BridgeSim — Bridge Load & Stress Simulator
User Manual

Version: v1.21 (or later)
Generated: February 21, 2026

What this app is for

BridgeSim is an interactive 2D truss/bridge simulator intended for training, demos, concept screening, and preliminary sizing. You can draw a truss, apply supports and loads (including a moving two‑axle truck), solve for member axial forces/stresses and support reactions, visualize utilization and factor of safety, scan for worst‑case conditions along a truck crossing, and export engineer‑friendly results.

Contents

1.       1. Safety & Intended Use

2.       2. Concepts & Solver Assumptions

3.       3. Installation & Requirements

4.       4. Getting Started (5‑minute walkthrough)

5.       5. User Interface Tour

6.       6. Building Geometry (Nodes & Members)

7.       7. Supports & Loads (Base and Truck)

8.       8. Solving & Interpreting Results

9.       9. Truck Crossing Tools (Autoplay, Auto‑Solve, Scan Max)

10.   10. Visualization Controls (3D look, Deformed shape, Grid/Snap)

11.   11. Results Table & Reactions Panel

12.   12. Exports (CSV / XLSX)

13.   13. Common Issues & Troubleshooting

14.   14. Appendix: Units & Conversions

15.   15. Appendix: Keyboard/Mouse Shortcuts

1. Safety & Intended Use

BridgeSim is not a certified structural design tool. Do not use it for final engineering sign‑off, public safety decisions, code compliance, or as the sole basis for fabrication. It is meant for:
• Training and demonstrations
• Concept screening (does this load path make sense?)
• Preliminary sizing and early trade studies
• Educational understanding of truss behavior

Always validate results with established engineering software, hand checks, and appropriate design codes before making real‑world decisions.

2. Concepts & Solver Assumptions

BridgeSim models the structure as a 2D pin‑jointed truss. Each member carries axial force only (tension/compression). Nodes have two translational degrees of freedom (Ux, Uy).

Key assumptions:

·         Linear elastic behavior (Hooke’s law).

·         Small displacements (geometry does not change during solve).

·         Members are axially loaded only (no bending/shear).

·         Supports are idealized as: Pin (Ux=0, Uy=0) and Roller‑Y (Uy=0).

·         Loads are applied at nodes as point loads (Fx, Fy).

·         Material properties are uniform per member: E (Pa), A (m²), and yield stress Sy (Pa).

·         If the stiffness matrix becomes singular, the structure is unstable (mechanism) or under‑constrained.

Outputs include axial member force (N), axial stress (Pa), utilization = |stress|/Sy, and support reactions computed from R = K·u − F.

3. What is Bridge Sim?

BridgeSim is a Python + PySide6 desktop application using OPENPYXL and other various python libraries

4. Getting Started (5‑minute walkthrough)

16.   Launch BridgeSim.

17.   Click “Reset Demo” to load the example bridge/truss.

18.   Enable the truck (right panel → Truck → Enable truck).

19.   Drag the Truck position slider or enable Autoplay to move the truck.

20.   Click “Solve” to compute forces/stresses/reactions for the current load state.

21.   Click “Scan max over crossing” to find the worst member utilization during a crossing.

22.   Review the Results table and Reactions panel, then export to CSV/XLSX if needed.

5. User Interface Tour

BridgeSim has three main areas:

·         Canvas (center): draw and edit your truss. Zoom with mouse wheel; pan with middle mouse or Space+drag.

·         Toolbar (top): choose tools (Select/Move, Add Node, Add Member, Delete, Pick Truck Path) and actions (Solve, Scan, Reset Demo, Clear).

·         Properties dock (right): global settings, grid/snap, truck controls, visualization options, results/reactions, and export actions.

Toolbar tools and what they do:

6. Building Geometry (Nodes & Members)

Nodes

Nodes are the joints of the truss. Each node stores its position in real units (meters internally) and may have support conditions and applied loads.

To add nodes:

·         Select “Add Node” tool.

·         Click on the canvas to place a node.

·         If Snap to grid is enabled, nodes will align to the grid.

·         If Snap to node is enabled, new nodes will avoid duplicating an existing node within the snap radius.

Members

Members connect two nodes and carry axial tension or compression. Each member has properties: E (Young’s modulus), A (area), and Sy (yield stress).

To add members:

·         Select “Add Member” tool.

·         Click the first node, then click the second node.

·         Duplicate members are automatically rejected.

Editing and deleting:

·         Use Select/Move to drag nodes. Member geometry updates automatically.

·         Use Delete tool to remove a member or a node (deleting a node removes its connected members).

·         Right‑click a node to edit supports and loads; right‑click a member to edit properties or delete it.

7. Supports & Loads (Base and Truck)

Supports

Supports constrain motion and prevent rigid‑body movement. In most bridges you need at least one Pin and one Roller‑Y.

Support types (right‑click node → Support):

·         None: free node (no constraints).

·         Pin: Ux=0 and Uy=0 (prevents translation).

·         Roller‑Y: Uy=0 (allows horizontal movement but prevents vertical).

