Grade 9 bridge school project: how to build a model arch bridge, step by step

The Grade 9 bridge project is the Technology project about structures. Learners design and build a model bridge, then explain how it carries a load: which parts are pushed, which are pulled, and why the shape holds up. The Slotbok kit builds a tied-arch bridge, one of the clearest bridges for that explanation, because every member does one job you can see.

This guide covers the main types of bridges, how a tied-arch bridge works, the build in eight steps, how to test it, and what to write up.

Watch the whole build first

The 3D animation that comes with the Slotbok kit: all 38 pieces going on in order, in just over a minute. Watch it once before you open the box.

The same build plays full-screen on the bridge assembly video page.

Types of bridges

Most Grade 9 bridge projects start by comparing the main types:

• Beam bridge — a beam resting on a support at each end. Simple, but the beam bends under load, so it only suits short spans.
• Arch bridge — a curved arch carries the load to its ends in compression. Strong for its weight, but its ends push outwards, so they need firm supports or a tie.
• Truss bridge — a frame of triangles, in which each member is either pushed or pulled.
• Suspension bridge — the deck hangs from cables draped over tall towers and anchored at both ends.
• Cable-stayed bridge — straight cables run from the towers directly to the deck. The Nelson Mandela Bridge in Johannesburg is one.
• Cantilever bridge — arms reach out from the supports and meet, or carry a short span, in the middle.

The Slotbok kit is an arch bridge with a twist: the arches stand above the road, and the road itself ties their ends together.

What is in the kit

The Slotbok arch bridge kit builds a bridge 41 cm long, 19 cm high and 16 cm wide, with a road deck 14.4 cm wide. In the box, 38 pieces:

• 26 laser-cut parts on four 3 mm MDF sheets (200 × 300 mm), every part labelled
• 10 wooden dowels: four for the deck joint, two for the arch feet and four for the top bracing
• 2 cotton twines, 1.35 m each, for the hangers
• Wood glue and sandpaper

The 26 parts are two deck plates, two end walls, four side rails, four splice plates, four legs, four beams, four arch halves and two crowns.

How a tied-arch bridge carries a load

Follow a load from the deck down to the table:

1. The deck carries whatever is put on it, and hangs from the twine.
2. The twine hangers are pulled tight and lift the deck towards the arches. They are in tension.
3. The arches are pushed along their curve and carry the load down to their feet. They are in compression.
4. The side rails tie the two feet of each arch together, so the arch cannot spread. They are in tension, like the string of a bow.
5. The stands carry the weight straight down to the table.

Why an arch?

An arch turns a downward load into a push along its own curve, so nearly all of it is carried in compression, and materials such as stone, concrete and MDF are strong when they are pushed. That is why arches have been used for bridges for more than two thousand years. The catch is that an arch's feet push outwards. Old stone bridges hold that push with heavy abutments in the ground; a tied-arch bridge ties its feet together with its own deck instead, so it can stand on simple supports.

Why the twine zigzags

The twine runs from a rail tab up to an arch hole and back down to the next rail tab, so the hangers make a row of triangles between the arch and the deck. A triangle cannot change shape unless one of its sides stretches, so the zigzag keeps the arch and the deck working together, even when the load is not in the middle.

What the top dowels do

A thin arch that is being pushed can bend sideways and buckle, the way a ruler bows when you squeeze its ends together. The four dowels across the top join the two arches, so each one holds the other straight. Real arch bridges have the same bracing between their arches.

The middle joint

The deck comes in two halves. The splice plates on both sides of each rail joint, held by dowels right through, carry the load across the join: the same idea as the bolted plates that join the steel sections of a real bridge.

How to build the bridge, step by step

The build follows the kit's 3D animation, in eight steps. Before you start, press the parts out of the sheets and sand any tab that feels tight, so that nothing has to be forced.

1. Deck plates

Lay the two deck plates end to end, road side up. The straight edges meet in the middle.

2. End walls

Push an end wall onto each end of the deck so its teeth sit in the deck slots, and add a drop of glue.

3. Side rails

Fit the four side rails onto the deck fingers, with the tabs on top and the single hole at the bridge ends. The end-wall tabs go into the rail notches.

4. Middle joint

Put a splice plate outside and inside each rail joint, then push four dowels right across the bridge.

5. Stands

Slot two beams into the bottom slots of two legs to make a stand. Make two stands and set them under the bridge ends.

6. Arch halves

Hold an arch half outside each rail end and push a dowel through the arch, the rail and the leg, right across the bridge. Do both ends.

7. Arch crown

Close each arch with a crown on the inside. Line up the holes and push the four top dowels through both arches.

8. Twine hangers

Tie the twine at the first arch hole, then zigzag: rail tab, arch hole, rail tab, all the way along. Tie off at the end, and repeat on the other side.

Done: let it dry

Let the glue dry for about an hour before you test the bridge.

The build as a flow chart

The eight steps make a ready flow chart for the making part of your project:

Deck plates → End walls → Side rails → Middle joint → Stands → Arch halves → Arch crown → Twine hangers → Glue dries (about an hour) → Test

Draw it with a box for each step and an arrow between them, and add what you checked at each stage.

