The Stage That Passed the Wrong Test
A school is building eight portable stage modules for its hall: each 2.4 m × 1.2 m, 600 mm high, with a 400 mm riser at the back. The brief requires each module to support 12 students (design load 100 kg per person, including a dynamic factor), to be liftable by two people (maximum 28 kg per module), and to cost no more than NZ$220 in materials. Deflection at the centre under a 1.2 kN point load must not exceed 4 mm. Three prototypes were tested. A: 18 mm plywood on a 50 × 50 mm pine frame, 24 kg, 9 mm deflection, one screw pulled out at 1.5 kN. B: 18 mm plywood on a 25 mm aluminium square-tube frame, 31 kg, 2.1 mm deflection, no visible damage at 2.0 kN. C: a 12 mm plywood/50 mm EPS foam/12 mm plywood sandwich on plywood ribs, 17 kg, 6 mm deflection, local crushing under the load foot. Budget for the whole project is NZ$1,760.
1. Recommend which prototype to develop and explain the trade-offs using the specifications.
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The three prototypes expose a classic stiffness-versus-mass trade-off.
- Prototype A fails both key specifications: 9 mm deflection against a 4 mm limit, and a screw pulls out at 1.5 kN, which is close to the 1.2 kN point load. The pine frame is not stiff enough and the joint is the weak point.
- Prototype B passes deflection (2.1 mm) and survives 2.0 kN, but at 31 kg it exceeds the 28 kg two-person lift limit by 3 kg. It is over-designed in strength and under-designed in mass.
- Prototype C passes mass (17 kg) but fails deflection (6 mm) and shows local crushing under the load foot. The foam core is not dense enough locally, even if the sandwich is globally stiff.
The best development path is C plus local reinforcement: replace the foam under the load points with a denser foam or plywood pads, and add one or two ribs to reduce centre deflection from 6 mm to under 4 mm. That might add 2–3 kg, leaving a comfortable margin under 28 kg. B could also be lightened by reducing the aluminium wall thickness or adding lightening holes, but it has only 3 kg to lose and the frame is already close to the deflection limit. C has 11 kg of mass margin, so it is the better platform to develop.
The student should not choose on one number alone. The specifications are a package: deflection, mass, cost, and safety. The next step is a full-scale test of the revised C with a distributed load, not just a point load.
2. Explain how the testing method might overstate or understate real performance, and what the school should do before using the modules.
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The current test is a single point load at the centre of one prototype. That is useful for comparing stiffness, but it does not represent a stage full of students.
It may overstate performance because a real crowd applies load over the whole deck, including near the edges, and people move and jump. Dynamic loads can be two or more times the static weight. It may understate performance because a single point load at the centre is a severe bending case for some frames, but it does not test the joints, the riser, or the connections between modules. It also ignores repeated setup and dismantling, which loosens fasteners, and moisture, which reduces plywood stiffness.
Before use, the school should:
- test a full module with sandbags totalling 12 × 100 kg, distributed in a realistic pattern;
- apply a dynamic drop or repeated loading, at least 50 cycles;
- inspect joints and deflection after each stage;
- test the connection between modules and the stability of the 600 mm height;
- use a safety factor of at least 2 on the design load;
- have a competent person check the design, because this is a public assembly structure.
The evaluation should record the actual test method, not just 'it held'. A module that passes a point load but fails under a moving crowd is not fit for purpose.
3. The board wants to use the modules for a community event with a drum kit and a 300 kg riser. Evaluate whether the current design evidence is sufficient.
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The drum kit and 300 kg riser are a different load case from the one the prototypes were tested for, so the current evidence is not sufficient. A 300 kg riser on a small footprint could produce a point load well above the 1.2 kN design point, and a drum kit introduces vibration and eccentric loading. The brief's 12 × 100 kg crowd load is distributed; it does not cover a concentrated riser or dynamic drumming.
The school should not use the modules for this event on the basis of the existing tests. Options are:
- commission a structural check from a chartered engineer for the new load case;
- prop the stage from below, so the load goes to the floor rather than through the deck;
- restrict the riser to a position over a support wall or beam;
- hire a certified stage for the event.
If the school wants to test, it should apply 1.5 times the proposed load (450 kg) to the riser area, check deflection and joint movement, and repeat after the event. It should also check egress, guardrails and the 600 mm height. The key judgement is that fitness for purpose is specific to the use: evidence from a student crowd does not transfer automatically to a heavy riser and a drum kit. Until the new load case is verified, the modules are not fit for that purpose.