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Use of Diaphragms

The influence of a diaphragm is outlined in the following diagram.

First of all the lateral stiffness S of the diaphragm is determined and compared to the required stiffness Serf.

The lateral stiffness S is calculated according to Ref.[19],3.5 and Ref.[20],3.3.4.

The required stiffness Serf is determined according to EN 1993-1-3 art. 10.1.1

In case S < Serf the member is seen as Inadequately braced. In this case, when the diaphragm is located on the compression side, the Lateral Torsional Buckling check is executed using the augmented torsional stiffness It. Reference is made to "Adaptation of torsional constant  ".

As specified in art. 10.1.1 the shear stiffness S is replaced by 0,2 S in case the diaphragm is connected every second rib only.

In case S ≥ Serf the member is seen as Fully braced. In this case, a first test is executed to evaluate if the special purlin checks according to EN 1993-1-3 Chapter 10 can be applied.

More specifically, this chapter is applied only in case the cross-section concerns a Z, C, Σ or U section:

The code specifies that the chapter is also valid for hat (Omega) sections however in all further paragraphs; no specific formulas are given for Omega sections. For example the free flange geometry is described only for Z, C and Σ sections, not for Omega sections. Therefore, Omega sections are not supported for this special chapter.

In case the cross-section does not match any of the above, the default checks are executed. Since the member is seen as fully braced, no Lateral Torsional Buckling check needs to be executed in case the diaphragm is located on the compression side.

In case the cross-section does match the list of set form codes, a second test is executed. More specifically, the special purlin checks according to EN 1993-1-3 Chapter 10 can be applied only in case:

• The dimensional limits of article 10.1.1(1) are satisfied
• The section is only loaded by N, Vz, My

Chapter 10 specifies only checks related to in plane effects N, Vz and My. Therefore, in case of other loading components, the special articles are not valid and the default checks will be applied.

For a section which meets all requirements, the following is done:

• Reduced default Checks are executed i.e. not all default checks will be executed.
• Special purlin checks according to Chapter 10

More specifically, the following ‘default’ checks will be executed:

The Torsional moment check will never occur in this case since the prerequisite is to have only N, Vz, My.

The combined axial and bending checks are not executed since they are replaced by the special purlin checks.

The flexural buckling check is executed for y-y buckling in accordance with EN 1993-1-3 art. 10.1.4.2(2).

Torsional buckling and Lateral-torsional buckling are prohibited by the fully braced diaphragm. The compression in the free flange is included in the special purlin checks.

The combined stability checks are not executed since they are replaced by the special purlin checks.

In contrast to art. 10.1.3.3(2) the Local Transverse Load Check and its interaction with the bending moment is executed even if the support reaction is a tensile force.

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