Question:
Answer:The lever arm is calculated from the ultimate bending analysis:
As per the Euro 1992 -1-1 6.2.3 the parameter z is the lever arm in the bending moment calculation, but if we don't have accurate calculations it is suggested to take 0.9*d .

In midas civil, we calculate the lever arm exactly as per the moment capacity and the compressive force in the section as shown below:
C = Total compressive force in the section for calculating moment capacity
M = MRx = Flexural moment capacity of the section
z = Inner Lever Arm = M/C