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Frequently Asked Questions (FAQ)
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AnswerBoth DLT and NLT are considered to behave more like single-axis beam elements stressed primarily in bending. Although realistically there is a small amount of stiffness perpendicular to the span direction, this is neglected in beam design. This is in contrast to cross-laminated timber (CLT) which includes two-way stiffness in both the directions parallel and perpendicular to the span direction. Therefore, RF-LAMINATE is not the suitable add-on module to NLT or DLT design but rather used for CLT design.The best approach in RFEM is to model a standard stiffness surface with modified orthotropic elastic 2D material properties. For the material model details, the modulus of elasticity in the local y-axis direction (Ey) should be set to a very small number (i.e. 0.001). The shear modulus in the local yz-plane (Gyz) should also be set to this small value. This will replicate little to no stiffness in the direction perpendicular to the DLT or NLT span direction.The surface can then be applied in the RFEM model for the full analysis. Note, design is not possible for these elements directly in RFEM. Internal forces, stresses, deflections, etc. from the RFEM analysis can be exported to alternative programs (i.e. Excel) for further design according to the various standards.
Because a net requires multiple cable segments to be modeled in a grid-type pattern, the cables are not one continuous member. This is not ideal for the RF-FORM-FINDING module as the cable prestress properties such as sag, target length, or force are best applied to a single cable segment. The program cannot easily apply these settings to a cable that is broken down into multiple segments along its length.
The best option to model the net in RFEM is to instead use a surface element. The material and stiffness properties of the surface are the equivalent properties to that of the net provided by the manufacturer. RF-FORM-FINDING can then be applied to the surface element such as a prestress force or stress.
A taper describes a member or a surface with a variable cross-section. The cross-section type must be consistent, for example, I-shaped cross-sections at both member ends.
In our example, we have a member with a PRO cross-section and a QRO cross-section.
To create a tapered member here, you should use a parametric cross-section for the member start and end:
This allows you to calculate the tapered member.
Temperature-dependent stress-strain properties of an elastic isotropic material can be defined in a diagram or imported from [Excel]. These properties are considered for member and surface elements subjected to thermal load (changes or differences in temperature).
The Reference temperature defines stiffnesses for the members or surfaces that have no temperature loads. For example, if a reference temperature of 300 °C is set, the reduced elastic modulus of this point of the temperature curve is applied to all members and surfaces.
The Options dialog section allows you to control if the Poisson's ratios that are applied to the complete temperature diagram are identical. Clear the check box to access the Poisson's Ratio table column for individual entries.
You have to select and define the Temperature/Modulus diagram by your own.
AnswerThere are two options for defining the failure:
- Assignment of member nonlinearity
For the member types "Beam" and "Rigid", you can define a member nonlinearity for each member. You can find the corresponding option in the "Settings" tab (see Figure 01).
- Assignment of nonlinear member hinges
Alternatively, you can define a member end hinge with failure criterion for the member. For the desired degree of freedom, you can assign the hinge condition with nonlinearity accordingly (see Figure 02).
- Assignment of member nonlinearity
Line supports are only intended for the lines that belong to a surface. For the support of members along their line, it is necessary to define member foundations (also applies to RSTAB), see the figure.
In this case, the following options are available:
1) Corresponding division of the beam (right-click Member → Divide Member) and set the parameterization in the way that there is only one central hole for one of the beams, see Figure 01.
2) Generating surfaces from the member (right-click Member → Generate Surfaces from Member), inserting a circular opening, defining a result beam, see Figure 02.
The load within this area is not taken into account, which can be clearly seen in the example in Figure 01. Here, a homogeneous temperature load (Tc only) was applied, which leads to stress peaks, although the surface can freely strain.
If it is important to consider the load, you can also model the support manually. To do this, simply create an opening of the size of the column and create a new surface corresponding to the surrounding surface (see Figure 02). The values for the spring stiffnesses can be found in the dialog box (see Figure 03).
A surface loaded in this way is then also without stress in the case of a temperature load, as shown in Figure 04.
The cross-sections of members are always related to the centroidal axis in RFEM/RSTAB. The rendering is adjusted accordingly. This has no influence on the calculation.
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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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