According to my information, the lumped mass is a diagonal mass that has six elements (u1, u2, u3, r1, r2, r3). u1, u2, and u3 have a value equal to (ml/2) while r1, r2, and r3 have a value equal to zero. In Figure (1-a), the matrix is arranged and constructed correctly, while in Figure (2-b), the matrix is unarranged and constructed differently why?
Dear Mahmoud Mohamed
This problem, usually happen with SAP2000 and ETABS models because the algorithms of CSI programs have a little bug during exporting the mass and stiffness matrix elements and it is not arranged truly according to the degree of freedoms and this matter have made results interpretation hard .
But what is the solution ??
if your model is simple and without too many elements or it is 2D you can approximately solve the equation, otherwise you must find which number is for which DOF by guessing it or you can model it in other software to ensure about arrangement of mass or stiffness matrix.
Note that CSI software calculate mass and stiffness matrix truly but the problem is that the software displaces the elements of matrix when it exports the text file.
Regards,
Hossein
Yeah one of the solutions is to write a small code in matlab so that you will be able to get the mass matrix for the structure, but as we are not sure that how big your model is and what exactly you want to achieve from this?
Hossein Safar Yousefifard, your answer is interesting. I have the same issue with the extracted matrix. My primary concern is to find the center of rigidity since SAP2000 doesn't have this option. Is there any workaround you suggest?
Thanks in advance.
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