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TR.31.7 Definition of Direct Analysis Members

This set of commands may be used to define the members whose flexural stiffness or axial stiffness is considered to contribute to the lateral stability of the structure.

These parameters are then used by the program to analyze the structure as per guidelines provided in Chapter C of the AISC 360-10/16 specifications (Appendix 7 of AISC 360-05).

Note: The yield strength of the steel is taken from the material definition. If the material definition is not provided for a member, then a default value of Fy = 36 ksi is used.

General Format

FLEX (f1) list-spec
AXIAL list-spec


list-spec = * { XR f4 f5YR f4 f5ZR f4 f5MEMBERS mem-listLIST memb-list | ALL }
Parameter Default Value Description
FLEX f1 1.0 The initial τb value (See Chapter C of AISC 360-10/16 or Appendix 7 AISC 360-05). Members listed with FLEX will have their EI factored by 0.80 times τb while performing the global solution.
AXIAL - Members listed with AXIAL will have their EA factored by 0.80 while performing the global solution. Although reduced stiffness is considered for analysis, the program considers the full value of E for both flexural and axial design.
XR | YR | ZR f4, f5 - Upper and lower range values, respectively, used when specifying a range for FLEX.

For specifying NOTIONAL LOADs, please see TR.32.13 Notional Loads. The notional loads and the factor used is specified entirely in the loading data.


τb is the value entered in the FLEX command. τb defaults to 1.0 if not entered.