Engineering Challenges for Students: Bridges, Catapults, and Load-Testing Prints

Give a student a worksheet about triangulated trusses and you'll get a shrug. Give that same student a printed bridge and a bag of coins to load onto it until it snaps, and you'll get their full attention. Engineering challenges work because they make failure visible, immediate, and fixable — which is a far better teacher than a diagram ever will be.

Here are three challenges that turn printed parts into real engineering lessons, plus the safety basics for running load tests in a classroom.

Challenge 1: Bridge Load Test

Print a simple truss-style bridge — a flat roadway supported by a triangulated lattice underneath, spanning a gap between two blocks or desks. Keep the span modest, somewhere in the 15–20 cm range, so it prints quickly and fits on a standard desk.

The activity: place the bridge across the gap and load it gradually with household weights — stacked coins, small washers, or bagged rice in measured increments — recording how much weight it holds before it deflects noticeably or fails. Then print a second bridge design that swaps the triangulated lattice for a simple grid of squares, and run the same test.

The lesson writes itself once students see the results side by side: triangles don't change shape under load without a member stretching or compressing, while squares can rack — deform into a parallelogram — much more easily. That's the actual engineering principle behind why real bridges, cranes, and roof trusses are built from triangles, and it's far more memorable demonstrated than described.

PLA Basic is the right material for this challenge. It's affordable enough to print several bridge iterations without budget anxiety, and its predictable, low-shrinkage printing means the geometry comes out consistent between design variants — which matters when you're comparing structural performance rather than print quality.

Challenge 2: Catapult or Trebuchet Build

A small tabletop catapult or trebuchet teaches leverage and stored energy in a way that's genuinely hard to fake understanding of — either the design launches a small projectile reliably, or it doesn't. Have students build a basic lever-arm catapult, then experiment with arm length, pivot point placement, and counterweight to see how each variable changes launch distance.

Material choice matters more here than in the bridge challenge, because the swinging arm takes repeated impact every time the mechanism fires. A catapult arm printed in standard PLA can develop stress cracks after repeated hard stops against a frame or stop-block — brittle materials don't handle sudden impact loading well, even if they're plenty strong under a steady, gradual load like the bridge test.

PETG Basic is the better choice for the swinging arm specifically. PETG has meaningfully better impact resistance than standard PLA — it flexes slightly and absorbs shock rather than cracking under it, which is exactly the property you want in a part that gets slammed against a stop dozens of times during testing. PETG Basic prints hotter than PLA, at a nozzle range of 230–250°C and a bed temperature of 70–90°C, so plan for slightly longer warm-up time and make sure your printer's bed can reach the higher temperature before you commit to it for a class set. You can print the static frame and base in PLA Basic to save cost, and reserve PETG Basic just for the arm — a good early lesson in matching material to the specific stress a part experiences, rather than using one material for everything by default.

Challenge 3: Material Comparison Test

This challenge turns material science itself into the subject. Print the same small bracket design — something simple like an L-shaped shelf bracket or a small hook — three times: once in PLA Basic, once in PETG Basic, and once in PLA+. Keep every other variable identical: same infill percentage, same wall count, same orientation on the bed.

Then break-test all three side by side. Apply steadily increasing force — hanging weight from a hook shape works well — until each bracket fails, and have students record and compare how much load each one held and how it failed. PLA Basic tends to fail more abruptly with a clean crack; PLA+ typically holds noticeably more load before failing and may show more deformation first; PETG Basic often bends significantly before it breaks, if it breaks at all under a reasonable test load.

This is also the moment to make the PLA+ vs. PLA Basic conversation concrete instead of abstract. Rather than telling students PLA+ is "tougher," let the test prove it. For the fuller technical comparison to reference when setting up the test or explaining results afterward, see our PLA+ vs. PLA Basic breakdown.

Recording and Comparing Results

None of these three challenges pays off fully without a simple, consistent way to record what happens. Have students keep a shared results table for each test — design name, material, load at which visible deflection or bending first appears, and load at which the part fails completely. A simple spreadsheet or even a poster-sized paper chart at the front of the room works fine; the goal is just making the comparison visible across the whole class rather than living only in each group's memory.

Test	What to Record	What It Teaches
Bridge load test	Weight at first visible deflection, weight at failure, which design (truss vs. grid) held more	Triangulated structures resist shape change under load better than square-grid structures
Catapult build	Launch distance across different arm lengths and counterweights	Leverage and stored energy directly change mechanical output
Material comparison	Load at failure and failure type (clean break, bend, no break) for each material	Different filaments trade off stiffness, toughness, and flexibility differently

Once you have results from a full class of groups, a short debrief works better than moving straight to the next challenge. Ask each group to explain one surprising result and one result that matched their prediction — this is where students actually process what the data is telling them, rather than treating the test itself as the whole activity.

Safety Notes for Load Testing

Load-to-failure testing is genuinely engaging, but it needs a few ground rules:

• Adult supervision during the actual failure moment. A part under tension or bending can release suddenly when it breaks, and small fragments can travel a short distance.
• Eye protection for brittle-failure tests specifically. Standard PLA Basic parts can shatter into small sharp pieces when they fail under load — safety glasses are a reasonable precaution any time you're testing to the point of breaking, not just a formality.
• Test over a table or tray, not over laps or open floor space, so fragments and weights have a controlled place to land.
• Set a clear stopping point in advance for weight increments, so nobody keeps piling on weight past the point where a controlled test becomes an unpredictable one.

None of this requires special equipment — a folding table, safety glasses from the science supply closet, and clear verbal expectations cover it. But treat it with the same seriousness you'd give any classroom activity where something is deliberately being broken.

Extending the Unit

If you have more than a week or two to work with, a few extensions keep the momentum going without introducing brand-new concepts:

• Design iteration on the bridge. After the first truss-versus-grid comparison, challenge students to design a third bridge that beats both, using what they learned about triangulation. This turns the first test into a design constraint rather than a one-off demonstration.
• Distance-versus-accuracy tradeoff on the catapult. Ask groups to optimize for the farthest launch, then re-optimize for the most accurate launch to a fixed target. These two goals usually pull the design in different directions, which is a genuine engineering tradeoff worth naming explicitly.
• A fourth material in the comparison test. Once students are comfortable with the three-material comparison, adding a fourth known reference — even something non-printed, like a wood or metal sample of similar size — gives them a sense of scale for how the printed materials compare to materials they already have intuition about.

Getting Started

These three challenges work well as a connected unit — bridge, catapult, then the material comparison test — building from structural geometry to material properties over a week or two. Stock PLA Basic for the bridge and comparison bracket, PETG Basic for the catapult arm and comparison bracket, and PLA+ for the third bracket in your comparison set. All three are available at Filazoo's collection, and having all three materials on hand means you're ready to run the full sequence without a mid-unit supply run.