RFEM – Online Manual

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RFEM – Online Manual

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6.14 Imperfections

General description

There are two ways how imperfections can be determined in RFEM:

    • Equivalent loads are applied to members.
    • A pre-deformed equivalent model is used.

This chapter describes imperfections in the form of equivalent loads. For detailed information on how to generate equivalent models using the RF-IMP add-on module, see Chapter 7.3.1.3.

To apply an imperfection, a member must already be defined.

Imperfections represent manufacturing deviations in model geometry and material properties. In EN 1993-1-1, clause 5.3, the application of imperfections is organized as precamber (deflection) and inclination (sway). Thus, imperfections are taken into account by equivalent loads.

Figure 6.52 Equivalent loads according to EN 1993-1-1

Equivalent loads are also taken into account by RFEM when calculations are performed according to the linear static analysis. Please note, however, that a pure imperfection load case does not produce any internal forces. The model must additionally have some "real" loads that induce axial forces in the imperfect member.

It is recommended to manage loads and imperfections in separate load cases. They can be suitably combined with each other in load combinations. Load cases with pure imperfections must be categorized as the action type Imperfection in the base data for load cases (see Figure 5.3). Otherwise, the plausibility check would display a message due to missing loads.

Generally, imperfections must be set affine to the lowest buckling eigenvalue in the most unfavorable direction.

Figure 6.53 New Imperfection dialog box
Figure 6.54 Table 3.14 Imperfections

The number of the imperfection is automatically assigned in the New Imperfection dialog box and can also be changed there. The numerical order is not important.

Reference to

Define the objects to which you want to apply the imperfection. The following options can be selected:

Members

The imperfection acts on one member or on each among several selected members.

List of members

The imperfection acts on all members, which have to be defined in a list. Thus, predeformations and inclinations are not individually applied to each member but as a total imperfection to all members of the member list. Load effects of an imperfection on single members in contrast to a member list are shown in Figure 6.55.

With a list of members, you can apply imperfections across members without having to define continuous members.

Figure 6.55 Imperfection with reference to members (left) and to a list of members (right)
Sets of members

The imperfection acts on a set of members or on each among several sets of members. Similar to the member list described above, parameters are applied to all members included in the member set.

Sets of members are divided into continuous members and groups of members (see Chapter 4.21). Imperfections for sets of members can only be applied to continuous members that lie on one line. They are not adequate for member groups or continuous members that are buckled.

Precambers are assigned along the total length of the set of members. However, the equivalent loads for inclinations refer to the contained single members.

On Members No.

In the text box, enter the numbers of the members or sets of members on which the imperfection acts. You can also select them graphically by using .

If you have selected the graphical input by clicking the button, you have to enter all imperfection data first. After clicking [OK], you can select the relevant members or sets of members one by one in the work window.

For imperfections referring to a list of members, it is possible to suitably arrange the member numbers by using the [Reverse Member Orientation] dialog button to, for example, reverse the inclination for the graphic display. However, the sequence is irrelevant for calculations because of the identical equivalent loads.

Direction

The imperfection can only be applied in the direction of the local member axes y or z. When asymmetrical cross-sections are used, the principal axes u and v are additionally available for selection (see Chapter 4.13). It is not possible to define a globally acting inclination or precamber.

The orientation of member axes is described in Chapter 4.17. For symmetrical sections, the axis y represents the so-called 'strong' axis, axis z accordingly the 'weak' axis of the member cross-section.

When the model type for plates or walls was selected in the general data, only the direction z can be selected.

Reference

The values for inclination and precamber can be defined in two ways: Relative allows for entering the reciprocal values of φ0 and e0 in relation to the member length, Absolute allows for directly specifying geometric dimensions.

Inclination 1/φ0

φ0 indicates the degree of inclination as it is described, for example, in EN 1993-1-1, clause 5.3.2. Enter the reciprocal value of φ0 or the absolute value into the text box. An illustration of parameters can be displayed in the dialog box by using the [Info] button.

In addition, the [Calculate inclination] button in the dialog box allows you to determine inclinations according to different standards in a separate dialog box.

Figure 6.56 Calculate Value of Inclination dialog box

The According to Standard dialog section controls, which text boxes are displayed in the Inclination Parameters dialog section. Based on the values entered in the dialog text boxes, reduction factors and inclinations are calculated in accordance with standards. Click [OK] to transfer the values to the initial dialog box.

Precamber L/e0

The precamber w0 or e0,d defines the degree of deflection to be applied according to the standard (e.g. DIN 18800 part 2, el. (204) or EN 1993-1-1, clause 5.3.2). The precamber depends on the buckling stress curve of the cross-section and is related to the member length L or entered as an absolute value.

Activity Criterion

The following options are available to select how precambers are handled in the interaction with member inclinations:

  • Always

Precamber is taken into account in all cases.

  • EN 1993-1-1 (5.8)

The influence of the precamber e0,d is applied to members with a slenderness ƛ determined according to EN 1993-1-1:2005, clause 5.3.2 (6), Eq. (5.8).

  • DIN 18800

w0 is only applied if the member coefficient ε exceeds a certain value. This regulation refers to DIN 18800, part 2, el. (207).

  • Manually

The activity criterion can be specified by the user.

To display the criteria in the dialog graphic, use the [Info] button.

Apply w0 from ε0

A precamber is considered in addition to the inclination if the member coefficient ε is higher than the value defined in this text box. DIN 18800-2 el. (207) specifies ε > 1.6 for most cases.