Stability Lab looks for mechanisms: any way the structure can move without a member stretching or bending. If there are none, it pushes the structure along X and Y to measure lateral sway, and applies a downward Z push to explore vertical load transfer.
Your structure
Key
Rigid end: no hinge drawn
Pinned end (hinge)
Pinned support
Fixed support
Cable (tension only)
Slack cable (doing nothing)
Line width: thin, regular, large
Design code
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About Stability Lab
Version 2.0 · 2026
Stability Lab is an interactive teaching tool for structural stability. Students build frames, braced frames, walls, floors and bridges on a 3D grid, choosing rigid or pinned joints and pinned or fixed supports, then check the result. The app runs a 3D elastic stiffness analysis to find mechanisms (any way the structure can move with nothing resisting) and animates them. If the structure is stable, it applies standard pushes along X and Y and grades the sway, with an additional vertical Z push for load-path exploration, so students can see the difference between unstable, flexible and stiff. Members, walls and slabs come in three sizes (thin, regular, large), cables work in tension only (so the structure is pushed both ways), and stable, stiff designs get an efficiency score based on the cost of bracing, rigid joints, fixed bases, panels and larger sizes.
New in v2: the load path explorer shows axial member forces, shear and bending end actions, equivalent diaphragm and wall bar forces, and foundation reactions. Explore all applied loads or one loaded joint’s contribution, and inspect elements by clicking the model or using the menus. Cable models retain the combined loading to keep their slack / taut state consistent.
Educational purpose
Developed for CENV2035 City Infrastructure Design Project at the University of Southampton, to help students connect structural systems, connection types, bracing and diaphragms with stability. It checks stability and relative stiffness only. It does not check member strength, buckling or code compliance, and must not be used for design.
Developer
Dr Ahmed Elkady, Associate Professor of Structural Engineering, University of Southampton · a.elkady@soton.ac.uk
Material, sections and reference loads
Computed forces and movements use a linear elastic model with steel-like properties: Young’s modulus E = 210 GPa and shear modulus G = 81 GPa. One grid step is treated as 1 m in the analysis. Sections are generic teaching values, with the same bending stiffness about both local axes, rather than named catalogue sections.
For the Regular size, beams and columns have area A = 5,000 mm², second moments of area I_y = I_z = 1,200 cm⁴, and torsion constant J = 20 cm⁴. Braces and tension-only cables have A = 1,200 mm² and pinned ends. Slabs and walls are represented by six equivalent in-plane bars per panel, each with A = 30,000 mm²; these are equivalent stiffness properties, not a physical slab thickness or concrete section.
Thin multiplies the section properties by 0.4, Regular by 1, and Large by 2.5. The factors scale A, I and J for members, and the equivalent bar areas for panels. They change stiffness without changing the displayed grid geometry.
The reference nodal load is 80 kN before tributary weighting. X and Y pushes act on the windward face; Z pushes act at elevated floor / roof joints. Outer edges and the top level receive half weighting, so individual joint loads can be smaller. Self-weight and distributed member loads are not included. Reported forces are in kN, moments in kN·m and movements in mm; animation magnifies movement by the displayed scale factor. Values describe the response to these reference loads and section assumptions.