Partial factor for prestressing Eurocode vs ACI

[Alvarobg]

While according to Eurocde 2, section 5.10.8, at ULS, it is necessary partial factors for preload (being 1.2 and 0.8). In the ACI code partial factor for preload is equal to 1.0 (only it is equal to 1.2 in anchorage zones).

Why is there such a difference? Does it exist any other factor according to ACI that involves the uncertainties of the preloading?

Thanks!

 

[rapt]

Eurocode is more logical as there is definitely variability in the prestress force due to inconsistent friction and wobble effects etc as well as creep and shrinkage variability. Normally extensions are accepted within +- 5-10% of the calculated value. At 10%, the force at a dead end would be 20% from the expected.

As well, the effects of prestress secondary moments are dependent on the indeterminancy (is that a new word) of the structure. After plastic hinges form there is doubt that any secondary effects will remain.

Some codes suggest that the designer allow for a a range of loss values to account for this. Some codes suggest that secondary effects that help the design should be ignored as their effect under overload are doubtful.

 

[Alvarobg]

Thank you.

It is really strange such a high difference between these normatives, right?

Applying or not those partial factor on preload force normally governs the design...

 

[rapt]

If you read the whole of the Eurocode note,

1 these are recommended values. Any member country can adopt their own values and I think some use 1.0.

2 The second sentence says if linear elastic analysis with un-cracked sections is used, the recommended value is 1.0.

And as I said, many design codes and guides suggest that you check for a range of values of prestress losses to account for the natural variability. And that would be in the order of 20 - 30%, so equivalent to about 1.3/.7

So, no it is nopt really strange. Some codes are requiring you to think for yourself!