The mass of the structure is input in the form of weights and internally converted by the program to mass units by dividing the weights by the acceleration of gravity. The mass may be provided by both specified nodal weights or by member weight/unit length or element material density. Nodal weights will contribute to only the diagonal terms of the mass matrix where the terms associated with the rotational degrees of freedom are often taken as zero. However, the user has the option of providing rotational inertias if this is felt to be appropriate. The x, y and z inertia quantities may be different in a structure particularly in a two-dimensional analysis where the frame being analysed is flanked by adjoining frames which carry vertical loads but have relatively insignificant lateral stiffness. In this case the vertical inertia is associated only with that of the frame being analysed while the horizontal inertia has to represent the sum of the inertia contributions of all frames being supported, in the lateral direction, by the frame being analysed.

The member contributions to the mass matrix may take one of three forms.

• The Lumped mass representation is where contributions are made to the diagonal terms associated with the three translational degrees of freedom at the nodes of a member with no contribution to the rotational degrees of freedom.
• The Diagonal mass representation is the same as the Lumped model above except that the contribution to the rotational degrees of freedom is equal to the diagonal term of the appropriate consistent mass matrix of the member concerned for beam, beam-column and wall elements. [see Cook, 1981]
• The third option is the Consistent mass representation using the kinematically equivalent mass matrix [Clough 1993] where inertia forces are associated with all degrees of freedom. This will result in a mass matrix for the structures with the same skyline form as that of the stiffness matrix. The Consistent mass model requires a greater computational effort in the multiplication by the nodal accelerations to get the inertia forces at each time-step in the analysis. It also bounds all natural frequencies and consequently gives a slight bias to the frequency content of the structure.

If the only masses are those from the input nodal masses then there is no difference in the actual mass matrices for the structure.