Investigating Science 11–12 · Year 12

Earthquake-resistant design on a shake table: resonance, bracing and base isolation

Module 6: Technologies

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

A building sways most when the ground shakes near its natural frequency; bracing raises that frequency and base isolation lowers it, both moving the structure away from resonance as Newton’s second law predicts for a mass on a spring.

What you need

a shake table: a tray resting on four tennis balls or rollers, moved by hand in time with a metronome app
model towers about 40 cm tall made from balsa or wooden craft sticks, with a 200 g mass fixed at the top; cross-braces; four rubber pads for base isolation
a phone running the phyphox acceleration spectrum experiment taped at the top of each tower

How to do it

Weigh the top load (the 200 g mass plus the phone), tap each tower once and record its free vibration; read its natural frequency from the acceleration spectrum.
Shake the table at a fixed amplitude at 1, 2, 3 and 4 Hz, adding frequencies around the tap-test natural frequency where the table can be moved that fast, and record the top acceleration at each frequency.
Add cross-bracing and repeat both steps.
Mount the unbraced tower on rubber pads and repeat.
Plot top acceleration against shaking frequency for each design and relate the peaks to the natural frequencies.
Explain how the technology of building design follows from the physics.

What you should see

Each tower sways most when the shaking frequency matches its own natural frequency, and short and tall towers swap which sways most as the frequency changes (the Earthlearningidea demonstration). Bracing stiffens the frame and raises its natural frequency; isolation pads lower the frequency of the whole system. The tap test gives each tower’s natural frequency f₀; with the weighed top load m (the 200 g mass plus the phone) the effective stiffness is k = 4π² f₀² m, and a tower whose f₀ lies above 4 Hz shows its sway rising across the 1 to 4 Hz sweep with no peak. In the undamped model, shaking at 1.5 times the natural frequency passes on 0.80 of the ground motion and at twice the natural frequency 0.33, which is the aim of base isolation.

What changes

What you change: design (unbraced, braced, isolated) and shaking frequency
What you measure: peak acceleration at the top of the tower
What you keep the same: same top mass
same shaking amplitude
same tower height
phone position

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The tallest building always sways most.
A stronger building is always a stiffer one.
Earthquake shaking is a single push.

Safety card

Low riskLearners carry it out

Hazards

collapsing towers and flying pieces
fingers caught between tray and rollers

Controls

safety glasses near collapsing models
hold the tray by its handles only

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS12-13 INS11/12-1 INS11/12-2 INS11/12-4
Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities