1 · Research
Identify how Pratt, Howe and Warren-type webs differ, what carries the load and what connection challenges appear.
Principles of Engineering • Unit 2 • Lesson 2.15
Bridge Design Research & Concepts
Engineering question: Which three bridge layouts are worth developing into detailed engineering designs under our manufacturing and performance constraints?
Project launch
Your challenge: generate three meaningfully different designs
Until now, students have studied mechanics and materials separately. This is the first lesson in which they apply that evidence to the real 400 mm bridge capstone.
2 · Sketch
Produce three structurally distinct candidate bridges, with dimensions, two side frames, lateral bracing and centered load transfer.
3 · Select
Screen against required limits, compare alternatives and recommend the best concept for calculations in Lesson 2.16.
Read criteria5 minutes
Research & videos10–15 minutes
Generate sketches20–30 minutes
Evaluate & choose10–15 minutes
Suggested pace: approximately two 45–50 minute periods if students complete all three labeled side/top concept views and evaluation in class.
01 / Nonnegotiable design rules
Buildable does not mean unrestricted
Your bridge design must account for both structural performance and the exact fabrication constraints. These values come from the Unit 2 challenge specification.
400 mm
Clear span between supports
65 mm MAX
Overall width including fittings
100 N
Centered qualification load
10 mm MAX
Midspan deflection at 100 N
150 g MAX
Finished mass, everything included
Approved materials
Wood-based main members only
Permitted primary structure
Balsa, basswood, craft sticks or plywood may form the primary structure. A complete laser-cut open-web plywood side truss is allowed, but not a solid plywood side plate. Cross bracing and loading connection must be designed.
Do not use: acrylic, steel/carbon fiber/aluminum/polyethylene/Delrin primary members from the materials lab, or fully 3D-printed bridge bodies.
Connections and manufacturing
Allowed: PVA or CA glue, interlocking wood joints, approved plywood gussets, approved PLA/PETG fittings and M3 nuts/bolts (washers allowed). No screws, hot glue, epoxy, wire or other bolt sizes.
Plywood local gussets ≤40 × 25 mm and ≤800 mm², with overlap ≤20 mm onto each member and at most one layer per side. Printed fittings must meet separate 50 mm/15 g each, 40 g total and specified slicer restrictions.
Design approval note: The concept explorer below is a geometry/mass planning aid; it does not calculate strength, buckling, deflection or real joint capacity. Final compliance requires drawings, physical inspection and teacher-controlled testing. Read the full specification here ↗.
02 / Bridge research
Three ways to organize a truss web
Every example below is a schematic side elevation for comparison, not a complete build plan. Joint and diagonal orientation changes the force distribution and connection requirements.
Pratt-type truss
Diagonals generally slope downward toward midspan in the typical configuration. Under common downward load cases, these diagonals often carry tension while verticals carry compression.
Ask: Are short compression members and many glued joints an advantage for our wood-and-gusset build?
Warren-type truss
Repeating triangles can reduce the number of vertical web members and create a simple repeated pattern. Diagonal force directions vary by location and loading.
Ask: Does the central 100 N load land at a suitable joint or need a specially designed crossmember?
Howe-type truss
Diagonal orientation is opposite to Pratt. Under common gravity loading, the diagonals are often in compression, while the verticals tend toward tension.
Ask: How will slender diagonals resist buckling, and can the joints transfer those compressive forces?
03 / Think beyond a side view
How do two truss sides become a complete bridge?
A single 2D truss sketch cannot explain the entire test setup. Students must plan two parallel side trusses, transverse cross-bracing, a centered load-transfer member or platform and actual joining details.
Where does the 100 N go?
- The loading nose presses onto a designated central load-transfer point.
- A transverse member or joint arrangement transfers that force into both truss sides.
- Each side transfers force through diagonals, chords and joints to the supports.
- The support interface resists reaction forces without rolling, sliding or local damage.
For a symmetric ideal bridge with a centered load, the total vertical reactions are approximately 50 N at each end. Equal load-sharing between side trusses depends on symmetry and connection stiffness.
