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LTB restraints are supports against lateral-torsional buckling (LTB) at the top or bottom flange of the beam. The topside is defined by the positive local z- axis of the section. It means that for a positive My (which causes compression at the topside) the LTB length (and the related moment factors - see Steel Code Check Theoretical Background, Calculation of moment factors for LTB) is calculated by the position of the stiffeners at the topside. The bottom side is defined by the negative local z-axis of the section. It means that for a negative moment My (which causes compression at the bottom side) the LTB length (and the related moment factors - see Steel Code Check Theoretical Background, Calculation of moment factors for LTB) is calculated by the position of the stiffeners at the bottom side. When no LTB stiffeners are defined, the values, introduced in the Buckling data dialog are used.
If required, it is possible to define the position of points where lateral-torsional buckling is prevented.
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Name |
Defines the name of restraint. |
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Position z |
Specifies the position in Z direction, i.e. either the topside or the bottom side. Note for IBC, AISC ASD and America (LRFD) codes: "Double LTB restraints" are inputted as LTB restraints with position z set to 'both'. This option is only available in case the following code flags are set: - IBC - AISC ASD - America (LRFD) A double LTB restraint specifies that both the top and bottom flange are held into position. As such, this point is seen also as a fixed point for weak axis buckling and torsional buckling. Therefore, during the code check (IBC, AISC ASD, America (LRFD) ) the following changes are made in case an LTB restraint with position ‘Both’ is detected on a member: - The buckling factor kz is set to 1,00 - The system length Lz is read between the LTB restraints with position ‘Both’ and the member ends - The torsional buckling factor kyz is set to 1,00 - The system length Lyz is read between the LTB restraints with position ‘Both’ and the member ends |
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Position x |
Defines the position of the restraint. If more than one weld is being used, this value specifies the position of the first weld. |
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Coord. definition |
The position can be input in absolute or relative coordinates. |
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Repeat |
Defines the number of restraint. This number includes the restraint at the beginning and at the end of the defined interval. If the "on begin" restraint or the "on end" restraint is not included, the actual number of defined restraint is lower (by one or two) than the value defined here. |
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Regularly |
If ON, the restraints are distributed regularly over the length of the beam. If NOT, the following parameter (Delta x) specifies the distance between the adjacent restraints. |
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Delta x |
This value specifies the distance between the adjacent restraints. |
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On begin |
If ON, the first restraint is defined. If OFF, the first restraint is not included. |
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On end |
If ON, the last restraint is defined. If OFF, the last restraint is not included. |
Open service Steel:
either using tree menu function Steel,
or using menu function Tree > Steel.
Select function Beams >Stability Check Data > LTB restraints and open it.
Confirm with [OK].
Select the member where the data are to be defined.
End the function.