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2021-08-04

Steel Design | Strength and Stability | Stability Analysis

  • Stability analyses for flexural buckling, torsional buckling, and flexural-torsional buckling under compression
    • Import of the effective lengths from the calculation using the Structure Stability add-on
  • Graphical input and check of the defined nodal supports and effective lengths for stability analysis
  • Lateral-torsional buckling analysis of the structural components subjected to moment loading
    • Depending on the standard, a choice between user-defined input of Mcr, analytical method from the standard, and use of internal eigenvalue solver
    • Consideration of a shear panel and a rotational restraint when using the eigenvalue solver
    • Graphical display of a mode shape if the eigenvalue solver was used
  • Stability analysis of structural components with the combined compression and bending stress, depending on the design standard
  • Comprehensible calculation of all necessary coefficients, such as the factors for considering moment distribution or interaction factors
  • Alternative consideration of all effects for the stability analysis when determining internal forces in RFEM/RSTAB (second-order analysis, imperfections, stiffness reduction, possibly in combination with the Torsional Warping (7 DOF) add-on)
Visualization of the critical mode shape for the lateral-torsional buckling design in the stability analysis.
Steel Design | Additional Results | Mode Shapes
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Considering Shrinkage
Shrinkage and creep are time-dependent deformation properties of concrete that usually have to be considered in the serviceability limit state design.
Layer Model in RF-CONCRETE NL
Different methods are available for calculating the deformation in the cracked state. RFEM provides an analytical method according to DIN EN 1992-1-1 7.4.3 and a physical-nonlinear analysis. Both methods have different features and can be more or less suitable depending on the circumstances. This article will give an overview of the two calculation methods.
Settings for Deformation Analysis with RF-CONCRETE Deflect
Performing serviceability limit state design also includes taking into account the allowable deformation. Calculating the deformation of reinforced concrete components depends on whether or not the observed cross-section cracks under the applied loading. The governing control parameter in RF-CONCRETE Deflect is the distribution coefficient ζ.
Illustration of a method for performing stability analysis according to DIN EN 1992-1-1 in RFEM 6
For reinforced concrete components and structures whose structural behavior is significantly influenced by the second-order effects, Eurocode 2 provides the general method based on a nonlinear determination of internal forces according to the second-order analysis (5.8.6), as well as an approximation method based on the nominal curvature (5.8.8).

The aim of this technical article is to perform a design according to the general design method of Eurocode 2, using the example of a slender reinforced concrete column.
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The deformation analysis according to the approximation method defined in standards (for example, deformation analysis according to EN 1992‑1‑1, 7.4.3) applies to the calculation of "effective stiffnesses" in the finite elements in accordance with the existing limit state of the concrete with or without cracks. These stiffnesses are used to determine the surface deformation by repeated FEM calculation.

The effective stiffness calculation of finite elements takes into account a reinforced concrete cross-section. Based on the internal forces determined for the serviceability limit state in RFEM, the program classifies the reinforced concrete cross-section as 'cracked' or 'uncracked'. If the tension stiffening at a section should be considered as well, a distribution coefficient (according to EN 1992-1-1, Eq. 7.19, for example) is used. The material behavior for the concrete is assumed to be linear-elastic in the compression and tension zone until the concrete tensile strength is reached. This is reached exactly in the serviceability limit state.

When determining the effective stiffnesses, creep and shrinkage are taken into account at the "cross-section level". The influence of shrinkage and creep in statically indeterminate systems is not taken into account in this approximation method (for example, tensile forces from shrinkage strain in systems restrained on all sides are not determined and must be considered separately). In summary, RF-CONCRETE Deflect calculates deformations in two steps:

  1. Calculation of effective stiffnesses of the reinforced concrete cross-section assuming linear-elastic conditions
  2. Calculation of the deformation using the effective stiffnesses with FEM
Display of Deformations in RFEM

After the calculation, the module shows clearly arranged tables listing the deformation analysis results. All intermediate values are displayed in a comprehensible manner. Graphical representation of design ratios and deformation in RFEM allows a quick overview of critical areas.

Since the design results are displayed by surface or by point including all intermediate results, you can retrace all details of the calculation. The complete integration of results in the RFEM printout report guarantees verifiable structural design.

Serviceability Limit State Design Point by Point
  • Deformation analyses of reinforced concrete surfaces without or with cracks (state II) by applying the approximation method (for example, deformation analysis according to EN 1992-1-1, Cl. 7.4.3 )
  • Tension stiffening of concrete applied between cracks
  • Optional consideration of creep and shrinkage
  • Graphical representation of results integrated in RFEM; for example, deformation or sag of a flat slab
  • Numerical results clearly arranged in tables and graphical display of the results in the model
  • Complete integration of results in the RFEM printout report
Material model Anisotropic | Damage, nonlinear behavior, concrete and reinforced concrete

The "Nonlinear Material Behavior" add-on includes the Anistropic | Damage material model for concrete structural components. This material model allows you to consider concrete damage for members, surfaces, and solids.

You can define an individual stress-strain diagram via a table, use the parametric input to generate the stress-strain diagram, or use the predefined parameters from the standards. Furthermore, it is possible to consider the tension stiffening effect.

For the reinforcement, both nonlinear material models "Isotropic | Plastic (Members)" and "Isotropic | Nonlinear Elastic (Members)" are available.

It is possible to consider the long-term effects due to creep and shrinkage using the "Static Analysis | Creep & Shrinkage (Linear)" analysis type that has been recently released. Creep is taken into account by stretching the stress-strain diagram of the concrete using the factor (1+phi), and shrinkage is taken into account as the pre-strain of the concrete. More detailed time step analyses are possible using the "Time-Dependent Analysis (TDA)" add-on.

Frequently Asked Questions (FAQ)
What is the maximum number of reinforcement groups that can be created in a design case in RF‑CONCRETE Surfaces?
When switching from the manual definition of the reinforcement areas to the automatic arrangement of the reinforcement according to window 1.4, the result of the deformation calculation changes, although the basic reinforcement has not been changed. What makes this change?
Is it possible to perform design without an additional reinforcement in RF‑CONCRETE Surfaces?
I obtain different results when comparing the deformation analysis in the RF‑CONCRETE add-on modules and another calculation program. What could be the reason for this?
Which tensile strength of concrete is used for the transition from State I to State II? fctm or fct;0.05?
Is it possible to define the damage parameter ζ manually for the calculation with RF‑CONCRETE Deflect? In such a way that it should always be at least 0.5?
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