Shake Table Bridge Challenge: Define, Build, Shake, Improve

Investigation question: How can a craft-stick bridge be designed to carry its load through three measured levels of shaking, meeting the performance targets its importance demands, within a materials budget?

Scientific purpose

Each group draws a bridge type (ordinary, important or lifeline), turns that type's performance targets into precise, measurable criteria and constraints, and builds a craft-stick bridge across a 25 cm gap on a hand-driven shake table. The table is 'measured': every shaking level has a fixed stroke distance, beat rate and duration, checked by counting strokes before testing, so different designs meet the same shaking. Groups test at three levels, pool results in a class table, identify the features shared by the best performers, redesign one feature and retest. The lab carries out the lesson's engineering spread: precise criteria, importance matched to hazard level, and a systematic comparison of designs.

Why this helps students with this chapter

Students usually build 'the strongest bridge possible'. Assigning a bridge type with its own targets and budget makes them face the real engineering question from the lesson: how good does this structure need to be, given how important it is and how often each level of shaking happens? Writing measurable criteria before building gives the test meaning, and the measured shaker makes the comparison fair enough that the class can argue from data about which features work.

Standards connections

MS-ETS1-1 (practice, turning performance targets into precise, measurable criteria and listing constraints before building): The design brief requires every criterion to be measurable (load stays on the deck, sag change in millimeters, joints intact, rating at each shaking level) and every constraint to be stated (budget, materials, time, no supports in the river). The problem frame and the targets are supplied, so students practice rather than fully define the problem.

MS-ETS1-2 (practice, comparing tested designs systematically against the criteria): The class results table puts every design's cost, features and ratings side by side so groups can judge which designs best meet the criteria and constraints and justify a redesign from that comparison.

MS-ESS3-2 (practice, the mitigation half: testing a technology meant to reduce earthquake damage): Earthquake-resistant structures are one of the mitigation technologies the PE refers to. The shaking levels stand for frequent, rare and very rare earthquakes, but students analyze test data from a model rather than data on real hazards, so this is practice.

Roles (rotate each session)

• Lead builder: Leads construction from the group's sketch and keeps the bridge within the design rules.
• Shaker: Practices the three shaking levels until the stroke count matches the target, then shakes the group's bridge (and checks one other group's calibration).
• Measurer: Measures mid-span sag with the ruler before and after each level, counts loosened or broken joints, and decides the rating with the recorder.
• Budget keeper and recorder: Tracks every material used and its cost, fills the test tables, and reports the group's results to the class table.

Materials

Estimated cost: about 35 to 60 to set up 8 groups (the low end with borrowed trays), then about 10 to 15 per class (estimates) (an estimate; Estimated U.S. discount-store prices: trays about 3 dollars each (or borrowed from the cafeteria), 1,000 craft sticks about 10 dollars, 4 rolls of masking tape about 8 dollars, latex-free rubber bands about 4 dollars, binder clips about 5 dollars, paper clips, string and index cards about 6 dollars, 320 pennies about 3 dollars (reusable). Cardboard, boxes, pencils, rulers and goggles are classroom stock. Sticks and tape are the per-class consumables.)

Safety

• Wear goggles while shaking and whenever rubber bands are stretched: a band can snap and a stick can flip up during shaking.
• Pinch point: keep fingers off the tray rim and away from the gap between the moving platform and the rim. The shaker holds the platform by its sides only and reverses at the tape mark instead of slamming into the rim.
• Rollers must stay inside the tray. Pick up any pencil, penny or paper clip that falls to the floor at once; round objects underfoot are a slip hazard.
• Broken craft sticks splinter. Break or cut sticks pointing down at the table, never toward a face, and put broken pieces in the trash.
• Cut away from your body with scissors. Use latex-free rubber bands in case anyone has a latex allergy. Wash hands after handling the pennies.
• No hot glue or other heated tools: glue guns run well above 50 degrees C. Tape, clips, string and bands are the only fasteners.

