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.
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
Could you, please, clarify the diffrence between LZ and UNT/UNB parameters for free-standing slender 18m-tall columns supporting the crane runway?
There is a disagreement among our engineers - how to set these parameters.
The columns have:
18.8m of unbraced length for the Strong axis >> LZ = 18.8
8m largest panel between the stringers that transmit forces along the wall to the bracing block >> LY = 8
KZ = 2
KY = 1
At 8m high the columns have full web stiffenerrs and 1m rigid cantilivers interconnected by light lateral trusses.
I attached FCSS_North-Runway.std file for you to look at kindly..
I believe that UNT & UNB should be set at 10.8m (18.8 - 8 = 10.8). but not at 18.8 (full unbraced length)
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.
On the other hand, UNT and UNB are defined as the “Unsupported length in bending compression of the …flange for calculating moment resistance” which means that as far as the Flange is restrained from losing its stability, it should be considered supported; the Flange that did not lose its stability still provides adequate ‘moment resistance’ as part of the cross-section of the ‘generally-buckled’ column.
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.