# Introduction

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Now we're going to make a suspension bridge!

Suspension bridges are suspended in the air from cables. To understand how they work we must learn about different types of stress and strength.

Strength refers to a material’s ability withstand stress, and stress is a force that bends, crushes, breaks, or stretches the material. Imagine a heavy weight on top of a marshmallow. The marshmallow would become compressed, but stone might support the weight.

Compressive strength is a material’s ability to keep its shape under pressure. A stone arch is a structure that only uses compressive strength.

Tensile strength is a material’s ability to withstand tension, or pulling. Cable is a material designed to have tensile strength.

In this lesson we will learn how suspension bridges work to support the weight of the bridge itself and the people and vehicles that travel over it.

Instructions

1. Continue to the next step.

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## Step 1: Learn the Parts

Before we start building our bridge lets take a moment to learn the names and roles of each part in our bridge.

Instructions

1. The towers are tall and sturdy, they will support the weight of the bridge and the travelers that cross it.
2. The suspension cables are long and very thick cables that run over the length of the bridge and hang between the towers. They are attached to anchor points on the ground at either end of the bridge.
3. The deck is the surface that is used to travel across the bridge. The deck is heavy, and it suspended from the suspension cable.
4. The suspenders are cables that run vertically and connect the deck to the suspension cables. The weight of the deck is transferred to the suspension cable, causing the suspension cable to inwards on the towers.
5. The struts are horizontal beams that connect the towers on either side of the bridge. When you travel over a bridge you pass under the struts.
6. Continue to the next step.

## Step 2: Site Survey

A site survey is when an architect gathers information about the place where a structure will be built.

Instructions

1. The span is 1600m, which will be 160mm in our model.
2. The bridge will be suspended 120m over the sea, 12mm on the model.
3. The towers will have a 900m span between them, 90mm in the model.
4. The distance between each tower and the shore on the other side will be 350m, 35mm in the model.
5. Continue to the next step.

## Step 3: Make the Towers

First we will build the towers.

The towers provide compressive strength for the bridge, all the other parts will hang from the towers.

The deck will be hung from the towers using the suspension cables. The towers will be 90 mm apart from one another, and 35mm away from the shore on either side.

Instructions

1. Make a box that is 4mm along the X axis, 50mm along the Y axis, and 2mm along the Z axis.
2. Duplicate this box and drag the new box 90mm along the X axis.
3. Group the boxes. These are your towers.
4. Continue to the next step.

## Step 4: Make the Deck

The deck is the surface that travelers use to cross the bridge.

The deck is made of heavy materials, its weight produces most of the stress that the bridge must bear.

The deck will have a gentle arch, so we will use a round roof to make it.

Instructions

1. Drag out a round roof for our deck and rotate it 90° about the X axis.
2. Scale the round roof so that it is 290mm along the X axis, 10mm along the Y axis, and 15mm along the Z axis. This will be the deck.
3. Center the roof to the grouped towers along the X axis, and align the roof to the towers' lower limits along the Y and Z axes.
4. Nudge the round roof in the positive direction along the Y axis 4mm.
5. Drag out a box, make it a hole, and size it to 65mm along the X axis, 50mm along the Y axis, and 20mm along the Z axis.
6. Align the box to the lower limits of the round roof along the X, Y, and Z axes so that it is clipping off one end of the deck.
7. Duplicate this box and align it to the round roof's upper limit along X axis, on the other end of the deck.
8. Continue to the next step.

## Step 5: Make Suspension Cables

Suspension cables hang between the tops of the towers, and from the tops of the towers to the anchor points on the shore.

The cables attached to the anchor points prevent the towers from collapsing inwards. The deck hangs from the suspension cables. The suspension cables carry the stress produced at the middle of the deck to the towers on either end.

Instructions

1. Drag out a round roof and rotate it 90° around the X axis so that the curve is pointing towards the negative side of the Y axis.
2. Size the round roof to be 90mm along the X axis, 30mm along the Y axis, and 2mm along the Z axis.
3. Center the round roof to the grouped towers along the X and Z axes, and along it to the tower's upper limit along the Y axis.
4. Duplicate the round roof and translate 90mm along the X axis in the positive direction, then scale it along the Y axis to 35mm.
5. Duplicate this new round roof and translate it 180mm along the X axis in the negative direction.
6. Continue to the next step.