Teacher preparation and setup

1. Build one shake table: lay the six rollers parallel across the tray, set the platform on them, and clip one rubber band from each short end of the platform to the tray rim so the platform returns to center. Put masking-tape marks on the tray rim at 1, 2 and 3 cm on each side of the platform's center line.
2. Try the three shaking levels yourself with an empty platform and a metronome, and count strokes for 10 seconds. A stroke is one move from one side mark to the other. Targets: Level 1 is 20 strokes in 10 seconds, Level 2 about 27, Level 3 about 33.
3. Prepare importance cards in equal numbers: Ordinary (budget 40 units), Important (55 units) and Lifeline (70 units), each printed with its targets from the performance table.
4. Fill and check the penny bags; pre-cut a supply of 10 cm tape pieces to make budgeting easy.
5. Tape the two bank boxes to each platform 25 cm apart (inside edges) before class, or have students do it as the first step, so every group has the same gap.

Investigation design

Student procedure

1. Session 1. Draw an importance card. Read the performance targets for your bridge type and the design rules. Complete the design brief: write each criterion as a precise, measurable statement and say how you will check it, then list the constraints. Answer q1 and q2.
2. Calibrate the shaker: with an empty platform, the shaker practices each level with the metronome while the recorder counts strokes for 10 seconds. Repeat until the count is within 2 of the target (20, 27, 33). Record both counts.
3. Sketch version 1 and label the materials, joints and how the load's weight is carried to the banks. Estimate the cost; it must not exceed your budget.
4. Build version 1 (25 minutes). The bridge must span the 25 cm gap, have a deck at least 6 cm wide, rest on the banks with nothing touching the platform between them, and be attached only with budgeted tape, clips, string or bands. Record the actual materials and cost.
5. Session 2. Test version 1. Measure the deck height at mid-span with no load, then place the penny bag at mid-span and measure again (sag = unloaded height minus loaded height). Shake at Level 1, inspect, measure and rate; then Level 2; then Level 3. Do not repair between levels. Use the rating scale exactly as written.
6. Report your bridge type, cost, main design features and three ratings to the class table. Study the table and answer q3: which features do the best-performing bridges share?
7. Redesign: change one main feature (answer q4 first, including your prediction), staying within budget. Build version 2 and test it with the same shaker, the same load position and the same levels.
8. Answer the analysis questions individually. Return pennies, rollers and boxes; recycle unbroken sticks.

Expected observations

• These are expected outcomes, not results from a tested run: flat plank decks of taped sticks usually pass Level 1, but the penny bag often slides off at Level 2 unless the deck has a curb or rail.
• Tall, narrow structures and side walls made of rectangles tend to sway and fold at Level 2 or 3; triangulated side walls and cross-bracing usually survive to Level 3 with minor damage.
• Tape joints are the usual first failure: tape peels or slips before sticks break, so wrapped or doubled joints perform better.
• Bridges attached rigidly to the banks may crack at the ends, while bridges that can shift slightly on the banks (or rest on rubber-band pads) sometimes ride out the shaking better, if they cannot slide off.
• Higher budgets do not guarantee higher ratings; the class table usually shows a well-braced ordinary bridge outperforming a heavily built lifeline bridge.

Likely difficulties

• Inconsistent shaking between groups. Insist on the calibration counts, and if time allows, have one trained shaker per station shake every bridge at that station.
• Slamming the platform into the rim at the end of each stroke adds jolts that are not part of the protocol. Coach the shaker to reverse at the tape mark.
• Measuring sag at different points. Mark mid-span on the deck with a pencil and always stand the ruler at that mark on the platform.
• Going over budget during the build. The budget keeper should approve every new piece of material before it is used.
• Rating disagreements. Read the rating scale aloud and decide by the written rules, not by how the bridge looks.

