Mass Point
Adds mass at a node without adding a force. It is how you represent something that has weight in a dynamic analysis but is not modelled as an element: a tank of water, plant on a roof, a façade, the participating fraction of an imposed load.
🔧 Grasshopper component
RotationalMassRotational mass (mass moment of inertia) about each global axis.
LoadPatterns input — it does not need a Load Pattern.Model View, with Show Loads on, draws each mass as a dot on its node, with its mass under it when Show Values is on.
Note
The input is in kg, and is converted to the solver's mass unit when the deck is written — the same conversion an element's mass density goes through. Added mass and element self-mass are therefore on the same footing, and you can check that by reading TotalMassOfStructure from the Modal Report menu of Natural Vibration.
Before 0.11 this input carried a stray factor of 9.81, which made every point mass 9.81 times too heavy and any model relying on it report periods about 3.1 times too long. If you have a model calibrated against that behaviour, its periods will now drop.
📈 When to use it
Use it when
- You are running Natural Vibration or a transient analysis, and some of the inertia is not in the elements.
- You are lumping a secondary system — equipment, a tank, cladding — onto the primary structure.
Do not use it when
- You want a static force → use Point Load. Mass alone produces no static action.
- The self-weight of the modelled elements is what you are after → that comes from the material density, applied by Gravity Load.
TransMass is in kg: a 4 t mass is 4000. The Response Spectrum from Sticks example puts one on top of each of thirty cantilevers, each its own model, to make single-degree-of-freedom oscillators. The towers of a hundred storeys and more shaken by El Centro - the Bundled Tube, Braced Tube, Buttressed Core, Outriggers and a Damper and Diagrid examples - share each floor's mass, in kg/m² of its area, between the floor's nodes, a Mass Point a node; the Outriggers one also hangs a 660 t damper from a Link.
🔗 Relation to OpenSees
mass $nodeTag $mx $my $mz $mrx $mry $mrz ;# 6 DOF node
mass $nodeTag $mx $my $mz ;# 3 DOF nodeThe rotational terms are dropped on a 3 DOF node — a node belonging only to bricks or tetrahedra.
mass sets a node's mass rather than adding to it, so Assemble writes one line per node, with all the Mass Points on that node added up. Before 0.12 each Mass Point had its own line, and of two on one node only the last counted.