 Frequently Asked Questions (FAQ)

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• Is it possible to design concrete solids with the add-on modules for RFEM?

New FAQ 004123 EN-US

In RFEM, you can design surfaces and members of reinforced concrete.

Members are designed in the add-on module RF-CONCRETE Members or RF-CONCRETE Columns.
Surfaces are designed in the RF-CONCRETE Surfaces add-on module (optionally with RF-CONCRETE Deflect or RF-CONCRETE NL).

Reinforced concrete solids can not be designed in RFEM. There is currently no add-on module for the reinforced concrete design of solids.

However, you can create solids with the material "Concrete" and select For example, you can determine the stresses within the solid. Optionally, you can insert a result beam into the solid that is used to convert the results of the solid into member internal forces.

The result beam can subsequently be designed in RF-CONCRETE Members or RF-CONCRETE Columns.
• Which concrete tensile strength is used for the transition from state I to state II? f ctm or f ct; 0.05 ?

New FAQ 004117 EN-US

The concrete tensile strength f ct, eff, wk = f ctm x crack width factor is applied.
• Is it possible to display the curvatures of the slab or the radius of curvature of the slab in the element nodes?

New FAQ 004116 EN-US

No, this is unfortunately not possible.
• Is it also possible to perform a deformation analysis in cracked state for a 2D position in RF-CONCRETE Surfaces?

New FAQ 003597 EN-US

In this case, the method of check and the type of the 2D position are important.

When using the analytical method (RF-CONCRETE Deflect), it is possible to perform a calculation in 2D positions. When using the nonlinear method (RF-CONCRETE NL), the calculation for 2D XY (u Z / φ X / φ Y ) is not possible. In the nonlinear calculation, among others Shrinkage is represented internally as an expansion load, which is not possible in this type of 2D position due to the limited degrees of freedom.

Convert 2D to 3D Position
In the general data, it is possible to simply convert a 2D position into a 3D position. For the supports, all degrees of freedom not contained in the 2D position are fixed when converting to a 3D position (see the video).
• How can I model a timber-concrete composite floor?

New FAQ 003589 EN-US

The detailed procedure can be found in the video. It indicates the most important features:
• How is it possible to design a reinforced concrete cross-section in SHAPE-MASSIVE?

New FAQ 003577 EN-US

In SHAPE-MASSIVE, the reinforced concrete design has to be activated in the General Data section. As soon as the design is active, it is possible to set the design accordingly in a separate tab (Figure 01).

There are three types for the design:

Strain-Stress Distribution (Example 01):
It is possible to determine an available design ratio by specifying the internal forces

Existing Safety (Example 02):
There is determined a state of fracture (ratio = 100%) and a safety in relation to it.

Design (Example 03):
By specifying a maximum and minimum diameter or a minimum and a maximum reinforcement, it is possible to increase the reinforcement within the design.

Irrespective of which of the three methods is used, it is necessary to specify the position of the reinforcement and an acting internal force (Figure 02).
• How can I create a rectangular timber profile in the COMPOSITE-BEAM program?

New FAQ 003574 EN-US

The program COMPOSITE-BEAM for the design of composite beams is effected in accordance with ENV 1994-1-1: 1992-10. This standard specifies a composite profile consisting of steel sections and reinforced concrete.

The design of composite beams consisting of timber and reinforced concrete is carried out according to EN 1995-1-3. In this case, the time-dependent load states t=0, t=3-7 years and t= unlimited have to be analyzed in particular. These states are usually taken into account by means of impressed strains. This is done manually in the RFEM program. For this and many other reasons, it is not possible to design timber-concrete composite beams in the COMPOSITE-BEAM program.

However, on our homepage you can find a lot of information about the design of timber-concrete composite beams, for example here.
• How to calculate a timber-concrete composite floor with CLT?

New FAQ 003553 EN-US

Two planar structural components can be defined in the RF-LAMINATE add-on module via the Hybrid material model (Figure 1).

In this case, the automatic input of a cross-laminated timber plate would according to the manufacturer's specifications also be possible (see Figure 2).

However, the disadvantage of the input in the RF-LAMINATE add-on module consists in the requirement for a rigid connection. This is not the case for a timber-concrete composite construction. Thus, the calculation is only an approximation.

Another possibility is to couple two surfaces via a surface release or a contact solid. The advantage is that you can define almost any shear transfer this way (Figure 3). In the RFEM model file attached here, this has been defined in the middle second model.

The third option would be to define a hybrid member as specified in the third model of the attached file. In this case, however, the biaxial load transfer of the structure will not be considered. However, this method has the advantage of highly automated design. This is also explained in this FAQ .
• Why is the crack moment Mcr smaller for a biaxial bending than for a uniaxial bending?

New FAQ 003533 EN-US

The crack moment of a concrete cross-section is calculated from the mean tensile strength of the concrete and from the ideal section modulus. The crack moment describes the internal force that occurs when the tension stress fctm is reached in the outermost fiber of the cross-section and crack formation occurs.

For uniaxial bending, it is possible to calculate the crack moment analytically. For biaxial bending, the introduction of a weighting factor k is helpful in order to determine from the components Mcr,y and Mcr,z Mcr.

Calculation for the attached example:

Bending moment My = 20 kNm
Bending moment Mz = 20 kNm

Ideal section modulus Wy = 3081 cm3
Ideal section modulus Wz = 3081 cm3

Mean tensile strength of concrete fctm = 0.290 kN/cm2

Member 1: Uniaxial bending My:

$\begin{array}{l}M_{cr\;}=f_{ctm}\times W_y\\M_{cr\;}=0,29\;\frac{kN}{cm^2}\times3081\;cm^3\\M_{cr\;}=893\;kNcm\;=\;8,9\;kNm\end{array}$

Member 2: Uniaxial bending Mz:

$\begin{array}{l}M_{cr\;}=f_{ctm}\times W_z\\M_{cr\;}=0,29\;\frac{kN}{cm^2}\times3081\;cm^3\\M_{cr\;}=893\;kNcm\;=\;8,9\;kNm\end{array}$

Member 3: Biaxial bending My and Mz:

$\begin{array}{l}M_{cr\;}=\sqrt{M_{cr,y}^2+M_{cr,z}^2}\\M_{cr,y\;}=k\times My\\k=\frac{f_{ctm}}{\sigma_M}\\\sigma_M=\frac{M_y}{W_y}+\frac{M_z}{W_z}=\\\end{array}$

• I recently purchased RSTAB with the CONCRETE module. Is it possible to perform a stability analysis of reinforced concrete columns by means of this?

New FAQ 003532 EN-US

Yes, it is because the CONCRETE module of RSTAB 8 also includes the nonlinear reinforced concrete design. Thus, you can activate the 'Nonlinear Analysis (State II)' in the 'Ultimate Limit State' tab.

In the detail settings for the nonlinear design, you can select the 'General Design Method for Members in Axial Compression acc. to Second Order Theory'.

It is important that you define the imperfections in RSTAB and apply load curves (CO) according to the second-order analysis for the design, no result combinations (RC)!

Note on RFEM 5:

In RFEM 5, the same procedure is possible in RF-CONCRETE Members. However, the add-on module RF-CONCRETE NL in RFEM is required for the non-linear reinforced concrete design.

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