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Frequently Asked Questions (FAQ)
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RF-/ALUMINUM checks the symmetry of general cross-sections and compares them with the SHAPE-THIN evaluation if activating the "Determine symmetry by module and compare with SHAPE‑THIN definition" check box (Figure 01).
If both methods provide different results, the corresponding error message appears (Figure 02).
Usually, there are small inaccuracies in the SHAPE‑THIN cross-section. Thus, the cross-section Sec‑1.du9 shown in Figure 03 is not absolutely symmetrical to the Z‑axis: The Z‑coordinates of Node 1 and Node 4 as well as Node 55 and Node 60 do not match in the second decimal place.
SHAPE‑THIN classifies the cross-section as asymmetrical, but RF‑/ALUMINUM as monosymmetric to the z‑axis, so the error message shown in Figure 02 appears.
The SHAPE‑THIN cross-section should be checked for symmetry. When modeling in SHAPE‑THIN, it is useful to only display one side of the cross-section and to create the other half by mirroring. This is also shown in the video.
If the cross-section consists of several unconnected partial sections, the sum of the moments of inertia is calculated without the parallel axis theorem components. The cross-section shown in Figure 01 consists of two angle sections that are not connected to each other.
The individual angle sections have the following moments of inertia:
Iy,1,2 = 180.39 cm4 (referred to the centroidal axes y, z)
Iz,1,2 = 65.05 cm4 (referred to the centroidal axes y, z)
The moments of inertia of the entire cross-section result in:
Iy,1+2 = 2 ⋅ Iy,1,2 = 2 ⋅ 180.39 = 360.78 cm4 (referred to the centroidal axes y, z)
Iz,1+2 = 2 ⋅ Iz,1,2 = 2 ⋅ 65.05 = 130.11 cm4 (referred to the centroidal axes y, z)
If the cross-section consists of several connected partial sections, the sum of the moments of inertia is calculated with the parallel axis theorem components. The cross-section shown in Figure 02 consists of two connected angle sections.
The individual angle sections have the following cross-section properties:
A1,2 = 16.25 cm²
yS,0,1,2 = ±2.30 cm (referred to the zero point)
zS,0,1,2 = 3.07 cm (referred to the zero point)
Iy,1,2 = 180.39 cm4 (referred to the centroid axes y, z)
Iz,1,2 = 65.05 cm4 (referred to the centroid axes y, z)
The cross-section properties of the entire cross-section result in:
yS,0,1+2 = 0.00 cm (referred to the zero point)
zS,0,1+2 = 3.07 cm (referred to the zero point)
Iy,1+2 = 2 ⋅ Iy,1,2 + 2 ⋅ A1,2 ⋅ (zS,0,1,2 - zS,0,1+2)²
Iy,1+2 = 2 ⋅ 180.39 + 2 ⋅ 16.25 ⋅ (3.07 - 3.07)² = 360.78 cm4 (referred to the centroidal axes y, z)
Iz,1+2 = 2 ⋅ Iz,1,2 + 2 ⋅ A1,2 ⋅ (yS,0,1,2 - yS,0,1+2)²
Iz,1+2 = 2 ⋅ 65.05 + 2 ⋅ 16.25 ⋅ (2.30 - 0.00)² = 301.46 cm4 (referred to the centroidal axes y, z)
AnswerThe reason is that there is no stability analysis for unsymmetrical, open cross-sections according to EN 1999‑1‑1 if the compressive normal forces and bending moments are effective.You can neglect the bending moments in Details, the Stability tab, by selecting the corresponding filter. Then, the flexural buckling design is performed without moments. However, this is under your own responsibility. Another way is to check the stability according to the second-order analysis, which would be possible by using the RF‑/FE‑LTB add-on module.
AnswerIn the "c/t-Parts and Effective Cross-Section" tab of the "Calculation Parameters" dialog box, you can define the settings for the automatic creation of c/t-parts.
