D. Structures and Mechanisms · Strong and Stable Structures

Struts and Ties: Push and Pull Parts

D2.7 D2.6 D2.3 A1.2 A1.5About 35 minutes

Learning goals

I can explain that a strut is a part that resists being squeezed (compression).
I can explain that a tie is a part that resists being stretched (tension).
I can find struts and ties in structures at home and in my community.
I can use struts and ties to make a wobbly frame strong.

Words to know

strut
A stiff part of a structure that is squeezed (in compression). It pushes back to hold things apart. Table legs are struts.

tie
A part of a structure that is stretched (in tension). It pulls back to hold things together. A rope, cable or chain can be a tie.

compression
A squeezing force.

tension
A stretching force.

brace
An extra part, often diagonal, that makes a frame stronger. A brace can be a strut or a tie.

Poles and ropes

Think about setting up a tent. The poles hold the tent up. The ropes (guy lines) tied to pegs in the ground hold the tent down and steady.

The poles are being squeezed. The ropes are being stretched. Engineers have names for these two kinds of parts: struts and ties.

A green dome tent stands on a forest floor beside a lake, held by poles and bright guy lines tied to metal pegs. — The tent poles are struts that are squeezed as they hold the tent up. The bright guy lines are ties that are stretched to hold it down and steady. AI-generated image

Struts are squeezed

A strut is a part that is being squeezed (in compression). It pushes back to hold things apart or up.

The legs of a table or chair are struts. The load presses down, and the legs push back up.
Tent poles are struts.
A column holding up a porch roof is a strut.

Struts must be stiff. You could never hold up a table with legs made of string!

Ties are stretched

A tie is a part that is being stretched (in tension). It pulls back to hold things together.

The chains or ropes of a swing are ties.
The guy lines on a tent are ties.
The big cables on a suspension bridge are ties. They hold up the road below them.
The string of a kite is a tie.

Ties can be stiff (like a metal rod) or bendy (like rope, cable or chain), because they only ever need to pull.

Two ways to hold up a shelf on a wall. On the left, a strut underneath is squeezed and pushes the shelf up. On the right, a tie above (like a cable) is stretched and pulls the shelf up.

A quick test

To tell a strut from a tie, imagine swapping the part for a rope. If a rope would still work, the part is being pulled: it is a tie. If a rope would just flop and the structure would fall, the part is being pushed: it is a strut.

Worked example

Let's find the struts and ties

Look at a swing set. Which parts are struts and which are ties?

The legs of the swing set hold up the top bar. The weight presses down on them, so they are squeezed. They are struts.
The chains hang from the top bar and hold the seat and the child. They are pulled down and stretched. They are ties.
Rope test: could the legs be made of rope? No, the swing set would fall. Could the chains be rope? Yes, many swings use rope. That confirms it.

Struts: the legs (squeezed). Ties: the chains (stretched).

A swing set: the legs are struts (squeezed) and the chains are ties (stretched).

Worked example

Let's fix a wobbly frame

You make a square from 4 craft sticks with a pin or paper fastener at each corner. When you push the top sideways, it leans over. How can struts or ties fix it?

Add a stiff stick from one corner to the opposite corner. It makes two triangles. Now the square cannot lean either way. When you push, the diagonal stick is either squeezed (a strut) or stretched (acting as a tie), and it is stiff enough to resist both.
Or add a string from corner to corner, pulled tight. When you push the square one way, the string is stretched and stops it. That string is a tie.
But push the other way and the string goes slack (loose), so the square leans! A string can only pull, never push.
To fix that, add a second string across the other two corners, making an X. Now whichever way you push, one string is stretched and holds the frame.

A stiff diagonal (strut) braces the square both ways. One string (tie) only works one way; two strings in an X brace it both ways.

Just for fun

What did the swing chains say to the swing set legs?

"You hold us up, and we will just hang in there!"

Hands-on activity

Strut and tie frame lab

You need: 6 drinking straws (paper or plastic), pipe cleaners or tape for the corners, string, scissors (a grown-up helps cut), and a small toy or eraser.

Make a square from 4 straws. Join the corners with short bits of pipe cleaner pushed into the straw ends (or with tape). Stand it up on its edge.
Gently push the top corner sideways. What happens? (It leans.)
Strut test: tape a fifth straw from one corner to the opposite corner. Push again from both sides. What happens now?
Take the diagonal straw off. Tie test: tie a piece of string tightly from corner to corner. Push one way, then the other way. Does it hold both ways?
Add a second string across the other corners to make an X. Test again both ways.
Draw each test and label the struts (squeezed) and ties (stretched).

What to look for: the plain square leans; the straw diagonal holds both ways; one string holds only one way; two strings in an X hold both ways. Safety: a grown-up does any cutting, and pipe cleaner ends can be poky.

Your turn

What is a strut?

What is a tie?

Are the legs of a table struts or ties?

Is the rope of a tire swing a strut or a tie?

Could you use a rope as a table leg? Why or why not?

Name the struts and ties in a tent.

How can you make a wobbly square frame stiff?

More practice: warm-up

Is a strut squeezed or stretched?

Is a tie squeezed or stretched?

Is a kite string a strut or a tie?

Is a porch column a strut or a tie?

Can a chain be a strut?

More practice: on your own

A hanging flower basket hangs from a hook by three chains. Are the chains struts or ties? Explain.

A suspension bridge has tall towers and long cables. Which are struts and which are ties?

When you push a folding chair down with your weight, which parts are struts?

In the strut and tie lab, why does one string only stop the square leaning one way?

A clothesline is held between two poles. Name the strut(s) and tie(s).

Challenge

A sign hangs from the end of a pole that sticks straight out from a wall. (a) A diagonal piece joins a spot on the wall above the pole to the pole's end. (b) Instead, a diagonal piece joins a spot on the wall below the pole to the pole's end. Is the diagonal piece a strut or a tie in each case?

Why can a tie be made of thin cable, but a strut usually needs to be thicker and stiffer?

Look at a bicycle frame. It is made of tubes joined in two triangles. Why are triangles of tubes a good design?

Check your understanding

Quiz

A strut is a part that is...

A tie is a part that is...

Which is a tie?

Which is a strut?

Why can't a rope be a strut?

One tight string across a square frame stops it leaning...

For parents and teachers

Big idea: In a structure under load, struts are parts in compression (squeezed) and ties are parts in tension (stretched) (D2.7). Struts must be stiff; ties can be flexible (rope, cable, chain) because they only pull. Diagonal struts or crossed ties turn wobbly rectangles into rigid triangles. The straw frame lab is an investigation, and labelling a drawing is communication (A1.2, A1.5).

How to help: At the playground or on a walk, play "Strut or tie?" Point to swing chains, slide ladders, fence posts, hammock ropes and bridge cables. Use the rope test: "Could this part be a rope?"

Common difficulties:

Thinking ties must be soft. A tie can be a steel rod; what matters is that it is being pulled.
Mixing up which is which. Try: "A tie is like a rope you tie: it pulls. A strut stands stiff and gets squeezed."
Expecting one string to brace both ways. This is a great "aha" moment: strings only pull.

Safety: a grown-up does the cutting; watch pointed pipe cleaner ends.