How can I model beams with SAFE using Euler-Bernoulli's theory?

[sarodriguezco]

Good evening, I was assigned the academic work of modeling a beam using different methods, including: finite difference, discontinuity functions and CSI SAFE 2016. However, I did not achieve the same results in SAFE as in the other two methods. I have already made the shear area equal to 0 (infinite). It does not work. For example, the reactions have an error in some cases of up to 18%. I know this is not the right software to perform this analysis, but I was still asked to perform it. Please, if anyone knows what the problem is, or what I have to consider to make it work, it would be very helpful for me. Note: It works in ETABS and SAP2000.

 

[IDS]

I don't use SAFE, but I thought maybe it might be including the shear deflection in spite of setting the shear stiffness to 0.

I checked the reactions in my spreadsheet and got exact agreement with your exact solution.

I then used Goalseek to adjust the shear stiffness until the R1 value was 90.543. The other reactions still differed significantly from your SAFE results.

I also tried setting the shear stiffness back to zero, and adjusting the EI value to get a match for R1. That gave a reasonable match for R1 to R4, but the last two were still a long way out.

So I don't know.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

From the CSI site:

https://wiki.csiamerica.com/display/kb/Troubleshooting+checklist

Avoid full fixity and full rigidity – Rather than applying full fixity to DOF and full rigidity to structural objects, restrained and rigid conditions should be modeled using sufficiently large stiffness values, perhaps on the order of 1e11, or 1e3 to 1e4 times the stiffness of nearby members. Stiffness values may be assigned using spring constants, property modifiers, or section properties. Full fixity or full rigidity may cause numerical sensitivity in linear solutions and singularities in nonlinear numerical formulation, leading to convergence problems or instabilities.

Might be worth a try.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Blackstar123]

I solved your problem in sap2000 (with very high shear modifiers) and got the approximately same results as your exact and FE solutions.

 

[Blackstar123]

Did a quick exercise in safe as well.
What I noticed is that sap and safe gives almost the same results for moment load (80 KN-m) and support settlement load. Differences in results arise from the "varying distributed load". I'm not in the habit of analysing beams in safe so I've never notice this behavior before. I will be looking forward to the answer if someone knows the reason for this behavior.

 

[Celt83]

Can SAFE do bar elements, my understanding was SAFE was CSI's version of Concept which if the beam is being meshed as a shell element and the distributed load is being turned into lumped loads at the mesh nodes there will be some deviation due to things like cross section averaging and the shape functions. SAP is likely a bar element where the between node behavior is likely being handled by exact equations.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[johnie134]

use a zero shear stiffness modifier.