Creep model in Eurocode 2 and ACI209

[asixth]

Hi guys,

What is the application of the creep factor (phi) that is calculated in the Appendix of Eurocode 2 and ACI209.

I have a prestressed bridge girder loaded at 1 day. Using the models, the creep factor at 30 days is 1.3 and after 100 days the factor is 1.9.

Does this mean that the deformations of the girder at 30 and 100 days will be increased by a factor of 1.3 and 1.9 respectivly.

I need to know that camber at these times to help with the setout of the deck slab. Has anyone got experience working in a precast yard and noticed the increase in camber?

 

[yakpol]

The creep factor represents a ratio of plastic and elastic deformations. Total deformation = Elastic(1+phi).

 

[asixth]

Thanks yakpol, I didn't think I was going to get a reply there for a moment. If that is the case, then the phi factor seems to be a bit excessive. I have a 60ft prestressed girder which by my calculations deforms upwards of 1.5" at release. The creep factors that I calculated is for a member loaded at 1 day, so after 30 days when the phi factor is 1.3, I should of had the 1.5" elastic deformation along with a further 2" plastic deformation due to creep. These numbers are starting to get large.

I am having trouble visualising a bridge girder continue to deform upwards such an amount after such a short period of time.

 

[yakpol]

That's about right. You may search for an alternative, more refined methods to calculate creep factor for high strength pretensione girders.

Yakov

 

[rapt]

asixth,

The creep factor modifies the stress distribution in the concrete. This does not result in a deflection ratio equal to phi.

Consider the simplified kcs factor approach to creep and shrinkahe. This assumes the long term effect of creep + shrinkage is 2 (over 30 years). This could come from a creep factor of 3-4 plus a shrinkage effect.

You will have to do a creep analysis of the section and determine the effects of it on decurvatures which will give you an indication on the deflection effect, or use RAPT which does all of this for you.