POE Unit 2

Bridge Structures & Materials

Driving question: How can engineers design a lightweight bridge that safely carries load while minimizing mass and deformation?

Start Lesson 2.1 Challenge Requirements Lesson Map POE Hub

Unit Challenge

Lightweight Bridge Structural Design Challenge

Design and manufacture a lightweight bridge that spans a fixed 400 mm gap between two supports. The focus is load paths, truss and beam behavior, stiffness, mass, materials, joints, fabrication quality, and test evidence.

400 mmRequired clear span between supports.

65 mm maximumMaximum overall bridge width.

100 NRequired centered design load.

≤ 10 mmMaximum midspan deflection at design load.

≤ 150 gMaximum finished structural mass.

Standard SupportsCommon teacher-provided support locations.

Fabrication + Testing Rules

Review the complete approved-material, laser-cutting, M3 fastening, and printed-connector limits below before fabrication. A one-piece laser-cut open-web plywood side truss is allowed; acrylic and fully printed bridges are not.

Instructional Sequence

Learn the Engineering, Then Build the Bridge

Lesson 2.1

Challenge Launch

Introduce the bridge goal and manufacturing constraints.

Lessons 2.2–2.10

Engineering Mechanics

Centroids, deflection, free-body diagrams, vectors, moments and truss analysis using examples beyond bridges.

Lessons 2.11–2.14

Materials & Testing

Materials properties, structural failure, experimental testing and efficiency.

Lessons 2.15–2.20

Bridge Engineering Capstone

Research, analyze, design, fabricate, test and present the engineering results to the class.

Lesson Map

Unit 2 Lessons & Deliverables

Lessons 2.2–2.14 teach foundational mechanics and materials using diverse examples. Lessons 2.15–2.20 apply that knowledge to the bridge capstone.

Lesson	Lesson	Deliverable	Open
2.1	Unit Challenge Launch: Build a Better Bridge	Bridge Mission Analysis	Open Lesson
2.2	Centroids & Center of Gravity	Centroid practice	Open Lesson
2.3	Beam Deflection	Beam deflection investigation	Open Lesson
2.4	Free-Body Diagrams	Free-body diagram practice	Open Lesson
2.5	Calculating Force Vectors	Force component calculations	Open Lesson
2.6	Calculating Moments	Moments and reactions	Open Lesson
2.7	Introduction to Truss Systems	Truss vocabulary and load paths	Open Lesson
2.8	Calculating Truss Forces	Method-of-joints practice	Open Lesson
2.9	Advanced Truss Analysis	Advanced truss calculations	Open Lesson
2.10	Truss Design & Simulation	Simulation comparison	Open Lesson
2.11	Structural Materials & Properties	Materials selection matrix	Open Lesson
2.12	Structural Failure	Failure case study	Open Lesson
2.13	Materials Testing Lab	Materials lab report	Open Lesson
2.14	Structural Efficiency	Efficiency and mass-budget exercise	Open Lesson
2.15	Bridge Design Research & Concepts	Three bridge concepts	Open Lesson
2.16	Bridge Engineering Analysis	Bridge engineering analysis	Open Lesson
2.17	Detailed Bridge Design	CAD + manufacturing package	Open Lesson
2.18	Bridge Fabrication	Build and inspection record	Open Lesson
2.19	Bridge Load Testing	Load-test report	Open Lesson
2.20	Bridge Design & Testing Presentation	Team presentation + slide deck	Open Lesson

Final Evaluation

What Success Means

Qualify

Carry the 100 N centered design load while keeping midspan deflection at or below 10 mm and meeting dimensional and mass rules.

Measure

Record load vs. deflection, maximum supported load, actual mass, failure mode, and structural efficiency.

Present

Deliver a team presentation using calculations, CAD and build evidence, measured test results and justified improvement proposals.

POE UNIT 2 · REQUIRED DESIGN RULES

Bridge Materials, Connections & Manufacturing Specifications

Use these limits when selecting materials, designing joints, creating toolpaths, estimating mass, and requesting fabrication approval.

Bridge performance

400 mm clear span between supports
65 mm maximum overall width, including connectors and hardware
100 N centered qualification load; deflection no more than 10 mm at 100 N
150 g maximum completed bridge mass including adhesive, M3 hardware, and printed parts

Approved construction

Balsa, basswood, craft sticks, and plywood for primary structural members
Laser-cut complete open-web side trusses from a single plywood sheet are permitted
No acrylic, solid plywood side panels, or fully 3D-printed bridges
Connections: PVA wood glue, CA glue, interlocking joints, plywood gussets, approved printed fittings, or M3 nuts and bolts (M3 washers permitted)
No screws, other-size bolts, nails, staples, wire, hot glue, or epoxy

Laser-cut reinforcement

Approved plywood only, up to 3.2 mm (1/8 in) thick
Local gussets/reinforcement plates: at most 40 × 25 mm and 800 mm² plan area
At most two gusset layers at one connection, one per side
Maximum 20 mm overlap onto each connected structural member
The gusset limits do not restrict primary member lengths or complete open-web side trusses

3D-printed connectors

PLA or PETG fittings for joints, gussets, brackets, or mounts only; no printed chords, beams, verticals, diagonals, or decks
Maximum 35% infill; 4 wall loops; 5 top and 5 bottom layers; 0.28 mm layer height; 0.4 mm nozzle
Maximum 50 mm length and 15 g per individual printed part; 40 g total printed-part mass
Do not join printed parts to form prohibited continuous members
Only M3 nut-and-bolt fastening; captive-nut pockets allowed, printed threads prohibited

Design evidence

Fusion CAD or dimensioned fabrication drawings, plus a bill of materials and mass estimate
Explain load transfer at two critical joints and identify all fasteners and adhesives
For printed parts: submit CAD models, slicer settings, and measured part masses
For laser-cut parts: submit cut profiles and verify minimum web/kerf suitability
Inspect against these requirements before official testing; no repairs after official testing begins