Base loads

Base loads are fixed nodal loads that stay on the model (e.g., dead load, equipment, a known point load). Right‑click node → Load (base) to edit, add preset downward loads, or clear.

Truck loads (two‑axle)

The truck is modeled as a moving two‑axle vertical load distributed to nearby deck nodes along the defined truck path. Controls in the Truck section allow enabling/disabling the truck, setting the total load, axle spacing, and axle load split.

Truck parameters:

·         Truck load total: total vertical load (displayed as kN or kip depending on units).

·         Axle spacing: distance between axles (in length units).

·         Front axle fraction: fraction of total load on the front axle (rear axle gets the remainder).

·         Truck position: slider for t from 0.0 to 1.0 along the path.

Note: If no explicit truck path is defined, BridgeSim can fall back to an automatic ‘bottom chord’ path heuristic.

8. Solving & Interpreting Results

Solve

Click Solve to compute:
• Nodal displacements u (Ux, Uy)
• Member axial forces and stresses
• Utilization and factor of safety versus Sy
• Support reactions (Rx, Ry)
After solving, member colors and thickness reflect loading and utilization.

Member force sign convention:

·         Positive force = tension (T).

·         Negative force = compression (C).

·         Stress uses the same sign as force.

Utilization and FoS:

Utilization (%) = |stress| / Sy × 100.
Factor of Safety (FoS) = Sy / |stress|. FoS → ∞ if stress is near zero.

Understanding colors:

Members are colored by relative force ratio and may be overridden if yielding is detected. Heavier/thicker lines indicate higher magnitude relative to the current max |F|. When a member is selected from the results table, it is highlighted without changing its base color.

9. Truck Crossing Tools (Autoplay, Auto‑Solve, Scan Max)

Autoplay

Autoplay moves the truck continuously. Use Autoplay speed to control travel rate. Autoplay does not automatically solve unless Auto‑solve is enabled.

Auto‑solve (throttled)

Auto‑solve recomputes results repeatedly as the truck moves. The FPS setting limits solve frequency to keep the UI responsive.

Scan max over crossing

Scan max over crossing sweeps truck position t from 0 to 1 in a number of steps and records the worst member utilization. The worst member is highlighted and reported in the status bar.

10. Visualization Controls (3D look, Deformed shape, Grid/Snap)

3D bridge look

When enabled, BridgeSim draws members with a pseudo‑3D depth effect (front/back chords). This is a visualization feature only; the solver remains 2D.

If you experience selection/highlight issues, try toggling 3D look off and back on, or reset the demo.

Back‑truss depth layer

This adds a secondary ‘back’ layer offset by (dx, dy) for depth. You can adjust offsets to tune the 3D appearance.

Deformed shape overlay

Show deformed displays the displaced member lines over the original geometry. Because real deflections can be small, BridgeSim uses a deformation exaggeration factor.

Grid and snapping

·         Show grid: toggles grid visibility.

·         Snap to grid: node placement snaps to grid intersections.

·         Grid size: spacing of grid.

·         Snap to node: prevents near‑duplicate nodes; uses Snap radius.

11. Results Table & Reactions Panel

After Solve, the results table lists each member with:
• Member ID and end nodes
• Length
• Axial force
• Axial stress
• Utilization (%)
• T/C flag
• FoS
Click a row to highlight the corresponding member on the bridge.

The Reactions panel lists support reactions (Rx, Ry) at supported nodes. Reactions are reported in the current force units.

12. Exports (CSV / XLSX)

CSV export

CSV export is the most portable format. It typically includes nodes, members, and reactions, and may include multiple ‘load cases’ such as Base only, Truck only, and Total (Base+Truck). Open CSV in Excel, LibreOffice, or import into analysis scripts.

XLSX export (engineer‑friendly)

XLSX export creates a formatted workbook with column widths and load case sections. It requires the openpyxl package in Python builds. EXE builds may bundle this automatically.

What engineers usually expect in an export:

·         Project meta: units, timestamp, truck state/position.

·         Node table: coordinates, supports, Fx/Fy by load case.

·         Member table: i‑j, length, force/stress by load case, utilization and FoS.

·         Reactions: Rx/Ry by supported node, by load case.

·         Clear units in headers.

13. Common Issues & Troubleshooting

14. Appendix: Units & Conversions

BridgeSim maintains internal SI base units:
• Length: meters (m)
• Force: Newtons (N)
• Stress: Pascals (Pa)

Display units depend on the selected Units System:
• SI: length in m (or cm/mm if implemented), force in kN, stress in MPa
• US: length in ft/in (if implemented), force in kip, stress in ksi

Common conversion factors:

15. Appendix: Keyboard/Mouse Shortcuts

·         Mouse wheel: zoom in/out.

·         Middle mouse button drag: pan view.

·         Hold Space + Left drag: pan (hand tool).

·         Rubber‑band drag (Select mode): select multiple items.

·         Right‑click node: support/load menu.

·         Right‑click member: member properties menu.