Grade by Grade
D. Structures and Mechanisms · Form and Function

Combination Structures: Form Follows Function

D2.1 D2 A3.3 A1.5About 45 minutes
Learning goals
  • I can identify combination structures and the type of structure in each part.
  • I can explain how the form of a structure suits its function.
  • I can describe how First Nations and Inuit builders use structural forms that suit the land, the climate and the materials available.

Words to know

form
The shape and build of a structure.
function
The job a structure is meant to do.
combination structure
A structure that uses more than one type of structure (solid, frame or shell) in different parts.
dome
A rounded shell shape, like half of a ball.
iglu
A dome-shaped shelter made of snow blocks, built by Inuit. Often spelled igloo in English.
longhouse
A long building with a frame of wooden poles covered with bark, built by the Haudenosaunee and some other First Nations.

Hook: what type is a house?

Is a house solid, frame or shell? Its foundation is a solid block of concrete. Its walls hide a frame of wooden studs. Its roof might be held up by triangle-shaped trusses (frames) covered with a skin of boards and shingles.

The answer is: all of them! Most real structures are combination structures.

A wooden house under construction with a grey concrete foundation, wooden stud walls and triangular roof trusses.
A house is a combination structure: a solid concrete foundation holds up a frame of wooden studs and triangle-shaped roof trusses. AI-generated image

Combination structures

A combination structure uses different structure types in different parts, choosing the best type for each part's job.

  • A house: solid concrete foundation + wooden frame walls and roof.
  • A bridge: solid concrete piers in the river + a steel frame truss + a concrete deck on top.
  • An umbrella: a frame of thin ribs + a fabric covering.
  • A canoe: a frame of ribs + a thin hull that works as a shell.

When you classify a combination structure, name the type of each part and the job it does.

concreteroof truss: frame of trianglesroof boards + shingles: coveringwall studs: framefoundation: solid(cut-away view, not to scale)
A house is a combination structure: a solid concrete foundation, frame walls and roof trusses, and a covering of boards and shingles over the frame.

Form follows function

Engineers and architects have a saying: form follows function. First decide what the structure must do. Then choose a form that does it well.

  • A bridge must span a gap and carry heavy, moving loads, so it uses long beams, arches or frames of triangles.
  • A helmet must spread out a hit and stay light, so it is a curved shell.
  • A dam must hold back the push of water, so it is solid, and often thicker at the bottom, where the water pushes hardest.

The environment matters too: available materials, the climate, the ground and what people need.

First Nations and Inuit structures: form that fits the land

First Nations, Métis and Inuit builders have designed structures that fit their land, climate and materials for thousands of years, and this knowledge is still taught and used today.

  • Iglu (Inuit): blocks of wind-packed snow are cut and stacked in a rising spiral to make a dome, a shell structure. Each block leans on its neighbours, so the snow is squeezed together and holds its shape. Snow is also a good insulator, so the inside stays much warmer than the wind outside. Some Inuit still build iglus today, for example on hunting trips, and Elders teach young people how.
  • Tipi (many Plains First Nations): long wooden poles leaned together form a frame shaped like a cone, covered with hides or canvas. The cone shape stands up well in strong prairie winds, and it can be taken down and moved.
  • Longhouse (Haudenosaunee and other First Nations): a frame of wooden poles, bent and tied together to make a curved roof, covered with sheets of bark. Many Haudenosaunee communities today have longhouses used for ceremonies and gatherings.
  • Birchbark canoe (Anishinaabe and other First Nations): a combination: a frame of cedar ribs inside a thin hull of birchbark that works as a shell, sewn with spruce root and sealed with tree gum. It is light enough to carry between lakes. Some First Nations builders still make birchbark canoes today and teach the skills to younger people.
StructureType(s)How the form suits the function
iglushell (dome)A dome of snow blocks uses only snow, sheds wind and keeps warmth in.
tipiframe with a coveringA cone of poles stands up to wind and can be taken down and moved.
longhouseframe with a coveringA long frame makes a big shared indoor space.
birchbark canoecombination: frame + shellRibs give shape; the thin bark hull is light and waterproof.
modern housecombination: solid + frameSolid foundation holds it steady; light frame walls are quick to build.
Forms chosen to fit their functions

Respectful words

Use the names communities use for themselves, like Inuit, Haudenosaunee and Anishinaabe. These are living communities today, not just history. Different Nations built different structures, so don't say "all First Nations lived in tipis".

Worked example

Let's break down a combination structure

A playground slide has a metal ladder, a curved plastic slide and concrete footings buried in the ground. Classify each part and explain its job.

