Design of the Stiffening Moment
After determining the design moments, the program analyzes the concrete compression strut. It is checked whether the moments used to stiffen the reinforcement mesh can be resisted by the plate.
In the design details, this analysis can be found under the Concrete Strut entry:
For the determined moments, the program performs a normal bending design at the plate's bottom and top sides. However, the design's aim is not to find a reinforcement: Rather, it is to verify that the concrete compression zone is able to yield a resulting concrete compressive force that, multiplied by the lever arm of the internal forces, results in a moment on the side of the resistance that is greater than the acting moment.
The design is not fulfilled if the moment on the side of the resistance is smaller than the governing design moment nsstrut even in the case of a maximum allowable bending compressive strain of the concrete and a maximum allowable retraction of an assumed reinforcement.
The current standards regulate the adherence to the allowable strains via the limit of the ratio between the neutral axis depth x and effective depth d. For this, the stress-strain diagrams for concrete and reinforcing steel as well as the limit strains of these standards are used (see the following explanations for EN 1992-1-1).
Stress-strain diagrams for cross-section design
The parabola-rectangle diagram according to Figure 3.3 of EN 1992-1-1 is used as the calculation value of the stress-strain curve.
The stress-strain diagram of the reinforcing steel is shown in Figure 3.8 of EN 1992-1-1.
The allowable limit deformations are shown in Figure 6.1 of EN 1992-1-1:
The ultimate limit state is determined through the limit strains: Either the concrete or the reinforcing steel fails, depending on where the limit strain occurs.
- Failure of concrete, for example C30/37:
- Limit strain for axial compression: εc2 = –2.0 ‰
- Ultimate strain at failure: εc2 = –3.5 ‰
- Failure of reinforcing steel, for example B 500 S (A):
- Steel strain under maximum load: εuk = 25 ‰
- Simultaneous failure of concrete and reinforcing steel:
- The limit compressive strains of concrete and steel occur simultaneously.