Stability check

What "unstable" actually means, and how to read the four checks Truzme runs on your structure.

Quickstart

1. Watch the stability icon in the toolbar — a balanced scale means "This structure is stable"; a tipped scale means "This structure is unstable. Click to see why".
2. Click it to open the Stability panel — it lists exactly what's wrong, in plain language.
3. The affected nodes, members, and supports are also highlighted red directly on the canvas: a red circle with a move icon on a node that can swing free, a red dashed line on a member that isn't held, red supports.
4. Fix whatever's flagged — usually adding a support or a member — and watch the panel clear once the structure is determinate again.
5. Once stable, the panel's Horizontal forces / Vertical forces sections let you double-check that loads and reactions actually add up to zero.

What it is

Solving a truss assumes the structure is a rigid, fully-supported shape — before computing any forces, Truzme checks whether that assumption actually holds. This runs continuously as you edit, with no button to press, and covers four different ways a structure can fail to qualify.

Support check

A 2D rigid body has 3 degrees of freedom — it can shift horizontally, shift vertically, and rotate — so it takes at least 3 independent restraints, from supports that aren't all pointing the same way and don't all cross at a single point, to pin it down completely. Truzme flags:

• Not enough supports — "There aren't enough supports to keep the structure in place".
• Single node, no pin — "A structure with a single node needs a pin support".
• Parallel supports — "All supports point the same way, so the structure can slide sideways".
• Supports that all meet at one point — "The support lines all meet at one point, so the structure can rotate around it" — the reaction lines need at least two distinct crossing points, or nothing stops rotation about that one point.
• No supports at all — "The structure has no supports — add at least one to hold it in place".

Solvability

Beyond having enough supports, the numbers have to work out too. Truzme compares the number of members plus restrained directions (m + r) against the total degrees of freedom (d): fewer members and supports than d means the structure is a mechanism, not a truss — it can move without any member changing length. This shows up as:

• "Part of the structure can move freely — try adding more supports or members"
• "Some nodes move too much — the structure isn't holding its shape" — a node's computed displacement blew up, which is what actually happens numerically when the underlying equations turn out to be unsolvable (a singular system) rather than just under-restrained.

Loose parts

This is the visual half of the solvability check: instead of only saying "something's wrong", Truzme points at exactly which node, member, or support. A node is flagged when everything connected to it — its members, its support — lies along the same line, so nothing stops the node from swinging sideways: "A node can swing freely — it needs another member or a support to hold it in place". A member is flagged the same way when the wider structure it belongs to is unstable: "Part of the structure can move freely — try adding more supports or members". If your model splits into more than one disconnected structure, the panel prefixes the list with how many of them have problems, e.g. "1 of 2 structures have problems:".

Equilibrium

Once a structure solves, Truzme adds up every applied load and every support reaction, separately for the horizontal (Fx) and vertical (Fy) direction — a basic sanity check, since a correct solve should always balance exactly:

Action + Reaction = 0

The panel shows each force as its own chip (filled for an applied load, outlined for a support reaction) under "Horizontal forces" and "Vertical forces", with the running sum at the end. A sum of 0 means the check passes; a non-zero sum is shown in red with "Not in equilibrium", which points at a genuine problem worth double-checking rather than something to fix by hand — it means the solve itself found an inconsistency.

Behavior notes

Runs continuously. There's no "check now" button — every edit re-evaluates all four checks immediately, so the toolbar icon always reflects the current state of the model.

Every problem is listed, not just the first one. If a structure has several issues at once, the panel lists all of them together rather than stopping at the first match.