## Question

In Structure Type option, what does off-diagonal masses imply, where and how should it be used?

## Answer

For any object , there is a unique direction along which the object has either small or large moment of inertia, accounting for the mass distribution. This in general is the mutually perpendicular x-y axis. But when the mass distribution is any arbitrary third angle has to be accounted for , off –diagonal masses comes into play.

From the main menu select Structure > Type >Structure Type

Consider Off-diagonal Masses :

When this option is checked on, all terms including off-diagonal terms in the lumped mass matrix are considered for mass calculations. The accuracy of results increases with a full lumped mass matrix, but the analysis time may increase.

For example consider this case where, the torsional frequency for a simple 2-D beam element has to be computed. In general, a frequency for the Rotational X degree of freedom can not be obtained from the eigenvalue analysis. This is because of all the masses lumped at the node, not accounting for the torsional distribution. Hence when the ‘Off-diagonal Masses’ option is used, this Rotational X DOF could be obtained.

However the following things are to be noted. When a section offset is considered, a node will be generated at the offset location and the loads, boundary conditions, masses, etc. to be applied to the node will be entered to the node at the offset location. However, structural characteristics related to elements (e.g., element stiffness, loads to be applied to the elements, masses converted from self-weight of elements, etc.) have to be entered at the centroid of a section. If this option is checked, masses converted from self-weight of elements are entered at the centroid of a section. Nodal mass and nodal load, which is entered at the node and has no relation to elements, will be entered at the offset node.