Fundamentals
Supports, joints and member end releases in structural models
Fixed, pinned and roller supports, soil springs, rigid and pinned joints, and member end releases: what each one means, how it changes the forces, and how to set it in Struxt3D.
A model is only as good as its boundary conditions: how it is held at the ground, and how its members are joined to each other. The same beam can have a midspan moment of wL²/8 or wL²/24 depending only on its ends. This tutorial explains supports and joints, shows how much they change the results, and how to set them in Struxt3D.
Degrees of freedom, again
Each node can move along X, Y, Z and rotate about X, Y, Z. A support stops some of these movements; in exchange it supplies a reaction — a force for each stopped translation, a moment for each stopped rotation.
| Support | Stops | Typical use |
|---|---|---|
| Fixed | All three translations and all three rotations | Column bases cast into a stiff foundation; moment-resisting base plates. |
| Pinned | The three translations; rotations free | Simple base plates with two or four bolts, bearings, bridge piers in one direction. |
| Roller | Vertical (Z) only | Bridge expansion bearings, sliding supports. Make sure something else stops the structure sliding sideways. |
| Spring | Resists movement with a stiffness k | Soil under a raft (Winkler springs), flexible supports. |
Fixed or pinned bases: the effect on a portal frame
For a single-bay portal frame under a sideways load, fixed bases share the overturning between base moments and the beam, while pinned bases push all the moment into the knees (column tops). The difference in sway is large: a fixed-base column is about four times stiffer sideways than a pinned-base one of the same section (12EI/L³ against 3EI/L³ for a column with a rigid beam on top).
- Fixed bases: smaller sway and knee moments, but the foundation must resist a moment — bigger footings, more anchor bolts.
- Pinned bases: simpler, cheaper foundations; larger column and rafter sections to control sway.
Choose the base that matches how the foundation will really be built. A "fixed" base on a small pad footing on soft soil will rotate; the model will flatter the frame.
Line supports and soil springs
Walls and slabs are meshed into many shell elements, so their supports must act along a line or over an area:
- A line support holds every mesh node along a wall's base. The retaining wall template fixes the stem along its base this way.
- A subgrade (Winkler springs) puts a vertical spring under every mesh node of a slab on the ground. Each spring's stiffness is the subgrade modulus (kN/m³) times the area the node carries. The raft foundation template uses this, so the raft settles more under heavy columns and bends accordingly.
Joints between members
Where members meet, the joint can be rigid (the members turn together and moments pass through) or pinned (shear and axial force pass, moment does not). In a model, a pinned joint is made by releasing the rotations at a member's end.
Worked example: one beam, three end conditions
A 6 m beam with w = 20 kN/m (wL² = 720 kN·m):
| Ends | Support moment | Largest span moment | Deflection |
|---|---|---|---|
| Pinned both ends (simply supported) | 0 | wL²/8 = 90 kN·m | 5wL⁴/384EI |
| Fixed both ends | wL²/12 = 60 kN·m | wL²/24 = 30 kN·m | wL⁴/384EI (5× less) |
| Fixed one end, pinned the other | wL²/8 = 90 kN·m | 9wL²/128 ≈ 50.6 kN·m | ≈ 2.4× less than pinned |
Real joints are in between: the end moment depends on how stiff the column or girder it frames into is. That is exactly what a 3D analysis captures when the joints are rigid.
End releases in Struxt3D
Each member has twelve end degrees of freedom — N, Vy, Vz, T, My, Mz at the start and at the end, in the member's local axes. Releasing one means that force or moment is zero at that end. Struxt3D offers four presets:
| Preset | What is released | Use for |
|---|---|---|
| Rigid | Nothing | Moment connections, cast-in-place concrete. |
| Pinned | My and Mz at both ends, and torsion at the start (so the member cannot spin) | Simple steel beams with fin plates or angle cleats; secondary beams. |
| Pin start | My and Mz at the start | A beam continuous at one end, pinned at the other. |
| Pin end | My and Mz at the end | The same, the other way round. |
Bracing, truss webs and cables belong to pin-jointed groups: they carry axial force only, and their releases cannot be changed.
Too many releases make a mechanism: a frame with every beam pinned and every base pinned has nothing to resist sideways load. Struxt3D warns when the model is unstable. Add bracing, a core, or rigid joints somewhere.
Try it in Struxt3D
- Start the Portal frame shed template. In How is it held and loaded?, the Bases buttons switch every base between Fixed and Pinned (templates that set their own supports, such as bridges, don't show this).
- Run the analysis with fixed bases, note the sway under a wind case (Displacement), then go back, choose Pinned, and run again.
- To change one support, use the Support tool in the workspace and click a node to add or remove its support. In the Full interface, select a node and use Assign › Fixed support, Pinned support, Roller support or No support.
- End releases are in the Full interface: select a member, then Assign › Pinned at both ends (or Rigid ends, Pinned at the start, Pinned at the end). The member's Properties also have an End releases grid where each degree of freedom can be released on its own (R = released).
- Compare the Moment diagram before and after: a pinned end drops to zero moment and the midspan moment grows.
- Check Results › Reactions to see the forces and moments each support provides.
Summary
- Fixed stops all six movements; pinned stops translations; roller stops vertical only; springs resist with a stiffness.
- Base fixity changes sway and foundation forces dramatically.
- End releases make pinned joints; four presets cover most cases.
- Model the connection you will actually build, and keep the structure stable.