Hello all,
Given that the new update of RAM Concept incorporates AS3600:2018 I think that the following becomes relevant. When determining the creep factor AS3600:2018 has an additional coefficient - k_6, which is intended to account for non-linear creep. Value of k_6 is dependent on stress in the concrete section and so I imagine its calculation must have been incorporated into internal workings of the program, however I have not been able to find any confirmation of this fact.
Can someone from Bentley please confirm whether k_6 is accounted for automatically. If not, can we have a method as to how the creep factor calculation changes (compared to what it was before for AS3600:2009).
Many thanks
The k6 parameter is not accounted for automatically by the program. The load history deflections continue to be based on the ACI 209 model for all design codes and the creep factor and shrinkage strain that the user inputs.
If the k6 factor is not 1 and needs to be considered, you would need to determine an appropriate value outside the program and use it to determine the final input creep factor. Note that the stress is likely different from load step to load step and even element to element, the best you can do is determine one factor that would be used for all areas and all load steps.
Answer Verified By: KONSTANTIN GONCHAROV
Thank you Karl, this makes sense within the general scheme of RAM Concept modelling.
I post this reply for the sake of others that may follow down this line enquiry. Given the response above one would naturally try to investigate variation of k_6 to try and gauge if some averaging of the value can be done in order to have a one-size-fits-all coefficient (for most design scenarios). I went back to AS3600:2018 and plotted how k_6 varies as a function of stress/f_cmi:
It looks like variation in this value can be quite significant, potentially almost tripling the design creep coefficient. However it is also instructive to put some numbers on this. Typical floorplates in AUS are currently designed with 40MPa concrete (f_cmi = 43MPa) this giving the 0.45f_cmi threshold value of 19.35MPa before k_6 "kicks in". In my personal experience it is not very common for compressive stresses to (locally) exceed 15-20 MPa thus indicating that k_6 can be left as 1 for modelling of global behaviour for most slabs.
Having said all of the above it is important to keep in mind that all designs have special cases and so special consideration is always worthwhile, especially when dealing with a coefficient that can increase a design value by a factor of 2.