Frequently Asked Questions (FAQ) - Application Area: Stress Analysis

The stability design checks according to EN 1999‑1‑1, 6.3, are not included in RSECTION 1.

According to EN 1999‑1‑1, 5.2.2 (5) a), the stability analysis may be performed as a cross-section design, provided that a spatial calculation according to the second-order analysis is performed while applying global and local imperfections. In order to model the lateral-torsional buckling, it is necessary to determine the internal forces according to the geometrically nonlinear torsional buckling analysis with regard to warping torsion. It is only necessary to perform the section design, as all stability effects are covered by the calculation.

In RSECTION 1, you can calculate the stresses due to the axial force, biaxial bending moments and shear forces, primary and secondary torsional moment, as well as warping bimoment for any cross-section shape. This way, you can use the design method described in EN 1999‑1‑1, 5.2.2 (5) a) in RSECTION 1.

Furthermore, the Effective Sections add-on allows you to perform a classification and calculate the effective cross-section according to EN 1999-1-1.

• Effective Sections (Extension for RSECTION 1)

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The stability design checks according to EN 1993‑1‑1, 6.3, are not included in RSECTION 1.

According to EN 1993‑1‑1, 5.2.2 (7) a), the stability analysis may be performed as a cross-section design, provided that a spatial calculation according to the second-order analysis is performed while applying global and local imperfections. In order to model the lateral-torsional buckling, it is necessary to determine the internal forces according to the geometrically nonlinear torsional buckling analysis with regard to warping torsion. It is only necessary to perform the section design, as all stability effects are covered by the calculation.

In RSECTION 1, you can calculate the stresses due to the axial force, biaxial bending moments and shear forces, primary and secondary torsional moment, as well as warping bimoment for any cross-section shape. This way, you can use the design method described in EN 1993‑1‑1, 5.2.2 (7) a) in RSECTION 1.

Furthermore, the Effective Sections add-on (extension for RSECTION 1) allows you to perform a classification according to EN 1993-1-1 and calculate the effective cross-section according to EN 1993-1-5.

• Effective Sections (Extension for RSECTION 1)

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Yes, it is. The Special Options of the Line Welded Joint Configurations provide you with the option to activate the consideration of the weld eccentricity. This is not always necessary. For example, according to EN 1993‑1‑8 [1], Section 4.12, the weld eccentricity can be neglected if a hollow section is connected uniformly over the entire perimeter, or if the eccentricity leads to an increase of compressive stresses in the weld root.

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The most likely cause of the different results is that you have probably not set the same smoothing of the surface internal forces.

This can be set separately in RFEM 6 and in the add-on.

• Result Smoothing in RFEM 6
• Smoothing Settings for Stress-Strain Analysis

If the smoothing is the same in both settings, the stresses are also the same.

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In the RSECTION program, you can analyze general steel or aluminum cross-sections and determine the effective section properties. For this, you need the Effective Sections program extension of RSECTION.

• Effective Sections

If you have licensed this add-on, you can activate the Effective Section option for the calculation in the Base Data of the cross-section.

Then, define the standard according to which the calculation should be performed. Currently, the following options are available:

• EN 1993-1-1 and EN 1993-1-5
• EN 1993-1-3 (Cold-Formed)
• EN 1999-1-1 (Aluminum)

The effective section properties depend on the internal forces of the cross-section. Therefore, create a load case and define one or more internal force constellations.

After the calculation, the effective section properties are displayed in the table. In the graphic, you can check the stresses on the effective section.

Once you have saved the cross-section, you can import it into RFEM or RSTAB to use it for further analyses.

The webinar Determination of Section Properties and Stress Analysis in RSECTION shows the modeling and calculation of a cold-formed section.

You can find further information there.

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RFEM or RSTAB are the ideal basis for the calculation of scaffolding or rack structures, such as high-bay warehouses. It is also possible to consider nonlinear properties of members and hinges, such as yielding, tearing, slippage, or special scaffolding diagrams. There are add-ons available for both programs that you can use to design steel or aluminum structures according to various standards.

