Measure
Record real force–deflection pairs from one supervised test, including an observation at exactly 100 N if safely reached.
POE • Unit 2 • Bridge Capstone • Lesson 2.19
Bridge Load Testing
Engineering question: Does the finished bridge carry the required 100 N with no more than 10 mm midspan deflection—and what do the real measurements reveal?
Learning outcomes
Replace predictions with evidence
Evaluate
Check bridge mass, span, outside width and the measured ≤10 mm displacement criterion without confusing estimated or actual data.
Explain
Use the plotted response and observed damage to compare the test with the structural model from 2.16 and prepare for the final class presentation in 2.20.
Suggested pacing: Teams rotate through the instructor-run test while others analyze evidence and prepare reports. Only the teacher selects safe test steps, gauge capacity and stopping rules; no force increments are imposed by this webpage.
01 / Qualification requirements
Every requirement matters
100 N
Total centered qualification load
≤10 mm
Measured midpoint deflection at exactly 100 N
400 mm
Clear distance between supports
≤65 mm
Outside width including M3 hardware
≤150 g
Completed mass including adhesive and connectors
Approved only
Wood/plywood structure and permitted joints
The official 100 N criterion applies to the whole bridge, not independently to each side frame. Meeting a mass or load-to-mass target does not override other dimensional, deflection or manufacturing restrictions. Check complete rules ↗.
02 / Understand the apparatus
Two separate measurements: force and midpoint deflection
Apparatus checklist
- Teacher-signed 2.18 pre-test inspection and a labeled finished bridge revision.
- Rigid fixed support fixture with the required clear span; no rocking or slip.
- Mounted calibrated digital force gauge and a teacher-approved rated loading mechanism.
- Independent fixed-datum midpoint displacement gauge, dial indicator or properly calibrated viewing setup.
- Digital scale and calipers for actual mass, clear span and maximum outside width.
- Guarding, eye protection and an exclusion area for failure debris.
Establish an accurate zero
- Teacher verifies fixture capacity, alignment and support conditions.
- Locate the intended central loading crossmember and ensure direct vertical loading without eccentricity.
- Set the force gauge zero according to its operating instructions and the approved preload procedure.
- Measure the bridge's unloaded midpoint reference with a stationary independent datum.
- Record all subsequent force and midpoint movement from the same baseline; do not confuse actuator travel with actual bridge deflection.
Do not improvise a press or push by hand in place of a rated mounting system. Gauge and fixture ratings must be suitable for the load being applied.
03 / Test safety
Teacher controls the force; observers stay clear
Before starting
- Verify that the finished bridge is approved, has cured, and has no unreconciled defects.
- Check rated capacity of force gauge, attachments, fixture and loading contact.
- Set teacher-defined force steps, ramp/hold time and maximum allowed force.
- Position the bridge securely and establish the fixed 0 mm reference.
- Assign one operator, a load reader, a deflection reader and a recorder.
Stop immediately for
- Unexpected crack, joint separation or progressive splintering.
- Sideways twist, buckling, loss of support contact or sliding fixture.
- Sudden uncontrolled deformation or force drop.
- Force gauge or fixture operating outside its rated range.
- Any unsafe condition or teacher stop command.
No students touch the loaded bridge. The instructor directs unloading and evaluates whether the test can safely continue.
Policy: No repair is permitted after the official test begins. If the bridge cannot be tested safely, record an incomplete or stopped test. Testing beyond 100 N for a greater demonstrated load or failure study is optional, instructor-approved and requires a suitably rated guarded rig.
04 / Repeatable procedure
Follow these six steps in order
1 · Record pre-test
Bridge ID/revision, team, actual mass, span, overall width, gauge ID and signed inspection.
2 · Position and zero
Set the supports to the real clear span, center the loading nose, zero force and record the fixed midpoint reference.
3 · Apply approved load
Instructor advances to the next authorized increment and allows the reading to stabilize as required.
4 · Log a paired reading
Record actual F (N), actual midspan δ (mm), reading order, visible damage and any measurement uncertainty.
5 · Document 100 N
If safely achieved, enter the actual simultaneous force and deflection values at 100 N; record whether deflection is at or below 10 mm.
6 · Stop, inspect and save
Teacher stops for completion, apparatus limit or structural damage. Record why, retain photos and export data without repairing the bridge.
05 / Learning resources
Watch a truss test and see how engineers use the data
DiscoverE: Truss Bridge Challenge
Watch the truss change under center loading. Identify the support points, the first visible bending and the differences between classroom demonstrations and our calibrated force-and-deflection test.
Open on YouTube ↗Vernier — Measuring load and deflection
An engineering educator explains instrumented bridge-testing strategies and why the force and midspan displacement must be recorded together.
Watch the Vernier webinar ↗University of Toronto — Bridge testing day
See structural-model loading, inspections and failure mechanisms in an engineering program. Their test capacity and loading positions are different from ours.
Watch university bridge testing ↗06 / Real experimental data
Live force–deflection recorder
There are 12 blank measurement rows; these contain no invented data. Record paired observations from the teacher-operated test. Enter the 100 N observation precisely if obtained. Browser values are temporary: export them before leaving the page.
Physical bridge and equipment
Official test outcome
A test that stops at 100 N proves the maximum demonstrated load is 100 N, but does not reveal ultimate strength or the force that would break the bridge.
Paired experimental observations
| Step | Force F (N) | Midpoint δ (mm) | Observed change |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 | |||
| 6 | |||
| 7 | |||
| 8 | |||
| 9 | |||
| 10 | |||
| 11 | |||
| 12 |
Enter measured readings once the instructor has started the official test.
