Effective Lengths for Steel Stability Design

Helpful Questions & Answers

  • Frequently Asked Questions (FAQs)

How do I define effective lengths when designing steel members in RFEM 6?

Answer

There are two options when it comes to defining effective lengths in RFEM 6. First, edit the member and navigate to the "Design Types" Tab. Second, create a new effective lengths definition.

Third, set whether you would like to calculate the critical moment according to an Eigenvalue method or Chapter F. from the AISC. Fourth, navigate to the "Nodal Supports and Effective Lengths" tab. Within this tab there are two different methods that can be used.

Method 1: Nodal Supports

Referencing Member No. 1 in the attached model, for this column you can see a how the effective lengths for the Start, End, and Intermediate nodes are defined. First, click on the Select Member or Member Set and then select the member. This will activate the intermediate nodes along the member length in the table. Next, check whether the node can move in the y/z axis (weak/strong axis), rotate about its local x axis (torsion), and about its local z axis.

Top and bottom flanges can also be restrained separately by releasing (unchecking) the rotation about the local x axis restraint (torsion).

Method 2: "K" Factors and Absolute Values

Referencing Member No. 5 in the attached model, the effective length factors can be used to define the unbraced length directly and/or apply the appropriate member end conditions. To adjust the unbraced lengths directly instead of utilizing the nodes on member (Method 1), the "K" factors can be entered manually in the table below. Or the "Absolute Values" can be entered by checking on the option "Absolute Values". Then the unbraced length itself can be entered manually instead. This method is best used when there are no intermediate node(s) currently present on the member.

Keywords

Effective lengths Steel design

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  • Updated 31 January 2023

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