Testing your bridge

1. Set the bridge on a flat table.
2. Put the load in the middle of the deck, a little at a time: coins, small books, or a bag of rice that you can weigh.
3. Watch the deck and the arches, and note where the bridge moves first.
4. Stop as soon as anything starts to bend or a joint starts to open, and write down the load.

Then repeat with the load near one end. Comparing the two results gives you a strong point for your evaluation.

What to write down while you build

Most of the marks for a Technology project are for showing what you did and what you worked out. Note these as you go:

• Which members are in compression and which are in tension, and how you can tell.
• Why the arch shape is strong, and what stops its feet from spreading.
• How many triangles the twine hangers make on each side.
• What the four top dowels do.
• Your test results: the load in the middle, the load near one end, and where the bridge moved first.
• Your design brief, specifications and constraints in your own words: who the bridge is for, what it must span and carry, and what limited your design.
• Anything that went wrong and what you did about it. This is the easiest part of a report to write and the one most learners leave out.

A photograph at the end of each step gives you pictures for the report without stopping to take notes.

Worth saying plainly: copying someone else's answers is the one way to lose marks you had already earned. Everything here is so you can explain your own model in your own words.

Frequently asked questions

How do you make a bridge for a Grade 9 school project?

Choose a bridge type, build a frame that uses an arch or triangles, and test it with a load you can measure. With the Slotbok kit the build takes eight steps: deck plates, end walls, side rails, the middle joint, the stands, the arch halves, the crown and the twine hangers.

What is a tied-arch bridge?

An arch bridge whose deck hangs from the arch and also ties the two ends of the arch together, so the arch's outward push is held inside the bridge instead of by heavy supports. It is also called a bowstring arch bridge.

What are the advantages and disadvantages of an arch bridge?

An arch is strong for the material it uses, because it works mainly in compression, and it can span a long way with no supports in the middle. On the other hand, its feet push outwards, so it needs strong abutments or a tie, and a real arch is harder to build because it has to be held up until it is closed.

What materials do you need for a bridge school project?

The Slotbok kit has everything: laser-cut 3 mm MDF parts, ten wooden dowels, two cotton twines, wood glue and sandpaper. Building one from scratch, the usual choices are ice-cream sticks or kebab sticks, cardboard, string and wood glue.

How do you make a model bridge stronger?

Use triangles, brace parallel arches or trusses to each other, and make every joint tight and fully glued. Then let the glue dry properly before testing: about an hour for the kit.

Where can I buy a bridge school project kit?

The Slotbok arch bridge kit is cut in our workshop in Gauteng and delivered across South Africa, with all 38 pieces, the glue and the sandpaper in the box.

How do I write the design brief of a bridge for Grade 9 Technology?

Start with the problem, then say what you will make to solve it. For example: a village is cut off every time the river floods, so it needs a bridge that people and vehicles can cross safely. Then say who the bridge is for, where it will stand and what it must do: span the gap, carry a load without bending, and stand up to wind and water. Keep it to a few sentences in your own words; the measurements go in the specifications.

What are the specifications of a bridge for Grade 9?

Specifications are the measurable things the bridge must meet: the span, or gap it must cross; the width of the road deck; the height above the water; the load it must carry; the materials and how they are joined; and any limits, such as cost or time. For the Slotbok kit they are a tied-arch bridge 41 cm long, 19 cm high and 16 cm wide, with a road deck 14.4 cm wide, built from 3 mm MDF, wooden dowels and cotton twine.

Can I make a cardboard bridge for my school project?

Yes. Corrugated cardboard is stiffest along its flutes, so cut strips with the flutes running along their length, fold them into beams or triangles, and glue every joint well. Cardboard is weak when wet and at the joints, so test a cardboard bridge with a small load first and add triangles wherever it bends.

What bridge designs work best for school projects?

Beam, truss, arch and suspension bridges all work as models. A truss or an arch is the easiest to make strong, because both turn the load into pushes and pulls along their members, and a tied arch like the Slotbok kit shows clearly which parts are in compression and which are in tension.

Afrikaans terme

If you are doing this project in Afrikaans: Graad 9 Tegnologie (Grade 9 Technology) · skoolprojek (school project) · strukture (structures) · brug (bridge) · boogbrug (arch bridge) · balkbrug (beam bridge) · vakwerkbrug (truss bridge) · hangbrug (suspension bridge) · boog (arch) · dek (deck) · pyler (pier) · drukkrag (compression) · trekkrag (tension) · driehoek (triangle).

Watch the bridge build on TikTok and Instagram

The same build as a short vertical video, made for your phone. Follow Slotbok on TikTok, Instagram and YouTube to see each new kit as it comes out.

Get the kit

The kit is cut in our own workshop in Gauteng, and every design is built once by hand before it is packed, so the slots and tabs are known to fit.

• Bridge School Project Kit — Grade 9
• All Grade 9 Technology projects

Other build guides: hydraulic arm kit, Grade 9 · mine headgear kit, Grade 8, another frame structure · cell phone tower kit, Grade 7.

Questions about a kit, a class set or delivery to your town: message us on WhatsApp.