How to Model a 3D Frame — Supports, Local Axes, Releases, Diaphragms and Modes
How to build, load and check a model in the free 3D frame analysis tool, from coordinates and supports to local axes, releases, diaphragms, P-Δ and modes. The habits here (checking reactions, knowing your local axes, meshing for modes) apply to any stiffness program, including RISA-3D, SAP2000 and STAAD.
Start from a generator
The Model menu builds four kinds of structure: a 2D frame (bays × stories), a 3D building frame (bays in X and Z, stories, fixed or pinned bases, rigid or shear-tab beam ends, optional X or chevron braces in the end bays), a continuous beam, and a cantilever. Generate the closest one, then switch the model to "Custom — keep my tables" and edit from there. Typing a whole model in by hand invites a typo.
Joints, coordinates and supports
Y is up. Gravity acts in −Y, and the floor plan lies in X and Z. In US units, coordinates are in ft, forces in kip, section dimensions in in and E in ksi. Supports: fixed, pinned, roller (restrained in Y only) and pin + Rx.
Sections and materials
Sections are defined by shape: I-shape (d, bf, tf, tw), box or HSS (H, B, t), pipe (D, t), solid rectangle, solid round, or custom properties (A, Iz, Iy, J). Properties are computed from the plate dimensions without fillets, so they run a few percent below the AISC tables. For an exact match, use custom and type the tabulated values. Materials carry E, G and the unit weight used for self-weight and mass.
Members, local axes and β
A member runs from joint i to joint j: local x points from i to j. For a sloping or horizontal member, local y lies in the vertical plane and points up, so a beam's strong-axis bending is Mz (Iz). For a vertical member, local y points along global +X. A column's strong axis therefore resists sway in X, and its weak axis resists sway in Z. Set β = 90° to turn a column. The building period example shows that one setting moving the first period by 16%.
End releases
A pinned end releases both bending moments at that end. A member pinned at both ends carries axial force and torsion only, which is how to model a brace or a truss bar. A shear-tab beam is pinned at both ends. If every member at a joint is pinned there, the joint has no rotational stiffness. The solver then locks that rotation and says so, but only after confirming the joint can't also translate. A real mechanism is reported as unstable, with the joint and the direction.
Loads, cases and combinations
- Joint loads: Fx, Fy, Fz, Mx, My, Mz at a joint, assigned to case D, L or W.
- Member loads: a uniform w along a member in global X, Y or Z, or in local x, y or z. Gravity on a sloping rafter goes in global Y, per unit length of the member.
- Self-weight can be added to D from the section area and the material unit weight.
- Combinations are rows of factors on D, L and W. Add the ASCE 7 set you need, for example 1.2D + 1.6L, 1.2D + 1.0W + 0.5L and 0.9D + 1.0W, plus service combinations for drift.
Analysis options
| Option | What it does | Use it when |
|---|---|---|
| Rigid floor diaphragms | Ties every joint at a floor level to move as one rigid body in plan (two translations and a twist) | Concrete-on-deck or slab floors. Leave it off for flexible diaphragms (wood, bare deck) and for roofs that are not one level. |
| P-Δ | Adds geometric stiffness N/L and iterates | Sway frames with significant gravity load. Split columns into 2–4 elements to capture P-δ; see the P-Δ example. |
| Modes | Lowest natural periods, mode shapes, mass participation in X, Y and Z | Seismic and vibration checks. Mesh beams into several elements for vertical modes. |
| Mass from | Self-weight only, or self-weight + D loads | Put the seismic weight into D, then use "self-weight + D loads". |
Check the model before you believe it
- Reactions = applied loads. The summary shows the sum of reactions; compare it to your own load total for each combination.
- Deformed shape. Animate it, or look at it in Front XY, Plan XZ and Side ZY. A floating joint or a released member stands out immediately.
- Diagram signs. Moments are drawn on the tension side. Click any member for its axial, shear, moment and deflection charts along the length.
- One hand check. A wL²/8, a portal drift or a column P-Δ amplifier. The tool's own verification table lists closed-form problems it reproduces, and the portal example works one through by hand.
- Mass participation ≥ 90% in each horizontal direction before you trust a modal result.
Save, share and export
Download the model as JSON and open it again later, copy a share link that rebuilds the model in a colleague's browser, or download member forces as CSV for a spreadsheet design check. Nothing is uploaded. The model and the analysis stay in your browser.
Worked examples
Portal frame, fixed vs pinned bases · P-Δ column · 3-story building, modal periods
Frequently asked questions
How are member local axes defined in a 3D frame program?
Local x runs from the member's start joint to its end joint. For non-vertical members, local y is usually taken in the vertical plane, pointing up, so beams bend about local z under gravity. Vertical members need a separate convention: in this tool, local y points along global +X. A rotation angle (beta) turns the section about local x. Always check the convention when you compare programs, because RISA, SAP2000 and STAAD differ in their defaults.
When should I use a rigid diaphragm?
Use it when the floor is much stiffer in its own plane than the lateral system, typically a concrete slab or a concrete-filled metal deck. A rigid diaphragm distributes lateral load to frames by relative stiffness and captures torsion. Wood and untopped metal deck diaphragms are usually flexible or semi-rigid, and then the load goes to the frames by tributary area instead.
Why doesn't the sum of reactions equal my load?
Usually a load is in the wrong case or combination, has the wrong sign, or a member load is per unit horizontal length when the program applies it per unit member length. Self-weight being on or off is the other common cause. The reaction check catches all of these, so make it before reading any member force.