RF-CONCRETE NL | Design

When designing reinforced concrete components according to EN 1992‑1‑1 [1], nonlinear methods of determining internal forces for the ultimate and serviceability limit states are possible. In this case, the internal forces and deformations are determined with respect to their nonlinear behaviour. The analysis of stresses and strains in cracked state usually provides the deflections, which clearly exceed the linearly determined values.

The fatigue design according to EN 1992-1-1 must be performed for the structural components subjected to large stress ranges and/or many load changes. In this case, the design checks for the concrete and the reinforcement are performed separately. There are two alternative design methods available.

Using an example of a steel fiber-reinforced concrete slab, this article describes how the use of different integration methods and of a different number of integration points affects the calculation result.

RF-CONCRETE Surfaces:

The nonlinear deformation analysis is performed by an iterative process considering the stiffness in cracked and non-cracked sections. The nonlinear reinforced concrete modeling requires definition of material properties varying across the surface thickness. Therefore, a finite element is divided into a certain number of steel and concrete layers in order to determine the cross-section depth.

The mean steel strengths used in the calculation are based on the 'Probabilistic Model Code' published by the JCSS technical committee. It is up to the user whether the steel strength is applied up to the ultimate tensile strength (increasing branch in the plastic area). Regarding material properties, it is possible to control the stress-strain diagram of the compressive and tensile strength. For the concrete compressive strength, you can select a parabolic or a parabolic-rectangular stress-strain diagram. On the tension side of the concrete, it is possible to deactivate the tensile strength as well as to apply a linear-elastic diagram, a diagram according to the CEB-FIB model code 90:1993, and concrete residual tensile strength considering the tension stiffening between the cracks.

Furthermore, you can specify which result values should be displayed after the nonlinear calculation at the serviceability limit state:

• Deformations (global, local based on non-/deformed system)
• Crack widths, depths, and spacing of the top and bottom sides in principal directions I and II
• Stresses of the concrete (stress and strain in principal direction I and II) and of the reinforcement (strain, area, profile, cover, and direction in each reinforcement direction)

RF-CONCRETE Members:

The nonlinear deformation analysis of beam structures is performed by an iterative process considering the stiffness in cracked and non-cracked sections. The material properties of concrete and reinforcing steel used in the nonlinear calculation are selected according to a limit state. The contribution of the concrete tensile strength between the cracks (tension stiffening) can be applied either by means of a modified stress-strain diagram of the reinforcing steel, or by applying a residual concrete tensile strength.

• Iterative nonlinear calculation of deformations for beam and plate structures consisting of reinforced concrete by determining the respective element stiffness subjected to the defined loads
• Deformation analyses of cracked reinforced concrete surfaces (state II)
• General nonlinear stability analysis of compression members made of reinforced concrete; for example, according to EN 1992-1-1, 5.8.6
• Tension stiffening of concrete applied between cracks
• Numerous National Annexes available for the design according to Eurocode 2 (EN 1992-1-1:2004 + A1:2014, see EC2 for RFEM)
• Optional consideration of long-term influences such as creep or shrinkage
• Nonlinear calculation of stresses in reinforcing steel and concrete
• Nonlinear calculation of crack widths
• Flexibility due to detailed setting options for basis and extent of calculations
• Graphical representation of results integrated in RFEM; for example, deformation or sag of a flat slab made of reinforced concrete
• Numerical results clearly arranged in tables and graphical display of the results in the model
• Complete integration of results in the RFEM printout report

After the calculation, the module shows clearly arranged tables listing the results of the nonlinear calculation. All intermediate values are included in a comprehensible manner. Graphical representation of design ratios, deformations, concrete and reinforcing steel stresses, crack widths, crack depths, and crack spacing in RFEM facilitates a quick overview of critical or cracked areas.

Error messages or remarks concerning the calculation help you find design problems. Since the design results are displayed by surface or by point including all intermediate results, you can retrace all details of the calculation.

Due to the optional export of input or result tables to MS Excel, the data remain available for further use in other programs. The complete integration of results in the RFEM printout report guarantees verifiable structural design.