# 常见问题和解答 (FAQ)

• ### Which explicit method is used in the RF-DYNAM Pro - Nonlinear Time History add-on module?

FAQ 002376 ZH 附加模块 RF-DYNAM Pro - Nonlinear Time History

#### 回答

The RF-DYNAM Pro - Nonlinear Time History offers, in addition to the implicit NEWMARK method of mean acceleration, also an explicit method. In the manual of this add-on module, it is mentioned that this is a solver which uses the central difference method.

It should be noted that not the "original" version of the central difference method is used here, but a modified form. The modified form is characterized by the fact that it is simply not a central difference when applying the speed difference. The following two equations show the applied speed and acceleration differences.

Speed: (no central difference)
${\dot{\mathrm x}}_{\mathrm n+\frac12}=\frac{{\mathrm x}_{\mathrm n+1}-{\mathrm x}_\mathrm n}{{\mathrm{Δt}}_{\mathrm n+{\displaystyle\frac12}}}$

Acceleration: (central difference)
${\ddot{\mathrm x}}_\mathrm n=\frac{{\dot{\mathrm x}}_{\mathrm n+{\displaystyle\frac12}}-{\dot{\mathrm x}}_{\mathrm n-\frac12}}{{\mathrm{Δt}}_\mathrm n}$

This approach leads to a faster convergence since it responds "faster" to changes in loading or structure (nonlinearities).
• ### For wall ends and wall corners, no soil contact stresses are deducted in RF-PUNCH pro, even though the type "Foundation" has been set. It is also nowhere possible to define that the soil contact stresses should be deducted. Why does this happen?

FAQ 002375 EN 附加模块 RF-PUNCH Pro RFEM 2D RFEM

#### 回答

If you perform a punching shear design for a wall corner or a wall end at a floor slab or foundation plate, this entry is not necessary.

Since the punching load is determined at a wall corner or a wall end from the shear forces in the control perimeter when performing the punching shear design, the ratio of the surface load is missing anyway within the control perimeter (up to the load application). Therefore, this entry is also not necessary in the 1.5 dialog box.

If you, however, perform the punching shear design for a single column at a foundation plate with the punching load from the normal force of this column, you can enter the "Deductible Surface Load" at the end of the dialog box "1.5 Nodes of Punching Shear - Node of Punching Shear - Details" which allows to determine ΔVEd.
• ### I have modelled a precast slab as "orthotropic surface". Can I use these surfaces also in reinforced concrete design?

FAQ 002374 EN 附加模块 RF-CONCRETE RF-CONCRETE NL

#### 回答

Yes, you can use "orthotropic surfaces" for the reinforced concrete design in RF-CONCRETE Surfaces.

If you only would like to carry out serviceability limit state designs, you do not have to define additional settings in RF-CONCRETE Surfaces concerning the surface type.

You need the RF-CONCRETE NL add-on module to perform the serviceability limit state designs. Select in the "serviceability limit state" tab (in the dialog box "1.1 General Data") the design method "Nonlinear...". See Figure 01.

You can then change the surface type to "Constant" in the dialog box "1.3 Surfaces" so that the serviceability limit state design is also possible here. See Figure 02.
• ### How can I enter a one-way continuous slab in RFEM?

FAQ 002373 EN 建模 | 结构 RFEM

#### 回答

It is possible to simulate one-way continuous slabs (e.g. precast slabs) by using the "orthotropy" of the surfaces.

You can set the stiffness type "Orthotropic" in the "Edit Surface" dialog box separately for each surface. See Figure 01.

In the "Edit Surface Stiffness - Orthotropic" dialog box, you can define the orthotropy type and select, for example "Effective thicknesses". See Figure 02.

You can then define the effective thickness in the local x and local y-direction of the corresponding surface in the "Effective thickness" tab. Moreover, you also have the possibility in this dialog box to define an effective thickness for the self-weight. See Figure 03.
• ### Is it possible to perform the structural analysis and design of containers with Dlubal Software?

FAQ 002372 EN 使用功能 RFEM

#### 回答

With the FEA Software RFEM, it is possible to perform the structural analysis and design of containers (shipping containers, office containers, mobile homes, etc.).

RFEM offers member, surface and solid elements for detailed modeling. All components can be designed in the corresponding add-on modules.

• ### Why are wind and snow load generator greyed out in my RFEM/RSTAB file?

FAQ 002371 EN 建模 | 荷载 RFEM RSTAB

#### 回答

The prerequisite for using both generators for wind and snow load are that you are working with the 3D model type because when working with a 2D model type, the program does not have the corresponding geomtrical parameters.
The model type has to be set in the "Edit Model - General Data" dialog box. The dialog is opened by right-clicking "Model Data", "General Data" in the Project Navigator - Data. The modeling may have to be adapted.
• ### Which resources are available to perform the pushover analysis?

FAQ 002258 ZH 附加模块 RF-DYNAM Pro RF-DYNAM Pro - Equivalent Loads RFEM

#### 回答

In RFEM, it is possible to calculate a pushover curve (also called capacity curve). The pushover curve data can be exported to Excel. Important steps in order to evaluate this nonlinear curve are listed below:

1. Comprehensive definition of nonlinear hinges
1. Plastic hinges in accordance to FEMA 356: This is a nonlinear hinge (elastic‑plastic or rigid‑plastic) with default values for yielding points in the hinge diagram and acceptance criteria, both for steel members (Chapter 5 in FEMA 356). The yielding limits of the members are dependent on the cross‑sections and are set automatically. For moment hinges, the diagram parameters and acceptance criteria are interpolated for different type of cross‑sections. You can adjust all the values of a 'FEMA' hinge using your own user‑defined values. Figure 1 shows the Plastic Hinge dialog box.

