Below is noted in Candaian steel code S16-09, it shall be similar to AISC LRFD 13 code as well.
Different Unbraced Length Definition in STAAD
UNT,UNB and UNL(obsolete)
For beam flexural code check as per CSA S16-09 clause 13.6.
UNT & UNB is unbraced length of
compressive flange for member flexural code check.
It applies to flexural code check, and the length definition is related to
compressive flange.
[red]
1) To decide which flange is top flange-->UNT and bottom flange --> UNB, turn on the memeber's local axis and the positive local y axis pointing direction is top flange UNT, the opposite direction is bottom flange UNB
2) One continuous beam may have top flange under compression in one segment and bottom flange under compression in other segment, so UNB and UNT might be different
3) Only when
compresive flange get lateral restraint shall a point be considered as lateral support to reduce unsupported length. If the secondary beam doesn't restrain the compressive flange's lateral movement, the beam shouldn't be considered as a lateral support
4) UNL is obselete, in the old time STAAD didn't differentiate top and bott. flange and it caused issue stated in item 2 above. Now UNL is no longer in use but still valid in STAAD [/red]
KY KZ, LY LZ
For doubly symmetric column compression code check as per CSA S16-09 clause 13.3.1
LY or LZ is the unsupported length to prevent column from flexural buckling. It’s more like a floor beam/strut framing to a compressive column and has nothing to do with compressive flange. Like a building column in the transverse (strong axis) direction LZ= full length and in the longitudinal (weak axis) direction LY=strut distance
KT, LT
For singly symmetric asymmetric member compression code check as per CSA S16-09 clause 13.3.2
LX or LT is the unsupported length to prevent compressive member from twisting. It’s normally the full length of member length.
Usage of LX or LT, LY, LZ in STAAD
For doubly symmetric wide flange and hollow sections, compression is governed by Flexural Buckling as per clause 13.3.1 of S16-09. In this case KY, LY, KZ, LZ are used to calculate KL/r value
For asymmetric and singly symmetric sections, besides Flexural Buckling check, compression needs to be further checked against Torsional Buckling or Flexural-Torsional Buckling as per clause 13.3.2 of S16-09. In this case KT or KX, LT or LX are used to calculate KT x LT, which shown as Kz x Lz in S16-09 clause 13.3.2 formula.
KT or KX can be conservatively taken as 1.0 as per clause 13.3.2 of S16-09. KT=1.0 is STAAD’s default value.
LX or LT, the torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis
LT or LX, torsional unbraced length can be taken as the length of segment where both top & bottom flange are braced.
In usual case, LT is taken as member full length unless at intermediate points both top & bottom flange are restrained from twisting.
In STAAD if secondary beams are framed to a main beam, the main beam is broken into several segments, and not both top & bottom flange are braced, the LT shall be re-defined as member full length for this member’s compression code check.
Why Defining of LX or LT Is Not Necessary in STAAD
From above we can see the usage of LT is only necessary for asymmetric and singly symmetric sections, such as single channel, single angle, WT and double angle.
Please refer to STAAD Pro-2007 Manual International Design Codes page 3-19 Steel Frame Design CSA S16-01, STAAD uses LT to check Torsional Buckling or Flexural-Torsional Buckling only when doing the member compression code check of single channel, single angle, WT and double angle.
STAAD’s code check of single channel, single angle, WT and double angle are NOT reliable due to the limitations of neglecting of eccentricity or flagging of class 4 section (class 4 doesn’t mean the member failing the design).
Using STAAD output for code check of single channel, single angle, WT and double angle is NOT safe and all these asymmetric and singly symmetric sections shall be code checked manually.
For more details on STAAD code check limitation on singly symmetric section, please check
For more details on singly symmetric section crane beam design crane girder crane runway beam design and anchor bolt design as per ACI 318-08, please check
For above reason, defining of LX or LT value in STAAD in not necessary as it will not be used for doubly symmetric wide flange and hollow sections code check.
Definition of LX, LY, LZ
LX or LT, the torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis
LY and LZ, the lateral unbraced length is the distance between braces that prevent relative movement of the
compressive flange
Setting of LX or LT in STAAD
LT or LX, torsional unbraced length is taken as the length of segment where both top & bottom flange are braced.
The lateral unbraced length is the distance between braces that prevent relative movement of the
compressive flange.
The torsional unbraced length is the distance between braces that prevent a member from rotation about its longitudinal axis.
Torsional unbraced length is taken as the length of segment where both top & bottom flange are braced.
