Creep Deformation (and fictitious force calculation due to creep)

Question:

Hi.

I have a question about the creep deformation and the relationship with the creep coefficient.

I did a simple staged modeling to see the deformation due to creep. The simple span structure is added to the model at age 30 with the axial load of 1000kN and the duration of 10000 days to see the creep deformation at 10000 days.

I used CEB-FIP 1990 model for the creep analysis and checked creep coefficient at 10000 days with the start loading age of 30-day (i.e. (t,t0) of (10000,30). The creep coefficient shows 1.3938. So, I expected that the creep deformation at 10000 days to be 1.3938 times higher than that of axial load deformation. But, when I check the result the creep deformation at 10000 days is only 0.989 of the axial load deformation. And, the fictitious creep primary axial force is also 989kN.

I am wondering why the creep deformation and the fictitious force does not match with the creep coefficient.

Is there an any age adjusted factor applied "internally" to consider the overall creep reduction? Otherwise, how are the creep fictitious force and creep deformation is calculated?

I have attached the model I did in this regard for your reference. If you need any other information in this regard please let me know.

        

Answer:
Hello,

Actually, the reason for this discrepancy is the way in which the material properties and the creep definition is done.  In the material property, elasticity is defined as 2.811e7 kN/m^2.  In Creep definition, the characteristic compressive strength is defined as 50000kN/m^2.  This leads to elasticity of about 3.86e7kN/m^2.
Thus, creep force  = Axial force x creep coefficient x Elasticity(Prop)/Elasticity(C&S) = 1000 x 1.3938 x 2.811e7/3.86e7 = 1014kN.
In the latest version of midas Civil, the midas creep primary force that we get is about 1001 kN.  The difference in results in version 2017 and 2019 is due to improvement in creep coefficient calculation algorithm.
The overall difference in manual calculation and midas creep primary is about 1.3%.
A simple excel sheet with calculations is attached.  Kindly let us know if further assistance would be required.

Regards,
Nandeep
Technical Manager, MIDAS
Creation date: 7/31/2018 3:00 AM (dklee@midasit.com)      Updated: 7/31/2018 12:30 PM (dklee@midasit.com)
Files   
Calcs.xlsx
Test for CRSH-5-Single Span.mcb