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2.4.7 Ultimate Limit State
Ultimate Limit State
Nonlinear analyses in the ultimate limit state serve to determine the limit of failure (mechanism) realistically. However, the design involves the following difficulty: Realistic estimations require realistic initial and computational parameters.
The material properties are not deterministic parameters. In contrast to the discrete cross-section design where the concept of "local defects" is always applied, mean material properties have to be used for the determination of deformations and internal forces.
Another aspect when determining the behavior of structural components realistically is the consideration of the concrete's effectiveness for tension between cracks (Tension Stiffening, see Chapter 2.4.3). The influence of creep and shrinkage is especially significant for compression elements.
According to EN 1992-1-1, clause 5.7, we have to use nonlinear methods leading to a realistic stiffness and considering uncertainties concerning failing. Design methods that are valid in the governing application areas may be used. An appropriate nonlinear method for determining internal forces including cross-section design is the approach with average values of material properties and the application of a global partial safety factor γr described both in the National Annex for Germany to EN 1992-1-1, clause 5.7 as well as the German DIN standard 1045-1, clause 8.5. This approach is described in the following as the method according to EN 1992-1-1, clause 5.7.
According to EN 1992-1-1, clause 5.7 (5), it is possible to use the approach as per EN 1992-1-1, clause 5.8.6 for structural components where effects according to the second-order analysis may not be neglected.
RF-CONCRETE Members provides both nonlinear methods of calculation (cf. Figure 4.4).