Creep non-linearity

[Anonymous]

Hello,

For pre stressed concrete beams, the check for creep non-linearity is false according to EUROCODE 2-1-1.

The clause 3.1.4 (4) only consider the "tranfer" and doesn't satisfy the clause 7.2 (3) for witch ALL "quasi-permanent" combinations must be checked in compression. (< 0.45 fck to be linear)

This change can make creep coefficient reach to 25 % higher and modify also the hyperstatics time-dependant effects.

Sincerely.

[david.geeves]

Hi gregorypajot,

Thank you for your post regarding non-linear creep effects in prestressed beams.

I completely agree with your comments that the check should be made for all quasi-permanent load combinations and this is what structural bridge design does.  If you consider the screenshot from the program you can see that I have analyzed for an SLS Quasi-permanent load case and looking at the stress calculations for the centre of the span:

In the results output the check for stress stress< 0.45fck is done and the non-linear creep coefficient calculated.

The stress being checked is the concrete stress at the centre of the tendons, due to the application of the full quasi-permanent load combination,  at time t0.  

This time, t0, is quite difficult to quantify as it is the time that the permanent load is first applied, and due to staged construction this is a variety of times. As the stress due to prestressing is generally a large portion of the permanent compressive stress, t0 is conservatively assumed to be the age of concrete at transfer (modified due to heat curing, atmospheric effects and cement type where appropriate using Annex B).  In my case this is 8.6 days.  The important thing is that the stresses being checked are not just the transfer stresses at t0 but the full quasi-permanent stresses at t0

So in conclusion I believe the software to be doing the check correctly, but please let me know if you think I have misunderstood something.

If this has answered your query could you please mark this post as "Solved" so that others can find this more easily.  Thanks

Kind regard

[Anonymous]

David,

Thank you for your response.

I does not agree with you BUT it's real that "EUROCODE" says 2 different things (about "t0" in 3.1.4 and about QUASI PERMANENT states in 7.2 (3)

[cid:image002.jpg@01D2A701.AC81CF80]

Nowadays, in France, we have to consider the 2nd clause in which creep coefficient is "non linear" (according equ 3.7) if the concrete stress exceed 0.45 fck under SLS QP combinations.

[cid:image007.jpg@01D2A701.AC81CF80]

May it be easier to consider MAX (w(inf,t) ; wk(inf,t)) for the creep coefficient ?

At your disposal

Regards

[Grégory PAJOT]

P Contribuez au respect de l'environnement, n'imprimez que si nécessaire

[Anonymous]

David,

Thank you for your response.

I does not agree with you BUT it's real that "EUROCODE" says 2 different things (about "t0" in 3.1.4 and about QUASI PERMANENT states in 7.2 (3)

[cid:image002.jpg@01D2A701.AC81CF80]

Nowadays, in France, we have to consider the 2nd clause in which creep coefficient is "non linear" (according equ 3.7) if the concrete stress exceed 0.45 fck under SLS QP combinations.

[cid:image007.jpg@01D2A701.AC81CF80]

May it be easier to consider MAX (w(inf,t) ; wk(inf,t)) for the creep coefficient ?

At your disposal

Regards

[Grégory PAJOT]

P Contribuez au respect de l'environnement, n'imprimez que si nécessaire

[Anonymous]

---

This time, t0, is quite difficult to quantify as it is the time that the permanent load is first applied, and due to staged construction this is a variety of times. As the stress due to prestressing is generally a large portion of the permanent compressive stress, t0 is conservatively assumed to be the age of concrete at transfer (modified due to heat curing, atmospheric effects and cement type where appropriate using Annex B). 

Dear colleagues,

Could you please specify what you mean by "age of transfer" in this case? Do you mean application of prestress forces to concrete?

It seems I missed something.

[Anonymous]

Deat Bjevtic,

Yes it's time of prestress application

The programs only check for this stage to determine if creep coefficient has to be calculated by non linear equation or not

It's clearly explain on his reports

It respect Eurocode 2-1-1 § 3.1.4 but does not respect Eurocode 2-1-1 5.10.2.2 (5) and 7.2 (3)

Regards