Boundary conditions

[Structural noob]

Hello All,

I am starting to model girders/beams in RISA and I am having a difficult time understanding boundary conditions. I have a basic idea but I am not fully confident about understanding the whole concept of Boundary conditions.

I want to be able to understand how changing a DOF in one direction changes the stability of a model. Are there any good online sources/videos/tutorials/articles that I can start reading/watching to get a better understanding of boundary conditions.

Thank you,
KY

 

[r13]

My understanding of boundary condition means how a element is supported, whether at its ends, or along its edges. The conditions can be either - fixed, pinned, rolling, guided, or mixed types in a structural system. Change a fixed support to the latter three types, the DOF is altered, thus the stability. You need to review your college textbook and notes.

 

[JoshPlumSE]

Hmmm. I don't know where to point you. The concept is really towards the beginning of structural analysis. So, maybe an introductory book on the subject will help.

On the other hand, it may be just the terminology that's giving you trouble. A boundary condition at a particular node merely describes how that node is connected to the outside world.

If you have modeled a cantilevered column, then you expect the base of that column to be fully restrained against translation and rotation in all degrees of freedom.

If, however, your structure has many columns, some may be gravity resisting columns and some maybe moment frame columns. Chances are the gravity only columns are bolted to the foundation (in reality), in which case you would model the boundary condition as "pinned" meaning it's restrained against translation in the X, Y and Z directions.

Is that starting to make more sense now?

 

[Structural noob]

Josh,

Yeah all that you said makes sense to me, I guess I am having a little trouble with the terminology, like I don't know what a moment frame column is so I would not know how to model it.

I will start looking into the different types of connections/supports. I know the basic ones, for example - pin, roller, fixed but I think there are other connections out there that are used in real world practice.

My line of work is Bridges so I will start looking into supports that are typically used in designing different types of bridges.

Thank you,

 

[BAretired]

http://www.geom.uiuc.edu/education/calc-init/static-beam/support.html

BA

 

[BAretired]

Use Dr. Google to find more examples. Search for "boundary conditions for beams"; the following is one example:

https://enterfea.com/what-are-different-boundary-conditions-in-fea/

If that doesn't help, check out a few more for yourself.

BA

 

[jseng9]

Hi,

When I first started working I found RISA to be the most difficult software to use. Risa uses 6 degrees of freedom as follows:

1) X Translation
2) Y Translation
3) Z Translation
4) X Rotation
5) Y Rotation
6) Z Rotation

Here are my tips: Pick an axis system that works for you and your office. Your office may stipulate that Y is always the vertical (up and down axis) while others may use Z.

Read this article here:

https://risa.com/risahelp/risa3d/Content/3D_2D_Only_Topics/Boundary Conditions.htm

If you find yourself getting instabilities in your models watch this video from RISA here:

https://www.youtube.com/watch?v=PTKodyfMJeQ

Know the difference between nodal fixities and member end fixities. You obviously can't pin a node and have a pinned member coming into that node as you will create an instability.

When I say Risa was the most difficult to learn, I mean it in the sense that most other software out there won't let you make an unstable model. If they do, you really have to try. RAM SS creates stable structures automatically. I don't think ETABS lets you release more degrees of freedom than is practical. Risa lets you release everything so you can end up with a node spinning in space - unrestrained from translation or rotation. If you take a course in advanced structural analysis or matrix analysis this will become more apparent. Having an instability is akin to having an unsolvable matrix operation.

 

[dold]

jseng9 is right about knowing the difference between nodal (joint) and member fixities. Say you want to model a cantilevered beam: one end is fixed to a wall (for example) and the other end is just hanging out in the air. The boundary condition at the 'wall' will need to be fully restrained AND the member end at the wall will also need to be fixed. Member end fixities dictate how the MEMBER connects to the NODE(joint). Nodal fixities (boundary conditions) dictate how the NODE(joint) connects to the world.

 

[r13]

Note, IMO, support fixity is boundary condition, nodal fixity is the discretized member/element connectivity (mode of connection). Check the default of your program, a node is usually default to fixed, until the user issues "release command".