Non-Structural Mass in Seismic Models

What constitutes non-structural mass

Non-structural mass is mass that contributes to inertia but is not part of the primary load-bearing structure. This includes architectural cladding, insulation, fireproofing, grout, cable trays, ducting, pipework contents, and equipment contents such as liquids or stored materials. Non-structural mass can represent a significant fraction of the total mass — in some industrial structures, non-structural mass exceeds structural mass.

Representation in the model

Non-structural mass can be represented in several ways: as additional density on existing elements (e.g. increasing the density of a floor slab to account for finishes), as lumped masses at nodes (e.g. equipment weight applied as a point mass), or as mass per unit area on shell elements (e.g. cladding mass distributed over a wall). The choice depends on the distribution of the mass and its effect on the dynamic response. Uniformly distributed mass is best handled by density adjustment; concentrated mass by lumped nodal mass.

Distribution

The distribution of non-structural mass affects the mode shapes and natural frequencies. Mass concentrated at the top of a structure lowers the fundamental frequency more than the same mass distributed uniformly. The distribution should reflect the physical layout. If the exact distribution is unknown, a conservative distribution should be assumed and the sensitivity assessed.

Verification

• Total model mass including non-structural mass should match the expected total
• Non-structural mass should be itemised in the analysis report so it can be reviewed
• If mass is represented by density adjustment, the adjusted density should be documented
• The effect of non-structural mass on the fundamental frequency should be checked against a hand estimate

Common errors

• Omitting non-structural mass entirely, producing a model that is too stiff and has frequencies that are too high
• Placing non-structural mass at the wrong location (e.g. at the base instead of the top), giving incorrect mode shapes
• Not accounting for liquid contents that may slosh, adding a fluid-structure interaction effect