Spring Link

A spring link is an element between two nodes that are apart. It carries one uniaxial spring per local direction, and nothing at all in the directions left out. It models a connection with a real stiffness: a bearing, a bolt, a damper, the interlayer of laminated glass.

🔧 Grasshopper component

Spring LinkAlpaca4d ▸ 02_Element

Inputs

LineLinelistOne line per link, from the node it starts at to the node it ends at, in m.

Both ends have to land on nodes the model already has.

MaterialMateriallistOne uniaxial material per direction, in the order of Direction.

A single material is used for every direction. Its E is read as a stiffness: kN/m for a translation, kN·m/rad for a rotation.

DirectionIntegerlistThe local directions that carry anything: 1, 2, 3 translation along local x, y, z; 4, 5, 6 rotation about them.

Anything left out is free.

PlanePlanealong the linkThe frame the directions count in.

Left empty, x runs along the link and y and z across it. Give the world XY plane to count along the global axes instead.

ElementIdText(empty)Optional name, shared by every link this component makes.

Outputs

ElementElementlistThe links, for the Elements input of Assemble.

They are on the branch their lines came from, as beams are on theirs, so links and beams wire into Assemble together without flattening.

The six directions

Direction	In the local frame	Acts as
1	translation along x	the axial spring, along the link unless a Plane says otherwise
2, 3	translation along y, z	the shear springs, across the link
4	rotation about x	torsion
5, 6	rotation about y, z	bending

A link on direction 1 alone pulls along its own axis and resists nothing else. On a solid's node only 1 to 3 exist, because a solid node has no rotations.

📈 When to use it

Use it when

• A connection has a stiffness you know: an elastomeric bearing, a bolt group, a flexible joint between two parts of the structure.
• Some directions should be stiff and others free: a slider, or a pin that carries force but no moment.
• Two parallel surfaces are joined through a soft layer. For laminated glass, give each node pair a shear stiffness k = G·A / t for the area of glass that node carries.

Do not use it when

• The join has no give at all → a Rigid Link is exact and has no stiffness to choose.
• Chosen degrees of freedom should simply read the same at both nodes → use Equal DOF.
• The node should be held against the ground → use a Support. A link cannot ground a node: its two ends have to be apart, and two points in the same place are one node.

💡 Notes

• A link makes no nodes. Assemble looks for a node within its Tolerance of each end, and stops with an error if an end has none. Put both ends on beam ends, mesh vertices or other nodes.
• A link has no mass. It is a stiffness between two nodes and nothing else. Weight at a bearing goes on a Mass Point at the node.
• The shear springs act at mid-length. A shear force on a link of some length also turns its nodes. That is not an error to tune away: it is what makes two offset panes act together.
• No result component reads a link's forces yet.
• Links do not survive a round trip through Tcl. Deserialise does not read twoNodeLink back, and says so in its warnings.

A tuned mass damper is one: the Outriggers and a Damper example hangs 660 t from a rod, a Truss, and swings it across on a Link along the same rod, with an elastic Uniaxial material across it each way, its E the spring's stiffness and its Eta the dashpot. The rod makes the node the damper's Mass Point sits on: a Link only joins nodes the model already has.

🔗 Relation to OpenSees

element twoNodeLink $eleTag $iNode $jNode -mat $matTag1 $matTag2 ... -dir $dir1 $dir2 ... -orient $yp1 $yp2 $yp3

When the frame runs along the link, only the y hint is written after -orient, and OpenSees takes x from the two nodes. A Plane that points elsewhere writes all six numbers, x first.