problem with modeling an IPE beam with shell elements @ sap2000

[ingthere]

I am modeling an IPE beam using shell elements in Sap2000. My boundary conditions are translational restraints(in all 1,2,3 directions) in one end, and translational restraints (in 2 and 3 direction)in the other end of the beam.

In such a case I don't understand why I have high positive moment values near to the ends of the beam, as you may see in the attached figure..

What am I doing wrong?

If needed I can also send the sap file

Thanks

 

[paddingtongreen]

I don't know SAP, but I liked to exaggerate the distortion, that usually gave me a clue.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Galambos]

Doesnt sound or look like there is a whole lot that could be changed in your model, however, this looks like Saint-Venant's principle in action.

 

[ingthere]

@Galambos, Could you detail a bit more on Saint Venant action?

 

[dcarr82775]

Try releasing movement perpendicular to the span on all but one of the supports.

 

[ingthere]

@dcarr82775: tried.. still have high moment values at those same points.

 

[ingthere]

In case someone would like to see, I attached the sap file. It s a very simple model for an I beam of 5 meters.

 

[dhengr]

Release all but the vertical restraint at the one reaction. You’ll probably still have some high stresses around that one node. If you are old enough and big enough to be using SAP2000 you should know a little about Saint-Venant’s Principle too. Twitter it.

 

[dcarr82775]

It surely has to do with the support conditions, keep messing with them. I find that sometimes SAP is a little too smart for its own good.

 

[ingthere]

@dhengr; what if I am not old and big enough to be using sap2000? should I stop trying and fixing the knowledge I miss or what?

I almost tried any boundary conditions which keep the structure at least stable (which don't, when I release all but the vertical restraint at the one reaction)

I can't find a detailed information of Saint Venant's effect on shell elements, could someone refer to a document or a book where I can throughly understand the problem?

Btw, my loads are point loads applied at the mid of the beam, on the top flange.. Do you think could this application as "point load" may lead to a problem?

 

[dcarr82775]

Why are you modeling the beam as shell elements and not just a frame element? Overkill for a simply supported beam.

SAP has decent tech support. Send them your file and maybe they can help.

 

[dhengr]

Then try only a vertical restraint at the bottom flange, and only a lateral restraint at the top flange, on this particular end of the beam. If Twitter doesn’t work, try Google; or better yet open your Strength of Materials text books or check out a Theory of Elasticity text book, RE: Saint-Venant’s Principle. You probably will see the same anomaly at the point loads on the top flange, at the middle of the beam. And, maybe your restraints need some adjusting of their spring stiffness, or the loads and reactions could be applied over 4" beam length instead of on only .000001".

If you’re not old enough and big enough to be driving, you sure shouldn’t be driving SAP2000, with shell elements on a simple beam. My point, and Galambos’ point, was that you should have a reasonable understanding of, and a working background in, the basics of engineering and Theory of Elasticity (Strength of Materials) before you try to do engineering with a black box which you don’t understand, or have any idea about how or why it works. Put another way, these damn FEA programs have gotten so complicated that understanding and manipulating their idiosyncrasies becomes the engineering endeavor, unto itself, rather than having a good fundamental conceptual understanding of the actual problem at hand. In fact, this has the potential of being down right dangerous if you don’t know that the computer is telling you the wrong thing, or you don’t know enough about the fundamentals of the problem to know that you can explain something the output is showing. Actually, you shouldn’t ignore these things, you should understand what is causing them and be able to explain why you don’t have to worry about them. I do believe that was the reason for your OP, but more importantly learn the fundamentals. Now, look up von Mises stresses and see what sense they make in the simple beam design world. Or, just try R=V=P/2 and M=Pl/4, but understand where they came from and how they were derived.

 

[BAretired]

I do not see high positive moment values near the end of the beam. Where do you see them? What is the meaning of the red coloring at the bottom flange near the end of the beam? What is the meaning of the green coloring?

What does the light brown shading around the red signify? What does the light brown shading over the web at the top flange signify?

This method of analysis is not familiar territory to me and I don't know why you are doing it when a straightforward solution seems clear.

Perhaps the red and green colors have something to do with the bearing stress at the support rather than positive moment. Sorry, I can't help you.

BA

 

[paddingtongreen]

I'm adding my voice to dcarr82775's. Why do this?

I don't speak "SAP" so my computer can't read your file. I don't know how to apply your end conditions but if they are what I see in the diagram, points at the bottom flange nodes, then there are bound to be local effects. When you put a load in the middle of a square plate, supported or fixed on four sides, the corners try to rise. Compare one quarter of this plate with your end condition. I think the problem is because of point supports on the very end of the beam. If you put the supports further in, or continued the beam one more element set, it would be different.

If you are checking the beam this way, the supports should be real. If they are at the edge, they must be welded and that would change the conditions considerably.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[paddingtongreen]

Another thought. If each of those reaction nodes is unmoving, all, or almost all of the reaction will be on the one under the web, it then has to distribute to the flange. Compressive strain there would case bending along the beam so there would be tension nearby.

Again, without a real model of the support, this level of FEA is not helpful.

Michael.
Timing has a lot to do with the outcome of a rain dance.