D. Structures and Mechanisms · Forces Acting on Structures

Nature's Powerful Forces: Danger for Structures

D2.3 A1.1 A1.4About 40 minutes

Learning goals

I can name natural phenomena that make strong forces, like earthquakes, strong winds, heavy snow, ice storms and floods.
I can describe the forces each one puts on human-built structures.
I can explain how these forces can cause severe damage.
I can research a natural event using reliable sources.

Words to know

natural phenomenon
An event that happens in nature, like an earthquake, a storm or a flood. (More than one are called natural phenomena.)

earthquake
A sudden shaking of the ground, caused by huge sections of rock deep underground moving.

tornado
A spinning column of air that reaches from a storm cloud down to the ground. Its winds can be very strong.

freezing rain
Rain that freezes as soon as it touches cold surfaces, coating them in ice.

flood
When water covers land that is usually dry.

foundation
The bottom part of a structure, usually underground, that holds it up and spreads its weight into the ground.

erosion
The wearing away and carrying off of soil or rock by water, wind or ice.

When nature pushes hard

Most days, the structures around us stand quietly. But sometimes nature pushes much harder than usual: a windstorm, a blizzard, an ice storm or a flood.

In Grade 3 you learned that wind, water, snow and ice push on structures. Now you will look more closely at which forces each natural event makes, and why they can cause severe damage to human-built structures like houses, bridges, towers and roads.

Tree branches and power lines coated in thick ice after an ice storm, bending under the weight. — After an ice storm, the heavy ice adds a big load to branches and power lines. Notice how the branches bend and sag under the extra weight. AI-generated image

Earthquakes

In an earthquake, the ground shakes back and forth and sometimes up and down. This is a dynamic load. The bottom of a building moves with the ground, but the top lags behind, so the building sways.

Walls are pushed sideways, which causes shear. Earthquakes often leave X-shaped cracks in walls.
Columns and beams bend back and forth.
Joints can break, and weak buildings can collapse.

In Canada, the west coast of British Columbia has the greatest chance of strong earthquakes. Earthquakes also happen in parts of eastern Ontario and Quebec, though they are usually small.

The ground moves the bottom of the building, but the top lags behind, so the walls are pushed into a lean. That shear often leaves X-shaped cracks. (Drawn more tilted than in real life.)

Strong winds

Tornadoes, windstorms and the strong storms that reach Atlantic Canada from hurricanes all bring very strong winds.

Wind pushes sideways on walls and can tip or slide structures that are not anchored.
Wind blowing over a roof can lift it, pulling it up like a lid. The nails and straps holding the roof on feel tension.
Wind can snap trees and power poles (bending), and throw flying objects into buildings.
Gusts come and go, so wind is a dynamic load.

Heavy snow and ice storms

Heavy snow piles up on roofs. Wet snow is much heavier than fluffy snow. The roof beams bend, and the walls and columns feel more compression. If the snow is too heavy, a roof can sag or collapse.
In an ice storm, freezing rain coats everything in ice. Ice is heavy. Power lines stretch (tension), towers and poles bend and can buckle, and tree branches snap and fall on houses and wires.
Freezing and thawing: water that seeps into cracks expands when it freezes and pushes the cracks wider. Over many winters this breaks up roads (making potholes), sidewalks and concrete steps.

Water expands when it freezes, so ice pushes the sides of a crack apart. After many freeze-thaw cycles, small cracks become potholes. (Not to scale.)

Floods and moving water

Floodwater pushes sideways on walls, bridges and docks. Fast water pushes very hard.
Water carries logs, ice and other heavy objects that smash into structures like battering rams (a dynamic load).
Moving water causes erosion: it washes away the soil under a bridge's supports or a building's foundation, so the structure loses its support and can tilt or fall.
Water pressing on the outside of a basement can crack the walls.

Natural phenomenon	Forces on structures	Possible severe consequences
earthquake	shaking (dynamic); shear and bending	X-shaped cracks, broken joints, collapse
strong wind or tornado	sideways push; upward lift on roofs (dynamic)	roofs torn off, structures tipped, trees and poles snapped
heavy snow	downward push (static, builds up)	sagging or collapsed roofs
ice storm	heavy ice adds weight; tension in wires	snapped wires, buckled towers, fallen trees
flood	sideways push of water; objects hitting; erosion	washed-out bridges and roads, damaged foundations
freezing and thawing	ice expanding inside cracks	potholes, cracked concrete and roads

Natural phenomena and the forces they put on structures

Worked example

Let's analyse the forces in an ice storm

In an ice storm, ice builds up on a power line strung between two tall steel towers. Describe the forces.

