 # Unbraced Length of Members w/ Intermediate Nodes

Say I have columns with intermediate nodes that are not stability bracing points (for example - nodes where cold formed girts are connected). When using SELECT command - how to instruct program to account for the actual height of columns and to ignore intermediate nodes?

I am using AISC LRFD so the design of physical members is not supported in this case.

thanks

• To each segment that makes up the physical member, specify the distance between points where the physical member is braced against flexural buckling, torsional buckling or flexural torsional buckling, using the parameters

LX, LY and LZ for calculating axial compression capacity

UNT and UNB for calculating bending capacity     • • • • • Definitions of LZ, UNT and UNB

In those equations of the code where a KL/r term is involved as part of a calculation for capacity in flexural buckling about the local Z axis (where the axis "Z" is as defined in STAAD), STAAD uses the value which you have specified for LZ. In the AISC 13th edition code for example, for a wide flange, the strong axis (the axis that is perpendicular to the web) is termed as x-x. So, for any equation containing the expression (Kx * Lx / rx), the means for specifying values for the terms Kx and Lx is through the STAAD parameters KZ and LZ. In Chapter E of the code, there are many instances of this expression.

UNT and UNB are used to specify the distance between points where the compression flange of a beam is braced against lateral displacement, or, the cross section of the beam is braced against twist. In the AISC 13th edition code, this term is denoted using the term Lb. in equation (F2-2), F(4-2), etc. in Chapter F.

UNT is for specifying the value of Lb when the top flange is in compression. The term "Top" signifies the flange on the positive side of the local Y axis. UNB is for specifying the value of Lb when the bottom flange is in compression.

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You wrote :

Every time we talking about ‘general column flexural buckling’ we should define KZ and LZ for the column strong axis and then KY and LZ for the column weak axis.

Agreed, except for a minor error in that sentence. For column weak axis, the parameter is LY, not LZ assuming that you are talking about an I-shaped section.

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You wrote :

That is why UNT and UNB should not be simply equated to Lu (unbraced length) - these lengths could be smaller than LY or LZ if the laterally-unsupported column has a point at the mid-height where the flanges are restrained by full web-stiffeners and by light interconnecting trusses hence preventing the flange distortion or column-section torsion.     • • • • 