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3.4.5 Design Method
When determining the required reinforcement, the principal internal forces are transformed into design forces (in direction of the reinforcements) and into a related concrete compression strut force. The sizes of these design forces depend on the presumed angle of the concrete compression strut that braces the reinforcement mesh.
In the load situations "tension-tension" and "tension-compression" (see Figure 2.18), the design force may become negative in one reinforcement direction for a certain inclination of the compression strut, which means that compressive forces would occur for the tension reinforcement. By optimizing the design internal forces, the program modifies the direction of the concrete compression strut until the negative design force is zero.
During the Optimization of the internal forces, the program finds the inclination angle of the concrete compression strut that produces the most favorable design result. The design moments are determined iteratively with adjusted inclination angles in order to find the energetically smallest solution with the least required reinforcement. The optimization process is described in an example in chapter 2.4.1.
The optimization for concrete components subjected to compression (such as walls) may result in non-designable situations due to failure of the concrete compression strut. Therefore, the optimization is not recommended for the load situation "compression-compression".