It is also possible to specify an angle from which a support should be created between two elements. In case that the angle for connection of elements is smaller, they are considered as an interconnected c/t-part. Stiffeners (longitudinal ribs, slopes (lips), or bulges, and so on) are not recognized by the program during the automatic generation of the c/t-parts. The c/t-parts have to be adjusted manually. You can make the changes in Table "1.7 Notional Flat Widths | EN 1993‑1‑3" or in the "Edit Notional Flat Width" dialog box.
The check box for Element is "significant" controls whether a curved element is considered as a c/t-part. If the length of the arc is larger than the diameter entered here, it cannot be neglected.
A corresponding error message appears before the calculation.
The option Element ist "straight" refers to curved elements. The arcs are usually excluded from the determination of the effective widths because the standards do not provide clear specifications. A curved element is assumed to be straight if the ratio of the connecting line (start/end node) to the element length is higher than the specified value.
AnswerIf you need to define different types of lateral intermediate supports, it is necessary to divide the specific member. After that, you can create a set of member and with that done, you can easily define different types of intermediate supports along this set of member, or you can use different nodal supports in the nodes of the set of member.
AnswerThis kind of result may occur if the limit internal forces of the cross-section cannot be determined. In most cases, the problem lies in a wrongly defined cross-section or in the selection of an unsupported cross-section. Please check if you have selected the cross-section allowed for the aluminum structure in the add-on module. These include the rolled cross-sections and parametric thin-walled cross-sections.The cross-section HK 120/40/5/5/5/5 shown in Figure 01 is not a valid cross-section as it has been selected from the area of solid cross-sections (concrete components).
In this case, it is necessary to change the cross-section to TO 120/40/5/5/5/5.In the case of the design in RF‑/ALUMINUM, please note that you have to select the material which also involves thicknesses used for the cross-sections. A material that is only allowed up to t=3 mm cannot be used for a cross-section with t=5 mm.
AnswerYou can usually set the standard and the National Annex in the top right corner of an add‑on module (see Figure 01). In most cases, it is also possible to display the factors of the National Annex and edit them, if necessary (see Figure 02).
For members (RSTAB) or members, surfaces, and solids (RFEM), you can display the material weight W. For this, simply select the entire structure or a part of it and right-click to open the shortcut menu. Here, you can find the "Center of Gravity and Info ..." option.However, the nodal masses are not apparent from this.To generate the nodal masses, you can use the RF‑/DYNAM Pro - Natural Vibrations add-on module. In this case, it is possible to create a natural vibration case where the mass only actsin the Z direction, for example. After the natural mode case has been calculated, the "nodal masses" are available in the result tables of RF‑/DYNAM Pro.In RF‑/DYNAM Pro, you can optionally convert the nodal loads into masses by applying the force components of the respective load case to the natural vibration case calculation.
AnswerThe cross-section class is defined according to EN 1993‑1‑1 and EN 1999‑1‑1 by the maximum width/width ratio c/t or b/t of the cross-section parts subjected to compression. EN 1993‑1‑1 or EN 1999‑1‑1 only cover various straight c/t or b/t parts. Therefore, the classification and determination of effective widths is not possible for the curved c/t or b/t sections.
AnswerBy default, the computation kernel of the cross-section program SHAPE‑THIN is used in the RF‑/ALUMINUM add‑on module to determine the stresses of the effective cross-section in an iterative procedure. This method is precise as all corners and edges of the cross-section are covered, but can be very time-consuming in the case of compound cross-sections.As an alternative, it is possible to determine the effective cross-section by using the simplified analytical method (see Figure 01), which is significantly faster. In the case of using this approach, the corners, roundings, and others, are neglected and then compensated by a factor. No iterative calculation is performed. Therefore, the effective cross-section values might be higher than with the SHAPE‑THIN calculation.In such a case, it is recommended to carry out the calculation using the analytical method and then to only design the governing structural component with the governing load combination by using the SHAPE‑THIN solution.
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Wind Simulation & Wind Load Generation
With the stand-alone program RWIND Simulation, wind flows around simple or complex structures can be simulated by means of a digital wind tunnel.
The generated wind loads acting on these objects can be imported to RFEM or RSTAB.
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