Hi,
I am writing because I need to verify Unsymmetrical Plate Girders with AISC 2010 and I do not why I have very high Unity Ratio under Lateral Torsional Buckling when I increase the Upper Flange width
in the picture you can see the same beam, with same loads. the only difference is the upper flange width. I do not why the beam with less material has lower Unity Ratios.
I have knowledge of the fact that:
- SACS provides an Allowable Stress, with some adjustments, no matter the code
- for Unsymmetrical Plate Girders, the Plastic Neutral Axis shall be at the level of the web
- AISC 2010 introduced significant modifications to the check of LTB and the formulas include the different inertia properties of the flanges
the problem is that the difference of Unity Ratio is sometimes very very high. Please see the model below and let me know.
thank you for your support.
OPTIONS MN 4A 1 1 DC A SECT SECT US1 PGU 20.0001.00020.0000.8004.0002.00 SECT US2 PGU 4.000 1.00020.0000.8004.0002.00 GRUP GRUP US1 US1 20.007.72223.50 1 1.001.00 1.00-3 GRUP US2 US2 20.007.72223.50 1 1.001.00 1.00-3 MEMBER MEMBER 00020005 US1 L MEMBER 00030004 US2 L JOINT JOINT 0002 0. -4. 0. FIXED JOINT 0003 0. -2. 0. FIXED JOINT 0004 3. -2. 0. JOINT 0005 3. -4. 0. LOAD LOADCNTES2 LOAD 0005 -1.0000 GLOB JOIN TES2 LOAD 0004 -1.0000 GLOB JOIN TES2 END
thanks for reporting this! I will look into it and reply you as soon as possible.
according to my check with AISC 14TH F4-2, the reason you get quite different UC results while increasing top flange width, is that the top flange you increased is too wide, which makes the inertial moment of flange about minor axis in compression (bottom flange) is less than 0.23 inertial moment of shape about minor axis. this will make J=0 according to equation F4-5, which make the allowable stress decrease sharply.
Answer Verified By: Fabio Galletti
Thank you!