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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 generate a set of lateral loads as defined in Appendix 7 of ANSI/AISC 360-05 when a PERFORM DIRECT ANALYSIS command is used.

General Format

FLEX (f1) list-spec
AXIAL list-spec
FYLD (f2) 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 Appendix 7 ANSI/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.
FYLD f2 36.0 ksi Yield Strength in current units.
XR | YR | ZR f4, f5 - Upper and lower range values, respectively, used when specifying a range for FLEX or FYLD.

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.