclature would be similar.

 

[Future_SE]

Ingenuity- My thought process is that the as-built plans show a working stress that the Contractor has to adhere to. I'd imagine this working stress has to be measurable for the Contractor to confirm that required stresses have been implemented onto the girder. Therefore, wouldn't it make sense that the Contractor is to verify the working stress that is achieved after immediate losses? Maybe the alternative norm was for them to calculate the long term PS loss but I can't see why a construction company would be required to do design work back in the 60's.

This is just my thought process since I can't find any manuals from the 50's with WSD for PS tendons. Any additional insights are appreciated!

 

[centondollar]

Future_SE:

It is not possible for the contractor to measure the stress in the tendons after the immediate (elastic shortening, anchorage and friction) losses and locking of tendons. They measure the elongation during stressing and ensure that the required elongation (specified by the EOR) exists in the tendons, and thus ensure that the structure functions as designed.

Ingenuity is probably on the right track. You are most likely referring to the stress, after immediate losses, that the EOR designed the structure for; these days, long-term losses would also be accounted for, and maybe the engineers back in the day also did that.

By the way: if the girder is approximate 70 years old and contains "normal" tendons (not low relaxation), there will be anything from 25-50% or possibly more losses in stress due to shrinkage, creep and relaxation. According to Eurocode calculation recommendations, standard concrete (cube strength somewhere around 25-45 MPa) with standard cement and standard tendons lead to over 20% in time-dependent losses at 50 years due to creep, shrinkage and relaxation, and the losses after that (or, if we´re being realistic, many years before 50 years have passed) cannot be accurately determined. In other words, you should not trust any figure given on the drawings, but instead instrument the bridge with strain or deflection gauges, load the bridge in a controlled manner and finally back-calculate the effect of prestressing based on the elastic response.

 

[BridgeSmith]

It is not possible for the contractor to measure the stress in the tendons after the immediate (elastic shortening, anchorage and friction) losses and locking of tendons. They measure the elongation during stressing and ensure that the required elongation (specified by the EOR) exists in the tendons, and thus ensure that the structure functions as designed.

I agree, except that really only the fabricator measures the stress in the strands, so unless the contractor is also the fabricator, they can't measure strand stresses either.

I also agree on the losses in tension over the long term. The serviceability of the girders for increased loads may be limited due to the losses, but the capacity of the girder is not. By that I mean if the loading on a 70 year old girder is increased, the concrete on the bottom may crack, but the ultimate capacity of the girder remains the same as it was.

Rod Smith, P.E., The artist formerly known as HotRod10