D. Structures and Mechanisms · Forces Acting on Structures

Design Challenge: A Structure That Withstands Shaking and Wind

A1.3 A1.4 A1.5 A2.1 A3.2 D2.1 D2.2 D2.3About 60 minutes

Learning goals

I can use an engineering design process to design, build, test and improve a structure.
I can use features like cross-bracing, a wide base and base isolation to resist shaking and wind.
I can record test data in a table, find the average, and read code that processes data.
I can explain my design using the words compression, tension, static load and dynamic load.

Words to know

engineering design process
The steps engineers follow: ask, imagine and plan, build a prototype, test, improve, share.

prototype
A first model of a design, built to test it.

criteria
What a design must do to succeed.

constraints
The limits you must work within, like materials, size and time.

shake table
A platform that shakes back and forth to model an earthquake.

average
A typical value found by adding all the results and dividing by how many results there are.

Your turn to be the engineer

You have learned about internal and external forces, static and dynamic loads, nature's forces and the features that help structures withstand them. Now you will design, build and test your own tower, and shake it like an earthquake and blow on it like a windstorm!

Criteria (what it must do)	Constraints (your limits)
be at least 30 cm tall	use only: 20 drinking straws, 1 metre of masking tape, 4 index cards, 1 metre of string, a small ball of play dough
hold a deck of cards on top (a static load)	build time: 30 minutes
survive the shake test (a dynamic load) for as many rounds as possible, up to 30	the tower may not be taped to the tray
survive the wind test: 5 fans with a piece of cardboard from 30 cm away	a grown-up does any cutting

Design brief

Engineering design process

Ask: What is the problem? What are the criteria and constraints?
Imagine and plan: sketch at least two ideas. Label struts, ties and braces.
Build a prototype.
Test it and record data.
Improve: find the weak spot, change one thing, test again.
Share: explain your design and your data.

A home-made shake table: a tray resting on four round pencils. Slide the tray back and forth to model an earthquake. The model tower has X-braces and holds a load on top.

Ideas from the unit

Cross-bracing (X-braces) turns rectangles into triangles. String works as a brace because it is used in tension.
A wide base and a low, heavy bottom make a tower harder to tip over. A little play dough at the feet adds weight low down.
Strong joints: wrap tape all the way around where straws meet.
Base isolation: try standing the tower on four small flat balls of play dough or pieces of sponge. Does it shake less?
Remember: the deck of cards is a static load; shaking and fanning are dynamic loads.

A model tower made of coloured straws and tape with string X-braces, standing on cardboard with play dough at its feet. — This straw tower uses ideas from the unit: string X-braces in tension, a wide base and play dough weights low down at the feet. AI-generated image

Using data and code

Test each version 3 times and find the average number of shake rounds it survives. To find the average, add the results and divide by how many there are. Here is code that stores each design's results in a list and works out the averages for you.

define average(results)

set total to 0

for each number in results

change total by number

report total ÷ (length of results)

set designA to [12, 15, 15] // shake rounds survived, 3 tests

set designB to [25, 30, 26] // after adding X-braces

say "Design A average: " + average(designA)

say "Design B average: " + average(designB)

if average(designB) > average(designA) then

say "The improvement worked!"

Output: Design A average: 14. Design B average: 27. The improvement worked!

Bars show the averages from the code (14 and 27). The dots show each of the 3 tests. Adding X-braces almost doubled the average.

Worked example

Let's trace the code

Work out what the code says for design A, step by step.

The list designA stores three results: 12, 15 and 15.
The average block starts total at 0, then adds each number: 0 + 12 = 12, 12 + 15 = 27, 27 + 15 = 42.
The list has 3 numbers, so it reports 42 ÷ 3 = 14.
For design B: 25 + 30 + 26 = 81, and 81 ÷ 3 = 27.
27 is greater than 14, so it also says "The improvement worked!"

Design A average: 14. Design B average: 27. The X-braces almost doubled the number of shake rounds the tower survived.

Worked example

Let's follow one engineer's design

Maya builds a square tower of straws. It holds the cards but falls after about 8 shake rounds. What should she do?

