Simply supported beam supported at base 5

[dl8860]

So at university we all learn that a simply supported beam with length L loaded with a point load P at mid-span will have a maximum bending moment at mid-span of PL/4 and always zero bending moment at the supports.

I have a case of a slightly complex piece of grillage, but one component is essentially an I beam that is supported at either end by bearing onto another I beam. The connection is simply some shear plates welded along the bottom flange of the top beam onto the top flange of the bottom beam, to restrain the overlying I beam along it's own axis.

When I model this (only the top beam) using structural software and offset the beam vertically upwards by half it's depth to have the support point at its base, I get moment resistance at the ends of the beam, yet it is simply supported.

This of course has the further effect of reducing my mid-span moment.

Is it OK to use this more beneficial bending moment profile? Is the bending only internal to the beam, and not passed on via the connection to the beam below? Note in my modelling the joint fixity at the ends of the main beam has no rotational restraint, there is only moment there because of the offset.

Thanks.

 

[Blackstar123]

I am using the software to give me my bending moment distribution, the issue is that in the model when I don't use an offset it gives me zero moment at the ends. When I use an offset it gains moment resistance.

As Celt had already pointd out that "If both ends are pinned, no translation allowed, you should be seeing end moments = Horizontal Reaction X Distance from Cross Section Centroid to Bottom of member."
The same thing is explained in the CSI knowledge base when you search for the effect of insertion point. I am sharing the screen shot. May be this can help you understand why you are finding end moments in your simply supported beam.

Euphoria is when you learn something new.

 

[human909]

I think this is a case where it would be a mistake to be too confident in your FEM analysis results. Let me explain...

My engineering judgment says that this is a simply supported beam. At the very least, I would design it that way. The FEM analysis showing how restraint at the bottom flange can result in beam end moments is something to think about. But, I don't think it should invalidate your engineering judgment. There are lots of little things that come into play when you start introducing rigid link / rigid offsets into a simple model like this. I don't know that this model is 100% accurate.

My recommendation would be to design it as a simply supported beam. Then, if you believe the end moments may be real, make sure that your design is capable of resisting these forces as well. But, DON'T allow the FEM results to reduce the design you'd get treating it as a simply supported beam.

Agreed, Especially with the last bit.

I do think it is always a good idea to second guess your pinned/fixed assumptions whenever there is possibly some doubt. And if there is doubt then do as Josh says run checks for both extremes pinned and fixed.

 

[r13]

the issue is that in the model when I don't use an offset it gives me zero moment at the ends. When I use an offset it gains moment resistance.

I think the problem was caused by the "member offset" command.

 

[dl8860]

@Blackstar123, you've got it spot on. I tried releasing one end of the member in the axial direction (to simulate a roller), under only vertical loading, and the end moments disappeared.

When I reinstate the x constraint, the end moments return, and because my structure will be welded at both ends, then I can theoretically make use of this beneficial effect.

I'm still slightly weary because it doesn't seem immediately intuitive, so will remain a bit cautious.

Thanks all for the help and for an interesting discussion.

 

[Celt83]

dl8860:
you need to understand the x constraint in your model is infinitely rigid, your real world condition will be dependent on the rigidity of the assembly which will not be infinitely rigid. I would follow JoshPlum's approach

My Personal Open Source Structural Applications:

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

Open Source Structural GitHub Group:

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

 

[rb1957]

we've interpreted your sect AA as being a sliding constraint, but you seem to think otherwise "because my structure will be welded at both ends".

It is good to be cautious when dealing with FEA.

The issue is how well our simple analytical models represent the real world. In your case, with simple tack welds at each end, how could the beam develop end moments ? The smarter (experienced) of us here (and there are many) would say prying (between the weld and some other point on the beam, like the end of the support). Whilst this could be considered pedantic (overly precise), and answer could be to weld shim plates to either piece so that the contact area is limited and the beam is free to deflect between the welds. This comes up looking at the insertion point sketches … if you used centroid you'd miss these "small" effects, but this is how we've analyzed structures for a very long time; if you used the bottom you'd detect these effects. But if this small end effect is so critical, how do you model this beam within the overall structure since it conforms to neither of these schemes (being on top of the support beams) ? do you add small rigid offset beams to position is precisely where it is ?

another day in paradise, or is paradise one day closer ?