What should your top-view sketch show?
- Both side trusses, separated by enough space for bracing and the loading point
- Overall maximum width 65 mm, including hardware protrusions
- Cross-members preventing lateral movement or twisting
- Location and design of the loading crossmember
- Connection method for braces, fittings and gussets
Don't solve by adding a solid deck: the bridge still must follow the permitted open-web design and material rules.
04 / Watch and apply
Instructional videos for informed design research
These tutorials explain the three reference web patterns and review how axial forces flow through an idealized truss. Sketch alongside the video instead of only watching it.
Examples of Bridge Trusses: Pratt, Howe, and Warren — Michel van Biezen
Watch for: Pause on each structure. Draw and label the diagonals, identify whether a central load reaches a node, and identify a plausible manufacture/joint challenge.
Open video on YouTube ↗Understanding and Analysing Trusses — The Efficient Engineer
Watch for: Distinguish tension from compression, and describe how support reactions and member forces would be checked on your own concept.
Open video on YouTube ↗If a school browser blocks embedded YouTube, use the direct watch links above. The webpage diagrams and explanations can be used without the videos.
05 / Reliable sources
Find examples, then cite what you actually used
For every research source, save its title, link and one useful fact or design observation. Do not simply paste a bridge picture without analyzing its structure.
FHWA · Historic truss types
Federal Highway Administration reference showing historic Pratt, Howe and other truss configurations, how they worked and why materials mattered.
Open FHWA truss reference ↗NCDOT · Truss bridge identification
Real photographs and identification guides for major truss types, including common diagonal arrangements.
Explore NCDOT bridge types ↗SkyCiv · Modeling a Warren truss
Modeling process and analysis workflow that can inform the next lesson; design simulations are a check, not a substitute for physical prototype evidence.
Read the illustrated SkyCiv tutorial ↗Review Lesson 2.14
Transfer measured material limitations, efficiency reasoning and a realistic mass allowance into your bridge concept study.
Open 2.14 Structural Efficiency →06 / From research to drawings
Draw three complete, dimensioned concepts
Use graph paper with a clear scale or a Fusion sketch. At this stage, three separate designs are required; full manufacturing CAD and final joint dimensions will be developed in Lesson 2.17.
Side viewWeb, chords, supports, height and 400 mm clear span
Top viewOverall width ≤65 mm, both side frames and cross bracing
Connection conceptShow joint type, material and 100 N load entry
Risk and massIdentify slender compression members, gusset use and estimate
Specific annotations every concept needs
- Truss-family name or an explanation of a unique web arrangement
- 400 mm clear span measured between actual support points
- Truss rise and overall width in mm
- At least one highlighted potential tension member and compression member, marked as a prediction
- Center load introduction and both support-contact locations
- Cross-bracing / lateral stability strategy in top view
- Proposed approved wood/plywood components and joint methods
- Preliminary mass estimate and at least two design risks or questions
07 / Interactive geometry planning
Compare three possible bridge concepts
Adjust each candidate's truss type, side-truss rise, overall width and rough member dimensions. The diagrams and mass proxy update automatically. Use these to help visualize and estimate a concept—not to replace your dimensioned sketches or later analysis.
Concept A
Design A · explore geometry
—Indicative completed-mass proxy, not measured mass
Calculated lengths and dimensional screening will appear here.
Concept B
Design B · explore geometry
—Indicative completed-mass proxy, not measured mass
Calculated lengths and dimensional screening will appear here.
Concept C
Design C · explore geometry
—Indicative completed-mass proxy, not measured mass
Calculated lengths and dimensional screening will appear here.
Review concept estimates and any screening issues here.
08 / Select with evidence
Weighted concept decision matrix
After research and sketching, assign an evidence-supported 1–5 rating for each criterion: 1 = poor, 3 = acceptable/plausible, 5 = promising. Empty scores are intentional until students decide, and do not count as zero.
| Criterion | Weight | Concept A | Concept B | Concept C |
|---|---|---|---|---|
| Stiffness potential | 30% | |||
| Low-mass potential | 25% | |||
| Joint / fabrication feasibility | 25% | |||
| Stability and robustness | 20% | |||
| Weighted total (out of 5) | 100% | — | — | — |
Complete all four criteria for any concept you want to compare.
