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• ### I have a model of a timber building, which I have transferred from Scia Engineer and adapted it into the RFEM program. The models should be the same; however, it can only be calculated in Scia Engineer while RFEM reports singularity. How do I edit the model to analyze it in RFEM?

Instability is caused by the fact that all member, which have a hinge at the member end, converge in one node. This means that the node can rotate freely. It is necessary to connect at least one member to a node without a hinge.

You could also solve this problem by setting a certain small stiffness at the hinge so that it is not freely rotatable. In that case, there can be hinges on all members at the node.

• ### Why are my steel members not being designed for stability in RF-STEEL AISC?

Curved members cannot be designed for stability within the RF-STEEL AISC module. These stability checks include flexural buckling (strong axis and weak axis) and flexural torsional buckling according to Chapter E. Lateral torsional buckling is also not checked according to Chapter F for these types of members.

A possible workaround to carry out stability design for a curved member is to convert the line element to a polyline and design as a straight member instead. Alternatively, a series of straight line segments can be modeled and convert to a set-of-members which can also be designed in the RF-STEEL AISC module including stability checks.

• ### How can I perform the stability analysis in RF‑/STEEL EC3 for a flat bar supported on edges, such as 100/5? Although the cross-section is rotated by 90° in RFEM/RSTAB, it is displayed as lying flat in RF‑/STEEL EC3.

In RF‑/STEEL EC3, the uniaxial bending stress is always related to the local y-axis of a member and the local y‑axis of a member is always assumed as the major axis in the case of symmetric cross-sections, therefore it is only possible in such a case to change the cross-section description from "100/5" to "5/100", see the image. Thus, the stability analysis is performed accordingly.

• ### In RF‑/STEEL EC3, is the "Elastic design (also for Class 1 and Class 2 cross-sections)" option under "Details → Ultimate Limit State" considered for a stability analysis when activated?

Yes, if activating the stability analysis in the "Stability" tab as well as the "Elastic design (also for Class 1 and Class 2 cross-sections)" option in the "Ultimate Limit State" tab (see the image), the stability analyses are also performed with the elastic cross-section properties.

• ### Why is there no stability analysis displayed in the results despite the activation of the stability analysis in RF‑/STEEL EC3?

If there is no stability analysis displayed in the results in RF‑/STEEL EC3 although the stability analysis has been activated, it is very likely not necessary.

For example, if the slenderness ratio λLT is less than or equal to 0.4, the lateral-torsional buckling analysis may be omitted according to Eurocode 3 and only cross-section design should be performed.

• ### How can I perform a stability analysis for a tapered member?

Tapered members must not be designed according to the simplified equivalent member method!

For steel structures, the design can be performed by considering the warping torsion or using the General Method. These methods are described in this technical article.

For timber structures, the design can also be performed by considering the warping torsion. The method for timber structures is explained in detail in thiswebinar.

According to the equivalent member method, the design can be performed if the provisions of the explanations for DIN 1052, Section E8.4.2 (3) for variable cross-sections are met. In various sources of technical literature, this method is adopted for Eurocode 5. An example of this can be found in the document on brettschichtholz.de, page 64 ff.

In the RX‑TIMBER program, the design of tapered members is performed according to the equivalent member method. This is briefly explained on a simple example.

Structural System (Figure 01):

• Span length: 8 m
• Beam height right: 80 cm
• Beam height left: 26 cm
• Roof inclination: 3.9°
No stiffening is defined. The lateral-torsional stability becomes governing with 99% (Figure 02) at the x‑location 1.598 m. The cross-section height is 36.8 cm. However, the slenderness ratio is based on the equivalent cross-section height of 60.9 cm (Figure 03).

The equivalent cross-section height results at the x-location 5.2 m about 0.65 × 8 m = 5.2 m.

If the stiffening is in the middle of the span, for example, the equivalent height for the x‑location changes to 45.3 cm.

Since the stiffening is usually applied over the member length, the height must be calculated according to a special algorithm. The supports are always applied as fixed points and the equivalent heights are calculated, based on the x-locations of the designs.

For the example, the following results: x0.65 = 0.32 x 4 m + 1.598 m = 2.878 m
• ### How can I change the details and the National Annex in the STEEL EC3 add-on module by using the COM interface?

The following code displays all elements of the STEEL EC3 add-on module that can be modified via the COM interface:

//  get interface to active modeliModel = iApp.GetActiveModel();//  get interface to STEEL EC3 moduleIModule module = iModel.GetModule("STEEL_EC3") as Dlubal.STEEL_EC3.IModule;//  get interface to module caseICase iStEC3case = module.moGetCase(1, Dlubal.STEEL_EC3.ITEM_AT.AT_NO);//  get ultimate limit state options (Details > Ultimate Limit State)ULS_OPTIONS optsULS = iStEC3case.moGetULSOptions();//  get options for stability design (Details > Stability)STABILITY_OPTIONS optsStab = iStEC3case.moGetStabilityOptions();//  get options for serviceability design (Details > Serviceabiltiy)SERVICEABILITY_DEFORMATION_TYPE optsServDef = iStEC3case.moGetServiceabilityOptions();//  get fire resistance options (Details > Fire Resistance)FIRE_RESISTANCE_OPTIONS optsFire = iStEC3case.moGetFireResistanceOptions();//  get other options (Details > General)OTHER_OPTIONS optsOther = iStEC3case.moGetOtherOptions();//  get national annex (e.g. DIN, CEN, ...)NATIONAL_ANNEX natAn = iStEC3case.moGetNationalAnnex();//  get interface for national annex detailsINationalAnnex iNatAn =  iStEC3case.moGetNationalAnnexOptions();//  get base data for national annexNATIONAL_ANNEX_OPTIONS_BASE natAnBase = iNatAn.moGetBaseOptions();//  get data for general method from national annexNATIONAL_ANNEX_OPTIONS_GM natAnGM = iNatAn.moGetGMOptions();//  get data for lateral-torsional buckling from national annexNATIONAL_ANNEX_OPTIONS_LTB natAnLTB = iNatAn.moGetLTBOptions();//  get data for stainless steel from national annexNATIONAL_ANNEX_OPTIONS_STEEL natAnSTEEL = iNatAn.moGetSteelOptions();

The corresponding elements in the parameter dialog box of the add-on module are shown in Figure 02.

• ### Why is it not possible to use result combinations in RF‑STABILITY? It is possible in RSBUCK indeed.

While the existing axial force in members is used for the eigenvalue analysis in RSBUCK, the external load is increased until failure for the stability analysis in RF‑STABILITY. Thus, in contrast to RSBUCK, no linear eigenvalue analysis is provided and you can only use load cases and load combinations, see the figure.

• ### I would like to perform a stability analysis of the upper flange in a long truss. What is the best way to proceed?

According to DIN EN 1993‑1‑1:2010‑12 [1], Annex BB.1.1, the buckling length may be used in the individual bracing under certain conditions. This means that in this case, the individual members, not a set of members, can be applied with the effective length factors specified in the standard.

Since this approach only considers the local failure, it is necessary to analyze the global failure of the entire structure. For this design, the set of members must have the corresponding imperfection. Under certain conditions, the design can be performed on single members, depending on the model (for example, a tower), or the set of members must be analyzed for a failure from the plane (the truss), as in the attached example.

• ### Is it possible to deactivate the stability analysis by member within a case in the RF‑/STEEL EC3 add-on module?

Yes, this setting can be made in Window "1.5 - Effective Lengths." Deactivate "Buckling Possible" or "Lateral-Torsional and Torsional-Flexural Buckling Possible", see the figure.

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