How to model Beams in Finite element software

[drasticxxxx]

Hey guys

I think I have one question ,that I suppose many people counter it before ,and I would like to see the best way to overcome it:

1.If we have columns in which their centers in Y direction are not on the same line,how I will be able to model the beam connecting them for example on etabs,the actual situation is that beam setting on the center of one column and the edge of the other columns ,however in etabs you can only model center to enter.(See the attached picture)

2.My first question is leading to my second question,if I model beam center to center ,I have to delete this part of slab that lies outside the beams

 

[mike20793]

I don't use etabs enough to know off the top of my head how it's done within that program, but in other similar programs, you can create a material that has a really large modulus of elasticity and has a 0 density. This creates a rigid link that is really stiff and has no selfweight. You can then use the rigid link to model the eccentricity.

 

[andriver]

I don't use etabs either, I use RISA 3D and SACS for offshore structural stuff.

That being said I agree with Mike, we call those "dummy" members. They are tools to work around the limitations of a program, you have to understand how the forces are transferred and then try to mimic these loading by getting clever with the program.

That being said, your model is not a true finite element model. If it were, you wouldn't have this problem.

 

[chekre]

i agree with mike20793.
have a look at the results once the analysis is performed. If it isnt as expected, model the beams center to center and that part of eccentricity as an additional moment carried by the column.

 

[drasticxxxx]

Thank you all .

Mike you hit exactly what I was thinking ,what I did I made very stiff element there to overcome the eccentricity ,hence I use this to transfer the load properly as the actual situation, but I don't read moment on it(not actual, since it is part of the column).

andriver

you are right it is not completely finite element model (I do agree)

I have a very common question that I keep thinking in my head

Suppose I have tie beams in the foundation(those tie beams above the foundations not on the same level of foundations, so do I need to model them when I study the building ,and what is the length of column I should consider? is it from top of footings to bottom of the first slab coming through slab on grade.

And this may lead to my second question what is difference between tie beams with foundation levels and tie beams above foundation levels(in terms of modeling and purpose)

 

[drasticxxxx]

I meant by tie beam above foundation as the following picture tell

 

[LargeDeflections]

In my opinion, if you want to model the eccentricity of the beam on the column, as other people have said you can use a zero-weight member that's infinitely stiff, length equal to the centroidal eccentricity between the columns in the beam line.

If you're wondering if there will be localized issues in the beam and there isn't a good equation in the applicable code, you could build the beam elements out of plates equal to the web/flanges of the beam. For example: your beam is a W16x40 with d = 16.0, bf = 7.0, tf = 0.5 and tw = 0.3. You would have two parallel layers of 7.0" x0.5" plates (16-0.5) = 15.5" apart connected by a 15.5"x0.3" plate. This could give you an idea of weird bending near the eccentric bearing area.

 

[drasticxxxx]

thanks

 

[chekre]

the tie beam showed in ur screenshot located above the foundaton is to tie the columns in order to reduce their unbraced height while tie beams at Foundation level is to tie the Foundation to catter for any differential settlement between your footings.