Guide to model analysis for steel frame design

[xing.kevin]

I have a case, mount compressor end and motor on a steel frame. The frame is supported by several vibration isolation pad. For model analysis, I did it like below

1.Add Ty constraint on bottom of frame,

2. Add two force on compressor end mounts and motor mounts  

3. Add gravity

4. Add rigid connector to bolts holes for connect compressor end to frame and connect motor to frame.

Because I don't have actual the motor and compressor end models, only know the weight, So I add connector on bolt hole. I don't know if these procedure is correct. If not, please help tell me the correct method.

[shigeaki.k · Accepted solution]

Hello @xing.kevin,

>1.Add Ty constraint on bottom of frame,

What about the constraints in the other 2 directions? You will get "rigid body" motions where the assembly is free "slide" on the ground.

>2. Add two force on compressor end mounts and motor mounts

Forces do not affect modal analysis. The ma+cd+ku=F becomes Ma+ku=0 as damping in generally ignored. You can try running a modal analysis on a simple model with and without a force applied, and compare the results between the two.

>3. Add gravity

Same as answer (2), it should not affect the results. You may wish to test it out yourself on a simple model.

>4. Add rigid connector to bolts holes for connect compressor end to frame and connect motor to frame.

You could try applying "concentrated mass" to the reference points of the rigid bodies to represent the compressor and motor using "manual". If you are in an assembly, you can select a part and convert it to a concentrated mass using the "Automatic" method. You will still need to connect it to the rest of the assembly using rigid connectors. 

Regards,

Shigeaki K.

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[John_Holtz]

Hi @xing.kevin

Can you let us know what type of analysis you are performing? Linear static stress? Normal Modes? Something else?

Shigeaki read your question as "modal analysis", as in a normal modes or natural frequency analysis, because many people type "model" instead of "modal". (I make the same mistake.) His comment for item 2 is correct for a modal analysis. If you are performing some type of linear stress analysis (or even a prestress normal modes), then applying forces is one way to do it. Using masses as Shigeaki suggested is better in some cases since the weight due to the mass always follows the gravity direction (in case you ever perform an analysis with the model in different orientations).

[xing.kevin]

Hi Shigeaki

See my reply in blue color

>1.Add Ty constraint on bottom of frame,

What about the constraints in the other 2 directions? You will get "rigid body" motions where the assembly is free "slide" on the ground.

The steel frame will be put on ground, no anchor bolt. That's why I only add Ty constraint on bottom plane. Do you means even if no anchor bolts, I have to add constraints on other 2 directions.

>2. Add two force on compressor end mounts and motor mounts

Forces do not affect modal analysis. The ma+cd+ku=F becomes Ma+ku=0 as damping in generally ignored. You can try running a modal analysis on a simple model with and without a force applied, and compare the results between the two.

Yes, the result is same. But I concern is I don't have actual compressor and motor model, but I trust it will affect the nature frequency, because it will be bolted together. Do you means I can use "concentrated mass" on simplified compressor and motor model, then add bolt connector on bolt holes. I attached a assembly picture as below to show the assembly relationships. 

>3. Add gravity

Same as answer (2), it should not affect the results. You may wish to test it out yourself on a simple model.

>4. Add rigid connector to bolts holes for connect compressor end to frame and connect motor to frame.

You could try applying "concentrated mass" to the reference points of the rigid bodies to represent the compressor and motor using "manual". If you are in an assembly, you can select a part and convert it to a concentrated mass using the "Automatic" method. You will still need to connect it to the rest of the assembly using rigid connectors. 

[shigeaki.k · Accepted solution]

1) In reality, friction and gravity would stop the part from moving across the floor. If you run the analysis and see natural frequencies that are zero or near zero, you have rigid body motions.
It will be up to you to decide if these can be ignored and look at the other results that you are encountering. 

2)I guess you can test both... If you assume that it is bolted so that it could be connected directly to the frame you could try connecting the concentrated mass to the frame using rigid connectors.
Or you could model the motor and compressor and connect using bolts. A bit of experience and engineering judgement will be required or confirmation with experimental data etc. might be needed so you are happy with the results.