Main Programs RFEM and RSTAB

The main programs RFEM and RSTAB are used to define the model with its properties and actions. RFEM proves to be the more versatile option, as it can be used to analyze structures with surface components in addition to pure member structures.

Available Standards

For the design of steel beam structures, the following standards are available:

• EN 1993 (Eurocode 3)
• BS 5950 (British standard)
• AISC 360 (US standard)
• SIA 263 (Swiss standard)
• IS 800 (Indian standard)
• GB 50017 (Chinese standard)
• CSA S16 (Canadian standard)
• AS 4100 (Australian standard)
• NBR 8800 (Brazilian standard)
• SP 16.13330 (Russian standard)

Aluminum structures can be designed according to the following standards:

• EN 1999 (Eurocode 9)
• ADM (US standard)
• GB 50429 (Chinese standard)

Analysis and Design Add-ons

Design add-ons supplement the functionality of the main programs. The Steel Design and Aluminum Design add-ons allow you to perform the ultimate and serviceability limit state design checks, as well as the stability analysis according to the standards listed above.

The Torsional Warping (7 DOF) analysis add-on allows for lateral-torsional buckling design with up to seven degrees of freedom. The Stress-Strain Analysis add-on provides the option for general stress design checks, where the existing stresses are compared to the limit stresses.

Dynamic Analysis

If you need to perform a seismic or vibration analysis, the corresponding Dynamic Analysis add-ons are the perfect tools for determining natural frequencies and mode shapes, or for the analysis of external excitations.

In case of any questions about the Dlubal solutions for scaffolding and rack structures, our sales team will be happy to assist you.

• Contact Our Sales Team

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RFEM allows you to perform structural analysis and design of laminate and sandwich structures. The same applies to the cross-laminated timber. Stress and deflection analysis of laminate and sandwich surfaces is performed according to the laminate theory, taking into account the shear coupling.

Programs and Add-ons

RFEM is the main program that you can use to define the model and actions. You can model planar and spatial structures, consisting of plates, walls, shells, and members.

For the stress and deflection analysis, you need the Multilayer Surfaces add-on. It allows you to define and analyze layer structures.

Use the Timber Design add-on to also design the member supporting elements of the structure according to Eurocode 5 or ANSI/AWC NDS, for example.

Dynamic Analysis

If you need to perform a seismic or vibration analysis, the corresponding Dynamic Analysis add-ons are the perfect tools for determining natural frequencies and mode shapes, or for the analysis of external excitations.

In case of any questions about the Dlubal timber design solutions, our sales team will be happy to assist you.

• Contact Our Sales Team

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Hydraulic steel structures, such as sluices, locks and lock chambers, roller water gates, sliding and sector gates, weir flaps, drum gates, flood protection gates, ship lifts, and canal bridges, are highly sophisticated structures. RFEM provides you with the possibility for structural design as well as static and dynamic analysis of these complex structures with surface and solid elements.

Main Program RFEM

Use the main program RFEM to define the model, including the properties and actions. You can model planar and spatial structures, consisting of members, plates, walls, and shells.

Add-ons for Steel Structures

Design add-ons supplement the functionality of the main programs. The Steel Design add-on allows you to perform the ultimate and serviceability limit state design checks, as well as the stability analysis according to the standards listed above. The Stress-Strain Analysis add-on provides the option for general stress design checks, where the existing stresses are compared to the limit stresses. The Nonlinear Material Behavior analysis add-on also enables plastic analyses.

The design checks can be carried out for a large number of standardized and parameterized cross-sections. If you use your own cross-sections, you can define them with the RSECTION program, and use them for further design in RFEM.

In case of any questions about the Dlubal solutions for hydraulic steel structures, our sales team will be happy to assist you.

• Contact Our Sales Team

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Both RFEM and RSTAB provide solution: There are numerous European and international standards as well as add-ons available for both programs that can facilitate your daily work in structural steel design.

Main Programs RFEM and RSTAB

The main programs RFEM and RSTAB are used to define the model with its properties and actions. In addition to spatial frame and truss structures, such as halls or scaffolding, it is possible to model plate, wall, and shell structures with RFEM. Thus, RFEM is the more versatile variant—especially if you work in other areas, such as solid construction.

