Online manuals, introductory examples, tutorials, and other documentation.
2.7.3 Critical Surface
For the determination of the critical surface, each reinforcement direction φ is considered separately. The state of stress is analyzed on both surfaces s – bottom surface (in the direction of the local +z-axis) and top surface (in the direction of the local -z-axis). The surface with the greatest tensile stress in the concrete is classified as governing. The internal forces on the critical surface are termed nΦ and mΦ.
The axial force nΦ,s transformed into the direction of reinforcement φ has the same value for both surfaces s (nΦ = nΦ,top = nΦ,bottom). The axial forces are therefore not relevant for determining the critical surface; only the moments are considered in order to find the governing surface. The algebraic signs for the bending moments nφ,s are determined regarding whether the moments cause tension or compression on the respective surface s. Therefore, the critical surface is the surface with the larger bending moment (that is, the surface that is more strongly subjected to tension).
For the calculation of the stiffness, only the internal forces nΦ and mΦ on the critical surface are taken into account. Until now, the term "bottom surface" referred to the local +z-axis. In the following, however, "bottom surface" refers to the critical side of the surface.