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
Dear Kris,
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.
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 :
Answer:
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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Agreed.
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I do not know enough about real world methods of bracing to confirm whether the web stiffener system that you are introducing is sufficient to act as a bracing point as defined by the code in its definition of Lb. But if you are sure that it is, then, I don't see a reason not to set UNT and UNB to 8.0 m.
Thank you Kris - the information you've provided is very helpful.
1. Yes, all this business with local axes and proper notations may look a bit confusing - yet, I believe, STAAD User should just remember that for I-beam the length for general buckling for the Strong Axis is always LZ; whereas this parameter for the Weak Axis is always LY. This must be consistent for any member under bending + axial compression (either a beam or a column) Am I right?
I could not find the 13th edition of the US AISC Steel Standard right away (I'm from Canada and working only on Canadian projects) but I managed to get ANSI/AISC 360-05 "Specification for Structural Steel Building" that apparently has very similar content and the same chapter & reference structure.
F2-2 is a formula similar to its Canadian counterpart and when I looked at notations, it reads that Lb is "Length between points that are either braced against lateral displacement of compression flange or braced against twist of the cross-section."
2. Thank you for correction - indeed LY (but not LZ) should be the parameter for Weak Axis; it was just a typo.
3. You wrote: “I do not know enough about real world methods of bracing to confirm whether the web stiffener system that you are introducing is sufficient to act as a bracing point as defined by the code in its definition of Lb”.
This sounds like "Catch-22"; I effectively prevented lateral displacement of compression flange as well as prevented twist of the cross-section at the given point, and yet, there is no certainty that this point "is sufficient to act as a bracing point" because....? ….because it is not laterally braced in the Strong Axis? But if the node were braced laterally in both directions, I wouldn't need to specify any parameters (STAAD defaults would be sufficient)
My structure is not something extraordinary; I can assure you that this is a very typical situation for the design of a tall industrial building (or an open yard) with a bridge crane.
There must be a reason why UNT & UNB have been introduced by the STAAD developers in addition to LZ, isn’t it?
Best Regards,
Len Bogdanov
Kris - proposed solution works well - thank you.
Len - the model you attached - is this stand alone structure or part of the building? What is the purpose of horizontal trusses (wind trusses, bracing or something else)?