# Engineering – Bridges, Tunnels, Complex Structures

In the first week of our Engineering Unit, we studied Towers – stacking one block on top of another block to build something tall. This week, we looked more at other more complex structures. We played with bridges, tunnels, beaver dams and more.

Concept Exploration

Index card bridges: We had a table with blocks, index cards, and glass stones. As children came in, we would demonstrate three types of bridges to them and tested how many “stones” it would hold. (We got this idea from Stay at Home Educator, whose post has lots of good pictures of her process.) We showed them a

1. Beam bridge – set two blocks up, put an index card spanning the distance between them. That will hold just three stones (unless kids were clever enough to rest the stones on top of the blocks rather than in the middle of the bridge)
2. Arch bridge – insert one index card BETWEEN the two blocks, to form a support arch, then lay another index card on top of it for the flat bridge deck. That generally would support 7 stones.
3. Accordion beam: take two index cards – fan fold them. Lay those on the two blocks, then put a flat index card deck on top of them. That would easily hold 20 or so stones.
We would demonstrate these three ideas, then challenge them to test as many other ideas as they could come up with. We walked away and let them tinker, exploring different materials and ways of using them in order to build stronger bridges. Our littler kids just played with the basics of bridges. Our older ones really took in more of the message about testing different methods. I only caught a few pictures of the process, but you can see a variety of methods were explored.

Tunnels and Bridges in the Sensory Tub. We made a cloud dough with flour, cocoa powder and vegetable oil. It smells good, it’s somewhat moldable (like kinetic sand), it looks like dirt, and it won’t hurt you if you eat it. We placed it in the tub with rocks, dishes, and tongue depressors, so children could build bridges and tunnels. Next time, we’ll do an illustration to hang next to it of how to do a basic cut and cover tunnel – dig a trench, then cover it over with the tongue depressors.
Marshmallow Structures: We put out mini marshmallows, big marshmallows, and toothpicks for kids to build whatever they wanted with. (Note: kids will want to eat marshmallows. For some of my kids, I didn’t bring it up unless they asked. Others, I knew to pre-empt. One I told on the way in that he could eat one marshmallow at the start of class and one marshmallow at the end of class and that was all – I knew if I gave that permission up front, he would absolutely eat those two, but that would be it. A mischievous little girl was eating marshmallows in front of me, as I told her not to, and I told her I would send her away from the table, so she ate one more, and I did. But it wasn’t really a punishment – it was her way of testing and accepting her limits.)
Pipe cleaners and straws: Kids could thread pipe cleaners through cut up straws, and then bend into various structures.

Arch Bridge. Just as we ask the kids to tinker in class, we are often tinkering ourselves as we plan the class. I made salt dough (1/2 cup salt, 1 cup flour, ~1 cup water), formed an arch, VERY carefully cut it into slices and baked it (250 for 2 hours). I was hoping it might be a stand-up arch bridge. It wasn’t. The pieces expanded just a little when cooking, but it was enough to distort the shapes so the bases would not stand. We did use it in class, but instead of being a cool upright structure, it was a math activity of put the pieces in order by number to make an arch. (We had a picture of a Roman arch bridge next to it so they saw what it related to.)

There are a few kits available of roman arches: Haba Roman Arch,
Our Amazing Bridges Model Building Kit, or the more expensive
Montessori Roman Arch. We generally try to build most of our own materials rather than buying specialty products, but this may be one of those places that’s worth buying a product someone else has engineered.

Igloos. We built igloos with sugar cubes and glue. Here’s the beginning of one. Place a good illustration of igloo building next to this for guidance.

Nature / Imagination

For next year, we’re planning spring semester as a Biology / Inventions unit, where we talk about things from nature and biology and also talk about how that inspires human builders and inventors. This week, we brought in the idea of an animal that builds structures.

Beaver Masks: Teacher Cym designed beaver masks on paper plates, and cut them out. In class, kids could decorate them.

Be a beaver: We built a set of tongs with paint stirrers, and kids could pretend that was their beaver mouth, and move around “sticks” made with newspaper rolls to build a dam.

