FEM Floor Vibration

[sauce_man]

I am currently working through a floor vibration study for a very sensitive imaging type program. I am targeting 1000 mips and there is some pretty significant difference in response for framing members and shells. Since we are working with a full SAP model, it seems logical to me that we should consider the vibration response in slab areas between supporting frame members.

There is so dated literature around (I assume related to v1 of DG11) stating that the response should only be measured at frames, and not areas. Is there any basis for this? In my mind, the position of equipment is not limited to the supporting frame members, so why would we neglect area elements?

I am also considering an increase in stiffness for slab elements in the strong deck direction (currently the slab is a uniform 3.5" thickness).

TIA

 

[JoshPlumSE]

There is a 2nd version of Design Guide 11 that has some very good discussions on how to do FEM vibration analysis.... Based largely on Brad Davis' research (I believe). In fact, if you're using the latest version of SAP there should be a "technical note" that discusses an example from that chapter of the design guide.

In my opinion, the 1st edition is the way to do when you've got a floor system that agrees with all the limitations of the method. But, the FEM method of the 2nd edition is superior when you have situations that don't easily fit into that method. Like cantilevers or stairs or such. The biggest problem with the FEM method (if you can call it a problem) is that it is pretty sensitive to how you model it.

Caveat: I work for the company that writes SAP2000 & ETABS and was involved in the development of the built-in implementation of the 2nd editions methodology (using frequency response functions and such.

 

[sauce_man]

Thanks Josh,

Can you share where I can find these technical notes? I have noticed the 2nd Ed seems much more developed, and the sensitive equipment example even indicates the equipment on the slab between framing members.

 

[JoshPlumSE]

If you're in the program, just go to the main toolbar at the top of the screen and select Help -> Documentation. This should bring up a window where you can choose to open any of the various manuals. One of the options will be Technical Notes. Expand this option and one of the options will be "Floor Vibration Analysis". It's about 9 pages long. If you've got that design guide in front of you, then look at section 7.2.

The model that we created to produce these results is in sub folder wherever SAP2000 is installed. SAP2000 / Verification / Design Examples / Floor Vibration / AISC-DG11 2ED EX001.sdb.

I should also point out that this is a relatively new feature.... maybe added a year or two ago. And, is meant to streamline the type of analysis that Brad Davis' put into that Design Guide. Though, the key to this is you need to model the floor area how it is described in the Design Guide.

This type of analysis can still be done manually... the way Dr. Davis did for most of his research. It just takes more effort to define the excitation forces and response points. And, to manually calculate the total response from the FRF results.

 

[sauce_man]

Wow thanks again, I found all this in EABS, not SAP. I had no idea any of this was there.

Still looking forward to what others doing as well!

 

[JoshPlumSE]

Sauce_Man: Wow thanks again, I found all this in EABS, not SAP. I had no idea any of this was there.

It's in both of the programs. It's a relatively new feature, so you may not have it if one of your programs is a little older. I think we added it to SAP2000 first because all the all the analysis infrastructure was there already and all we had to do was put in an interface that automated the input and automated the FRF interpretation / checks.