different results (bending moments/shear) for different cross sections (FEM)
different results (bending moments/shear) for different cross sections (FEM)
(OP)
Im using software Tower 6 for FEM.
I have modeled 3 frames (same geometry but different cross sections/material for each frame):
MODEL 1: concrete 300/300 mm
MODEL 2: concrete 200/200 mm
MODEL 3: steel H section

Im wondering why do I get so different results for each model? Looks like software takes into account the stiffnes of columns/beam (based on geometry and material)when calculating internal forces in elements?
I dont like it since you can design elements based on lets say model 1, but when you choose to change a section in the middle of design - lets say from 300/300 cm to 300/350 mm and you have different internal forces (moments, shear)...
How do you guys model stuff like this?
As far as I know, internal forces should be the same for each model (if we dont consider self weight of elements) - forces should depend on geometry of model and loads only...
I have modeled 3 frames (same geometry but different cross sections/material for each frame):
MODEL 1: concrete 300/300 mm
MODEL 2: concrete 200/200 mm
MODEL 3: steel H section

Im wondering why do I get so different results for each model? Looks like software takes into account the stiffnes of columns/beam (based on geometry and material)when calculating internal forces in elements?
I dont like it since you can design elements based on lets say model 1, but when you choose to change a section in the middle of design - lets say from 300/300 cm to 300/350 mm and you have different internal forces (moments, shear)...
How do you guys model stuff like this?
As far as I know, internal forces should be the same for each model (if we dont consider self weight of elements) - forces should depend on geometry of model and loads only...






RE: different results (bending moments/shear) for different cross sections (FEM)
You must re run the program with the new section and check if the section is sufficient for taking all the forces
RE: different results (bending moments/shear) for different cross sections (FEM)
Are the column bases fixed against rotation, or is the stiffness less than infinite?
RE: different results (bending moments/shear) for different cross sections (FEM)
Yes, but I get different results even when self weight is taken as zero for all models.
Are the column bases fixed against rotation, or is the stiffness less than infinite?
columns are fixed - stiffnes is infinite.
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)
I'm not familiar with the software. Is it the type that needs to be broken into a sufficient number of elements to get accurate results (and that hasn't been done)? Or is it the type where beam elements don't need to be sub-divided to get accurate results?
Can you check the sum of reactions? Do they match the applied loads (ie is it in equilibrium)?
RE: different results (bending moments/shear) for different cross sections (FEM)
You're showing us shear (on the RH graph). These shears (equal and opposite) are driven by the model stiffness.
another day in paradise, or is paradise one day closer ?
RE: different results (bending moments/shear) for different cross sections (FEM)
Changes in *relative* stiffness should change the results. GMD255 has not done that.
I've back-calculated the span of the horizontal member from the shear forces and bending moments:
300*300: 6.886 units (metres?)
200*200: 8.201 units
HEB220: 8.24 units (exactly)
However GMD255 says the geometry is the same. 200*200 and HEB220 are similar (though not exactly the same) but the 300*300 results are significantly different.
Also the 300*300 hogging moment is smaller than the sagging moment which is the opposite of the other two analyses. Without knowing the software and seeing the inputs, I lean to some sort of analysis error (convergence, discretisation?).
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)
The frame on the left is made from 610UB125 steel I-beams (610mm deep, 125 kg/m weight). The frame on the right is 150UB14 (150mm deep, 14 kg/m). This is the biggest Australian rolled I-beam versus the smallest: stiffness varies by a factor of 49.
This shows what GMD255 expected to see: deflections are of course affected by the change to cross-section, but shear forces and bending moments are virtually unaffected.
Analysis error or unexpected difference in inputs (input error) are the most likely culprits IMO.
EDIT: I think I misread the corner bending moment on the 300*300 output diagram in the original post. So, the results for that cross-section are not as far off the other two cross-sections as I thought - please read my other comments with this in mind.
RE: different results (bending moments/shear) for different cross sections (FEM)
Quick checks you can do are the difference in shear force between beam ends should be equal to WL, and the difference in bending moment between beam ends and mid-span should be WL^2/8.
As others have pointed out, if the columns and beam have different stiffness, or if there is a spring restraint at the base rather than fixed condition, the bending moment diagrams will be very different.
Coming back to your original point, if you are doing a computer frame analysis, and you change the section for any or all members, then yes, you do need to do the analysis again, unless you have done a separate calculation and are certain that the differences in results are not significant.
You should also always review computer results to make sure the results are consistent with mechanics and your intended loading.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)
If the boundary conditions are identical and ratio of beam stiffness to column stiffness is constant for all three cases, you should have identical results.
BA
RE: different results (bending moments/shear) for different cross sections (FEM)
RE: different results (bending moments/shear) for different cross sections (FEM)