Find the weak spot: she watches a slow-motion video. The tower leans into a slanted shape before it falls.
Think about forces: a square frame can be pushed into a lean. Shaking is a dynamic sideways load.
Change one thing: she adds string X-braces to every side.
Test again 3 times: it survives 22, 25 and 25 rounds. Average: (22 + 25 + 25) ÷ 3 = 72 ÷ 3 = 24.
Share: "The X-braces turned my squares into triangles. When the table shakes, one string on each side is always in tension."

Adding X-braces fixed the weak spot. Her average rose from about 8 rounds to 24.

Just for fun

Engineer's secret: most great designs fail first. Maya's first tower fell after just 8 shake rounds! Every wobble and crash is free information (and sometimes very funny to watch).

Hands-on activity

Build and test your tower

You need: the materials in the design brief, a baking tray, 4 round pencils, a deck of cards, a ruler, a piece of cardboard for fanning, a timer, scissors (for a grown-up to use), and paper for your plan and data table.

Ask: read the criteria and constraints aloud.
Plan: sketch two ideas. Label the struts, ties, braces and base. Choose one and explain why.
Build your prototype (30 minutes).
Test: measure the height. Gently place the deck of cards on top and count to 10. Then do the shake test: set the tray on the 4 pencils on the floor or a low table, with the tower in the middle of the tray. One shake round means sliding the tray 5 cm one way and back. Do one round each second, and count rounds until the tower falls or the cards fall off (stop at 30). Do the shake test 3 times. Last, do the wind test: fan the tower 5 times with the cardboard from 30 cm away. Record every result in a table and find the average number of shake rounds.
Improve: find the weak spot. Change one thing. Test 3 more times and compare averages.
Share: explain to your family which parts are in compression and which are in tension, what the static and dynamic loads were, and what your data shows.

What success looks like: a labelled plan, a tower that meets most criteria, a data table with averages, one improvement tested fairly, and a clear explanation.

Safety: test on a low table or the floor, keep the shake gentle (5 cm), and let a grown-up do any cutting. Pick up the pencils afterwards so no one slips on them.

Your turn

What are the six steps of the engineering design process?

What is the difference between criteria and constraints?

In this challenge, which load is static and which loads are dynamic?

Why can string be used for X-braces?

A tower survives 10, 14 and 12 shake rounds. What is the average?

Why should you change only one thing when you improve your design?

More practice: warm-up

What is a prototype?

What does a shake table model?

Name one feature that helps a tower resist tipping.

How many times should you repeat each test?

In the code, what does the list designA store?

More practice: on your own

A design survives 18, 21 and 24 shake rounds. Another survives 30, 9 and 30. Find both averages. Which is more reliable?

Your tower is tall but tips over in the wind test. Suggest two improvements.

Why does a tall, thin tower sway more in the shake test than a short, wide one?

In the code, what would the average block report for the list [20, 20, 23]?

Explain one part of your tower using the word compression or tension.

Challenge

Add a design C with results [27, 29, 28] to the code. What would its average be, and is it better than design B?

Explain why standing the tower on soft play dough or sponge pads might help it survive the shake test. Connect it to real buildings.

An engineer says, "My tower survived all 30 rounds in every test, so it's perfect." What else should be tested? Name two things.

Check your understanding

Quiz

What is the first step of the engineering design process?

In this challenge, the shaking of the tray is a...

A tower survives 15, 18 and 21 rounds. What is the average?

Which change would most likely help a tower that leans and falls when shaken?

Why test each version 3 times?

Which is a constraint in this challenge?

For parents and teachers

Big idea: This design challenge pulls the unit together through the engineering design process (A1.3), done safely (A1.4) and shared clearly (A1.5). Your child applies internal and external forces and static/dynamic loads (D2.1, D2.2) and features that withstand natural forces like earthquakes and wind (D2.3). The data table and average connect science with math (A3.2), and the code shows how a program can store results in a list and process them (A2.1).

How to help: Let your child struggle productively: ask questions rather than fixing the tower. Useful questions: "Where did it fail first?" "Which part is being squeezed? Stretched?" "What one thing will you change?" Help with the shake test by keeping the slides the same size each time.

Common difficulties:

Changing many things at once. Then it's impossible to tell what worked.
Averages: remind your child to add all results, then divide by how many results there are.
Safety: test on the floor or a low table, keep the shaking gentle, and collect the pencils afterwards so no one slips.