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Frequently Asked Questions (FAQ)
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AnswerThe internal forces and deformations are determined according to the second-order analysis for flexural-torsional buckling taking into account 7 degrees of freedom. For a linear calculation of deformations, a vertical/horizontal load results in only one vertical/horizontal deformation. Since the internal forces refer to the deformed system and it is a nonlinear analysis, this is not valid for the second-order analysis for flexural-torsional buckling.
The deformations in the shear center can be checked in RSTAB and RFEM with the RF-/FE-LTB add-on module (see Figure 02). The deformations that additionally result from the displacements or rotations can only be controlled with a surface model in RFEM.
AnswerFrom the formula apparatus of EC 3-1-8, it is clear that the limit tensile forces of the cross-section's end plate are introduced into the column flange and thus directly into the column web.
Therefore, the upper end plate is not fully stressed by these forces.
For purely structural reasons, the column top plate is available for the stiffening and the panel boundary and serves for the transmission of the proportional force flowing into the stiffener in case of a required diagonal stiffener.
AnswerWith the time history monitor, you can view all results over time. In this case, it is also possible to select several parts of the structure and then export the results directly to Excel.
When designing general cross-sections in the RF-STEEL AISC module, a "Non-designable" message is given for the design ratio. General cross-sections are defined in the program as user-defined sections or shapes which are not included in the specification. Though, some user-defined cross-sections can be designed under specific conditions. The warning message is displayed because shear buckling is not provided for general cross-section in the AISC 360-16  Chapter G. More info on this topic can be found in the RF-STEEL AISC manual linked below.This warning can be turned off in RF-STEEL AISC add-on module under the "Details" button > Shear Buckling > "Shear buckling warning for general cross-section."
The cross-section selection error is most likely to occur:
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 for 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 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 resistance moment W t and a correspondingly high shear stress.
The solution is therefore the selection of a flat steel within the main program as described above.
AnswerTrapezoidal sheeting is already available in the cross-section library of RFEM. These cross-sections are primarily used for stabilization measures in the RF- / STEEL EC3 add-on module and as a template for modeling surfaces. A design as a member is thus not possible.The easiest way is to first model the trapezoidal sheeting as a member. Subsequently, this member can be automatically converted into surfaces (see Figure 01). Thus, the geometry is obtained as a surface model. Curved trapezoidal sheets are also possible. If a curved member is split into surfaces, it is represented by straight segments. If you still want to use curved lines, you can simply rotate the contour lines accordingly. The exact procedure can be found in the video.
AnswerTo enter a taper on a beam, it is necessary to select the Layout dialog section. A taper can be modeled either on the bottom side or on the top side of the beam.The plate thicknesses, as well as the height of the taper, are defined by selecting a tapered cross-section, since, for example, repeatedly cut rolled cross-sections are used. If any plates are used, a parametric T-section can be selected. The length of the taper, as well as the material, are entered in the same dialog section. The weld thicknesses for connecting the taper to the beam can be defined in the Welds dialog section.If a tapered member is already used as a beam (entered in the 'Nodes and Members via Different Cross-Sections and Lengths' section), it is not possible to model an additional taper.
AnswerSince 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.
AnswerThe analysis of a rigid column footing is performed according to EN 1993-1-8 in the RF-/JOINTS Steel - Column Base add-on module. The standard only provides for simple bending in the design format. The design can be performed for compression only, tension, simple bending, or a combination of bending and tension, or bending and compression. If the note is confirmed with 'Yes', the add-on module performs all designs neglecting the unsupported internal force Mz.
In this case, the slab should rest on the downstand beam without any bond (see Figure 1).
First, assign the member type 'Beam' to the downstand beam because the rib is not effective here due to the missing joint.
To implement the non-linearity 'Only Transfer of Compression Forces in Z-Direction', you can use the 'Line Release' function. In this case, all forces and moments must be activated, that is, released, within the 'New Line Release Type' dialog box. Moreover, for uz, the non-linearity "Fixed if vz Is Positive" must be set, see Figure 2.
The line of the member within the corresponding area should be selected as a line to be released, and the corresponding surface like a released object.
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