Available Standards

• EN 1993 (Eurocode 3)
• BS 5950 (British standard)
• AISC 360 (US standard)
• SIA 263 (Swiss standard)
• IS 800 (Indian standard)
• GB 50017 (Chinese standard)
• CSA S16 (Canadian standard)
• AS 4100 (Australian standard)
• NBR 8800 (Brazilian standard)
• SP 16.13330 (Russian standard)

Add-ons for Steel Structures

Design add-ons supplement the functionality of the main programs. The Steel Design add-on allows you to perform the ultimate and serviceability limit state design checks, as well as the stability analysis according to the standards listed above. The Torsional Warping (7 DOF) analysis add-on allows for lateral-torsional buckling design with up to seven degrees of freedom. The Stress-Strain Analysis add-on provides the option for general stress design checks, where the existing stresses are compared to the limit stresses.

Use the Steel Joints add-on if you want to analyze hinged and rigid steel connections.

The design checks can be carried out for a large number of standardized and parameterized cross-sections. If you use your own cross-sections, you can define them with the RSECTION program, and use them for further design in RFEM or RSTAB.

In case of any questions about the Dlubal steel design solutions, our sales team will be happy to assist you.

• Contact Our Sales Team

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As of the version 6.03 (RFEM) and 2.03 (RWIND), the results calculated in RWIND can also be displayed in RFEM.

To do this, you just need to activate the "Wind Simulation" add-on in RWIND and start the corresponding calculation.
RWIND appears and performs the simulation. Once the calculation has been completed, the following dialog box appears.

By clicking "Yes", RWIND is closed and RFEM loads the result data. In the result display in RFEM, you can now display the properties related to the surface, as shown in the following image. You can use the navigator in the top left to switch between the results of the structural analysis and the wind simulation.

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Corrugated web beams allow for further optimization of steel structures. They are often used to reduce the self-weight of main load-bearing elements and to prevent buckling failure of webs due to the corrugation.

This example will be based on a WTA – 1250 – 300x20 Beam from the Zeman catalogue.

To create a model of this beam, it's best to start by modeling a sine wave that will be used to edit the web. This consists of the steps shown below and in the attached images.

• Create nodes representing sine function
• Copy nodes to desired length of beam
• In RFEM's Lines table, manually create spline from 1st to last node

After these steps, it's recommended to create a parametric section with the dimensions of the corrugated beam. Members using this section can later be transformed into surfaces.

• Create a parametric section with the geometry based on desired beam
• Generate surfaces from members

With this set of surfaces, it is recommended to copy the sine spline into the position of the top and bottom edge of the surface representing the web.
Later, it is required to change the geometry type of the web surface to Quadrangle and redefine the boundary lines of this element.

• Copy sine into top and bottom of web
• Change web geometry type to Quadrangle
• Exclude straight lines from web surface and add sine lines

After preparing the geometry, it's required to focus on finishing the model for the calculation. Select the start and end lines of the member and create a member with the type Rigid. After that, it's possible to define nodal supports on the beam and loads in the model. In order to check the resulting internal forces of a member, it is recommended to define a result beam on the main center line. Finishing all of these steps allows us to start the analysis of the beam.

• Define rigid members on external lines of surfaces
• Add nodal supports
• Add loads – use line loads, not member loads for continuous loading
• Define a result beam on the center line
• Perform the analysis

Remember to define proper mesh settings with an adequate target length of the finite elements and to use a line mesh refinement on the sine lines.

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The fastest way to create a contact solid between two surfaces is to first copy the original surface by using the Step Links option.

In the new Step Links tab, select the "Link surfaces with solid" option.
Select the template for this solid to be a standard Solid Type.

After copying and creating the new elements, select the Solid type to be a Contact. In the new tab, define the Contact Solid Type (for example, Full force transmission). Additionally, you will be able to select a pair of surfaces to have this contact. A list of available surfaces is generated automatically.

After defining the Contact Solid, it’s possible to change the side Surfaces to the type Without Thickness.

