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AASHTO Created Edited

Unbraced Length


  • Input the Lateral Unbraced Length that is used to calculate lateral torsional buckling resistance in compression flange of I Girder or top flange of Tub Girder.

If the lateral unbraced length is not added, the program will use span lengths. If span lengths are not defined either, the lateral unbraced length is applied for the corresponding element length. The value input here has higher priority than the value calculated from Span Group and element length.



From the main menu, select [Rating] tab > [Type : Steel Bridge] > [Steel Bridge : AASHTO] >  [Rating Input Data] group > [Rating Parameters] > [Unbraced Length]



First, select the members in the model, and then enter the following:


Select Add/Replace or Delete.

Add/Replace : Add the newly entered values or update the previously entered values.

Delete : Delete the entered values. Data entry is not required when deleting.

Unbraced Length

Enter the unbraced lengths for the selected members' strong and weak axes. (Refer to NOTE.png (1))

Ly : Unbraced length for buckling about strong axis (determined by braces in z-dir.)

Lz : Unbraced length for buckling about weak axis (determined by braces in y-dir.)

Ly and Lz are used for calculating the slenderness ratio due to axial forces.

Laterally Unbraced Length

Enter the laterally unbraced lengths.

Lb : Laterally unbraced length [Default = Lz] (Refer to NOTE.png (2))

Do not consider : Select this item, if the allowable bending stress computation about the member`s strong axis is to neglect the laterally unbraced length. (Refer to NOTE.png (3))

apply.png : Apply the entered values to the selected members.

close.png : Close the entry Dialog Bar.

NOTE.png (1)
For members where the values of Ly and Lz have not been entered, the member lengths calculated by the program based on the nodal coordinates of the relevant beam (truss) elements are applied as the values of Ly and Lz.

NOTE.png (2)
For members where the values of Lb have not been entered, Lz values are set to Lb.

NOTE.png (3)
If "All Beams/Girders are Laterally Braced" option is selected in the Steel Design Code dialog box, this item is automatically selected and Lb=0 is automatically set by default.
(Refer to "Steel Design Code")

NOTE.png (4)
If the values of Ly, Lz and Lb are repeatedly entered for the same member, the values will be updated to the last values entered.

NOTE.png (5)
The entered unbraced lengths and laterally unbraced lengths can be reviewed in the data table arranged in the order of element numbers. The user may Modify/Add and Delete items in the data table.
Access the data table following the procedure below.

from the Main Menu select Design > General Design Parameter > Unbraced Length (L,Lb)
From the Tables tab in the Tree Menu select Design Tables > General Design Table > Unbraced Length (L,Lb).

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