RF-CONCRETE Surfaces – Online Manual Version 5

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RF-CONCRETE Surfaces – Online Manual Version 5

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2.6.4.6 Selection of concrete compression strut

Selection of concrete compression strut

With the design internal forces mend,-z,φ1 = 38.49 kNm/m and mend,-z,φ2 = 25.25 kNm/m, we obtain the strains εφ1 = 0.735 ‰ in the first reinforcement direction and εφ2 = 0.527 ‰ in the second reinforcement direction. Thus there is a strain ratio Rs,-z of 0.717.

The assumed strain ratio of 1.00 therefore does not correspond to the actual strain ratio. Hence, the inclination of the stiffening compression moment is increased from 75.0° to 79.746°.

Geometrically, this inclination of the stiffening compression moment can only appear if the geometric ratio Rs,geo,-z of the strain in the reinforcement direction φ2 to the strain in the reinforcement direction φ1 is approximately 0.717. This is the case in our example.

When determining the crack width wk, it is shown that with the design moments, strains result in the individual reinforcement directions for an inclination of the stiffening compression moment of 79.746°, which lead to the strain ratio Rs,geo,-z of 0.717.

Figure 2.88 Direction of the concrete compression strut and ratios of deformation

The selected inclination of the stiffening compression moment of 79.746° results in modified design moments in the individual reinforcement directions. This corresponds to the method for determining the design internal forces in the serviceability limit state that is used here, which takes the deformation ratio of the longitudinal reinforcement that was selected in the Settings for Analytical Method of Serviceability Limit State Design dialog box into account (see Figure 2.89).

Figure 2.89 Settings for Analytical Method of Serviceability Limit State Design dialog box