Torsional stiffness for concrete bridge deck - EQ

Hello,

According to Euro code as shown in the attached picture

How I can reduced the torsional stiffness of a bridge deck (concrete box section) for EQ design

I work with SOFIPLUS environmental. How I can reduced the torsional stiffness of the concrete box section to 50%

Is option grp2 stit available for Sofistik 2018?
I can’t find it as shown in the attached picture
Another question, please use your text grp2 stit for linear analysis only, What about the traffic loader and CSM tasks? How can I reduce the torsional stiffness?

There is a better way directly in AQUA:

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In cross-section, properties (Cross section editor). Can I change the torsional moment of inertia to 0.5 as shown in the attached picture?

It looks like this. (I have nearly no experience in SSD/Sofistik-GUI).

Thank you