With the design internal forces for the reinforcement directions φ1 = 0° and φ2 = 90°, you can determine the lever arm of the internal forces.
As described in chapter 2.5.2, a preliminary design is carried out with the determined internal forces for both reinforcement directions. It serves to determine the lever arm of the internal forces. The lever arm is determined from the state of strain due to the design internal forces.
The value 1.239 m is obtained for the smaller and therefore governing lever arm zmin,+z.