3.1.2.1 Analytical detection method

Analytical method of check

The Analytical method is preset for the check. This method uses the equations given by the standards for reinforced concrete. This method is described in chapter 2.6 of the manual.

Click the button to open a dialog box for checking and, if necessary, adjusting the design parameters.

This dialog section allows you to control, which strain ratio of the reinforcement directions is applied for the serviceability limit state design.

With By assuming an identical deformation ratio of the longitudinal reinforcement, the program assumes the same strain ratio of the provided reinforcement: All rebars are subjected to the same strain in the individual reinforcement directions. This approach is a fast and exact procedure. The selection of the compression strut inclination plays a significant role in this. This method is based on a purely geometrical division (see chapter 2.6). It is applicable if the provided reinforcement more or less corresponds to the required reinforcement.

The By classifying the structural element as a plate or wall option provides a simplified solution that you can use for a non-rotated, orthogonal reinforcement mesh: For each design point, the program checks if the tensile stresses from axial forces or bending moments do not exceed a certain stress. The limit value of the stress is defined in the Classification Criterion section. It is used to control whether the surface is designed as a plate (axial forces are set to zero) or a wall (moments are set to zero).

By neglecting small internal force components, it is possible to use the flowchart from ENV 1992-1-1, Annex A 2.8 or 2.9. The design internal forces correspond to the values shown in RFEM Table 4.16 (see RFEM manual, chapter 8.16).

Should the classification criterion for a design point of the surface not be fulfilled, an error message will appear during the calculation.

The By taking into account the deformation ratio of the longitudinal reinforcement option is only enabled for 2D model types (see Figure 2.1). This method considers the effective strain ratios due to the selected reinforcement and takes them into account for the serviceability limit state design.

In this dialog section, you can specify whether to analyze stresses and/or cracks in the design. You must select at least one of the check boxes.

If you select Cracks, it is possible to check the minimum reinforcements a_s,min, the limit diameters lim d_s, the maximum crack spacings max s_l, and the crack widths w_k. The settings for the individual checks can be specified in window 1.3 Surfaces (see chapter 3.3).

The analysis of the stresses can be differentiated with regards to the Concrete stresses σ_c and Steel stresses σ_s.

Furthermore, it is possible to calculate the Deflection with RF-CONCRETE Deflect considering creeping, shrinkage, and tension stiffening (see chapter 2.7). To do so, you need a license of the add-on module RF-CONCRETE Deflect.

The Increase the required longitudinal reinforcement automatically check box allows you to control if the longitudinal reinforcement is to be dimensioned to fulfill the serviceability limit state designs. If this check box is clear, only the specifications entered in the Longitudinal Reinforcement tab of window 1.4 (see chapter 3.4.3) are used: basic reinforcement, required reinforcement from ultimate limit state design, or basic reinforcement with provided additional reinforcement.

The dimensioning of the reinforcement for the serviceability limit state design is determined by increasing the reinforcement iteratively. As the initial value for the iteration, the program uses the required ULS reinforcement to resist the given characteristic load. The dimensioning of the reinforcement has no result if the rebar spacing s_l of the applied reinforcement reaches the rebar diameter d_sl. In this case, the results windows indicate that the respective point cannot be designed.

In the design according to EN 1992-1-1, it is possible to Find the most economical reinforcement for crack width design. Click the [Info] button to display information about this option (see Figure 3.11). The Information dialog box describes when the check of crack width can be considered as being fulfilled. Moreover, clause 7.2 of EN 1992-1-1 describes the conditions under which the stresses should be limited.

This means that not all design ratios shown in window 3.1 have to be less than 1 in order for the serviceability limit state design to be fulfilled!

The dimensioning of the reinforcement with regards to the concrete and steel stress, the limit diameter, and the maximum bar spacing is done separately for each reinforcement direction. However, if the resulting crack width w_k,res is governing to fulfill the crack width check, the reinforcement amount is increased equally for each direction.

Check criteria that do not have to be fulfilled for economical reasons are indicated by message 236) in the result windows of the serviceability check: "The check of the reinforcing layers does not need to be fulfilled for economical reasons". The governing check of crack width for the most economical reinforcement is marked with message 235): "The check restricts increase of reinforcement for economical reasons". This message applies to the designs for lim d_s, lim s_l, and w_k, but not to a_s,min.

If the Find the most economical reinforcement for crack width design check box is selected, you cannot specify a user-defined additional reinforcement for the SLS design in the Longitudinal Reinforcement tab of window 1.4 Reinforcement.

This dialog section (see Figure 3.10) is only available for 3D model types. The check boxes allow you to control if small normal forces and/or bending moments may be neglected in order to design surfaces as pure plates (upper box selected) or walls (lower box selected) in an idealized way. As the limit value, the mean value of the axial tensile strength f_ctm is respectively preset with 2.9 N/mm² of concrete C30/37: It is assumed that the tensile strength of concrete compensates for a crack formation due to minor tensile stresses, which is why they can be neglected.

If you have selected the classification as a plate or wall in the Method dialog section, you have to select at least one of the two check boxes.