Boundary conditions influence on static design - case study
?ukasz Skotny
? Nonlinear FEA Enthusiast ? Structural Steel Expert ? Engineering Tutor and Blogger
Recently I have finished a project for a Customer who became a friend long time ago. Thanks to his kindness I can use the project as a case study. From all interesting topics I decided to use it to discuss boundary conditions issues. I think those are one of the most popular problems in structural engineering.
The model I will use has deceptively simple support conditions. This makes it a perfect case to discuss potential mistakes!
What we are dealing with
Hopper stands on a concrete slab. This means that its support looks relatively easy: circumferential beam is supported in vertical direction on entire surface. This support zone is marked in black on the drawing below. In such arrangement making montage connections is tricky, so I have not used any. There will be a lot of load on the hoppers - friction will deal with any possible horizontal movement... and they won't fly away either :)
Let's start with the correct boundary conditions
It is quite obvious that support is a typical contact case. The bottom of the beam simply stands on the steel plate embedded in concrete. Support easily transfers compression forces, while there is no capacity due to tension.
Contact in nonlinear analysis may mean convergence issues, some additional work etc. However I always do design accurately and I have never regretted the invested time!
This is a von Mises stress distribution in the model. I have checked several places for plastic strain capacity etc., but this is not all that important here. Just notice how the stress is distributed in the structure, especially near the support zone. There will be few more models with not so lucky boundary conditions. You will be able to see how things will turn out :)
To make comparison easier I will also show a stability path for a node in the corner of the support beam:
Outcomes for wrong boundary conditions and discussion (with video)
Read the full article on my blog!
Want to learn more?
This is really awesome! I have a free nonlinear course just for you!
Continuous Improvement Manager at Permobil
7 年Learned that lesson the hard way, everyone should have to build something that they designed and did an FEA on to prove their results.
Research engineer at SDTools, PhD
7 年Nice and very demonstrative example ! Not just regarding boundary conditions, but also how structural interactions are treated in general : overlooked. The consequences are often hidden behind a good looking result, although sometimes completely wrong.
Structures DER, Rotorcraft & Fixed Wing, Damage Tolerance & Seating; Expert Witness; Commercial Pilot, SEL, SES, MEL, CFII
7 年FEMs make it too easy to produce wrong answers that look right, which is why the FAA has placed increased emphasis on validating analyses with independent methods.
Consulting Electric Motor Designer | FEA Developer | D.Sc.
7 年Nice timing - I was just about to write a cheat sheet about boundary conditions in magnetics!