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If no angle can be defined in the "Rotation" column, there is an isotropic material model selected for the material, where stiffnesses are identical in all directions and it is not necessary to define an angle.

If you use materials with anisotropic behavior (for example, timber), it is necessary to ensure that the "Orthotropic | Linear Elastic (Surfaces)" material model is selected.

Note: The "Orthotropic | Timber | Linear Elastic (Surfaces)" material model cannot be currently used in combination with the "Layers" thickness type.

As soon as switching to the orthotropic material model, the individual layers can be rotated accordingly.

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If member loads are generated from an area load, extremely small concentrated loads may result, depending on the nature of the surfaces. If these are too small to be displayed with the number of decimal places in the output, the load is rounded to the value 0.
Image 01 illustrates this effect.

In the load wizard, there is a "Smooth concentrated load" check box. If it is selected, such concentrated loads are converted into variable loads. Image 02 shows this option in the load wizard "Member Load from Area Load".

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Where the geometries become complicated, it is difficult to use analytical methods for the design. The weld elements in RFEM 6 are particularly helpful for such applications.

RFEM 6 allows for design checks for different types of welds.

In the Joint type list, you can select how the surfaces to be connected are related to each other.

Then, select a weld type.

Finally, it is necessary to define the weld parameters.

The welds do not affect the stiffness of the model. You use the stresses from the surface elements and evaluate them according to the regulations.

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Some materials have multiple limit stress limits for compression, tension, and so on. For these materials, the limiting stress must be input manually by the user.

The limit stress values are listed under the Material Values tab.

These values can be added in the Member/Surface Configurations under the User limit stress type.

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Yes, it is. If a weld is defined on a line in the surface model, the stresses of the line weld can be calculated with the "Stress-Strain Analysis" add-on.

The following joint types and design variants are available:

• Butt Joint
• Single Square
• Double Square
• Double Bevel
• Single V
• Double V
• Single U
• Double U
• Single J
• Double J

• Corner Joint
• Single V
• Fillet Weld
• Double Fillet

• Lap Joint
• Fillet Weld

• Tee Joint
• Double Bevel
• Double J
• Double Fillet
• Single-Bevel Weld
• Single J
• Fillet Weld

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To perform the deformation analysis of a surface, make sure that the Stress-Strain Analysis add-on is activated. Then, select the Strain Analysis check box by right-clicking Stress-Strain Analysis in the Data navigator.

By using the color scale, it will be possible to display the deformation areas above the 0.5‰ limit.

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The value $\gamma,_{sat}$& corresponds to the weight of the saturated soil and can be adjusted in the settings of material parameters, if necessary. If there is groundwater in a layer, the weight of the saturated soil $\gamma,_{sat}$ is automatically reduced from this layer by the weight of the groundwater (10 kN/m³). Above this layer, the stresses are calculated with the specific weight of the soil.

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The material model according to Tsai-Wu unifies the plastic with the orthotropic properties. In this way, it is possible to specifically model the materials with anisotropic properties, such as plastics or timber. If the material is plastified, the stresses remain constant. The redistribution is carried out according to the stiffnesses available in the individual directions. The elastic region corresponds to the "Orthotropic - 3D" material model. For the plastic area, the yielding according to Tsai-Wu applies:

${\text{f}}_{\mathrm{crit}}\left(\mathrm\sigma\right)=\frac1{\mathrm C}\left[\frac{\left({\mathrm\sigma}_{\mathrm x}-{\mathrm\sigma}_{\mathrm x,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm x}{\mathrm f}_{\mathrm c,\mathrm x}}+\frac{\left({\mathrm\sigma}_{\mathrm y}-{\mathrm\sigma}_{\mathrm y,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm y}{\mathrm f}_{\mathrm c,\mathrm y}}+\frac{\left({\mathrm\sigma}_{\mathrm z}-{\mathrm\sigma}_{\mathrm z,0}\right)^2}{{\mathrm f}_{\mathrm t,\mathrm z}{\mathrm f}_{\mathrm c,\mathrm z}}+\frac{{\mathrm\tau}_{\mathrm{yz}}^2}{{\mathrm f}_{\mathrm v,\mathrm{yz}}^2}+\frac{{\mathrm\tau}_{\mathrm{xz}}^2}{{\mathrm f}_{\mathrm v,\mathrm{xz}}^2}+\frac{{\mathrm\tau}_{\mathrm{xy}}^2}{{\mathrm f}_{\mathrm v,\mathrm{xy}}^2}\right]$

where:

