Pump foundation on piles using Ansys. Can Risa3d be used as well?

[AskTooMuch]

Anyone done this to get the resonance frequency? I feel only the vertical frequency maybe valid if I use risa3d and other frequency (rocking, etc) may not be checked properly.

 

[Klitor]

What kind of dynamic spring values did you get from geotech?

 

[AskTooMuch]

Goetech only provided static pile capacity. Can i even input "dynamic spring" in risa 3d boundary condition.

 

[Klitor]

Its just ordinary springs (dynamic values are just much higher than typical statical values)
I cant see you doing any meaningfull analysys with just pile capacity as input in any software.
I would try to get some dynamic soil properties from geotech and just check rocking/sliding using literature equations(arya/prakash, fema...)

 

[bones206]

In RISA you can model plates, solids and frame elements. Boundary conditions can be defined as springs in all DOF’s. And RISA can do an eigen analysis for the frequencies. So given those 3 tools, I think you can get some meaningful results. But I would recommend validating/calibrating the model with hand calcs. SAP2000 has some more sophisticated spring types than what‘s available in RISA.

RISA can do time history loads as well, but it’s a newer feature and I found it to be a little buggy when I tested it out.

 

[Klitor]

Bones, are you aware of acrony GIGO?
Risa/sap will for sure churn out some results. But what value of springs will you enter?
How will risa cope with the fact that soil is compression-only medium?
Do you know is modal analysys possible using compression-only springs?

 

[AskTooMuch]

Is there a similar books or guide like Arya's if on piles.

 

[bones206]

Bones, are you aware of acrony GIGO?

Yes, I’m well aware. That’s exactly the reason I recommended validation with hand calcs. My overall point is that RISA has tools that you can do something with. Of course you have to have an understanding of the underlying mathematics and principals involved if you have any hope of getting meaningful results.

The spring values would be calculated based on geotechnical considerations using classical methods. RISA is limited in that dashpots can’t be defined, but I do believe it can do compression only springs. If SAP2000 was used, I would approach the problem using the Richart and Whitman method.

 

[JoshPlumSE]

For something like this, the concept of damping is pretty important too. Is that what you're getting at when you talk about a "Dynamic spring"? The newer versions of RISA do have a spring with a damping value associated with it. So, it should be possible.

Though I would reiterate bones206's comments about this being a new feature (I was the project lead on the feature!). There were a couple of significant bugs with the first release that we (me and RISA) got worked out pretty quickly. However, it wasn't much longer after that when we had less than amicable parting of the ways. I'm honestly not sure there is anyone left over there that knows much about Time-History, or cares much about the feature. So, use with caution and check / verify your work.

 

[AskTooMuch]

How can this be checked/verified when there isn't a book or guide how to do this (on piles) manually. There are several on soil but none on piles that I could find. The only thing I saw when I googled was a video to design it using Ansys.

Anyway, I just want to learn how to calculate or model this properly. I don't have a project currently that has this.

 

[JoshPlumSE]

AskTooMuch -

One thing I should point out is that you should never blindly trust your software. Especially for features that are lesser used or items that may be of lesser importance to the software company.

For something like this there are lots of ways to verify. My initial thoughts.
1) Start out with a simple springs that have no damping of their own. Impose an enforced displacement in each direction and rotation (individually, not all at once). You know the mass, and mass moment of inertia. You should be able to determine the period of movement should be something like T = 2*pi*Sqrt (M/k). So, when you look at the response you should be getting something along those lines.

2) Start introducing some damping. The damping should change the natural frequencies a little f_damped / f_undamped = sqrt (1- (damping ratio)^2). Make sure the damping affects the model like you know it should.

If all those things are correct, then the bigger issue is going to be to make sure you have used good values for stiffness and damping.

 

[Klitor]

Asktoomuch,
Principle is the same as without piles.for block foundations, piles are basically just soil improvement (you will probably not connect pile rebar with foundation)
You need to work with competent geotech to get you spring values,use arya and obey some time tested rules of thumb (foundation weight ~5 times the machine weight, machine frequency +-25% of foundation etc) and you will be fine

Damping (while it reduces amplitudes significantly) usualy changes block foundation frequency by only ~5% so you can disregd it (if you are out of +-25% interval you will not even have amplitude problems)

Om the other hand if it was a table top turbine foundation.......

 

[bones206]

Can you give us an overview of how you are doing the analysis in ANSYS?

 

[Klitor]

Bones, I dont think anyone in the thread said he/she uses Ansys for this.
Asktomuch only said he found a video showing the usage.

 

[AskTooMuch]

I'm not doing in ANSYS. The video tutorial how to do this was using ansys. I have not seen using Risa or staad.

This is not a real project I'm doing right now. This is just for my information and education.

 

[bones206]

Do you have a link to the video?

 

[bones206]

Link to a good paper with case study.

 

[JoshPlumSE]

I don't have access to many of the reference books that I purchased back when RISA started work on Time History analysis. Here are the ones that I started from. If I find the others, I'll post them as well.

351.3R-18: Report on Foundations for Dynamic Equipment
Design of Structures and Foundations for Vibrating Machines by Suresh C. Arya , Michael O'Neil, et al

Foundations for Industrial Machines: Handbook for Practising Engineers by K.G. Bhatia

Dynamic Analysis and structural design of turbine generator foundations This one, I think, has an Ansys example.