Mode shapes explained and demonstrated

[@mmhrakin8630]

Thanks for nice explanations with examples.

[@chinannabella1165]

Well explained. Thank you Sir

[@navinmech540]

Sir, Can you guide us how to do numerical simulation for modal analysis of a parts in a projectile ?

[@TheShavarin]

Great. Thanks for your efforts.

[@chinannabella1165]

Thank you for the lesson Sir. I am actually working on the dynamic analysis of a reinforced concrete bridge structure. But when I carried out numerical modal analysis considering 30 modes I realized the sum of my mass participation ratio for each mode is not even up to 80% of total mass, highest was at 77%. My main problem is: about the first 20 mode shapes had really small mass participation and instead of the first modes contributing a lot to the mass, I had the last modes contributing instead (to have up to 77% mass participation). From my knowledge of modal analysis, the first modes are usually the most important and the ones contributing a lot, but that's not the case with my analysis results. What could possibly be the problem Sir? (I assigned boundary conditions to both ends of the bridge)

[@SrinathSrinivasan]

You're welcome. As I am not from Structural engineering background but rather Mechanical, I cannot guide you accurately but will try to answer generally. If higher are modes are dominant in your structure, it could be either that higher modes are really contributing to the overall deformation or the lower modes have been suppressed by choice of boundary conditions (ex if the structure became too stiff). For example, consider a plate with too many accelerometers mounted on it. This certainly increases the system stiffness to such an extent that the lower modes are unable to make an impact. So the important thing to note here is that accelerometers need to be mounted in such a way that it does not alter the physical properties of plate (do not add mass or increase stiffness). Higher modes usually have complex shapes and are more localized. In contrast, lower modes have simple shapes and are more globalized. Also the excitation frequency range is important. Usually broad band excitation is given so as to check how the system responds overall. Certain frequencies will get excited which indicates resonance. But if the excitation frequency range is limited to high frequencies then the lower frequencies were not excited in the first place. So ensure all possible frequencies of interest are excited. These are the possible scenarios I could think of.

[@chinannabella1165]

@@SrinathSrinivasan Thank you very much for your response Sir. Will try to follow your advice and see if there are improvements

[@panem7527]

Great video thank you