Bridge Strength Engineering Lab
Choose a bridge type, pick a construction material, set the span length, then incrementally add load until the structure fails. Explore how design decisions and material properties determine how much weight a bridge can carry.
Guided Experiment: How Does Material Choice Affect Bridge Strength?
If you keep the bridge type and span the same but change the material, which material do you predict will hold the most load? Why?
Write your hypothesis in the Lab Report panel, then click Next.
Bridge View
Controls
Design Your Bridge
Bridge Type
Simple horizontal span resting on two end supports
Material
Longer spans reduce load capacity due to increased bending moment.
Load Test
Design Capacity
45.7 kN
Target: 20 kN - Passes target
Add Load
Data Table
(0 rows)| # | Bridge Type | Material | Span(m) | Max Load Held(kN) | Failed At(kN) | Passed 20 kN Target |
|---|
Engineering Reference
Types of Bridge Structures
- Beam. The simplest form - a horizontal deck on two supports. Load causes bending stress. Economical for short spans only.
- Arch. A curved structure that converts bending into compression. Stone, concrete, and brick work well because they are strong in compression.
- Truss. A network of triangles. Triangles cannot change shape without changing the length of a side, making them rigid. Very efficient for medium spans.
- Suspension. Towers support long main cables. Vertical hangers carry the deck. Cables work in tension, which steel handles extremely well.
- Cable-Stayed. Cables run directly from a tower to the deck. Stiffer than suspension bridges and easier to build for medium-to-long spans.
Compression and Tension Forces
When a bridge carries a load, different parts of the structure experience different types of internal force.
Compression is a pushing or squeezing force. Arch bridges convert downward load into compression that travels along the arch to the ground. Concrete and stone resist compression well but crack under tension.
Tension is a pulling or stretching force. Suspension cables and truss members in the bottom chord are in tension. Steel wire can carry enormous tension loads without breaking.
The most efficient bridges route forces into the type of stress that their material handles best.
How Engineers Test Bridges
Before a bridge opens, engineers verify it can safely handle the expected traffic. They use several methods:
- Physical load testing. Trucks with known weights drive across the bridge while sensors measure deflection and stress.
- Computer simulations (FEA). Finite Element Analysis software predicts how every part of the structure responds to load.
- Scale models. Small replicas help visualise failure modes before construction begins.
- Safety factor. Real bridges are built to carry 2-4 times the maximum expected load. A safety factor of 2 means the bridge can hold twice the design load before failure.
Famous Bridges and Records
- Akashi Kaikyo Bridge (Japan). The world's longest suspension bridge at 1,991 m main span. Steel cables 1.1 m in diameter carry the deck.
- Millau Viaduct (France). A cable-stayed bridge whose tallest pier reaches 343 m. It crosses a valley at highway height.
- Firth of Forth Rail Bridge (Scotland). A cantilever truss bridge opened in 1890. Uses 55,000 tonnes of steel in its triangular structure.
- Lupu Bridge (China). World's longest steel arch bridge at 550 m. The arch rises 100 m above the deck.
- Ponte Vecchio (Italy). A medieval stone arch bridge from 1345, still carrying foot traffic across the Arno River in Florence.