External force: the ice is heavy, so gravity pulls the wire down harder. Wind may also push on the ice-coated wire.
The wire: it is stretched more and sags lower: tension increases. Too much and it snaps.
The towers: the heavy wires pull down and inward on the towers' arms, and the legs are squeezed: more compression and bending.
Result: if the load is greater than the towers were built for, the steel parts can buckle, and a tower can collapse. Then it may pull down the next tower too.

Heavy ice adds a large load. The wire feels more tension and the towers feel more compression and bending. Too much, and wires snap or towers buckle and collapse.

Worked example

Let's identify the forces in a flood

A river floods during the spring melt. Ice chunks and logs float down and hit a small bridge. Name three ways the flood can damage the bridge.

The fast water pushes sideways on the bridge's supports.
Ice chunks and logs smash into the supports (a dynamic load).
The water erodes (washes away) the soil around the supports' foundations, so they can tilt or sink.

Sideways push of water, hits from ice and logs, and erosion of the soil under the foundations.

Stay safe in natural events

After a storm, stay far away from fallen power lines: at least 10 metres (about the length of a school bus). Tell an adult to call for help.
Never walk or play near fast-moving or flood water, or on river ice in spring.
Stay out of damaged buildings until experts say they are safe.

Just for fun (and science)

Sneaky but true: a pothole starts as a tiny crack. Water creeps in, freezes, gets bigger and pushes. Every winter, ice wins a little wrestling match against the road!

Hands-on activity

Ice is a pusher! (freezing demonstration)

You need: a small plastic container with a lid that snaps on (like an old yogurt tub), water, a marker, and a freezer.

Fill the container right to the top with water. Snap the lid on loosely.
Put it on a plate in the freezer overnight.
Predict: what will happen to the ice and the lid?
The next day, look closely. Draw what you see.

What to look for: the ice bulges up above the rim and may push the lid up or off. Water expands (takes up more space) when it freezes, and it pushes hard on whatever holds it in. That is the force that widens cracks in roads, sidewalks and rock every winter.

Research part (with a grown-up): use a reliable source, like a government weather website, a library book or a news report your grown-up chooses, to find one natural event that damaged structures somewhere in Canada. Write: what happened, which forces acted, and what was damaged.

Safety: use a plastic container, never glass (glass can crack and break in the freezer).

Your turn

What is a natural phenomenon? Give two examples.

What kind of crack do earthquakes often leave in walls, and which internal force causes it?

How can strong wind damage a roof?

Why is wet snow more dangerous for roofs than fluffy snow?

How can moving floodwater damage a building's foundation without touching the building itself?

Why do potholes appear in spring?

More practice: warm-up

Which natural phenomenon makes the ground shake?

Is the wind in a storm a static or dynamic load?

What does freezing rain do?

What happens to water when it freezes?

How far away should you stay from a fallen power line?

More practice: on your own

During an earthquake, why does the top of a tall building sway?

In an ice storm, why can one collapsing tower cause the next one to fall too?

Explain why logs and ice in a flood are especially dangerous to a bridge.

A tornado can lift a shed and drop it far away. Which two things did the wind have to overcome?

Choose one natural phenomenon. Name one external force it puts on a house and one internal force that results.

Challenge

Two houses get the same deep snow. One roof is flat and the other is steep. Which roof is more likely to collapse? Explain.

Why might climate change make some kinds of damage to structures more common in Canada? Give one example and use the word 'force'.

You read two reports about the same storm. One is from a government weather service. The other is a social media post with no name. Which is more reliable, and why?

Check your understanding

Quiz

Which natural phenomenon most often leaves X-shaped cracks in walls?

How does heavy snow mainly push on a roof?

Why does freezing and thawing make potholes?

Which is a way a flood can damage a bridge?

In an ice storm, the wires between towers feel more...

How can strong wind damage a house?

For parents and teachers

Big idea (D2.3, first part): Natural phenomena such as earthquakes, strong winds, heavy snow, ice storms, floods and freeze-thaw cycles put large external forces on human-built structures. Your child should be able to link each event to the forces it creates (sideways push, uplift, extra weight, shaking, erosion) and the damage that can result. The next lesson covers the features and materials that help structures withstand these forces. The research step practises A1.1; the safety tips practise A1.4.

How to help: After a storm, look (from a safe distance) for effects: broken branches, cracked sidewalks, potholes. Ask, "Which force did that?" Help your child choose reliable sources for research, such as Environment and Climate Change Canada or a library book.

Common difficulties:

Thinking snow is light. A roof covered in deep wet snow can carry a very large load.
Forgetting erosion. Water damages structures by removing the ground beneath them, not only by pushing.
Worry: some children find disasters frightening. Reassure them that engineers design buildings and building rules to keep people safe, and that weather warnings give people time to prepare.
Safety: use a plastic container (not glass) in the freezer. Talk about staying far from downed power lines and flood water.