Hi,
I'm trying to calculate the flexural torsional buckling moment of a cantilevered I section (by modelling the section as a series of shells), in RAM elements.
for a check, i modelled a simply supported beam and used the linear buckling calculator to compute the buckling load and compared it to the reference buckling moment for such a beam:
I modelled the 200 UC 46 (I or H section) as a series of appropriate shells.
I applied a the equivalent to a F restraint to both ends (end plates and torsional restraint fixity).
I applied a 1kN point load to the shear centre of the beam, at the centre length of the beam.
Accounting for the moment distribution (alpha-m=1.35), the two results were quite close (approx 4% difference).
in summary, is this an appropriate method for calculating the flexural torsional buckling moments of beams (both simply supported and cantilevered)?
What design code? If you want to include your file that's always helpful. Ram Elements Zipping a Model- https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/39895/zipping-a-model
The verification manual (C:\Program Files\Bentley\Engineering\RAM Elements\Documentation\en\RAM Elements Verification Manual.pdf) has some similar examples that you might compare with.
Hi Seth i uploaded 2 files.
in regards to AS 4100.
the one called '200 UC 46 3d buckling check MOS' is the one i performed a check on check, based on a 4m simply supported beam. It showed a 3% difference to the approx hand solution (attached as a pdf).
the one called '220711 200 UC 46 3d buckling check cantilever' is the member i wanted to calculate the flexural torsional buckling load on (and then back calculate the moment).
I briefly checked the verification manual, the manual seems to only checks for the flexural buckling mode (axial load condition) ( see section 1.8), i'm trying to calculate the flexural torsional buckling moment.
As discussed, just wanted to get a bit of guidance on the appropriateness of using this for calculating the flexural torsional buckling moment.
PDF
Hello Callan,
We have been checking the models you provided in order to determine the elastic moment for lateral torsional buckling using the buckling analysis in RAM Elements for AS4100. In the case of the simple supported beam, we agreed the way you modelled the beam and the boundary conditions imposed. However, for the cantilever case we did several tests. As this is mainly a boundary condition problem in order to get the best results we have concluded that the free end needs to be freed to displace and rotate so we deleted the plate at the end, then as the elastic effect is more preponderant for slender beams we increased the beam length to 5 m and get much better results, in fact, the slender (longer) the beam, the better the approximation from the buckling moment to the theoretical moment.
Using these conditions the difference between the elastic moment obtained from the buckling analysis and the theoretical one is just 7%.
Attached the model we used and the spreadsheet to perform the comparison. Please let us know your thoughts.
buckling check cantilever.zip
Answer Verified By: Callan Ward