Grade by Grade
D. Structures and Mechanisms · Forces Acting on Structures

Built to Last: Features and Materials That Withstand Nature's Forces

D2.3 A1.2 A2.2 A3.1About 45 minutes
Learning goals
  • I can identify structural features that help buildings and bridges withstand earthquakes, wind, snow, ice and floods.
  • I can explain why materials like steel, reinforced concrete and wood are chosen for strong structures.
  • I can explain how cross-bracing works, using the words tension and compression.
  • I can describe how computers, sensors and skilled trades help keep structures safe.

Words to know

structural feature
A part or design choice that makes a structure stronger or steadier, like a brace or a deep foundation.
cross-bracing
Two diagonal braces that cross in an X inside a frame. They turn the frame into triangles so it can't lean.
shear wall
A strong, stiff wall (often covered with plywood or made of concrete) that stops a building from being pushed sideways.
base isolation
Setting a building on flexible pads so the ground can shake underneath while the building moves much less.
reinforced concrete
Concrete with steel bars inside. Concrete is strong when squeezed; the steel is strong when stretched.
building code
The rules that say how structures must be built so they are safe.
skilled trades
Jobs that use special hands-on training, like carpenter, ironworker, electrician and welder.

Engineers plan for the worst

In the last lesson you saw how earthquakes, wind, snow, ice and floods can damage structures. So why do most buildings stay standing, storm after storm?

Because engineers and builders plan for those forces before they build. They choose structural features (shapes and parts) and materials that let the structure withstand (stand up to) the forces of nature.

Remember from Grade 3

In Grade 3 you learned that triangles keep their shape, that a diagonal brace makes a wobbly square frame strong, and that reinforced concrete has steel rods inside. Now you will see how engineers use these ideas, and many more, against nature's biggest forces.

A tall building under construction with large diagonal X-shaped steel braces in its frame.
See the big X-shaped braces? They turn the frame's rectangles into triangles, helping the tower resist wind and earthquake shaking. AI-generated image

Standing up to earthquakes and wind (sideways forces)

  • Cross-bracing: diagonal braces in an X turn rectangles into triangles, so the frame can't be pushed into a lean. When the frame is pushed one way, one brace is stretched (tension). When it is pushed the other way, the other brace is stretched. So even thin steel rods or cables work as X-braces.
  • Shear walls: stiff walls covered with plywood or made of concrete resist being pushed sideways (they resist shear).
  • Strong connections: metal straps and brackets tie the roof to the walls and the walls to the foundation, so wind can't lift the roof off.
  • Base isolation: some buildings sit on flexible pads made of layers of rubber and steel. When the ground shakes, the pads bend, and the building above moves much less.
  • Flexible materials: steel and wood can bend a little without breaking, so a building can sway and spring back.
  • Smooth, rounded or tapered shapes: wind flows around them more easily, so it pushes less.
no brace: leansX-braced: stays square
Push the top of a square frame that has no brace and it leans over (the dashed square shows where it started). Add an X-brace and the frame is made of triangles, so it stays square.
fixed to groundon isolator pads
Base isolation. Left: a building fixed straight to the ground shakes along with it. Right: a building on flexible isolator pads; the pads bend as the ground shakes, so the building moves much less.

Standing up to snow and ice (downward forces)

  • Steep roofs let snow slide off instead of piling up.
  • Stronger roof beams or trusses (frames made of triangles), placed closer together, carry heavy snow.
  • Building codes say how much snow a roof must hold. The amount is bigger in snowier places.
  • Power towers and poles can be built stronger with more bracing, and trees near power lines are trimmed so branches can't fall on the wires.

Standing up to floods

  • Raise the building: build on tall posts or a raised foundation so floodwater flows underneath.
  • Deep foundations: bridge supports (piers) go deep into the ground or down to solid rock, so erosion can't easily wash out the soil holding them.
  • Leave room underneath: bridges are built high enough for floodwater, ice and logs to pass under.
  • Flood walls and dikes (long banks of earth) hold water back.
  • Water-resistant materials in basements, like concrete instead of wood, are less damaged by water.
MaterialWhy it is useful against nature's forces
steelstrong in tension and compression; bends a little before it breaks, so it can sway in wind and earthquakes
reinforced concreteconcrete is strong when squeezed; the steel bars inside carry the stretching (tension)
woodlight, strong for its weight and a bit flexible; a light building pushes less when shaken
rubber and steel layersused in base isolation pads; bend sideways but hold up the building's weight
Materials engineers choose

People, computers and sensors

  • Engineers use computer models to test how a design will behave in a strong wind or an earthquake before it is built. This is faster and safer than building and breaking real ones.
  • On some big bridges, small electronic sensors measure how much the bridge bends, stretches or shakes, and send the data to computers. Engineers watch the data to spot problems early. Code tells the computers how to sort and store all those numbers.
  • People in the skilled trades do the building. Ironworkers put up steel frames and braces, carpenters build frames and shear walls, concrete workers place reinforced concrete, and welders join steel parts. Each of their careful connections helps the whole structure stand up to nature.
Worked example

Let's choose features for a house in a flood area

A family wants to build a cottage near a river that floods some springs. Choose three features to help the cottage withstand floods.

