Hi, as part of a baseline truss design we wanted to verify our model. The maximum compression force is being calculated correctly. I defined an S355 140x140x5 SHS. According to the blue book the sections capacity is 662kN at the 4m buckling length. This would imply a utilization of 477kN/662kN = 72.1%. Karamba is giving a utilization of 104.9%.
Why is it calculating a much lower capacity and how do i correct this in my script?
Please se the pdf attached for script and blue book screenshots.
The primary reason for the difference in utilization between the Karamba3D results and those from the Blue Book lies in the buckling imperfection coefficient, alpha. You can find the relevant definition here: EN6472-Project 3-Baseline-Utilization_Error_cp.gh (63.1 KB).
For hot-rolled SHS cross sections, the coefficient corresponds to line “a,” resulting in alpha = 0.21. In contrast, cold-formed SHS sections fall under line “c,” where alpha = 0.49. These lines represent the influence of imperfections caused by residual stresses from the manufacturing process. Karamba3D defaults to the more conservative value of alpha = 0.49.
I’ll need to review whether this default should be adjusted for certain cross sections in the future. Thank you very much for pointing this out!