Balsa Bridge Competition

A hybrid design-model-build-test competition in truss analysis, structural efficiency, and experimental validation

Overview

This activity challenges student teams to design, analyze, fabricate, and test a balsa-wood truss bridge. Each bridge must span 24 inches and have a width of 6 inches. Teams prepare full-scale AutoCAD construction templates, develop a structural model in SAP2000, predict the bridge’s failure load, and fabricate the physical model in the Structures Laboratory.

On competition day, each bridge is weighed and loaded to collapse. The measured load-carrying capacity is compared with the analytical prediction, allowing students to evaluate both the structural efficiency of their design and the accuracy of their computational model.

View Full Activity Details in the Handbook

Why This Activity Matters

Successful structural design is not simply about producing the strongest possible structure. Engineers must balance strength, stability, stiffness, material use, constructability, workmanship, and cost.

The Balsa Bridge Competition brings these considerations together in one integrated project. Students see how truss geometry, member forces, compression buckling, connection quality, and fabrication accuracy affect actual structural performance. The activity also demonstrates that even a sound analytical design may behave differently when material variability and construction imperfections are introduced.

Connection to Structural Engineering

This activity connects structural-analysis theory with the design and behavior of a physical truss system. Students apply concepts including:

structural idealization and load paths
support conditions and applied loading
axial forces in truss members
tension and compression behavior
Euler buckling of compression members
material stiffness and strength
structural efficiency
analytical prediction and experimental validation

The project also introduces students to the relationship between a computational model and the physical structure it represents.

Activity Status

First Implemented, Spring 2026

Main focus

Truss Behavior, Axial Forces, Buckling, Structural Efficiency, Prediction vs. Test

Course Connection

Structural Dynamics, Structural Analysis

Tools / Platforms

AutoCAD, SAP2000, Excel, Balsa-Wood Fabrication, Physical Load Testing

Activity Type

Design-Model-Build-Test Competition

Team Format

Small Teams — typically 3 students

Mode

Hybrid

Key Deliverables

AutoCAD Drawings, SAP2000 Model, Capacity Prediction, Fabricated Bridge, Presentation, Test Comparison and Reflection

Technical Snapshot

Bridge type: Balsa-wood truss bridge
Required span: 24 in
Required width: 6 in
Construction drawings: 1:1 scale AutoCAD templates based on member centerlines
Structural analysis: SAP2000 truss model
Reference loading: 100 lb total load represented by four 25 lb joint loads
Capacity prediction: First predicted member failure through tension rupture or compression buckling
Material calibration: Effective balsa modulus determined through small-scale bending tests
Measured responses: Load-carrying capacity at collapse and bridge self-weight
Efficiency measure: Load-carrying capacity divided by self-weight, LCC/SW

The Design Challenge

Student teams must develop a truss bridge that satisfies the required geometry while achieving high structural efficiency and reliable performance.

Each team selects its truss form, member arrangement, member dimensions, bracing configuration, and joint details. The proposed bridge is first documented through full-scale AutoCAD drawings showing the side, plan, and front views. These drawings become the templates used during fabrication.

Teams then create a corresponding SAP2000 model using the physical support and loading conditions. Member axial forces are obtained from a reference-load analysis, and the predicted failure load is calculated by comparing member demand with tension and compression capacities. Students must identify the governing member and state whether the predicted failure is caused by tension rupture or compression buckling.

After completing the analysis and prediction, teams fabricate their bridges from balsa wood, verify dimensional compliance, measure self-weight, present their design, and load the bridge until collapse. The final evaluation compares the predicted and measured capacities, observed failure location, fabrication quality, and overall structural efficiency.

The goal is not merely to build the bridge that carries the greatest load. Teams must create a design that is efficient, accurately modeled, carefully fabricated, and supported by sound engineering judgment.

Full details regarding geometry limits, loading points, AutoCAD templates, SAP2000 modeling, material calibration, capacity calculations, fabrication requirements, scoring, testing, and final deliverables are provided in the PolySDL Activities Handbook.