STAAD.Pro: how to input Lb for AISC 360 steel beam design

Dear all,

I am modelling a simple roof steel frame and the steel beam design is not passing the design check for AIS-360 LRFD LBT

I have defined UNT and LY for the steel beam and it should meet the condition of Lp < Lb < Lr, however the design results shows the nominal flexural strength Mn is calculated for the case of Lb > Lr (F2-3)

I thought STAAD would consider UNT or LY  as Lb, but it didn't?

Could someone please help me to understand what I did wrong here~? Thanks.

Below please find the STD file:

steel roof.STD

Parents
  • UNT=Unsupported length of the top flange for calculating flexural strength. Will be used only if compression is in the top flange.

    UNB=Unsupported length of the bottom flange for calculating flexural strength. Will be used only if compression is in the bottom flange.

    Top and Bottom represent the positive and negative side of the local Y axis (local Z axis if SET Z UP is used)

    If UNT and UNB is not specified then default length of the member will used as Lb value

    For the attached model, for member 10, UNT is 1.5m

    For Load case 134, the top flange is in tension and bottom flange is in compression. As no UNB value is specified for this member. Program considers the default length of the member as Lb that is 8.031 m. Lr is 4.879m. That is why as per equation F2-3 (Lb>Lr), the Mn value is calculated

    Answer Verified By: Chuen Han Lee 

Reply
  • UNT=Unsupported length of the top flange for calculating flexural strength. Will be used only if compression is in the top flange.

    UNB=Unsupported length of the bottom flange for calculating flexural strength. Will be used only if compression is in the bottom flange.

    Top and Bottom represent the positive and negative side of the local Y axis (local Z axis if SET Z UP is used)

    If UNT and UNB is not specified then default length of the member will used as Lb value

    For the attached model, for member 10, UNT is 1.5m

    For Load case 134, the top flange is in tension and bottom flange is in compression. As no UNB value is specified for this member. Program considers the default length of the member as Lb that is 8.031 m. Lr is 4.879m. That is why as per equation F2-3 (Lb>Lr), the Mn value is calculated

    Answer Verified By: Chuen Han Lee 

Children
No Data