07 / Analyze and qualify
Measured graph, qualifying reading, and structural efficiency
Midpoint deflection vs. force
Blue line: measured points in recorded order. Orange vertical: 100 N. Red horizontal: 10 mm. These thresholds remain visible even before measurements are entered.
Live experimental calculations
—Deflection at exactly 100 N
—Highest verified observed load
—Measured finished mass
—Approximate initial slope k
—100 N per gram
—Observed load per gram
Qualification not yet verified.
An estimated value between readings cannot replace a real measured 100 N observation.
Whole-bridge stiffness
kbridge ≈ ΔF / Δδ [N/mm]
The calculator reports a through-origin approximation based on the first three-to-five positive paired loading readings at or below 100 N. This assumes correctly zeroed instruments and an approximately linear initial region. If that assumption is not reasonable, discuss the graph without using a single stiffness value.
Performance per mass
ηqualification = 100 N / finished mass (g)
ηobserved = highest verified observed F (N) / mass (g)
These are supported load per gram calculations. Neither is a measured ultimate failure efficiency unless failure was actually observed and its force verified. A heavier or noncompliant bridge can have high N/g and still fail qualification.
08 / Structural behavior
Classify visible changes and damage
What can fail?
- Compression diagonal or top chord bows or buckles.
- Plywood gusset peels or adhesive separates, or M3 bearing zone splits.
- Whole bridge twists due to weak lateral bracing.
- Local center loading surface or end support crushes.
- Bridge remains upright but exceeds the 10 mm displacement limit at 100 N.
Document for 2.20
Mark the actual defect on a photograph or sketch; record the first observed load if known; compare its location to predicted compression forces and weak joints from 2.16; and propose two evidence-driven revisions.
Do not repair during or after the official test. Lesson 2.20 is a redesign analysis, not permission to invalidate the tested result.
09 / Independent check
Test your reasoning
1 · Exact qualification reading
You recorded 95 N at 8 mm and 105 N at 11 mm, but nothing at 100 N. Does the software certify a pass?
Check answer
No. An interpolated estimate might be informative, but qualification requires observed data at the test load. Record the missing measurement; do not call it a pass.
2 · Actual bridge mass
The Fusion mass estimate was 140 g but the actual completed bridge weighs 153 g. Which value counts?
Check answer
153 g, the measured finished mass. It exceeds the 150 g maximum regardless of the CAD estimate.
3 · Qualification deflection
A bridge carries 100 N with a measured 9.2 mm midpoint deflection. What else must be checked?
Check answer
It meets the numeric deflection criterion. Also verify true supported span, total width, mass, approved construction and actual teacher-controlled test evidence.
4 · Demonstrated vs. ultimate
Testing stops without failure at exactly 100 N. Is the bridge's ultimate strength 100 N?
Check answer
No. 100 N is the highest demonstrated force if it was the highest measured. Failure force remains unknown.
5 · Ratio calculation
A tested bridge weighing 125 g supports 100 N. Calculate qualification load per mass.
Check answer
100 N / 125 g = 0.800 N/g. This does not compensate for any failed dimensional or deflection requirement.
6 · Actual stiffness
Measured force changes by 60 N while deflection changes by 4 mm in a linear region. What is the approximate slope?
Check answer
ΔF/Δδ = 15 N/mm, a whole-bridge stiffness under the actual support and load configuration, not the wood's Young modulus.
7 · Safety
A joint begins opening and the gauge is close to its rated force. Should students continue until the bridge breaks?
Check answer
No. The teacher stops and documents the safe maximum observed force; deliberate failure testing requires separate approved capacity, guarding and direction.
8 · After the run
The bridge's central gusset peels at 80 N during official testing. May the team re-glue and retest immediately?
Check answer
No. No repairs are allowed once the official test begins. Record the failure and plan an evidence-based redesign in 2.20.
10 / Further learning
Instrumented bridge experiments
Vernier: Testing the Limits
Educational video on collecting force and displacement data during bridge competitions.
Open Vernier video ↗University of Toronto: Bridge Testing Day
Student design–build–test examples, including measurement and failure, on larger laboratory rigs with different rules.
View university bridge testing ↗ASCE Engineering Education (2026)
Research discussing a load–deflection bridge laboratory and failure analysis.
Read case study ↗Class bridge rules and test preparation
Return to the manufacturing constraints and 2.18 inspection if any qualification issue is unclear.
Open Unit 2 specifications →Open 2.18 fabrication record →Assignment deliverable
Exactly what students must submit for 2.19
Bridge Load Test & Structural Performance Report
- Team ID, revision, test date, approved test setup and instrument information.
- Actual bridge mass, clear span and overall width from physical measurements.
- An annotated diagram or photograph of center loading and the independent deflection reference.
- All genuine paired force–deflection data, including exact 100 N reading if achieved.
- Labeled graph with the 100 N and 10 mm thresholds.
- Clearly documented numeric qualification result and any unverified requirements.
- Highest observed load, test stop reason, photos and the earliest observed damage.
- N/g efficiency calculation where actual force and mass are known.
- Comparison with 2.16 predicted compression/joint risks and two recommendations for 2.20.
- CSV/summary export or printed four-page packet plus at least three labeled photos/stills.
Scientific integrity check
- No fabricated or preset force–deflection data.
- At 100 N, deflection is a genuine simultaneous reading—not an interpolated estimate.
- Highest observed load is not automatically ultimate failure strength.
- Instrument and fixture limits are respected; stopped tests are reported honestly.
- No repairs once official testing starts.
- Units are N, mm, g and N/g; clear span and total width are checked.
- Actual physical results are separated from 2.16 predictions.
- Teacher confirms qualification and manufacturing compliance.
Due: See the Daily Agenda and your teacher's submission instructions.