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Frequently Asked Questions (FAQ)
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AnswerRF-Glass distinguishes between two different types of calculation. On the one hand, the so-called 2D calculation. The glass structure is represented as a surface element. Considering the shear coupling, a substitute cross-section is determined by laminates theory. In contrast to this, the 3D calculation is available. In this case, the layer structure is represented as a solid element in the calculation and thus the stiffness effectiveness between foil and glass is precisely determined when considering the shear composite.More information about the calculation method is also described in the manual of RF-Glas, Chapter 2.
The stiffness modifications can be controlled separately for the following elements:
The first option "Materials" is only activated for load combinations by default (see Figure 02) because the second-order analysis is preset for this. When the function is activated, the stiffness of all elements is reduced by the partial safety factor of the material (see Figure 03). This is especially important in timber construction in Europe. If the automatic load combination was selected for the standard EN 1990 + EN 1995, SIA 260 + SIA 265 or DIN 1055-100 + DIN 18008, the result is different default settings. If the partial safety factor of the material is defined as 1.0, it does not matter if the function is activated or not.
Use this option to control the multiplication factors for individual cross-sections. In the "Modify" tab of the "Edit Cross-Section" dialog box, you can adjust the moments of inertia as well as surfaces of the cross-section. This affects the stiffness of the cross-sections.
When editing a member, the "Modify Stiffness" dialog tab is available. There are different definition types (see Figure 05). The "Multiplier factors" option allows you to modify the stiffnesses of individual members in analogy to the cross-sections.
For surfaces of the "Standard" and "Without Tension" type, you can adjust the stiffness of the surface in the "Modify Stiffness" tab of the "Edit Surface" dialog box. There, you can modify the elements of the stiffness matrix with a factor (as with orthotropic surfaces).
Further possibilities of stiffness modification
In addition, you can select another option in the Calculation Parameters to specifically adjust stiffnesses for other elements (see Figure 07). When you select the "Modify stiffnesses" option, a new tab opens (see Figure 08). In addition to the member and surface stiffnesses, it is also possible to adjust the stiffnesses of supports and hinges individually.
Interaction of individual factors
If several factors have been defined for an element (for example, cross-section and member), they are multiplied by each other. The result for the example shown in Figure 09 is as follows:
Global control of stiffness modification
In the global calculation parameters (see Figure 10), you can deactivate all options mentioned above at once. The local settings in the calculation parameters of the load cases or of the load combinations are ignored.
AnswerIn this case, there is the member type called 'Stiffnesses', and the member stiffnesses can be specified directly in a dialog box that you open with the [Edit] button. Thus, the assignment of a cross-section is unnecessary.
AnswerThese factors reduce the torsional stiffness D33 as well as the shear stiffness D88 of the corresponding stiffness matrix elements of a surface. Since cross-laminated timber is generally not laminated at the narrow side, it is also not possible to transfer shear stresses to narrow sides of a timber. Thus, the stiffness would be overvalued in this case. For this reason, the stiffness must be reduced accordingly.Some manufacturers have already informed us about these values when transmitting the layer structures. These result from internal analysis. An explanation for determining the correction factors is presented in . The analysis of this work has also been involved in the Austrian Annex to EN 1995-1-1  . The result is shown in Figure 02. The ratio of board width (a) to board thickness (ti) can be taken from the corresponding authorization.
AnswerTo only consider the stiffness modification (see Figure 01) for certain load combinations, it is necessary to deactivate this option for the corresponding load combinations in the Calculation Parameters (see Figure 02).
RSTAB is a FEM program that uses trigonometric trial functions for the members. For this reason, members do not have to be subdivided for sufficiently accurate results and the calculation speed is correspondingly higher.
RSBUCK determines the eigenvalues of the stiffness matrix and can thus linearly calculate the critical load and buckling mode of the structure.
AnswerMembers with the member type 'Spring' or 'Dashpot' have only axial stiffness, that is, in the direction of the local x-direction. The bending stiffness and the stiffness perpendicular to the longitudinal member axis are negligibly small and are not considered in RFEM and RSTAB.If a spring should be applied in the structure, it acts only in one direction; in the other two directions (local y- and z-direction), this member type must receive additional support, otherwise, the system becomes unstable.
AnswerThe RF-/JOINTS add-on module is divided into several joint groups. For this, see the following FAQ.Therefore, there is no straightforward answer to the special aspects of the design as in this FAQ.In contrast to the RF‑/TIMBER Pro add-on module described in the mentioned FAQ, however, it is obvious that the RF‑/JOINTS add-on modules cannot design EC2, even if manually changing the LDC, for example in the RF‑/JOINTS Timber - Steel to Timber add-on module (see Figure 01).Furthermore, this also applies to the add‑on modules RF‑GLASS and RF‑/CONCRETE NL.The reason for this is that there are stiffnesses exported in the program in the case of some joint groups of the RF‑/JOINTS add‑on module. For nonlinear calculations, the superposition with result combinations is not allowed. In the case of the second result combination mentioned above, there is the special feature that the superposition is no longer conservative, even in the case of simple structures. The design cannot be also performed correctly by manually changing the LDC.Nevertheless, if a result combination should be superimposed with constant and alternative additive, it is necessary to split EC2 in the attached file into load combinations as follows.
- RC2*=CO1 or CO2
AnswerThe reduction of the shear stiffness results from the fact that the fibers have a very small stiffness and a low strength perpendicular to each other. For this reason, the rolling shear strength of CLT panels is also very small.For cross-laminated timber panels, glued sidewalls on boards are assumed. The boards are applied in the longitudinal direction along the x-axis of the coordinate system (Figure 2). For the shear stresses in Figure 2, the shear stress τyz is thus juxtaposed to the rolling shear strength.The same applies to the shear stiffnesses. In the direction of the minor axis (yz plane), the stiffness of the individual boards is considerably smaller than in the direction of the principal axis (xz plane) and also larger than in the xy plane. The coordinate system shown in Figure 2 should be placed on the board in Figure 3.
The stiffness matrices of orthotropic surfaces are saved in the MatrixConstants.cfg file for the user-defined matrices in the folder C:\ProgramData\Dlubal\Global\Master Data. The folder requires a standard installation!
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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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