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• ### For a buckling analysis, FE‑BUCKLING determines the governing shear stress of τ = 7.45 kN/cm², while RF‑/STEEL gives the result of the maximum shear stress of τ = 8.20 kN/cm². Where does this difference come from?

In most cases, slender beams receive a parabolic shear stress in the web of the cross-section, which has the maximum value in the centroid of double-symmetric cross-sections.

According to DIN 18800, Part 3, Section 403, the following applies:

Shear stresses that are variable over the width b of the buckling panel should be considered with the larger of the following two values:

• Mean value of τ
• 0.5 max τ

In this case, the mean value of the shear stress is used for the buckling design.

Since the variable shear stress τz depends on the statical moment Sy, there is a table with the details of the c/t-parts of the cross-section in FE‑BUCKLING. This also includes the average statical moments which are used to determine the corresponding shear stresses for the buckling design according to the usual formula, but with the average statical moment, see the formula and Image 01.

Accordingly, the following shear stress results in FE‑BUCKLING, see Image 02.

The respective statical moments that are used to determine the shear stresses in RF‑/STEEL for the stress analysis can be displayed in the result window by clicking the "Show Cross-Section Values and Extended Stress Diagram" button, see Image 03.

• ### What are the StandardID and AnnexID of various National Annexes for the processing using the COM interface?

StandardID and AnnexID can be easily displayed at any time by using the following macro:

cominterfaces-en\SDK\Examples\Modules\Excel\RF-STEEL_EC3.xls

You can find this macro in the archive of the product website (see Links).

Here is an overview of the current attachments:

StandardID AnnexID Name

DIN 0 Germany

ÖNORM 1 Austria

CSN 2 Czech Republic

STN 3 Slovakia

PN 4 Poland

SIST 5 Slovenia

DK 6 Denmark

UNI 7 Italy

NEN 8 Netherlands

SFS 9 Finland

SS 10 Sweden

NF 11 France

BS 12 United Kingdom

CEN 13 European Union

BDS 14 Bulgaria

CYS 15 Cyprus

LST 16 Lithuania

SR 17 Romania

SS 18 Singapore

NBN 19 Belgium

NP 20 Portugal

UNE 21 Spain

MAL 22 Malaysia

NS 23 Norway

LU 24 Luxembourg

ELOT 25 Greece

• ### How can I display the stresses in the stress points in RF‑STEEL Members?

The "Extended Display" button allows for a targeted evaluation of the results for each stress point. It opens the "Cross-Section Properties and Stress Distribution" dialog box (Figure 01).

In the "Position" section, the current member number and location x on the member are preset. You can also select other members or x‑locations in the list.

The "Stress Points" section lists all stress points of the cross-section. The "Coordinates" columns show the centroidal distances y and z, and the "Static moments" columns show the surface moments of the first degree Sy and Sz. The "Thickness t" column shows the thickness of the cross-section part that is required for the determination of shear stresses. In the case of closed cross-sections, the cell area A* is specified, which is required for the determination of the stress due to torsional moment.

The "Stresses" section shows all stresses at the current stress point (selected in the section above). In this dialog box, it is also possible to select a stress type by a mouse click to display the stress diagrams in the graphic.

• ### What is the difference between the RF‑/STEEL and RF‑/STEEL EC3 add-on modules?

Even if the description of the add-on modules is very similar, the calculations performed are different.

###### RF-/STEEL

This add-on module performs a general stress analysis by calculating the existing stresses and comparing them with the limit stresses. The designs are performed elastically. The designs do not depend on a standard.

###### RF-STEEL EC3

This add-on module, on the other hand, performs all typical designs of ultimate limit state, stability, deformation, and fire resistance for steel members according to Eurocode 3 (numerous National Annexes are available). There is a range of module extensions available within this add-on module. These include: warping torsion analysis, plasticity analysis, designs for cold-formed sections.

###### Comparison of Both Add-on Modules

The results of RF‑/STEEL can be compared with the results of the cross-section design of RF‑/STEEL EC3.

Why the results can differ is explained in FAQ 003489.

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

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.
• ### In the RF‑/STEEL add-on module, I am trying to design a cross-section created in SHAPE‑MASSIVE. However, the cross-section is classified as invalid. What is the reason?

In this case, take a look at the stress points in the cross-section details. If they are not accessible (grayed out), no stress points have been defined in SHAPE‑MASSIVE and the design is not possible. In SHAPE‑MASSIVE, it is necessary to activate the "Stresses by Stress Points" option in General Data. After recalculating and saving, the cross-section can be designed in the RF‑/STEEL add‑on module.
• ### The design ratio of the cross-section check is different for the RF‑/STEEL and RF‑/STEEL EC3 add-on module. What is the reason?

Cross-sections assigned to Class 1 or Class 2 are designed plastically in RF‑/STEEL EC3 by default. In order to be able to compare the results with RF‑/STEEL, please activate the elastic design the cross-sections of Class 1 and Class 2 (Figure 02) in Details of RF‑/STEEL EC3.

Please also check whether the same partial safety factor γ for the resistances of the cross-sections is defined in both add-on modules (Figure 03 and Figure 04).

• ### Where can I see the stresses in RSTAB?

RSTAB is a pure framework program and only determines internal forces, deformations, and support reactions.

On the other hand, stresses are variable depending of the cross-section and are calculated on the stress points of a cross-section. This stress determination is performed in the RF-/STEEL add-on module by calculating the existing stresses and comparing them with the limit stresses.
• ### I compare the flexural buckling design according to the equivalent member method and the internal forces according to the linear static analysis with the stress calculation according to the second-order analysis including imperfections. The differences are very large. What is the reason?

While the influence of imperfection increases significantly with increasing axial force and the ratio increases exponentially in the case of the second-order analysis,, the influence of imperfection only increases linearly in relation to the axial force in the case of the equivalent member design. Therefore, there is usually a greater difference between the design ratios according to the linear static analysis by using the equivalent member method and the second-order analysis by using the stress analysis for the structural systems with very high or very low design ratio.
• ### For a cross-section from the cross-section library of RFEM/RSTAB, the stresses calculated with RF‑/STEEL differ from the stresses of the same cross-section calculated with SHAPE‑THIN. What is the reason?

In RF‑/STEEL, the stresses are calculated on stress points of a cross-section. The stress points defined for the cross-section can be displayed in the details of the cross-section. For the cross-section shown in Figure 01, the stress points are defined along the cross-section. Thus, the stress points (here, Stress Points 1 and Stress Point 9) are also available in the roundings.

In SHAPE‑THIN, you can only calculate the stresses on elements. Stresses cannot be calculated on point elements with which irregularities in the cross -section geometry such as fillets are displayed. The stress points of the SHAPE‑THIN cross-section are displayed in Figure 02.

In the case of the cross-sections with point elements, minor deviations in stresses may thus occur due to the different stress points.
Furthermore, SHAPE-THIN allows you to calculate the stresses for the most unfavorable element edge or only for the element center lines. In RF‑STEEL, the stresses are calculated exclusively on the stress points.

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