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4.2.1 Analysis Method
The tab sections differ according to the selected design standard (ACI 318 does not provide for any nonlinear calculation). The following description refers to EN 1992-1-1.
As described in chapter 2.4.7, two methods for the nonlinear calculation are specified in EN 1992-1-1. The Method with mean values according to EN 1992-1-1, clause 5.7 is preset.
The procedure has been modified in order to consistently use one safety concept only. According to EN 1992-1-1, clause 5.7 (NA.10) for Germany, the global partial safety factor on the ultimate load side is to be applied as follows:
- γR = 1.3 for permanent and temporary design situations and analysis for fatigue
- γR = 1.1 for extraordinary design situations
The concrete's modulus of elasticity can be reduced for the analysis by the factor 0.85. This is recommended for cross-sections that are fully compressed.
The General design method according to EN 1992-1-1, clause 5.8.6 is mainly suited for the design of slender compression elements. In most cases, the determination of deformations and internal forces using verified mean values leads to more efficient designs. Chapter 220.127.116.11 offers more information about this method.
The check box is enabled for both analysis methods (EN 1992-1-1, clauses 5.7 or 5.8.6). The reason for it is that clause 8.6.1 (5) of the German DIN standard 1045-1 does not allow plastic releases (curvatures (1/r)m > (1/r)y) for structural components stressed by longitudinal compression. Because of the abrupt stiffness decrease when plastic zones or releases are created, the result is often a loss of stability for slender compression elements resulting in failure of the column.
If the check box is clear, no plastic curvatures are possible in the calculation of cross-section curvatures.
If this option is activated, the longitudinal reinforcement will be increased if the cross-section's load bearing capacity is exceeded. This is the case when the design ratio in the results window of the nonlinear calculation (see chapter 5.5.1) is greater than 1.
If you Apply linear elastic shear rigidity, the shear areas will be calculated linear-elastically. Reduction due to cracking is not taken into account.
Alternatively, you can Reduce shear rigidity affine to flexural rigidity. In this case, the linear-elastic shear stiffness diagram will be reduced in line with the diagram of bending stiffness. The theoretical basis is described in chapter 18.104.22.168.
The Global reduction of torsional rigidity allows you to reduce the stiffness for cracking to a user-defined residual value. A residual stiffness of 10 % is preset, which is based on the relatively high decrease of torsional stiffness (see Figure 2.27).
In this dialog section, you can save the stiffness from nonlinear calculations (considering reinforcement and cracked state) to use it later in RFEM. This way, it is possible to also consider the reduced stiffnesses of reinforced concrete components in the cracked state for the determination of internal forces and the design of remaining structural components consisting of steel or timber. This is useful, for example, if the stiffening components of a model are designed in reinforced concrete.
In the Extra Options tab of the Edit Load Cases and Combinations dialog box in RFEM, you can also find settings for considering the nonlinear stiffness from RF-CONCRETE Members.
The two options in the add-on module's dialog box control the assignment of stiffnesses for RFEM: The stiffnesses of load combinations calculated linearly with RF-CONCRETE Members can be saved Individually (separately). Then, in RFEM, they will only be useable for the respective load combinations. With the Consistent for reference load option, however, the stiffness of a reference load is saved, which can then be assigned to any load combination in RFEM.