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Direct Transient Analysis

Valued Contributor
Valued Contributor

Hi everybody!

 

I prepared Direct Transient Analysis in FEMAP but I´m not sure about correctness of this analysis.

 

I made a simply model of beam (11.JPG). And I made a coinstraints - this beam is fixed at the top.

Next, I applied a load at the foot of this beam. I am not sure about value of this load, and I applied 1  (122.JPG). After that I prepared a function of excitation. It isn´t sine because I can´t prepare this function but I made quite similar to the function from the norm (133.JPG). The vaule of peak in function i 981 because I applied a excitation 0,1g. The units of acceleration is mm/s2. My next step was preparing analysis -(144.JPG) and (177.JPG). I have got a few questions to the last picture. Because I choosen damping ratio 0,2 becasue this is the damping ratio for steel. After that I applied 6 in a gap W3 because this is the first frequecy of bending mode in modal analysis. And shall I check ''gap as linear contact''? After calculating, I made a chart (188.JPG) with excitation function in a green colour. I checked a few points, that's why there are so many funcions.

 

I would like to ask You, is this analysis is correct because I am not sure about it at all. what about damping ratio, is this way is ok? what about my function and my load?

 

 

Best regards

J. Krzesimir

4 REPLIES

Re: Direct Transient Analysis

Siemens Phenom Siemens Phenom
Siemens Phenom

Look at Example 15 in the Femap Help for a direct transient example problem. It covers all of the setup required. Next look at the Nastran documentation for the Basic Dynamics Users Guide to see  discssion of direct transient from the solver point of view.

 

Regards,

Joe

Re: Direct Transient Analysis

Valued Contributor
Valued Contributor
I did it. Unfortunatelly I dont't understand everything, that't why I am looking for any informations here. For exemple, Which value of damping function shall I apply and what does it depends on? I am making this kind of analysis for the first time in my life. I am asking for indulgence.

Re: Direct Transient Analysis

Creator
Creator

Hello,

I afraid it's too late fro answer, but...

I guess there is a mistake in your setting:

"I choosen damping ratio 0,2 becasue this is the damping ratio for steel"

No, this typical damping ratio 2% must be written as 0.02 - you used therefor 10x higher damping...

 

Regards, Jan

 

 

Re: Direct Transient Analysis

Valued Contributor
Valued Contributor

 

Hi Jakub

 

Regarding your damping value. The input G is the overall structural Damping Coeff, which is 2*Critical damping ratio.

 

For solid steel structures this can be 0.025, large welded increases this to say 0.03-0.04, large bolted joints increase this further. These are guidelines - not definitive values. Ideally you should conducted a FFT on your structure to establish your Q factor which can be used to calculate the critical damping ratio where Q = 1/(2*Critical).

 

For your model you should use a G value of (0.02-0.025)*2.

 

Below are the inputs I have used on a cyclic loading model:

 

DAMP.JPGLOADING.JPG

 

With the constant loading curve - although is important the value of G does not change the results a lot - when the loading is similar to yours where is it a shock load and the structures response is long etc. it becomes more critical.

 

Also, you mention that you have applied a load at the foot of the beam, but it looks like you have applied a body load to the entire beam?

 

Run the analysis again with the damping G corrected - then make a number of similar runs increasing the Damping value.