${\mathrm\sigma}_{\mathrm x,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm x}-{\mathrm f}_{\mathrm c,\mathrm x}}2$

${\mathrm\sigma}_{\mathrm y,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm y}-{\mathrm f}_{\mathrm c,\mathrm y}}2$

${\mathrm\sigma}_{\mathrm z,0}=\frac{{\mathrm f}_{\mathrm t,\mathrm z}-{\mathrm f}_{\mathrm c,\mathrm z}}2$

$\mathrm C=1+\left[\frac1{{\mathrm f}_{\mathrm t,\mathrm x}}+\frac1{{\mathrm f}_{\mathrm c,\mathrm x}}\right]^2\frac{{\mathrm E}_{\mathrm x}{\mathrm E}_{\mathrm p,\mathrm x}}{{\mathrm E}_{\mathrm x}-{\mathrm E}_{\mathrm p,\mathrm x}}\mathrm\alpha+\frac{{\mathrm\sigma}_{\mathrm x,0}^2}{{\mathrm f}_{\mathrm t,\mathrm x}{\mathrm f}_{\mathrm c,\mathrm x}}+\frac{{\mathrm\sigma}_{\mathrm y,0}^2}{{\mathrm f}_{\mathrm t,\mathrm y}{\mathrm f}_{\mathrm c,\mathrm y}}+\frac{{\mathrm\sigma}_{\mathrm z,0}^2}{{\mathrm f}_{\mathrm t,\mathrm z}{\mathrm f}_{\mathrm c,\mathrm y}}$

The stress criterion can be imagined as an elliptical surface within a six-dimensional space of stresses. If one of the three stress components is applied as a constant value, the surface can be projected onto a three-dimensional stress space.

If the value for f_y(σ) is smaller than 1, the stresses rest within the elastic area. The plastic area is reached as soon as f_y(σ) = 1. Values higher than 1 are not allowed. The model behavior is ideal-plastic, which means there is no stiffening.

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If there are member sections connected by welds, you can define the arrangement and design of the transverse welds in Window 1.5. This allows you to determine the cross-section softening in the heat-affected zone.

According to EN 1999‑1‑1, Clause 6.1.6.2, NOTE 2, there are two options to consider the reduced strength:
In the case of the reduced stress method, the strength values _f,o,haz and f_u,haz are reduced in accordance with Table 3.2. By contrast, the reduced cross-section method uses reduced cross-section properties (area A, section moduli W, moments of inertia I) for the design checks.

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

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

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In SHAPE-THIN 9, you can model cold-formed sections and effective cross-sections according to EN 1993‑1‑3 and EN 1993‑1‑5. This is not possible in SHAPE‑THIN 8.

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Contrary to the graphic display, the CRANEWAY add-on module shows the resulting support forces or the centroid.

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

For steel design, a thin-walled flat bar cross-section should be selected instead of a rectangular solid cross-section; see Image 01.

The high shear stress of a solid cross-section is due to the existing stress points of the cross-section, or the corresponding thickness of this stress point.

In the case of a thin-walled flat bar cross-section, there are four stress points at the corner points of the cross-section with the corresponding thickness t = 10 mm; see Image 02.

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 Image 03.

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

The solution is, as described above, to select a flat bar in the main program.

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For the calculation of members, it is possible to define the cross-sections in RFEM and RSTAB that create a member with the openings arranged regularly (circular or honeycombed).

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No, it is not possible to set user-defined values when viewing stress results on a solid. These unfortunately do not work like results on a surface. It is possible to activate the result values on each FE Mesh Node using the settings shown below under Values on Surface. To access the filter settings you must right click on the label "Values on Surfaces."

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