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• ### Why do I get much higher design ratios in RF-/STEEL compared to cross-section design in RF-/STEEL EC3?

New FAQ 004154 EN-US

In the RF-/STEEL add-on module, an equivalent stress design is performed according to von Mises. An elastic stress design (EL-EL) is to be made. In RF-/STEEL EC3, a classification is carried out before the design. If the cross-section is classified as class 1 or class 2, the design is performed against plastic limit internal forces. An EL-PL design is 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 cross-section design of a flat steel, I receive unusually high shear stresses due to torsion in the STEEL EC3 add-on module, which can be refuted by means of a simple manual calculation. What is the error?

New FAQ 003612 EN-US

Most likely, the error is in the selection of the cross section:

For a steel design, a thin-walled flat steel cross-section should be selected instead of a rectangular solid cross-section, see Figure 1.

The reason for the high shear stress of a solid cross-section is caused by the existing stress points of the cross-section or by the corresponding thickness of this stress point.

In the case of a thin-walled flat steel cross-section, there are four stress points at the corner points of the cross-section with the corresponding thickness t = 10 mm, see Figure 2.

For a solid cross-section, however, there is another stress point in the center, where the maximum of height h or width b is assumed as the thickness t for this cross-section type. In this case, the width b is 200 mm, see Figure 3.

This results in a small torsional section modulus Wt and a correspondingly high shear stress.

Therefore, the solution is, as described above, to select flat steel within the main program.

• ### Why are the equivalent member designs grayed out in the Stability tab when the plastic designs are activated by means of the partial internal force method (RF-/STEEL Plasticity)?

New FAQ 003582 EN-US

Since the equivalent member designs of Eurocode 3 have different interactions than are the case for the designs according to the partial internal forces method and a mixture of these different designs is not desired for reasons of clarity, RFEM deactivates the equivalent member designs when using the RF-/STEEL Plasticity add-on.
• ### In the RF- / STEEL EC3 add-on module, I have selected two braids of equal size for one beam to be designed in the "Parameters" window as the shear panel type. Accordingly, the beam should be supported laterally in the middle. Why is the eigenvector arbitrary?

New FAQ 003562 EN-US

The term "shear panel" indicates that the translational spring, which is created along the beam length by means of the shear panel type including the corresponding parameters, is smoothed, see Figure 1.

This is also the case for the shear panel type "Bracing", so that the mode shape always appears to be arbitrary at this location, see Figure 2.

In order to obtain accurate results, it is recommended to manually define a lateral support by means of a nodal support according to the general method (Figure 3) or to define the effective lengths according to the equivalent member method, including intermediate restraints, if necessary. Finally, a eigenvector with visible lateral restraint in the mid-span is created (Figure 4).

• ### In RF- / STEEL EC3, I receive the error message saying that a node with supports does not exist in the set of members. What is the reason?

New FAQ 003524 EN-US

One support was defined at a node that is not part of the set of members in the window ‘1.7 Nodal Supports’ of RF-/STEEL EC3. In this case, it is necessary to define support at a node existing in the set of members or delete this nodal support in the window '1.7 Nodal Supports' of RF-/STEEL EC3.

In Figure 1, for example, a set of member supports was defined for the set of members 1 on nodes 2 and 7. When the calculation starts, the message 'Node No. 7 with support does not exist in the set of members No.1. Please correct this in Table 1.7.' appears (Figure 2, Figure 3).

In window ‘1.7 Nodal Supports’ of the RF-/STEEL EC3, it is possible to get displayed only the selected set of members by clicking the 'Show Only Current Object' button and the node numbering can be displayed by clicking the 'Local Coordinate System' button (Figure 4). It is obvious that the node 7 is not contained in the set of members. Instead of at node 7, the support must be defined on a node contained in the member set (for example, node 6).

• ### I would like to define lateral supports along the component´s axis that sometimes act on the top chord, sometimes on the bottom chord. However, only one item can be selected for each member. How can I create the input?

FAQ 003494 EN-US

To provide a component with different supports, it must consist of several members and be designed by a set of members. Different parameters can then be assigned to the individual members. The procedure can be found in the video.
• ### What is the meaning of the warning message ER061) Minimum amplifier of design loads <1?

FAQ 003491 EN-US

This message indicates that the critical load of the structure or the cross-section has been exceeded.

The causes for this are very diverse. Often, insufficient lateral supports have been defined in the STEEL EC3 add-on module.

It is also possible that the used cross-sections or the structure itself cannot be calculated according to the general method in EC3. This FAQ provides information about such a case.
• ### 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?

FAQ 003489 EN-US

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

Please also check whether the partial safety factors γ for the resistances of the cross-sections are defined identically in both add-on modules (Figure 3 and 4).

• ### When designing a set of members by means of the equivalent member method in RF-/STEEL EC3, the calculation fails. The system is unstable delivering the message 'Non-designable — ER055) Zero value of the critical moment on the segment'.What could be the cause?

FAQ 003440 EN-US

Please check if all sets of members selected for the design are sets of members. The equivalent member method is only applicable for straight sets of members with a uniform cross-section involving, for example, no taper. In this case, use the preset General Method.
• ### According to which formula is the elastic critical buckling load for the torsional buckling Ncr,T calculated in RF-/STEEL EC3?

FAQ 003376 EN-US

The elastic critical buckling load for torsional buckling Ncr,T is calculated as follows:

${\mathrm N}_{\mathrm{cr},\mathrm T}\;=\frac1{{\mathrm i}_{\mathrm M}^2}\;\cdot\;\left(\frac{\mathrm\pi^2\;\cdot\;\mathrm E\;\cdot\;{\mathrm I}_{\mathrm w}}{{\mathrm L}_{\mathrm T}^2}\;+\;\mathrm G\;\cdot\;{\mathrm I}_{\mathrm t}\right)$

${\mathrm i}_{\mathrm M}\;=\;\sqrt{{\mathrm i}_{\mathrm u}^2\;+\;{\mathrm i}_{\mathrm v}^2\;+\;{\mathrm u}_{\mathrm M}^2\;+\;{\mathrm v}_{\mathrm M}^2}$

with

 E Modulus of elasticity G Shear modulus Iw Warping resistance It Torsion moment of inertia iu, iv Pricipial radius of gyration um, vm Shear center coordinates in the principal axis system LT Torsional buckling critical length

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