GEO5 Logiciel

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In this frame are the settings that relate to the buckling analysis of the members in compression. The frame is divided according to the buckling orientation into two parts " In-plane buckling" and " Out-of-plane buckling".

Out-of-plane buckling

This section is focused on the out of plane buckling properties of the truss, i.e. in the direction where the stability of the structure is provided by the lateral bracing. The settings located here therefore affect the positioning of the lateral braces.

For the "Top chords" and "Bottom chords" (which include the collar beams) it is possible to select whether the lateral bracing to prevent buckling should always be inserted ("Apply to all members") or only to members that are subjected to compressive forces ("Apply only to compressive members"). The style of inserting the bracing can then be selected:

Braces in given spacing

  • In this mode, a fixed distance of the longitudinal braces is entered (input field "Spacing"), which will be applied to the entire length of the chords. The drawing of the braces on the workspace corresponds to this value. The entered distance is the input for determining the buckling length out of the truss plane. The resulting buckling length, which enters the calculation, is also influenced by the option "Bracing style", which can be found in the "Standard" tab.

Continuous brace

  • If this mode is selected, it is assumed that out-of-plane buckling is prevented along the entire length of the chords. This is particularly the case when full-plane sarking is used.

Automatic design of braces

  • This style is only available for the bottom chords. In this mode, the lateral braces are automatically inserted into all the joints on the bottom chord. Subsequently, if any of the bays between the joints do not satisfy the out-of-plane buckling analysis, the program adds intermediate braces to that bay until the out-of-plane buckling is satisfied.

For " Infill members" (diagonals and inner verticals) it is possible to switch on the automatic design of lateral braces. Within this automatic design, the program adds braces to the member if the member fails due to the out-of-plane buckling stresses. The maximum number of braces per member can be adjusted by the user, the default value is 4.

The "edge verticals" are considered as standard outer members and share the setting (method of brace insertion and distance between braces) with the top chords. If the setting "Design as infill members" is checked, the edge verticals are treated as diagonals and internal verticals. In this mode, the reinforcements on the edge verticals are then determined by number, not by distance.

In-plane buckling

In-plane buckling is the direction of buckling where the buckling characteristics are determined by the shape of the truss and cannot be influenced by the addition of lateral braces. This section allows user to enter the coefficients that are used to determine the buckling length of the members when checking the in-plane buckling of the truss. The coefficients are multiplied by the basic length of the segment, resulting in the buckling length. This procedure substitutes the exact calculation of buckling lengths from the inflection points of the deflection curve, which is not feasible under the conditions of a conventional structural program. With a value of 1.0, the buckling length is equal to the basic length of the member or member part. This value is mainly used for members whose connection to the structure is close to pinned. However, it can be used almost anywhere besides overhangs and cantilevers as a relatively safe value. Values less than 1.0 find use in cases of members or their parts with significant end moments and result in a buckling length that is shorter than the basic length. This may be the case, for example, for single bays of continuous outer members with linear transverse loading, which prevents different orientation of deformation in odd and even bays. Values greater than 1.0 are mainly used in the case of missing or insufficient connection at one of the ends, the resulting buckling length being longer than the basic length. Typical applications are cantilevers and overlaps. The coefficients are entered separately for subsequent members or parts thereof:


  • These are members with member code 2 (inner vertical) or 3 (web). These members usually do not have any joints on them, the total length of the member is considered as the basic member length. If there are joints on these members, it is not assumed that in-plane buckling is prevented in these joints. The recommended value of the buckling length factor is 1.0.

Chords including collar beam

  • For outer members, the basic length of the segment is the length of the bay, i.e. the distance between two adjacent joints. All members that do not have code 2 (inner vertical) or 3 (web) are considered as outer members. The recommended safe value for the buckling length factor is 1.0, but lower values may be used (e.g. according to clause 9.2.1(4) in EN 1995-1-1) in some cases.


  • Overhangs are parts of members that have one free end. Therefore, it is necessary to specify a buckling length factor greater than 1.0. The recommended value in this case is 2.0.

Upper parts of rafters in attic trusses without supported collar beam

  • Although the connections of the collar beam to the rafters form relative joints and divide the rafters into individual bays, they do not always provide sufficient support for the rafters with respect to in-plane buckling. In fact, due to the lack of triangulation in the attic, the collar beam is not sufficiently rigidly anchored to the substructure, and the left and right rafters in the truss may buckle simultaneously. These cases should be treated with a coefficient value greater than 1,0. This coefficient is only used for the top chords in cases where the collar beam is not supported in the horizontal direction. If it is supported in the horizontal direction (the support may simulate bracing truss or spatial bracing in the plane of the collar beam), the coefficient value for the bays of the outer members shall be used. The default value of the coefficient is 1.0.

Chords with one bay including collar beam

  • If the outer members are made up of only one bay (there is no relative joint), they behave more like infill members. The recommended value of the coefficient in these cases is 1.0.

Part "Buckling"