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• ### According to DIN EN 1995-1-1/NA, the crack factor kcr may be increased by 30% for softwood in areas at least 1.50 m from the end of the timber grain. How to implement it in RF-/TIMBER Pro?

The increase of the crack factor kcr still has to be done manually because the program does not know where the end of the grain is defined. To do this, divide the member by 1.5 m from the end of the grain so that the affected areas can be designed as a separate member (see Figure 01).

Two design cases are now required (File → New Case ...). In case 1, members within the 1.5 m are selected for the design. In case 2, it is necessary to select the members where the 30% needs to be considered. Then, in case 2, the kcr value is adjusted manually in the settings for the National Annex. Thus, a kcr of 0.65 results for C24, which is entered as shown in Figure 02. The design is carried out this way with an increased kcr value.
• ### I want to design a crack width of 0.1 mm in RF‑CONCRETE Surfaces. But when entering "0.10 mm" as the limit value, I get an error message saying that a value between 0.2 to 0.4 mm can only be designed.

Please deactivate the "indirect calculation" using the limit diameter "lim ds" or the member spacing "lim sl."

The calculation of crack widths smaller than 0.20 mm is only possible by using the direct calculation according to EN 1992‑1‑1, 7.3.4.
• ### Is it possible to define the damage parameter ζ manually for the calculation with RF‑CONCRETE Deflect? In such a way that it should always be at least 0.5?

No, this is unfortunately not possible.

If the "Tension Stiffening" option is not applied for the calculation, the damage parameter ζ is either "0" for "uncracked sections" or "1" for "cracked sections." For this, see the technical article about the distribution of the damage parameter ζ under Downloads below.

Nevertheless, the manual specification of ζ = 0.5, for example, is not possible.
• ### Which tensile strength of concrete is used for the transition from State I to State II? fctm or fct; 0.05?

The concrete tensile strength fct, eff, wk = fctm × crack width factor is applied.
• ### Why is the cracking moment Mcr smaller for biaxial bending than for uniaxial bending?

The cracking moment of a concrete cross-section is calculated from the mean tensile strength of the concrete and the ideal section modulus. The cracking moment describes the internal force that occurs when the tension stress fctm is reached in the outermost fiber of the cross-section and crack formation occurs.

For uniaxial bending, it is possible to calculate the cracking moment analytically. For biaxial bending, the introduction of a weight factor k is helpful to determine Mcr from the components Mcr,y and Mcr,z.

Calculation for the attached example:

Bending moment My = 20 kNm
Bending moment Mz = 20 kNm

Ideal section modulus Wy = 3,081 cm3
Ideal section modulus Wz = 3,081 cm3

Mean tensile strength of concrete fctm = 0.290 kN/cm²

Member 1: Uniaxial bending My:

$\begin{array}{l}M_{cr\;}=f_{ctm}\times W_y\\M_{cr\;}=0.29\;\frac{kN}{cm^2}\times3,081\;cm^3\\M_{cr\;}=893\;kNcm\;=\;8.9\;kNm\end{array}$

Member 2: Uniaxial bending Mz:

$\begin{array}{l}M_{cr\;}=f_{ctm}\times W_z\\M_{cr\;}=0.29\;\frac{kN}{cm^2}\times3,081\;cm^3\\M_{cr\;}=893\;kNcm\;=\;8.9\;kNm\end{array}$

Member 3: Biaxial bending My and Mz:

$\begin{array}{l}M_{cr\;}=\sqrt{M_{cr,y}^2+M_{cr,z}^2}\\M_{cr,y\;}=k\times My\\k=\frac{f_{ctm}}{\sigma_M}\\\sigma_M=\frac{M_y}{W_y}+\frac{M_z}{W_z}=\\\end{array}$

• ### In RF‑CONCRETE Surfaces, the crack depth is displayed in connection with the nonlinear calculation. What is behind this variable?

