Direct Analysis Method and Stiffness Reduction

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Standard AISC 360, 2016 Edition, defines the requirements for stability designs in Chapter C. In particular, the provisions for the direct method of analysis are described in detail (previously contained in Annex 7 of the 2005 edition). This method is considered an alternative to the equivalent member method, which in turn helps to avoid that effective length factors (K) are not required, with the exception of 1.0.

Clause C2.3 (1) requires a factor for reducing the stiffness of 0.8 for axial and bending stiffnesses of all structural components that contribute to the structural stability. Moreover, the standard suggests that the reduction factor be applied for all members, regardless of whether they contribute to the stability or not, in order to avoid artificial strains. In addition, the factor τ b should be applied in the calculation for the bending stiffness of steel components. This factor τ b is a function of the ratio of the required axial axial and axial compressive strength (P r) and axial yield strength (P r).

With RFEM and RSTAB, it is possible to consider these considerations effectively and precisely for the reduction of member stiffnesses. In the " Edit Member" dialog box , the " Modify Stiffnesses" tab, you can select the definition type According to AISC 360-16 C2.3 (2) (see Figure). It lists the variables mentioned above in connection with the reduction of axial stiffness and bending stiffness.

The factor τ b can be set to "iterative". Thus, the program automatically determines this value based on the equations C2-2a and C2-2b. You can also select the "Set to 1" setting. The user must also specify whether the factor α used in these equations should be based on the design method according to LRFD or ASD. Finally, the multiplication factors of the bending stiffness and axial stiffness are shown. They are set to 0.8, as defined in Section C2.3 (1).


Direct design method Stiffness Reduction Modify Stiffness bending stiffness Axial stiffness Member stiffness


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RFEM Steel and Aluminium Structures

Add-on Module

Design of steel members according to the American standard ANSI/AISC 360

Price of First License
1,480.00 USD
RSTAB Steel and Aluminium Structures

Add-on Module

Design of steel members according to the American standard ANSI/AISC 360

Price of First License
1,480.00 USD