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3.4.5 Design Method
When determining the required reinforcement, the principal internal forces are transformed into design forces (in the direction of the reinforcements) and into a developing concrete compression strut force. The sizes of the 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 a reinforcement direction for a certain compression strut inclination, 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 becomes zero.
During the Optimization of the internal forces, the program therefore analyzes, which inclination angle of the concrete compression strut leads to 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 by example in chapter 2.4.1.
For concrete components subjected to compression such as walls, the optimization may lead to non-designable situations due to failure of the concrete compression strut. The optimization is therefore not recommended for the load situations "compression-compression".