The 3D button in the View group takes the model into space. It is still Basic mode, with the same tools and the same member logic, but each node goes from three degrees of freedom to six: three displacements (ux, uy, uz) and three rotations (θx, θy, θz).
The Z axis is vertical, pointing up; X and Y are horizontal. Gravity points to −Z. A 2D model, which lives in the XZ plane, stands upright in that same plane when moved to 3D.
Besides the global axes, each member has its own local axes:
- local x runs along the member, from node I to node J.
- local z is the global vertical projected perpendicular to the member. In a horizontal beam it points up.
- local y completes the triad (y = z × x).
- In vertical members, where the vertical cannot serve as a reference, global X is used instead.
Member forces are expressed in those local axes. That is why, in a horizontal beam loaded by gravity, the main moment is My (bending about the local y axis) and the shear that goes with it is Vz.
Settings lets you choose a right- or left-handed local triad. It changes the sign of some member forces and which side the diagrams are drawn on, not their magnitude.
| 2D | 3D | |
|---|---|---|
| Degrees of freedom per node | 3 | 6 |
| Member forces | N, Vz, My | N, Vy, Vz, T, My, Mz |
| Matrix of one member | 6 × 6 | 12 × 12 |
| Stresses in the section | bending in one plane | biaxial bending and torsion |
The Results buttons add Mz, Vy and T (torsion).
Nodes are drawn on a working plane (XY, XZ or YZ) at a given level: for instance the XY plane at first-floor height.
In 3D a support is defined by ticking what it restrains: three displacements (Fx, Fy, Fz) and three rotations (Mx, My, Mz). The shortcuts Fixed (all six) and Pin. (the three displacements) cover the usual cases. Every unrestrained degree of freedom can be given a spring stiffness. Inclined rollers and prescribed displacements are options of 2D mode.
- Nodal: Fx, Fy, Fz, Mx, My, Mz.
- Distributed: in the member's local y and z directions, with a value at each end.
- Self-weight: as in 2D.
Watch the default direction. In 3D the point load starts on Fy and the distributed load on local y, which are horizontal on a horizontal beam. For gravity loads use Fz on nodes and qZ (local z) on members, and set qYI and qYJ to 0; otherwise the member also gets the horizontal load that comes by default.
Thermal loads and point loads within a member's span are entered in 2D; when the model moves to 3D they are kept and solved.
In 3D a joint releases any combination of the six relative movements between the member end and the node. It is set with the Joints mode of the Node tool: tick the movements to release (dx, dy, dz, θx, θy, θz) and click the member near the end. Sliding joints belong to the 2D model.
In a model with 3D geometry, the Hng I and Hng J columns of the members table release only the Mz moment. In a horizontal beam the gravity moment is My, so to hinge it for gravity loads release θy with the Joints mode. In a model that is still flat, such as one brought over from 2D, those columns release the in-plane moment, as in 2D.
A 3D member can work in torsion, and for that it needs the torsion constant J of its section. Catalogue sections and sections built from a shape have it computed. If you define an amorphous section, enter J by hand, so that torsion is computed with the value that belongs to the section.
The same button, which now reads 2D, takes the model back to the plane. If the model really is flat, the switch is immediate. If not, the program asks what to do:
- The plane: XZ, YZ or XY.
- What to take:
- One frame, cut at a distance. Takes only what lies in that plane, at the chosen distance: "give me the frame on grid line 3". The program lists the possible cuts with their members, supports and loads, and warns if one would be left without supports or without loads.
- The whole structure, flattened. Projects everything onto the plane. It is useful to see the structure from the side, but it stacks frames on top of each other.
- With the plane and the option chosen, Take it to 2D makes the switch. There are also Stay in 3D, to leave things as they were, and Erase model and switch to 2D, to start from scratch in the plane.
The original 3D model is kept: pressing 3D again brings it back exactly as it was. Changes you made to the 2D cut are not carried back into the 3D model.