Build a beaver dam: We put out paste (glue and flour mixed), spaghetti sticks, and “pebbles” for kids to sculpt a beaver lodge or beaver dam. We had an illustration of a beaver dam nearby.

Big Motor Play – Trusses

Months ago, I’d found a blog post on Modern Parents, Messy Kids that showed how to build what they called “Life Size Building Blocks” – triangular trusses made from newspaper. I’ve been looking forward to this ever since. To make the paper tubes, they recommend “Take two sheets of newspaper, lay them out flat and start rolling it from one corner.” The advantage of this method is it’s quick and easy, and you need just one little square of tape to fasten each one. Their 7 and 9 year old were able to roll these by themselves. When I made mine, I made 45 (enough for 15 triangles) in the time it took us to watch Kung Fu Panda. But – the ends of these are very thin – that means they’re easy to staple together, but they also are weak and floppy and can collapse in on themselves. So, my husband developed a new method. Take two pieces of paper, and off-set them a bit. Then lay those two diagonally across a third. Roll from there. This results in ends that are still thin enough to be stapled, but sturdier than the basic method.

This method was a little slower. My husband and I working together probably rolled about 45 in the time it took us to watch Little Mermaid.

In the blog post, they say their 7 and 9 year old did all the assembly. In our class, the parents did the assembly, using duct tape to fasten the joints, then the kids played in the big structure we had built.

In the book Build It: Invent New Structures and Contraptions, there are more detailed instructions on how to build a free-standing geodesic dome with newspaper trusses. It looks to be about 4 feet tall and 8 – 10 feet across.

Big blocks. It’s just fun to build with really big blocks – these are heavy and moving them around feels like an accomplishment. You have to warn the kids not to knock structures over and to be very careful when building up, because these blocks are really heavy and really hurt if a corner of one falls on your toes!

Note, if you have access to big blocks (wood, foam, or cardboard) I love the idea from Not Just Cute of taping out a pattern of blocks on the wall and having kids match that pattern.

Circle Time

Key concepts: A good place to start would be to ask why people build bridges. Like our discussion of towers last week, we could have a collection of pictures of bridges, from a simple log over a stream on up, to help them think up reasons. Reasons would include: to go across water without having to swim or get wet, to go across a valley without having to go down the hill and up the hill (think railroad trestle), to go over a road. Different obstacles present different challenges. For example, the log across the stream works great, but what if the water you want to bridge is wider than any log you have? Building a bridge on pilings is great, but if the water is REALLY deep, that doesn’t work. (We have floating bridges in Seattle that all our kids are very familiar with.)

So we need to be able to build bridges with a variety of materials and a variety of methods. We reviewed what they had learned with the blocks and the index cards. Then did this demo:

What shape holds the most weight: Take four pieces of paper. Fold one in half to make a tent. Fold one into a triangle tower, one into a squat square, and one into a round column. Ask your child which will hold the most weight, then test them. Watch this video from PBS’ Zoom to see this demo in action, except… they use a heavy book each time which crushes three out of four shapes. I would start with a thin lightweight book, which would crush the tent, but maybe not the triangle or the square. Then test the heavier book on that shape – it will crush it. Then test light book on round column, then heavy book. We did this with newspaper, and then ended by showing them a truss made of newspaper, and pointing at the structure we had made using trusses.

Suspension bridge: We demo’ed a suspension bridge, built with a cardboard deck, string, and paper clips. If you’re working with 10 – 12 year olds, try this suspension bridge project.

Song: Teacher Cym wrote a song to the tune of “Johnny Comes Marching Home Again”

We’re going to build a bridge today, hurray! hurray!
We’re going to build a bridge today, hurray! hurray!
We’re going to build a bridge today,
We’ll make it strong, we’ll make it straight,
We’ll see if it will hold the weight.
And we’ll all be learning together while we play.
Hurray!

Books and more songs: In opening circle, we read Iggy Peck, Architect, which is a delightful story of a boy who likes to build, who is discouraged by his teacher, until he saves the day by guiding his class in building a suspension bridge from shoelaces, fruit roll-ups and things.

