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no convergence of foundation with uplift

8 REPLIES 8
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Message 1 of 9
eralma
856 Views, 8 Replies

no convergence of foundation with uplift

Hello, I have this foundation slab with 3 columns and uplift and I keep getting “no convergence of nonlinear problem”. I have search this forum and follow most recommendations and no luck, I am using Z+ uplift, Full Newton Raphson, UX and UY blocked, “first case as starting for nonlinear cases” checked… don’t know what else is missing, could somebody help me? Attached model

8 REPLIES 8
Message 2 of 9
Artur.Kosakowski
in reply to: eralma

As far as I can see the lack of convergence is reported for cases 3 and 4 only which do not act alone on the model - they are part of combinations with other load cases. If so you can set them as auxiliary (or just ignore their results) and focus on results obtained for combinations instead (all of them converge). In addition there is no point to set case 1 as the starting one when you use it in the definitions of the combinations.

 

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Artur Kosakowski
Message 3 of 9
eralma
in reply to: eralma

Thank you Artur for your answer, so it doesn’t matter that 3 and 4 doesn’t converge the combinations still manage to use those cases and converge? It seems odd that those cases doesn’t converge but the combinations that contain them does it. This is a foundation for a cell tower so those cases are the more important because they represent the wind effect.

Message 4 of 9
Artur.Kosakowski
in reply to: eralma

Let me try to explain this again. My understanding is that wind load cases do not act alone but they are combined with other load cases e.g. self-weight. If so it is not important that they don't converge as long as combinations do. Mind that if you apply a wind load then you have the overturning moment about the foundation's edge. This moment has to be compensated by some stabilizing moment which is due to the vertical loads. If you have no vertical loads then the structure collapses -> lack of convergence.

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
Message 5 of 9
eralma
in reply to: Artur.Kosakowski

So on those cases the foundation elasticity it's not working??? because the overturning moment casued by the wind load it is supposed to be compesated by the elastic foundation coefficient, simulating the soil preassure

Message 6 of 9
Artur.Kosakowski
in reply to: eralma

It is working but against the pressure only as you defined is as unidirectional. Imagine a situation that you laid a piece of paper on an table. If you grab it at one edge and then you try to lift this edge nothing is going to prevent the paper from being separated from the surface you placed it on. Then put the other hand on this piece of paper and try again Smiley Happy

 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.



Artur Kosakowski
Message 7 of 9

And to give some more explanations about the convergence : 

Load case not converging and combination OK, it is weird because you have in mind that Robot it calculating all the load cases and then making X + Y etc .... so how is it possible if you don't have Y ???

 

Remember that due to your support, your are performing non linear analysis so :

 

Robot take all the loads case and step by step increases the loads till the value you set

if case one you set load X, then robot is doing 0.1X, 0.2X, .... till X

if case two you set load Y, then robot is doing 0.1Y, 0.2Y, .... till Y -> crashes

 

AND

 

comb 1 A+Y, it is doing 0.1 (X+Y), 0.2(X+Y) ........ no crash because X+Y is stable but Y alone not! 🙂

 

 

Message 8 of 9
eralma
in reply to: t.sautierr

Thanks Artur and t.sautir for your answers, I guess I have to do some more excersices with non-linear analsis in Robot...

Message 9 of 9
marko.glavinic89
in reply to: eralma

i had the same problem with the foundation uplift (steel structure with high wind loads). Just as Arthur mentioned, i set the wind cases to auxiliary, while i kept the combinations "normal", of course. Turns out the calculation protocol just skipped the cases alone and did the combination calculations just fine.

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