Coming In
What You Already Know
Unit 1 showed that engineers use mechanisms to control force and motion. Unit 2 shifts the focus to structures: systems that must carry forces safely without moving very much.
POE Unit 2 • Lesson 2.1
Unit Challenge Launch: Build a Better Bridge
Engineering question: How can a bridge span a gap, carry a required load, and remain lightweight and stiff?
Phase 1 · Understand the StructureLearnApplyDocumentCarry Forward
Learning Targets
By the End of This Lesson
- Explain the difference between strength, stiffness, and structural efficiency.
- Identify the criteria and constraints of the bridge challenge.
- Generate initial ideas for how chords, verticals, diagonals, deck members, and joints could carry load.
The Unit Challenge
Lightweight Bridge Structural Design Challenge
Design and manufacture a lightweight bridge that spans a fixed gap between two supports. The engineering problem is to carry a centered load efficiently through members and joints into the supports.
400 mmRequired clear span between supports.
65 mm maximumMaximum overall bridge width.
100 NRequired centered design load.
≤ 10 mmMaximum midspan deflection at 100 N.
≤ 150 gMaximum finished structural mass.
Standard SupportsEvery team uses the same teacher-provided support locations.
Manufacturing Rules
Wood and plywood primary members, open-web single-piece laser-cut plywood side trusses, localized gussets, and approved PLA/PETG connectors are permitted. Only M3 nuts/bolts for mechanical fastening. No acrylic, screws, or fully printed bridges. See the complete Unit 2 specification.
Left Support→Chords + Verticals + Diagonals→400 mm Span→Right Support
Learn the Concept
What You Need to Know
Use these ideas as your lesson notes. Focus on explaining the behavior, not memorizing vocabulary by itself.
Structure vs. mechanism
A mechanism is designed to move; a structure is designed primarily to carry loads while maintaining its shape. A bridge still flexes, but excessive deformation can make it unusable long before it actually breaks.
Strength
Strength describes how much load a material or structure can resist before yielding, fracturing, buckling, separating at a joint, or otherwise failing.
Stiffness
Stiffness describes resistance to deformation. A bridge may be strong enough not to break but still fail the design if its midspan deflection is too large under the required load.
Structural efficiency
Structural engineers cannot solve every problem by adding more material. A useful structure carries the required load with as little mass as practical.
Why mass alone does not determine success
Imagine Bridge A has a mass of 70 g and carries 45 N before failure. Bridge B has a mass of 45 g and carries 40 N. Bridge A is stronger in absolute load capacity, but Bridge B may be more structurally efficient because it carries nearly as much load with much less mass. Later in the unit you will quantify this tradeoff.
structurestrengthstiffnessdeflectioncriterionconstraintstructural efficiency
Team Project Kickoff
Launch the Bridge Design Project
- Review the challenge parameters as a team and separate criteria from constraints.
- Create at least two initial internal bridge concepts. These are starting ideas, not final designs.
- Label likely chords/girders, verticals, diagonals, deck/cross members, and critical joints.
- Create a need-to-know list of at least three engineering questions the team must answer before fabrication.
Student Resource
Bridge Mission Analysis
Use the printable mission-analysis sheet to capture criteria, constraints, initial bridge concepts, and need-to-know questions.
The resource opens in a clean print-friendly layout and can be printed or saved as a PDF from the browser.
Project Kickoff Record
Bridge Mission Analysis / Design Brief
This is the official starting record for the Unit 2 project, not a routine worksheet.
- Criteria and constraints summarized in your own words.
- At least two labeled initial bridge-structure concepts.
- A need-to-know list with at least three engineering questions.
Format: Use the provided Bridge Mission Analysis template or complete the same evidence in your engineering notebook when directed.
Carry Forward
Why This Matters Next
In Lesson 2.2 you will identify the forces the bridge must resist and predict where early concepts are most likely to fail.
Self-Check
Can You Explain It?
1. Why is “does not break” an incomplete success criterion?
Because the bridge must also meet the stiffness/deflection requirement and mass limit. A structure can remain intact yet deform too much or be too heavy.
2. Why can adding material be a poor structural-engineering solution?
Added material usually increases strength or stiffness, but it also increases mass. Efficient structures must balance performance with mass.
Engineering thinking · Lesson 2.1
Start with requirements, not a shape
Sketch two different open-web configurations and label how the midspan load reaches both supports.
Check your understanding: How will the 65 mm width limit affect cross-bracing and connector placement?
Bridge challenge connection: Apply this result to your planned 400 mm span, 65 mm maximum-width bridge; record evidence in your engineering notebook rather than assuming a design is compliant.