Nonstructural Mass Definition

The mass contribution from nonstructural features can be included in the model even if the features themselves are omitted. The nonstructural mass is smeared over an element set that is typically adjacent to the nonstructural feature. This element set can contain solid, shell, membrane, surface, beam, pipe, or truss elements. The nonstructural mass can be specified in the following forms:

• a total mass value,

• a mass per unit volume,

• a mass per unit area (for element sets that contain conventional shell, membrane, and/or surface elements), or

• a mass per unit length (for element sets that contain beam, pipe, and/or truss elements).

When a total mass is spread over an element set region, it can be distributed either in proportion to the underlying element “structural” mass or in proportion to the element volume in the initial configuration.

A “structural” mass is defined as the sum of all the mass contributions to an element outside of the nonstructural features. This may include the mass due to any material definitions associated with the element; any “mass per unit area” given on the section definition for shell, membrane, and surface elements; mass from any rebars included in shell, membrane, and surface elements; and any additional inertia given on the section definition of beam/pipe elements. A nonstructural mass contribution to an element is not allowed if that element has no structural mass.

A given element in the model can have contributions from multiple nonstructural mass specifications. The nonstructural mass in a given element will participate in any mass proportional distributed loads, such as gravity loading, defined on that element. When a nonstructural mass is added to a shell, beam, or pipe element with active rotational degrees of freedom, the nonstructural contribution affects both the element mass and the element rotary inertia. The element stable time increment increases with a positive nonstructural mass and decreases with a negative nonstructural mass. In general, it is easier to use a nonstructural mass definition to bring an additional mass into the model than to do the same with a group of point masses. It is also more beneficial in an Abaqus/Explicit analysis due to a possibly higher time increment.

Any mass proportional damping specified as part of the material definition (see Material Damping) will also apply to the nonstructural mass contribution assigned to the element or element set using that material definition.

Nonstructural mass contributions associated with an element set are not imported when transferring model data between Abaqus analyses (see About Transferring Results between Abaqus Analyses). These contributions need to be redefined in the import analysis if they are to be included in the model.

To define a nonstructural mass contribution to the model mass, you must first identify the region over which the contribution must be added. You then specify the value of the nonstructural mass using the appropriate units and, if the total mass from the nonstructural features is known, determine how the nonstructural mass is distributed over the region.