The crack depth is used to specify the height of the cross-section referring to damage, or, in other words, to a stiffness modification. The nonlinear calculation and the associated layer arrangement of the FE element (see Figure 02) is the basis for the results of this value.

The results of the crack depth are displayed by side and in both principal directions of the cracks. The direction is also able to relate to the trajectories ϕ,hw.
• ### When selecting the nonlinear method of check in the serviceability limit state design, there is the option to export the stiffness of the already cracked structure. Is it possible to recalculate the internal forces with these stiffnesses?

You can activate the export of nonlinear stiffness of a cracked structure in Settings for Nonlinear Calculation of the RF‑CONCRETE Members or RF‑CONCRETE Surfaces add-on modules. In the Edit Load Cases and Combinations dialog box, select "Extra options" for the selected combinations. A new tab appears where you can activate the transfer of stiffnesses from the add-on modules.
• ### How can I consider the partial safety factor γQ,T in the automatic load combinatorics with 1.0 in compliance with DIN EN 1992‑1‑1/NA: Chapter NCI 2.3.1.2 (3) or DIN EN 1990/NA Chapter NDP A.1.3.1 (4)? The value γQ,T = 1.5 is used by default.

To apply this in the program, it is necessary to copy the "Temperature" load case (see Figure 01). All loads contained in the load case will be copied as well. One of the load cases is used for the ultimate limit state (ULS) load combinations, the other one for the serviceability limit state (SLS). The loading in the load case for ULS is now multiplied by 1.0/1.5 = 0.667 (see Figure 02).

In order to ensure that both load cases do not occur simultaneously, but only for the specific design situation, the exceptions are determined in the respective combination expressions. For ULS, it is quite simple to define the dead load case in the way that it is not combined with the temperature load case for SLS (see Figure 03). SLS applies exactly the other way around, that is, the load case of dead load is not combined with the temperature for ULS (see Figure 04). In this case, the "Differently for each combination expression" option must be activated.

The desired load combinatorics is then obtained (see Figure 05).

• ### Why do I ge the error message 226 in RF&#8209;CONCRETE Surfaces? The crack width analysis is not performed or the design ratio is "0.00."

Message No. 226 appears in the result windows 3.1 to 3.3 in RF‑CONCRETE Surfaces if the concrete tensile stresses from the defined load for the surface to be designed are smaller than the concrete tensile strength.

In this case, the message 226 is displayed with the corresponding information.

By clicking the the [i] button, you can open the design details for the respective non-designable location and display the intermediate values for the determination of the present tensile stress.

• ### When starting the calculation in RF‑CONCRETE Surfaces, I get the error message "Failed to calculate the deflection for the result combination." Why? What can I do to solve this problem?

For the calculation of deformations in cracked sections (state II), the RF‑CONCRETE Deflect extension is available in the RF‑CONCRETE Surfaces add-on module.

RF‑CONCRETE Deflect requires an explicit load situation for the analytical calculation of deformations in cracked sections (state II), which is only given by using the load combinations (CO). Result combinations do not provide an explicit load situation, no matter if an additive or an enveloped OR combination. Therefore, when applying an RC for the calculation of deformations in RF‑CONCRETE Surfaces by using RF‑CONCRETE Deflect, you receive the mentioned error message. See Figure 01.

To avoid this problem, you can simply generate load combinations (CO) instead of result combinations (RC). If you still want to perform the ultimate limit state design with RCs, you can manually create a LC in addition to the existing RCs, for which you want to calculate the deformations by means of RF‑CONCRETE Deflect. See Figure 02.

It is important that for the calculation of the deformations, RF‑CONCRETE Surfaces applies the loads from a quasi-permanent design situation by default. See Figure 03. This means that the LC, for which the deformations are to be calculated, must be defined as "quasi-permanent." As an alternative, it is also possible to user-define the check boxes for the settings of design situations (see Figure 03).

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#### First Steps

We provide hints and tips to help you get started with the main programs RFEM and RSTAB.

#### 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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