# Frequently Asked Questions (FAQ)

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• ### When displaying the result diagrams on a member (the "rib" type), there is the option to display the internal force VL. What is this value and how is it calculated?

#### Answer

The force VL is the longitudinal shear force between the top surface and the member. It is calculated as an integrated shear flow between the plate and the member at a particular point on the member.

For the simplified example provided here, the resulting cross-section values for the integration width of 10 cm are as follows:

• $I_y=\frac{b\times h^3}{12}=\frac{10 cm\times20 cm^3}{12}=6,666.67 cm^4$
• $S_y=h_1\times b\times((h-e_z)-\frac{h_2}2)=10 cm\times10 cm\times((20 cm-10 cm)-\frac{10 cm}2)=500 cm^3$
• $\tau=V_L=\frac{V_z\times S_y}{I_y\times b}=\frac{5.53 kN\times500 cm^3}{6,666.67 cm^4}=0.415 kN/cm=41.5 kN/m$
The integration width has been set to the total of 10 cm.

Values:
• Iy second moment of area
• Sy statical moment
• h1 height of the upper cross-section part
• h2 height of the lower cross-section part
• ez centroidal distance
• h total height
The values can be adjusted for a T-beam.
• ### Is it possible to use the RF‑/STEEL AISC add-on module itself to specify the internal forces, for example, from another calculation or program?

#### Answer

Unfortunately, this is not possible. The design module can only be used together with the internal forces from RFEM or RSTAB.
• ### Is it possible to use the RF‑/STEEL EC3 add-on module itself to specify the internal forces, for example, from another calculation or program?

#### Answer

Unfortunately, this is not possible. The design module can only be used together with the internal forces from RFEM or RSTAB.
• ### Why are there different equivalent stresses in RFEM and RF‑STEEL, although the same CO is considered?

#### Answer

It is possible to display or calculate the stresses in RFEM as well as in the add-on module by means of the following smoothing options:

• Constant on Elements
• Non-Continuous
• Continuous Within Surfaces
• Continuous Total
• Continuously by Groups / Continuous by Groups

To compare the results, the same display type and calculation type must be selected in RFEM and RF‑STEEL Surfaces.

In RFEM, it is possible to do this in Project Navigator Display → Results → Surfaces → Distribution of Internal Forces/Stresses (Figure 02). In RF‑STEEL Surfaces, this can be displayed or changed in Details → "Options" tab (Figure 03).

• ### In spite of having defined average regions, they are not considered in the design in RF‑LAMINATE Surfaces. Which setting did I overlook?

#### Answer

In order to consider the average regions in the design in RF‑LAMINATE Surfaces, it is always necessary to activate them in the detailed settings of the add-on module. See Figure&nbso;01 with the detailed settings in RF‑LAMINATE.
• ### In the model, there are two governing internal forces, Vz and My. In the printout report selection, I have only activated these two internal forces using the "Extreme Values" option. Why are the result lines under "Members - Internal Forces" displayed twice in the printout report?

#### Answer

In this case, the reason for four result lines (see Figure 01) is that there are the results displayed for the minimum and the maximum including the associated remaining internal forces for each selected internal force.

• First line: Numerical maximum of Vz (3.54 kN) with the corresponding internal forces
• Second line: Numerical minimum of Vz (0 kN) with the corresponding internal forces
• Third line: Numerical maximum of My (0 kNm) with the corresponding internal forces
• Fourth line: Numerical minimum of My (-12.37 kNm) with the corresponding internal forces

• ### In spite of having defined smooth ranges, they are not considered in the design in RF‑STEEL Surfaces. What setting have I missed here?

#### Answer

In order to consider smooth ranges in the design in RF‑STEEL Surfaces, they must always be activated in the detail settings of the add-on module. See Figure 01 with the detail settings in RF‑STEEL Surfaces.
• ### Why are the imperfections taken into account in RFEM and RSTAB when calculating according to the linear static analysis, even though the definition says that no deformations of structural components are considered according to the linear static analysis?

004193

#### Answer

In general, the imperfection describes the incompleteness of a structure or the deviation from an ideal shape caused by the production. There are different ways to simulate the imperfection. In RFEM and RSTAB, the imperfections are modeled as equivalent loads. The definition of the equivalent loads is shown in Figure 01, and is taken from [1]. The same is described in EC 3 [2]. Since these are the equivalent loads that are dependent on the axial force, they are also considered in the calculation according to the linear static analysis. It is recommended to manage loads and imperfections in separate load cases. They can be combined in an appropriate way with each other in load combinations. The load cases with pure imperfections have to be categorized as the "Imperfection" action type in General Data of the load cases (see Figure 02).
• ### I have modeled a curved member with straight member segments. However, this results in discontinuities in the distribution for the diagrams of internal forces. How can I optimize it?

004171

#### Answer

In the case of modeling curved members by using member segments, discontinuities in the diagram of internal forces occur at the transition of the individual members because of the different inclinations of the members as the nodal equilibrium must be fulfilled (see Figure 01). If the jumps are too great, it is recommended to increase the number of member segments to minimize the buckling between the members. Thus, you obtain more consistent and accurate distribution of the internal forces.

In Figure 02, the different variants are compared. At the top, a curved member has been modeled in RFEM and is considered a reference. Below, the variants with 150, 50, and 10 member segments (n) are displayed with the result. The finer is the division of the member segments, the more accurate is the result.

In the menu [Tools → Generate Model - Members → Arc], you can also generate these member segments (see Figure 03).

• ### Why do I obtain much higher design ratios in RF‑/STEEL than for cross-section design in RF‑/STEEL EC3?

#### Answer

In the RF‑/STEEL add-on module, the equivalent stress analysis is carried out according to von Mises. The elastic stress analysis (EL-EL) is to be performed. In RF‑/STEEL EC3, a classification is done before the design. If a cross-section is classified as Class 1 or Class 2, the design is carried out against the plastic limit internal forces. The EL-PL design is to be performed. If you do not want to use the plastic load reserves, you can switch the design to EL‑EL in the details of the RF‑/STEEL EC3 add-on module. The results are then comparable with RF‑/STEEL.

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#### First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

#### Wind Simulation & Wind Load Generation

With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.

The generated wind loads acting on these objects can be imported to RFEM or RSTAB.

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