Decision rule: Scores reflect preliminary engineering judgments, not calculated member forces or proved capacity. A concept that violates a mandatory manufacturing or dimension rule cannot win, regardless of its matrix total. Verify all predictions in Lesson 2.16 and revise the decision if analysis contradicts the assumptions.
09 / Self-check
Work the question before revealing the explanation
Use a sentence, diagram or quick calculation for each answer.
A · Support reactions
Under an ideal centered 100 N vertical load and symmetric supports, what total vertical reaction would you expect at each end?
Reveal an explanation
50 N upward at each end from force and moment equilibrium. This does not guarantee the two side trusses share loads equally; crossmember design matters.
B · Design differences
Two sketches have exactly the same chords and web, but one has a different logo. Are these separate concepts?
Reveal an explanation
No. The structural configuration, loading, cross-bracing or connection engineering needs to change meaningfully, not decoration.
C · Width
Two side trusses are 55 mm apart measured outside-to-outside, but hardware adds 7 mm to each side. Does the bridge satisfy the 65 mm maximum?
Reveal an explanation
No. Overall width is 55 + 7 + 7 = 69 mm, which exceeds 65 mm.
D · Carbon fiber
Why can't the carbon-fiber strip from the 2.13 lab automatically be used as a bridge chord?
Reveal an explanation
It is a laboratory comparison material. Primary members must be approved wood/plywood under this challenge, regardless of strength or stiffness.
E · Truss drawings
Why is a side-view Pratt truss sketch not enough to begin construction?
Reveal an explanation
It does not show the second side truss, lateral stability, overall width, load-introduction crossmember, or detailed joins. The full 3D load path must be conceived.
F · Force predictions
Can you assume every diagonal in a Warren truss is in tension under the class test?
Reveal an explanation
No. Members may carry tension or compression depending on exact geometry, load location and support constraints. Perform structural analysis before labeling forces as definitive.
G · Mass estimate
An initial wood frame mass is estimated at 118 g and connections/bracing add 38 g. Is it below the 150 g limit?
Reveal an explanation
Total = 156 g, which is 6 g over the limit. The design needs a change.
H · Connection choice
A concept uses hot glue and M4 bolts because they are convenient. Can it pass the manufacturing review?
Reveal an explanation
No. Hot glue and M4 bolts are not permitted. The bridge specifies approved adhesives or joints and M3 nuts/bolts if using mechanical fasteners.
Required student evidence
Exactly what to submit
Submit a POE 2.15 Bridge Research and Three-Concept Design Packet as a PDF, teacher-provided template or organized engineering-notebook pages. This is a concept review, not final manufacturing approval.
Your packet must include
- Research notes from at least two credible sources, including source titles/links and one useful technical observation from each.
- Three distinct structural concepts (Pratt, Warren, Howe or defensible alternatives), each with a labeled side elevation and top-view sketch.
- Each side view marked with 400 mm clear span, truss rise, central load and support locations.
- Each top view showing total width ≤65 mm, lateral bracing, two side frames and centered load transfer.
- For each design, a proposed approved material and joint/fastener scheme, a preliminary mass estimate and two possible failure or fabrication risks.
- A completed weighted decision matrix for all three designs, with one sentence explaining the rationale for every major score.
- A four- to six-sentence recommendation naming the preferred concept and at least two evidence-backed trade-offs.
Before turning it in
- Concepts differ structurally, not cosmetically.
- Dimensioned drawing views are legible and use millimeters.
- The load path to both bridge sides and supports is clearly indicated.
- Construction plan uses approved materials and fastener sizes; plywood and printed fitting limits are acknowledged.
- Mass estimates include bracing and connections, not only chords.
- Speculative performance and mass estimates are labeled as assumptions, not tested results.
- The preferred design is a candidate for Lesson 2.16 analysis, not a claimed guaranteed winner.
Due date: Follow the Daily Agenda and current teacher submission directions.