Plastic hinges in accordance to EN 1998‑3: This is a nonlinear hinge with a bilinear definition. The bilinear hinges include predefined values for the yielding points in the hinge diagram, acceptance criteria, and yielding limits of the cross‑sections. These values can also be adjusted manually.

The plastic hinges are displayed with various colors when viewing the results of several load steps. The colors depend on the state of plasticity. This is very useful to identify exceeded acceptance criteria.

2. Instead of using hinges, you can use the 'Plastic Hinge' type of member nonlinearity in RFEM. In the relevant dialog box, you can define perfectly plastic behavior and set the yielding limits manually. The main advantage of this option is that the location of the plastic hinge is found automatically during the iterative calculation. See Figure 2 for further explanation.

2. Definition of load pattern for nonlinear analysis

To achieve load distribution in accordance to a mode shape of your structure you can use RF‑DYNAM Pro - Equivalent Loads. This add‑on module calculates eigenvalues, mode shapes and equivalent loads by using the multi‑modal response spectrum analysis. For each selected eigenvalue, the program automatically exports the equivalent loads into a load case in RFEM.

In the calculation parameters of load cases, you can define an incrementally increasing load. The results of each load step can then be analyzed. When using 'Plastic Hinges', you can easily evaluate the plasticity states due to the color marking of the hinges. It is important to scale the exported equivalent loads from RF‑DYNAM Pro to avoid load increments in too large steps.

Figure 3 shows a load case exported from RF‑DYNAM Pro and the recommended calculation parameters.

4. Calculation diagrams for creating pushover curve

These diagrams are available in the 'Global Calculation Parameters' dialog box. Here you can set the sum of support forces on the vertical axis and the deformation in a roof plane on the horizontal axis and thus obtain the required pushover curve. The data can easily be exported to Excel. The pushover curve is displayed in Figure 4.

Figure 5 shows the color representation of the plastic hinges. The color scale can display the hinge diagram parameters or the acceptance criteria.

The continuing pushover analysis (determination of the inelastic spectrum and performance point) can further be carried out in an external program (for example Excel).

• ### My structure is unstable. What can be the reason?

FAQ 002257 ZH 建模 | 结构

#### 回答

There can be different reasons for an unsuccessful calculation due to an instable. On the one hand, this can indicate a “real” instability due to an overloading of the system. On the other hand, the error message can result from inaccuracies in the model. Below you find a possible procedure for discovering the reason for this instability.

First of all, you should check if there are errors in the model. For example, you can calculate the structure only with its self-weight in a load case according to the linear static analysis. If results are displayed afterwards, the structure is stable concerning the model. If this is not the case, the most common cases are the following (see Video 1):

- Supports are missing or have been defined incorrectly
- Members are twisted about their own axis (torsional releases are defined at both member ends)
- Members are not connected with each other (Tools --> Model Check)
- Nodes seem to be in the same place, at a closer look they deviate minimally from each other (common cause at CAD import, Tools --> Model Check)
- Member end releases/line hinges cause a "chain of releases"
- Stiffening of the structure is not sufficient
- Failure of nonlinear structural elements (for example tension members)

Figure 2 shows the latter point. You can see a hinged frame which is stiffened by tension members. Due to the column contractions as a result of the vertical loads the tension members receive minor compressive forces during the first calculation. They are removed from the structure (because only tension can be absorbed). During the second calculation, the model is then unstable without these tension members. There are several ways to solve this problem. You can assign a prestress (member load) to the tension members to "eliminate" the minor compressive forces, allocate a little stiffness to the members (see Figure 2) or have removed the members successively during the calculation (see Figure 2).

The RF-STABILITY add-on module (RFEM) may be useful if you want to display graphically the reason for instability. Use the "Calculate eigenvector for unstable model ..." option (see Figure 3) to calculate supposedly unstable structures. On the basis of the structural data, an eigenvalue analysis is performed so that the instability of the structural component in question is displayed graphically.

First of all, the load of the concerned load combination should be reduced until the load combination becomes stable. The load factor in the calculation parameters of the load combination is a useful tool here (see Video 2). This also corresponds to manually determining the critical load factor if the RSBUCK and RF-STABILITY add-on module is not available. Based on this load combination, you can then calculate the buckling modes in the RSBUCK and RF-STABILITY add-on module and display the results graphically. By displaying the results, you can detect the "weak point" in the structure and then optimize it systematically.

Attachments
Video 1-en.wmv (16.60 MB)
Video 2-en.wmv (18.98 MB)
• ### Where can I find examples and help files for the COM interface?

FAQ 002230 ZH 附加模块

#### 回答

For one thing, there is a zip archive with examples and help files available for download on our product page of RF-COM, which can be found at:

The archive can be found on the left in the box labeled "help files and example macros" by clicking "Download".

In the newer versions of RFEM, there is also an SDK folder in the "Dlubal" folder. It is located under "C:\Users\Public\Documents\Dlubal" in the standard installation. It contains additional and newer help files.

• ### How can I transfer a softlock to another Computer?

FAQ 002229 ZH 授权 & 许可

#### 回答

In this case, use the RUS tool for transferring the software license.

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