structure 2

[Engr ZANIA]

hello
For dynamic analysis of machine foundation, i want to use a software to Interpret my manual results, please recommended a software.
I have to find the natural all 6-frequencies of foundation

 

[klaus.]

RFEM

 

[Denial]

From the fact that you have already produced some "manual results", and your comment you are looking for "ALL SIX frequencies", I am assuming that your machine and your foundation are each (in effect) rigid blocks, and that they are connected rigidly to each other.

If this is correct, then you could do a lot worse than refer to the book "Design of Structures and Foundations for Vibrating Machines", by S.[ ]Arya, M.[ ]O'Neill & G.[ ]Pincus, published in 1979 by the Gulf Publishing Company, Houston.[ ] In particular see the book's chapter[ ]6.[ ] If this is appropriate to your problem, there are spreadsheets around that implement the method presented in the book.[ ] For example, you can find one on my web site (

http://rmniall.com).

 

[Engr ZANIA]

these are the frequencies i extracted manually, i want to verify it from any software, i ll try on above mentioned software

 

[WARose]

STAAD Pro is what I typically use.

 

[WARose]

By the way, looking at some of your frequency ratios, I'd be sure I had some accurate geotechnical info. You aren't that far off from the amplitudes being greatly magnified, so it's something to look out for. Typically I vary the spring constant I get from the geotech's info by about +/- 25% (depending on the test done) to see if there are potentially any issues.

Remember: the spring constant you come out with will not be exactly that number (neither will the damping, stiffness, etc)....so keep that in mind. There are good rules of thumb for initial sizing of these foundations that help you to avoid most problems.

 

[Engr ZANIA]

DEAR WARose!
Please guide me how to model machine foundation in STAAD Pro. as I'm not getting the desired results in all 6 modes, i have assigned springs constants to plate model and assigned time history. Whats your method of calculating spring constants Or can you please share your model for my ease. I'll be very thankful.

 

[WARose]

Please guide me how to model machine foundation in STAAD Pro. as I'm not getting the desired results in all 6 modes, i have assigned springs constants to plate model and assigned time history. Whats your method of calculating spring constants Or can you please share your model for my ease. I'll be very thankful.

If you want, you can post your model and I can look at it and give my opinion. If you want to see an example (in STADD), see Example #22 (in the help manual). It's not a foundation, but it gives a pretty good set up for a time-history analysis.

As far as calculating spring constants go.....Denial has a good reference. Another good one is:

'Vibration of Soils and Foundations', by: Richart, Woods, and Hall. (1970)

It's pretty much the bible of foundation vibrations.

 

[Engr ZANIA]

here is the model

 

[WARose]

Ok, a few things I noted:

1. There is no pedestal/housekeeping pad modeled. I don't know if you actually have one......but if you do and you don't model it, you could be missing some important rocking displacement (from the horizontal loads).

2. Me personally, I would organize the edit file a bit differently. (I'm sort of old school that way.) There is a lot of extra stuff that doesn't need to be there. I'd eliminate the static case......and I saw a AISC code check as well.

3. I'm not sure if you have inputted cracked section properties for the slab (usually done by adjusting the modulus of Elasticity value)....but if your static check indicates the slab will crack under static loads.....be sure to account for that. Stiffness is critical in a dynamic analysis.

4. In your time loads you have only 5 cycles (at 1200 rpm) which means that the dynamic load will be applied only briefly (less than a second) and you will not get a proper response. You'd need it to run for at least a minute or so. (Long enough to get through the transient phase.)

You also have a issue in your forcing function in that no phase is called out when you have the same forcing function applied in different directions. Properly, there would be a 90 degree difference. (Assuming we are talking about a reciprocating machine here.)

5. Your cut off mode shape is too low. You are only getting 95%+ mass participating in one direction. You need to have enough modes so that 95%+ of the mass is participating in all three orthogonal directions (X, Y, and Z).