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Weird FEM issue (nat freqs) 
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#1
Nov811, 01:56 AM

P: 3

Hello,
I recently used Matlab to find the natural frequencies of a clampedclamped beam. It was fairly simple, as I construct the global mass and stiffness matrices. Then it's just a matter of using the eig() function. (For sake of simplicity I put beam properties all to 1.) When I choose how many elements (lets call it N) to use, it seems to give me natural frequencies which, seem to be smaller than usual. I somehow stumbled upon the fact that these frequencies are the literature values I have divided by N^2. Anyone got any ideas as why this is so? Here is my code. Thanks!! Edit: For example when N = 10, my first natural frequency is 0.2237 when the exact value is 22.37. Jeff



#2
Nov811, 07:23 AM

Engineering
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Thanks
P: 6,951

I haven't checked every line of the code, but I think you have each element of length 1, so the length of your beam depends on the number of elements in the model.
If the length of the beam is [itex]l[/itex], the global mass is proportional to [itex]l[/itex] and the global stiffness proportional to [itex]1/l^3[/itex], so the frequences would be proportional to [itex]\sqrt{k/m} = 1/l^2[/itex]. 


#3
Nov811, 10:20 AM

P: 3

Jeff 


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