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Frequently Asked Questions (FAQ)
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The first natural frequency is required to determine the structure coefficient. It is not determined by using a generalized formula, but the integrated eigenvalue solver RF‑/DYNAM, taking into account the real mass distribution and displaying the results in Column A of Table 2.3.
In RFEM and RSTAB, the simplified design from , Chapter 2.2.3, have been implemented for the automatic load combinations. This means that, strictly speaking, the structures concerning the final deformation may only be analyzed, in which the materials with identical creep behavior occur since the creep deformations are considered in a simplified way on the load side. If the structure is a combined structure made of timber with different creep behavior or in combination with steel, the final deformations must be determined according to  Amendment to 2.2.3 as follows:
"(4) If the structure consists of members or components having different creep behaviour, the long-term deformation due to the quasi-permanent combination of actions should be calculated using final mean values of the appropriate moduli of elasticity, shear moduli and slip moduli, according to 22.214.171.124(1). The final deformation ufin is then calculated by the superposition of the instantaneous deformation due to the difference of the characteristic and quasi-permanent combinations of actions with the long-term deformation."
However, this requires the superposition of the results from different load combinations, which cannot be implemented automatically in RFEM and RSTAB. If the different creep behavior should be taken into account, it is necessary to create the load combinations manually, and reduce the stiffness according to the creep coefficient. The procedure is described on an example of a timber-concrete composite floor presented on the Info Day 2017. Below this FAQ, you can find the link for this video.
AnswerThe internal pressure coefficients do not need to be considered with in the RWIND Simulation program.RWIND Simulation always outputs the net pressure on the surfaces in RFEM. When it comes to a simulation with a building that has open windows in RWIND Simulation, there is an internal pressure acting on the inside of the building. The program uses this information to determine the resulting pressure based on the external and internal surfaces. This can be seen in Figure 1.A comparison cannot be made between this coefficient in the standard and a CFD calculation because there is no direct correlation.
RWIND Simulation is a program for numerical simulations of wind flow. It is a tool for describing a problem of wind flow around an object, consisting of a system of partial differential equations with the output of an approximate solution on the basis of the finite volume method. Similarly to a simplified model in a real wind tunnel, such a mathematical model provides the information about the wind velocity field and the pressures acting on the surfaces of the wind flow object.
The standards describing the wind effects on buildings and structural components (for example, EN 1991‑1‑4, ASCE/SEI 7, and others) are based on different principles. The rules and standards specify the corresponding instructions for determining wind loads for specific situations and application cases. These formulas are undoubtedly correct for the assigned situations and the resulting values have been confirmed in practice.
However, these guidelines do not describe all situations that occur in the real world of engineers. Here, infinitely different model shapes in the wind flow are processed, whereby each shape has its own high-grade influence on the resulting surface pressures caused by the wind load. However, for all model shapes not mentioned in the respective directives, the equivalent load from wind effects remains unclear.
RWIND Simulation can help here as an auxiliary tool for determining the resulting forces from the wind action. However, despite the use of RWIND Simulation, all requirements of the valid engineering standards must be met. The technology used in the simulation program may also provide further useful insights for sufficiently controlled object shapes.
Figure 01 - Wind Flow Around Complex Antenna System
Figure 02 - Surface Pressures due to Wind Load on Complex Antenna System
For surface supports, this option is only available if there is also the nonlinearity defined in the local z direction (failure if contact stress in z is negative/positive). In the dialog box where you can edit the nonlinearity, you can find the "Friction in plane xy" option.
This option works as shown in the graphic dialog box: The support in the x and y direction is fully accepted only when reaching the contact stress Tau (contact stress Sigma multiplied by the friction coefficient). It is necessary to reduce this linearly in advance.
In order to use this option, a support must be in the horizontal directions. It can be defined as fixed or with an elastic spring. If the spring is defined with 0, no support is considered even though a friction coefficient has been entered.
AnswerIn Table 1.3 Surfaces, you can specify the parameters for the automatic determination of the creep coefficient and shrinkage value in the corresponding tabs. It is also possible to enter user-defined values, if necessary.
In both RFEM and RSTAB, it is possible to consider friction for the translation at nodal support.
You can select from different options that take into account the different directional components. For example, the friction force for the z‑direction can only be calculated from the y‑component or just from the x‑component, but also from both together or even from the addition of both forces.
In addition to the friction coefficient, you can also define the spring constant. It determines the behavior of the support before reaching the maximum friction force or the transition from static friction to sliding friction. The higher is the value of the spring constant, the less the support can deform before it changes to the sliding friction.
AnswerIn the result tables of the add-on module, you can find all intermediate values for each wind direction, face and sector. This feature is also available in the long version of the printout report, though it can significantly increase the size of the report.
AnswerFor duopitch roofs with a roof inclination of > 5°, the roof areas F, G, H, I and J have to be separately classified according to the windward and leeward side. For the wind direction of 0° (wind in longitudinal direction), positive as well as negative aerodynamic coefficients have to be taken into account for roof inclinations of up to 45°.For these cases, this results in a total of 4 possible wind combinations, depending on the building side (see Figure 1).For the wind direction of 90° (wind on gable side), however, there are no positive external pressure coefficients for a roof inclination of > 15°. For a building with a roof inclination of 45° you would receive 10 possible wind load cases (0° = 4*2, 90° = 1*2).LC w+:Only positive (pressure) aerodynamic coefficients per roof area are used.LC w-:Only negative (suction) aerodynamic coefficients per roof area are used.LC w-/+:Negative (suction) aerodynamic coefficients for the windward side and positive (pressure) coefficients for the leeward side of the roof are used.LC w+/-:Positive (pressure) aerodynamic coefficients for the windward side and negative (suction) coefficients for the leeward side of the roof are used.If there are, for example, only negative coefficients for a load position, then only negative loads are applied to the roof surface. Consequently, there is no pressure -> therefore these values are set to 0. A load case, which thus contains only values with the size 0, could also be deselected during the generation.For example, this is always possible, as already described, for the LC w + with a wind direction of 90° (gable-sided wind) and a roof inclination of > 15°.
AnswerFor example, the standard EN 1990 + EN1991-2; Road bridges could be used (see picture 1). Alternatively, one could of course also determine the combinations by hand. Further information can be found i.a. in our manual.
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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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