At closing circle, we demonstrated the basic idea of a draw bridge (using our suspension bridge model), then read The Bridge Is Up! which is a nice cumulative read-along for three to six year olds about all the vehicles who have to wait while the draw bridge is up. Then we taught them the rhyme Zoom Down the Freeway. (the lyrics on this page and the hand motions are the same we use, but I say it as a rhyme instead of singing it to the tune he uses.) Many of the parents and kids learned this rhyme in our toddler class, and recognized it from there.

We also read London Bridge Is Falling Down which includes all the verses of the song, illustrated, plus music notation, plus (for adults or older kids) a section on the history of London Bridge. We read/sang it – it’s an old nursery rhyme, so the verses are a little odd in places… for example, they’ve put a guard on the bridge they need to keep awake, so the verse is “give him a pipe to smoke all night.”

After reading that, we played London Bridge. I like to bring in classic kids’ games where we can, and this was a nice opportunity. Two children make an arch, the rest of the kids line up and start going under the arch, then circling around to go under again. We sing the song, and whenever we get to “my fair lady”, the bridges drop their arms and capture someone, who then replaces one of the bridge supports and the game begins again.

Other Explorations of the Day

Creation Station: We have an area stocked with tape, glue, and lots of recycled and found materials where kids can always work to create anything they want. The oldest child in our class (who just turned 8) often does fabulous stuff over there. This week’s creation: a U-Scan checkout like you find at the grocery store.

Symmetry Game: One of our 6 year olds was playing with blocks, and I noticed he was setting up a symmetrical arrangement.

I challenged him to a game (that I invented on the spot.) I would place a block, and he had to put one to mirror it to keep the structure symmetrical. Then he would place a block that I had to mirror. At first, I stayed on the vertical or horizontal axis, which is easier. But then I started placing things on the diagonal and he had to figure out the radial symmetry. We had a GREAT time playing the game. It was at the edge of his capabilities. Hard enough to be challenging, and to feel really good about his successes. Great learning in the math and spatial realm. Here’s two of our creations:

Other Ideas

PVC pipes: At one of our classrooms, we have a collection of PVC pipes and joints that allow kids to build a variety of structures.

Animal Tunnels: You could offer pictures of various animal tunnels and burrows: prairie dogs, rabbits, ants, and so on. You could also create a worm habitat and watch it for several weeks to see the tunnels develop.

More books:

Cross a bridge is a nice age appropriate and engaging non-fiction about bridges.

Bridges: Amazing Structures to Design, Build & Test is aimed at much older kids, but has some fun ideas for advanced bridge building projects.

Dig a Tunnel is a engaging preschool read about tunnels. Digging Tunnels and Tunnels are also fine non-fiction choices, but Dig a Tunnel is my favorite of the 3.

Monsters Under Bridges. I wanted to love this book, a guidebook to the bridges of the Pacific Northwest. I only know one of these bridges well, but when I looked at those pages, they were not what I hoped for. In Seattle, under highway 99, we have a statue known as the Fremont troll. Where it appears in the book, it is mentioned as being one of many imaginary monsters here. I’d rather they just talked about the Fremont troll and made it “real” instead of making up others that aren’t here.

Look at That Building!: A First Book of Structures. A nice story that introduces fundamentals of buildings, like foundations, beams and frames. Too long to use in circle time, but worth a read.

Bridges Are to Cross. Very nice artwork depicting famous bridges around the world. Each page has a description of what bridges are for: “some carry llamas loaded with firewood across deep canyons, this bridge lets boats float under the road”. Has more details on each type of bridge in fine print. A fine book for the shelf.

[affiliate links – learn about all these books on Amazon and buy if you choose, but remember most of these can be checked out from your library!]

Videos to Preview or Review the Lesson. For 3 – 5 year olds, Peep and the Big Wide World: Bridge the Gap. For 4 – 6 year olds, What Make Bridges So Strong? (3:44). For 6 and up: Bill Nye’s Structures episode.