D. Structures and Mechanisms · Strong and Stable Structures

Forces and Loads on Structures

D2.6 D2.1 A1.2About 35 minutes

Learning goals

I can name forces that act on structures, like gravity, wind, moving water, snow and people.
I can tell the difference between a dead load and a live load.
I can describe how forces can change a structure's shape, balance or position.
I can use the words squeeze (compression) and stretch (tension).

Words to know

force
A push or a pull.

gravity
The force that pulls everything down toward the Earth.

dead load
The weight of the structure itself. It is always there. A bridge's dead load is the bridge's own weight.

live load
A load that comes and goes or moves, like people, cars, furniture, snow or wind.

compression
A squeezing force that pushes the parts of something together.

tension
A stretching force that pulls the parts of something apart.

balance
Staying steady and upright without tipping over.

Why do structures fall down?

In a big winter storm, heavy, wet snow piles up on roofs. Strong winds blow. Sometimes a carport roof sags, or a tree branch snaps, or a garbage bin blows down the street.

What happened? Forces pushed and pulled on these structures. Engineers must think about every force a structure will feel, so it stays strong and steady.

A house roof and an evergreen tree are covered with a thick layer of heavy snow after a winter storm. — Snow presses down on the roof and the tree branches. The roof must be strong enough to hold this heavy load. AI-generated image

Remember from Grade 2

In Grade 2 you learned that a force is a push or a pull, and that gravity pulls things down to the ground. Forces can make things start moving, stop, or change direction. Structures feel forces too, even when they stand still!

Forces that act on structures

Gravity pulls every structure, and everything on it, down. It never stops.
Wind pushes on the sides of structures.
Moving water and waves push on docks, bridges and boats.
Snow and ice press down on roofs, trees and power lines.
People and animals push and pull when they climb, sit, lean and play.
In some parts of Canada, like British Columbia, the ground can shake in an earthquake, and buildings there are built to handle shaking.

Two forces on a tower: the wind pushes on its side, and gravity pulls it down.

Two kinds of loads

A dead load is the weight of the structure itself. A bridge must hold up its own steel and concrete. A dead load is always there and does not change.
A live load is a load that comes and goes or moves. The cars on a bridge, the people in a school, the snow on a roof and the wind pushing on a tower are live loads.

A good structure must hold both: its own weight plus the live loads.

What can forces do to a structure?

Forces can change a structure's shape, its balance or its position.

A force can change its...	What happens	Example
shape	It gets squeezed, stretched, bent, twisted or broken.	Heavy snow makes a tent roof sag. A shelf bends under too many books.
balance	It leans, tips or falls over.	Strong wind tips over a patio umbrella. A tall stack of blocks falls when bumped.
position	It slides, gets pushed or blown to a new place.	A wave pushes a dock away from shore. Wind blows an empty garbage bin down the street.

How forces affect structures

Squeeze and stretch

Two special words describe forces inside a structure:

Compression is a squeeze. It pushes the parts together. When you stand on a stool, its legs are squeezed between you and the floor.
Tension is a stretch. It pulls the parts apart. When you swing, the chains or ropes of the swing are stretched.

Things can also bend (squeezed on one side and stretched on the other) and twist.

Worked example

Let's find the forces

A ruler is laid across two stacks of books like a bridge. You put a juice box in the middle. What happens, and why?

Gravity pulls the juice box down. That is a live load on the ruler bridge.
The ruler bends down in the middle. A force changed its shape.
If you look closely, the top of the ruler is squeezed (compression) and the bottom is stretched (tension).
If the load is too heavy, the ruler could slide off the books (position) or snap (shape).

Gravity pulls the load down, so the ruler bends. The force changed the bridge's shape.

The ruler bridge bends under the load. Its top is squeezed (compression) and its bottom is stretched (tension).

Worked example

Let's predict: shape, balance or position?

A big windstorm is coming. What might happen to (a) a tall, thin flagpole, (b) a backyard trampoline, (c) a tent with no pegs?

