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Frequently Asked Questions (FAQ)
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If one or more members with a circular cross-section have been dimensioned, A s, above and A s, are no longer indicated below: "Top" and "bottom" can not be defined for this cross-section; the reinforcement is inserted "peripherally".
Solution: Create a separate design case for the circular cross-sections (select the "File → Add New Case" module menu) and design the corresponding members separately. In the other design case, you delete the members with circular cross-sections. After the design, A s, top and A s are displayed at the bottom of the Results navigator.
AnswerIn most cases, these new elements only have to be added in the relevant add-on module. In Figure 1, you can see that the tick for "all" is not set and therefore new elements have to be added manually.
This is because the effective lengths or buckling lengths of members and sets of members differ. While the effective length is used for the stability analysis for members, RFEM takes the length of the summarized members for the set of members.
The frame shown in Figure 01 consists of a horizontal beam that is divided into four equally long members. In addition, a set of members is created for the four members. The stability analysis is carried out for both cases according to the equivalent member method.
For the design of members, the program calculates with a length of 1.00 m in each case. In contrast, the set of members has a length of 4.00 m (see Figure 02). This difference in length naturally affects the stability design, which means that the capacities are also different (see Figure 03).
In addition, it is not recommended to calculate all members and sets of members in a single design case because this leads to falsified results.
AnswerThe method according to EC 3 can only be used for the temperature curves from EN 1990-1-2. In addition, with the simplified calculation of the steel temperature in the EC 3-1-2, it is only possible to take into account increasing temperature profiles; a drop in the temperature is not available in this simplified calculation. However, it is also possible to use direct steel temperatures in the program (see Figure 1).
AnswerThe deduction of the precamber in the serviceability limit state design is actually not regulated in steel structures. However, in response to customer requests, we have adopted the corresponding add-on module according to the timber or concrete add-on module. However, according to the standard specification, this value should only be considered in the quasi-permanent design combination.This does not apply to the characteristic and frequent design situation. Thus, as soon as the design situation in the add-on module is adjusted accordingly (see figure), the applied precamber is also subtracted from the resulting deformation.
AnswerIf you want to use a user-defined material in the design modules, make sure that it has the same properties as a standard-specific material.Example: You want to design a user-defined material in RF- / Concrete according to the standard EN 1992-1-1. To create the material, proceed as follows: Select a material from the standard series EN 1992-1-1 in the material library and select "Create New Material ...". Here you can change all specified values. All values are required for a design in the add-on module.
The proof of stability for tensile structures can be converted into a pure stress analysis, if the theory is considered to be 2-fold and the imperfection required by the standard has been applied to the system.
With the help of the modules RF-STABIL and RF-IMP imperfection (resp. a preformed FE mesh). The type of imperfection depends heavily on the component and the standard used. For bars, which were modeled as a tensile structure, the values from DIN EN 1993-1-1: 2005 5.3 can be used. For flat surfaces, for example, the values from DIN EN 1993-1-5: 2006 Appendix C can be used. For trays, the problem is much more complex and there are different approaches. From a generation of imperfections I would advise against this and perform the buckle proof by means of MNA / LBA concept according to DIN EN 1993-1-6, which does not require an approach of imperfection.
If, for example, the surface model of a steel girder is to be detected, you can proceed as follows, for example:
First A burden me comparatively (compared to. other internal forces in the load case) select high normal forces, in most cases the self-weight load case or a load case combination with the corresponding own weight is suitable. It may be necessary to provide each load combination with an individual imperfection.
2. Calculate load combination according to 1-order theory and use as the basis for RF-STABIL
3. Using RF-STABILITY to find the first eigenstate of a global failure
4. Using RF-IMP, use the calculated eigenmode as the basis for an imperfection. In this case, for example, 1/300 of the carrier length can be used as the amplitude.
5. Create a load case combination that uses the generated imperfection as a basis and is calculated according to 2-order theory.
6. Perform a proof of tension on the basis of this load case combination, which at the same time is also proof of stability of the structure.
In the add-on module there are options for the reinforcement that has to be considered for the shear force design. For the shear force resistance V Rd, c , the provided longitudinal reinforcement has an influence. With the options, you can define the longitudinal reinforcement with which the shear design check should be performed. The shear design can be performed by applying the following reinforcement:
- Calculated required reinforcement
- Maximum value from required or provided reinforcement
- Automatically increase the longitudinal reinforcement so that shear reinforcement is avoided
For example, if the second option is selected and a shear reinforcement can be avoided with the basic reinforcement, the basic reinforcement is selected and output as required.
For the stability design of compression elements, you need the combination of RF-CONCRETE Members and RF-CONCRETE NL. The reason is the following:
First, the internal forces of the individual load combinations (second-order analysis + imperfection) are subjected to the linear-elastic calculation. For this, you basically only need RFEM.
Then, the cross-section design is performed in RF-CONCRETE Members with these internal forces determined linearly-elastically, and the required bending reinforcement is determined from these internal forces.
This bending reinforcement is then compared with the user-defined entries concerning the existing basic reinforcement or the minimum reinforcement and based on this, the reinforcement concept is generated (dialog box '3.1 Existing Longitudinal Reinforcement' of the module).
This existing longitudinal reinforcement is then used for the nonlinear design.
According to Section 5.8.6 (1), geometric nonlinearities must be taken into account according to the second-order analysis. However, the general rules for nonlinear methods according to 5.7 also apply.
In Sec. 5.7(1), 'an adequate non-linear behaviour for materials is assumed'. According to 5.7(4)P, the use of material characteristics which represent the stiffness in a realistic way but take account of the uncertainties of failure shall be used when using non-linear analysis.
This requires the RF-CONCRETE NL add-on module. Thus, the geometric and material nonlinearities are considered and the requirements of EC 2 regarding the ultimate limit state design are fulfilled.
Similarly, this method is also available in RSTAB in the CONCRETE add-on module.
Even simple structures such as continuous beams or 2D frames can be quickly and effectively modelled, calculated and designed according to various standards in RFEM and RSTAB. For such models, several model generators are particularly recommended (see figures).
After entering the cross-section parameters, loads and geometry details, the model generator automatically creates a model including load cases (see video file).
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