Fictitious axial forces and element capacities

[Giosmi]

Dear Support Team,

I'm trying to check the chord rotation capacity by Italian NTC2018 Code for a very simple 3D RC frame (two frames, two storeys) by using a Pushover analysis and checking the output values when the first chord rotation performace criteria is reached.

I've been having issues in replicating the results of chord rotation capacity. I noticed that all the beams (all elements are modelled with InfrmFP Class type) suffer a very high value of axial force. I understand this is a particular interaction between fibre modelled elements and rigid diaphragms.
I do not believe this fictitious force change the overall behaviour of the structure (dynamic behaviour, equilibrium, inter-storey drift, etc.), but it does affect the calculation of element capacity: in the specific the chord rotation capacity (in my case, calculated by the NTC2018 Italian Code) changes in function of the inner axial force of the element, since it is not considered by the "Check: Do not consider the axial force contribution in the shear capacity of beams" in the Element Settings.

It is a known problem?

Which values does Seismostruct use for the calculation of the Chord Rotation Capacity by NTC2018? In the specific, how can i determine the confined nucleus of the beam section for the calculation, if the section is partialized?

Thank you

[seismosoft]

Contrary to the shear capacity, the effect of the axial force on the calculation of the chord rotation capacity is generally small. Hence, the axial loading is always considered in the calculation.
The chord rotation capacity calculations are carried out according to the expressions in NTC2018. For the details of the calculation, please refer to Appendix H3 of the Users Manual.
Seismosoft Support

[Giosmi]

Dear Support Team,
Thank you for the answers

I have two more questions:

1) Why is there a very high axial force on the horizontal elements modelled with InfrmFP and rigid diaphgram? For example, in my very simple frame (two RC frames, two storeys), at the peak of shear of the pushover analysis, the axial force of the first story beams is 1350kN. In the same frame whithout rigid diaphgram the first floor horizontal elements have no axial force.

2) Is this value (1350 kN) used for the calculation of the chor rotation capacity of the element?

Thank you
Giovanni

[seismosoft]

1) Without knowing the details of your model, it seems that this is because of the spurious axial forces that you mentioned above (I guess that with infrmFP you mean the infrmFB frame model)
2) Yes
Seismosoft Support

[Giosmi]

Dear support Team,
Thank you for your answer.

What does cause these fictitiuos axial forces?

Thank you
Giovanni

[seismosoft]

As you mentioned above 'this is a particular interaction between fibre-modelled elements and rigid diaphragms.' For further details, please refer to relevant papers in the bibliography.
Seismosoft Support

[z.gronti]

Dear Support Team,

I'm trying to check the chord rotation capacity by Italian NTC2018 Code for a very simple 3D RC frame (two frames, two storeys) by using a Pushover analysis and checking the output values when the first chord rotation performace criteria is reached.

I've been having issues in replicating the results of chord rotation capacity. I noticed that all the beams (all elements are modelled with InfrmFP Class type) suffer a very high value of axial force. I understand this is a particular interaction between fibre modelled elements and rigid diaphragms.[...]

Dear Giosmi,

I would just like to mention a note written in SeismoStruct's User Manual:
The main difference between the Code-based Checks and the Performance Criteria is that the latter are checks against the 'expected' values of the response quantities, whereas the former follow the conservative assessment methodologies as defined by the corresponding Codes and Standards. Hence, in Code-based Checks the expressions employed for the calculation of the threshold value, when the different performance limit states are reached, employ conservative (e.g. characteristic or nominal) material strengths, and are based on the safety and confidence factors, as specified in the Codes. On the contrary, mean material values and no safety or confidence factors are used in the Performance Criteria calculations.