Eurocode defines the lever arm as:
Let´s explain it in all possible cases:
1) Biaxial bending moment
Lever arm z is defined as a projection to the plane of bending and is decreasing while increasing the angle between the resultant force and the plane of bending.
In the case that the angle between the resultant force and the plane of the strain is over 20 degrees, this warning appears.
2) Cross-section fully subjected to compression
No zones under tension appear in cross-sections fully subjected to compression. As the response can’t be found, lever arm value can‘t be determined from it and default value z = 0.9 d will be used. Although, this value is defined by the code with the assumption, that no axial force is introduced.
3) Shear force is perpendicular to the direction of bending
The following case can exceptionally occur. According to the response and the projection of the lever arm to the plane of strain, the value of the lever arm would be zero.
In case we are unable to determine the lever arm value, we recommend using the user-defined value of z. This value can be set as the distance of main layers of rebars (compressed layer and tensioned layer).