(a) The wind pushes on the flagpole. It might bend (shape) or, if it is not fixed well, lean or fall (balance).
(b) A trampoline is light and wide, so a strong wind can lift and push it across the yard (position). That is why people tie them down.
(c) A tent with no pegs is light. The wind can blow it away (position) or flatten it (shape).

Wind can change the shape, balance or position of a structure. Tying down and pegging help.

Just for fun

Why does the bridge stay calm during rush hour?

It knows the traffic is only a live load. It will pass!

Hands-on activity

Forces test lab

You need: a kitchen sponge, a hair elastic or rubber band, a ruler, two equal stacks of books, a small toy, a paper cup, and a piece of cardboard to use as a fan.

Squeeze the sponge (compression). What happens to its shape?
Stretch the elastic gently (tension). What happens to its shape? Let go slowly.
Twist the sponge. What happens?
Make a ruler bridge between the books. Put the toy in the middle. Does the ruler bend? Move the books farther apart. Does it bend more?
Stand the paper cup upside down on a table. Fan it with the cardboard, gently, then harder. Does it slide, tip, or stay put? Now put a coin inside, under the cup, and try again.
Record each test: force, what changed (shape, balance or position).

What to look for: squeezing and twisting change the sponge's shape; the ruler bends more when the gap is wider; the empty cup slides or tips in a strong wind, but a weight inside helps it stay. Safety: stretch the elastic gently and point it away from faces. Keep coins away from little children.

Your turn

What force is always pulling down on every structure?

Is the weight of a bridge itself a dead load or a live load?

Are the people walking across a bridge a dead load or a live load?

Snow on a roof makes it sag. Did the force change its shape, balance or position?

A strong wind knocks over a garbage bin. Did the force change its shape, balance or position?

Is compression a squeeze or a stretch?

When you hang from monkey bars, are your arms in compression or tension?

More practice: warm-up

Is tension a squeeze or a stretch?

Name a force that pushes on the side of a tall building.

Is the snow on a roof a dead load or a live load?

A wave pushes a dock away from the shore. Did its shape, balance or position change?

What do forces do: push and pull, or only push?

More practice: on your own

Name three live loads on a school building.

When you sit on a stool, are its legs being squeezed or stretched? What is that called?

Why do people tie down trampolines and patio furniture before a windstorm?

A rope holds a tire swing. Is the rope in compression or tension? How do you know?

You make a ruler bridge, then move the two stacks of books farther apart. Predict what happens to the bend when you add the same load.

Challenge

A builder in Nunavut and a builder in southern Ontario both build houses. Name two forces they both must plan for.

A shelf bends in the middle when it holds many books. Which side of the shelf is squeezed and which side is stretched?

Your tower of blocks falls when your little brother bumps the table. Explain what happened using the words force and balance.

Check your understanding

Quiz

Which force always pulls structures down?

The weight of the structure itself is called a...

Which is a live load on a bridge?

Compression is a...

A strong wind blows a tent across a field. Which changed?

The chains on a swing are...

For parents and teachers

Big idea: Forces such as gravity, wind, water, snow and people can change a structure's shape, balance or position (D2.6). Grade 3 students learn dead loads (the structure's own weight) and live loads (things that come and go), and the words compression (squeeze) and tension (stretch). The forces lab is a hands-on investigation (A1.2).

How to help: After a storm, look around together: "What did the wind or snow do to that tree, fence or bin? Did it change shape, balance or position?" When your child sits, swings or climbs, ask: "Is that part being squeezed or stretched?"

Common difficulties:

Thinking structures feel no force when nothing moves. Gravity is always pulling, and the structure is pushing back.
Mixing up compression and tension. Compression = squeeze (like a spring pushed together); tension = stretch (like a rope pulled tight).
Thinking snow is a dead load. Snow comes and goes, so it is a live load.

Safety: stretch elastics gently and away from faces; keep coins away from young children.