Structured debrief

1. Display the class comparison table and sort it by Level 3 rating. Ask which features the top bridges share. Typical answers: triangles in the side walls, diagonal cross-bracing under or across the deck, a low, wide structure, a curb that keeps the load from sliding, and joints wrapped so tape cannot peel. Ask for the physics: a triangle cannot change shape without a side changing length, so triangulated frames resist the sideways sway that folds a rectangle.
2. Compare bridge types. Did lifeline teams with 70 units always beat ordinary teams with 40? Usually not: where bracing goes matters more than how much material is used. Connect to the lesson: engineers match performance to importance, and money spent well beats money spent everywhere.
3. Discuss the measured shaker. Did everyone's Level 2 feel the same? The stroke counts were the evidence that it did, roughly. Point out what the model cannot do: real shaking comes in many frequencies and directions at once, including up and down, real bridges are far larger and sway much more slowly, and the two ends of a long real bridge can move differently.
4. Close on mitigation: the shake table stands in for the tests engineers run on large shake tables and computer models before building. Japan's base isolators and the Bay Bridge's energy-absorbing parts are ideas students may have stumbled on with rubber bands and flexible joints.

Recording sheet: what students fill in

Lab 2 recording sheet: Shake Table Bridge Challenge

Your county needs a new bridge in earthquake country. How well the bridge must survive shaking depends on how important it is. Your group has drawn one bridge type. Your job is to write precise criteria, build a bridge that meets them within your budget, test it with measured shaking, and improve it using the class's evidence.

Design rules: span the 25 cm gap and rest on top of both banks; deck at least 6 cm wide; nothing may touch the platform between the banks (it is the river); only the listed materials, no glue; 25 minutes to build each version; every piece of material counts toward the budget, including tape used to attach the bridge to the banks.

Analysis questions: answer key

Individual evidence of learning

Each student submits their own copy of the design brief, the version 1 sketch, the test tables and individual answers to q5 through q10. The design brief and q1 are the evidence of defining precise criteria and constraints (MS-ETS1-1); q3, q5 and q6 show systematic comparison of designs using test data (MS-ETS1-2); q8 and q10 connect the design to earthquake mitigation.

Assessment criteria

Accessibility adaptations

• Provide pre-cut 10 cm tape pieces in a cup and pre-sorted material kits so budgeting is counting, not measuring.
• Offer a starter photo card of three basic bridge forms (beam, truss, arch) for students who freeze at a blank build.
• Let a student with limited fine-motor control take the shaker, measurer or recorder role; use a large-print ruler and an enlarged rating scale.
• Use a visual timer for the 25-minute build and give a two-minute warning; accept a labeled photo plus a spoken explanation in place of the written sketch.

Low-cost alternative

Without trays, set a sheet of stiff cardboard on six round pencils directly on a desk and tape two books to the desk as stops at 1, 2 and 3 cm. Rolled newspaper tubes or index-card strips can replace craft sticks, and a juice box or a cup of washers can replace the penny bag, as long as every group uses the same load.

Model limitations

This lab has not been physically tested as written; the stroke distances, beat rates and rating thresholds are reasonable starting values that a teacher may need to adjust after a trial run. Hand shaking only approximates a steady back-and-forth motion: at the stated rates the estimated peak accelerations are only about 0.04 g, 0.14 g and 0.34 g, and the stops add jolts. The table moves in one horizontal direction only, while real ground motion mixes many frequencies and directions, including vertical motion, for seconds to minutes. A craft-stick bridge is tiny and light compared with a real bridge, so it vibrates much faster and its resonance does not scale to real structures; tape and wood behave differently from steel and concrete. Both banks sit on the same platform and move together, whereas the two ends of a long real bridge can move differently, and soft ground can settle or liquefy beneath real supports. The ratings are classroom stand-ins for real performance levels.

Teacher notes

This lab has not been physically tested; run one bridge through all three levels yourself before class and adjust stroke distances or tempos if every bridge survives Level 3 or none survives Level 1. A useful target is that a simple taped plank survives Level 1 but loses its load at Level 2. The rating scale's thresholds (2 mm and 5 mm of added sag, 2 loosened joints) can also be adjusted, but announce any change before testing. For a math extension, the estimated peak accelerations (assuming smooth back-and-forth motion) are about 0.04 g, 0.14 g and 0.34 g for Levels 1, 2 and 3, from (2 x pi x frequency) squared x stroke half-distance, with frequencies of 1.0, 1.33 and 1.67 cycles per second. The performance-target table matches slide 24 and Lab 3, and is this chapter's own simplified version of performance-based design, not a copy of the textbook's Figure 5.