## Step 6: Trim the Deck and Cables

We have defined the shapes of our suspension cables and deck, but there is a lot of excess material.

In this step we will trim these parts down to size.

Instructions

1. Select the round roof that represents the deck, duplicate it, turn the duplicate into a hole, and nudge the hole 2mm along the Y axis in the negative direction. Group it with the original.
2. Select the three round roofs which define the suspension cable parabolic shapes, duplicate them, turn them into holes, and nudge them 2mm along the Y axis in the positive direction.
3. While holding down Alt on the keyboard, individually scale each of the holes for the cables to 86mm along the X axis.
4. Continue to the next step.

## Step 7: Make the Suspenders

The deck will be connected to the suspension cables by the suspenders.

Suspenders are thinner cables that run vertically from the suspension cable down to the deck.

Suspenders have tensile strength, and carry the weight of the deck to the suspension cables.

Instructions

1. Drag out a box and size it to 2mm along the X and Z axes, and 40mm along the Y Axis.
2. Center this box to the tower group along the X and Z axes, and align this box to the towers' upper limit along the Y axis.
3. Duplicate this box and move it 9mm along the X axis.
4. Smart duplicate it 8 more times.
5. Now make 8 more in the negative direction along the X axis using the same steps.
6. Delete the copies that overlap the towers.
7. Continue to the next step.

## Step 8: Make the Second Half

To avoid overhangs we will make this object in two halves that will assembled after printing.

Instructions

1. Select all and then shift click the tower group to deselect them.
2. Group the remaining selection.
3. You should now have a tower group and a group with the deck surface and the suspension cables.
4. Select both groups and duplicate them, drag the copies to an empty space on the workplane.
5. Ungroup the duplicate of the suspension cable group.
6. Continue to the next step.

## Step 9: Make a Lip for Gluing

Since we are printing our bridge in halves we will need to glue them together afterwards.

To make this easier we’ll add a part called a lip. The lip will connect the sides and provide plenty of surface area for the sides to bond.

Instructions

1. Select the deck and scale it along the Z axis to 3mm.
2. Duplicate the deck and nudge the copy 2mm along the Y axis in the negative direction.
3. Select the original deck and scale it along the Z axis to be 2mm.
4. Select the round roof shaped hole, also for the deck, and nudge it 2mm along the Y axis in the negative direction. You should be left with a thin, 1mm lip. This will receive the deck from the other half of the bridge.
5. Continue to the next step.

## Step 10: Make a Foundation

Next we will make a foundation for our bridge.

This will represent the surface of the earth beneath the water.

Instructions

1. Drag out a box and scale it to be 160mm along the X axis, 2mm along the Y axis, and 17mm along the Z axis.
2. Center this box to the tower group with the deck along the X axis, and align it along the Y axis to the bottoms of the towers.
3. Select this box and nudge it 2mm in the negative direction along the Y axis.
4. Continue to the next step.

## Step 11: Make the Struts

Struts are the horizontal beams that connect and stabilize the towers on either side of the bridge.

The ends of your struts can be glued to the tower on the opposing side.

Instructions

1. Select all of your parts except the deck and duplicate them. Drag the duplicates to the other end of the workplane. Group these parts.
2. Make a box that is 4mm along the X axis, 4mm along the Y axis, and 15mm along the Z axis.
3. Ungroup your original towers.
4. Align this box to the top of one of your original towers along the X axis, and align the box to the center of the tower on the Y axis.
5. Nudge this box 1mm along the Y axis in the negative direction.
6. Duplicate this box and nudge it another 9mm along the Y axis in the negative direction.
7. Smart duplicate a third evenly spaced box.
8. Select all three boxes, duplicate them, and align the copies along the X axis to the center of the other tower.
9. Group all of the parts
10. Continue to the next step.

## Step 12: Printing Your Bridge

Both sides of your bridge are complete.

Now we will prepare the file for printing and print it. Once the sides of the bridge have been printed you will apply glue along the lip of the deck, and gently but firmly hold both sides in place until it is dry.

Instructions

1. From the design menu in the menu bar, click the 'Download for 3D Printing.'
2. Select the STL print option.
3. Upload the file to your 3D printer.
4. Print!

Congratulations, you have completed this project!

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