  1. Think about the forces: water pushes sideways, carries ice and logs, and erodes soil.
  2. Feature 1: raise the cottage on tall posts or a high foundation so water flows underneath.
  3. Feature 2: put the posts on deep foundations so erosion can't wash out their support.
  4. Feature 3: use water-resistant materials (like concrete) for the lowest parts.
  5. Even better: build farther from the river or on higher ground.

Raise it on posts, use deep foundations, and use water-resistant materials (or build on higher ground).

Worked example

Let's explain cross-bracing with forces

A steel frame has an X-brace made of two thin steel rods. Wind pushes the frame from the left. Which rod does the work?

  1. Wind pushes the top of the frame to the right.
  2. As the frame tries to lean to the right, the top right corner moves away from the bottom left corner. So the rod from the top right corner to the bottom left corner gets longer: it is stretched, so it is in tension and pulls back.
  3. The other diagonal (top left to bottom right) would get shorter. A thin rod that is squeezed can buckle, so it doesn't help much.
  4. If the wind blows from the right instead, the two rods swap jobs.

The rod that is stretched (top right to bottom left) does the work, in tension. That is why X-braces need two diagonals: one for each wind direction.

Just for fun

Riddle: Why did the wobbly square frame feel so much better after getting an X-brace?

It finally had some triangles to lean on... so it didn't have to lean at all!

Hands-on activity

Brace test: which brace works?

You need: 5 strips of thin cardboard about 2 cm wide (cut from a cereal box; four about 15 cm long and one about 22 cm long), 4 paper fasteners (brass fasteners) or a pushpin and 4 short pieces of pipe cleaner, a strip of paper about 2 cm wide and 25 cm long, tape, and scissors (for a grown-up to use).

  1. Join the four 15 cm strips at their corners to make a square frame that can pivot at the corners. (A grown-up can make the holes with a pushpin.)
  2. Stand the frame on the table and gently push the top sideways. What happens? (It leans and folds flat.)
  3. Tape the paper strip across the frame as a diagonal brace, corner to corner. Push the top sideways, first one way, then the other way.
  4. Record: in which direction did the paper brace hold? In which direction did it crumple?
  5. Now replace the paper with the stiff 22 cm cardboard strip, taped on as the diagonal. Push both ways again.
  6. Last, use two paper strips in an X. Push both ways.

What to look for: one paper strip holds only when it is stretched (tension) and crumples when it is squeezed (compression). The stiff cardboard strip works both ways because it can be a strut and a tie. Two paper strips in an X hold both ways, because one of them is always in tension. That is why many steel buildings and towers use X-braces made of thin rods or cables.

Safety: a grown-up should make any holes with the pushpin, and do any cutting with scissors.

Your turn

1

What does cross-bracing do to a rectangular frame?

2

How do steep roofs help in snowy places?

3

What is base isolation?

4

Why is reinforced concrete stronger than plain concrete?

5

Give two ways to help a building withstand floods.

6

Name one skilled trade that helps build strong structures and what that person does.

More practice: warm-up

1

What shape do braces create in a frame?

2

What is a building code?

3

Which material bends a little before it breaks: glass or steel?

4

Why are some buildings near rivers built on tall posts?

5

What do metal straps between a roof and walls do?

More practice: on your own

1

Why do engineers test designs with computer models before building?

2

Why do building codes ask for stronger roofs in some parts of Canada than in others?

3

Explain why a bridge should be built high above a river that floods.

4

A tall steel tower sways in a strong wind and then goes back to normal. Is this a problem? Explain.

5

How do sensors on a bridge help keep people safe?

Challenge

1

In the brace test, a single paper strip held when the frame was pushed one way but crumpled when it was pushed the other way. Explain why, using the words tension and compression.

2

A school in a place with earthquakes is being designed. Suggest three features or materials and explain how each helps.

3

Why do you think engineers make structures stronger than the biggest load they expect?

Check your understanding

Quiz

1.

Which feature stops a frame from leaning when it is pushed sideways?

2.

Why are roofs in snowy places often steep?

3.

In base isolation, what does the building sit on?

4.

In reinforced concrete, what job do the steel bars do?

5.

Which is a good way to help a house withstand floods?

6.

An X-brace made of thin rods works when wind blows from either side because...

7.

Who puts up the steel frames and braces of a tall building?

For parents and teachers

Big idea (D2.3, second part): Structures withstand natural forces through features (cross-bracing, shear walls, strong connections, base isolation, steep roofs, raised and deep foundations, flood walls) and materials (steel, reinforced concrete, wood). The lesson also touches on how computer models, sensors and the skilled trades help (A2.2, A3.1). The brace test is a hands-on experiment (A1.2).

How to help: Look for X-braces on hydro towers, bridges, parking garages and playground equipment. Ask which brace would be stretched if the wind blew from the left. If you know someone in construction or a skilled trade, invite your child to ask them how they make buildings strong.

Common difficulties:

  • Thinking stronger always means stiffer. In earthquakes and wind, a bit of flexibility (steel, wood, base isolation) helps a structure survive.
  • Getting the X-brace direction wrong. Have your child mark the corners and watch which diagonal gets longer when the frame is pushed.
  • Safety: a grown-up should cut the cardboard and make holes with the pushpin.