  1. Ladder: metal bars joined together, open space between: frame. Its job: hold up the weight of climbers and the platform.
  2. Curved plastic slide: a thin curved layer that holds its own shape: shell. Its job: carry the sliding child with a smooth, light surface.
  3. Concrete footings: filled blocks of concrete: solid. Their job: anchor the slide so it can't tip or move.

The slide is a combination structure: frame (ladder), shell (slide) and solid (footings), each chosen for its job.

Worked example

Let's match form to function

Why is a concrete dam usually much thicker at the bottom than at the top?

  1. Function: the dam must hold back water without breaking or sliding.
  2. Water pushes harder the deeper it is, so the bottom of the dam feels the biggest push.
  3. So the form puts the most material where the force is biggest: at the bottom. The top can be thinner and save material.

The form (thick bottom) matches the function (resisting the biggest water push near the bottom).

waterdeeper water pushes harderthin topthick bottom(not to scale)
Water pushes harder the deeper it gets (longer arrows), so a solid dam puts the most material at the bottom, where the push is biggest.
Hands-on activity

Build three forms, test one load

You need: 3 sheets of paper, tape, scissors, a piece of cardboard, a few identical small paperback books, and a grown-up to help (a grown-up should help with the scissors).

  1. Make a solid column: roll one sheet tightly into a solid rod and tape it. Stand it up (trim the ends flat so it stands).
  2. Make a shell: roll a sheet into a hollow tube about 4 cm across and tape it.
  3. Make a frame: cut a sheet into 6 strips, roll each strip tightly into a thin stick, and tape them into a small triangle-sided tower (a tripod with crossbars works).
  4. Predict which will hold the most books placed gently on top (use a piece of cardboard as a platform).
  5. Test each one. Add books one at a time until it buckles. Record the number.
  6. Write a conclusion: which form held the most for the paper used? Which was easiest to build?

What to look for: many students are surprised that the hollow tube (shell) holds a lot of books because the load is spread evenly around its wall. Results vary, so the conclusion should be based on the student's own data. Keep fingers and toes clear when it collapses.

Just for fun

A house is solid, frame and shell all at once. It is the over-achiever of the structure world.

Your turn

1

What is a combination structure?

2

Name the type of each part of a house: the foundation and the walls.

3

What does "form follows function" mean?

4

What type of structure is an iglu? How does it hold together?

5

What type of structure is a tipi?

6

Why is a birchbark canoe called a combination structure?

More practice: warm-up

1

True or false: most real buildings are combination structures.

2

The covering on a tipi is hides or canvas. What holds up the load?

3

Which Nation builds longhouses?

4

Is an umbrella's fabric or its ribs the frame?

5

Name one function of a dam.

More practice: on your own

1

A bridge has concrete piers in the river and a steel truss above. Classify each part.

2

Why is a dome a good form for a snow shelter?

3

Why might a family on the move choose a tipi instead of a log cabin?

4

A bike helmet has a hard outer layer and foam inside. Explain how its form matches its function.

5

Explain why a cottage on a rocky lakeshore might be built on posts instead of a full concrete foundation.

Challenge

1

Pick an object at home that is a combination structure. Name each part's type and explain why the designer chose it.

2

Snow is the only material available in some places in the Arctic in winter. Explain how the iglu's design shows that form follows function and fits the environment.

3

An engineer says a shell roof is "strong but unforgiving". What might she mean?

Check your understanding

Quiz

1.

Which is a combination structure?

2.

"Form follows function" means...

3.

An iglu is mainly which type of structure?

4.

What carries the load in a tipi?

5.

Why is a dam thicker at the bottom?

6.

A birchbark canoe is a combination of...

For parents and teachers

Big idea (D2.1, D2): Most real structures combine solid, frame and shell parts, and good designs match form to function. Students also see that First Nations and Inuit builders designed structures (iglu, tipi, longhouse, birchbark canoe) to suit local land, climate and materials (A3.3, contributions from various communities).

How to help: Pick any object (a lamp, a bike, a chair) and ask your child to break it into parts and name each part's type and job. Talk about why a design suits its place: a cottage dock, a farm barn, a northern building.

Common difficulties:

  • Forcing one label on a combination structure. Encourage "this part is ... and that part is ...".
  • Talking about Indigenous structures only in the past. Point out that these are living communities and that some of these skills are still practised and taught today.
  • Safety: collapsing paper towers can drop books; keep hands and feet clear, and use light books.