huangyhg

warning message about "eigen value" in ram advanse
dear all,
i have just started using ram advanse after having used staad-pro, strap, and other structural engineering software for the last few years.
i have just completed my first model. it is a simple two level rack, say 100 x 100 square feet, braced at the perimeter. i have put in some preliminary sizes, loaded it, run it and it seems to be working fine. i have release moments in all members except for the columns. all my supports are pinned.
however, i keep getting the following warning:
only 1 of 10 required eigen values found, it might be the case that only 1 degree of freedom with main exist.
a line above that reads, "active degrees of freedom 6 (tx ty tz rx ry rz).
can anyone tell me what the warning means, and how can i resolve it? i don't think the warning has any impact on my structure or the design of it.
also, what is an "eigen value"fff">? i know it is a multiplier, but not really sure how it ties in to my situation.
hope, one of you experienced in ram advanse, or someone having had the same problem before, can help me out, ok?
thanks, y'all.
the eigenvalue and eigenvector are used in matrix analysis and are therefore in structural analysis involving the same. usually in relation to a characteristic solution of a linear differential equation. you will find them in stability analysis (buckling) and modal analysis (vibrations, seismic).
your structure may lack the appropriate degrees of freedom for overall stability since all beams are pinned as all supports.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
the eigenvalue isn't a multiplyer, it is related to the frequency of a natural mode of vibration (in a frequency analysis) or a value belonging to a buckling mode.
the eigenvector represents the mode shape (i.e. relative displacements of the individiual nodes of the model) of the related eigenvalue (also called: natural frequency).
in your case, the warming may be related with improper boundary conditions (insufficient restraining of the model). if possible, activate the "find rigid body moves" option, or properly constrain the model.
if your boundary conditions are wrong as rob suggests then your results are likely to be meaningless, or at least, will require a great deal of manipulation to render meaningful.
cheers
greg locock
eigenvalue has many meanings, including the ones discussed above. the primary meaning is principal values of any square matrix. the interpretation of those principal values is where it gets interesting. the eigenvalues of a stiffness matrix k in the ku=f you are solving with fea is the natural mode of vibration as rob768 suggests; the natural mode of a tuning fork is the sound you hear when you give it a good whack. physically, if you excite a structure at its natural frequency, the response of the structure is amplified, which is why you want to avoid exciting a structure at its natural freq.
the eigenvalues of the 3x3 matrix of 3d stresses (sxx, sxy, etc.) are the principal values s1, s2, s3, which you can also find from the mohr's circle by rotating the diagonal so that the shear stresses are zero. to compute the eigenvalues of a square matrix, check almost any college level calculus or matrix algebra book.
well, do not concern by this warning, you just change the number of modes to calculate to 1.
the eignen values are directly related to the vibration modes, then you have only one degree of freedom (mass in one direction for example) therefore ram advanse only could found one eigne value in mass matrix.
regards

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