# How a Suspension Bridge Works

A table\-top model of a suspension bridge that shows how towers, cables, hangers and anchorages share the load

By sunny\-koala\-2b0c8f48

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/rWhobts3v2zV)

## Transcript

### 1. A road that hangs in the air

This is a model of a suspension bridge, based on the Golden Gate Bridge in San Francisco\. Its road does not sit on a row of pillars\. It hangs from cables, a bit like a giant swing\. Let us find out how\.

Drag the scene to look around\. Pinch or scroll to zoom\.

### 2. Two tall towers

Two towers rise from the water on concrete feet called piers\. The road passes between the two legs of each tower without resting on them\. The job of the towers is to hold the cables up high\.

The Golden Gate towers stand 227 metres above the water\.

### 3. The main cables

Two main cables drape over saddles, curved steel seats on top of the towers\. Between the towers they sag in a smooth curve\. Beyond the towers, each cable runs down to the shore\.

Each Golden Gate cable is almost a metre thick and is made of 27,572 steel wires bundled together\.

### 4. Hangers hold the road

Hundreds of thin ropes called hangers drop from the cables to the edges of the road\. They are short in the middle and long near the towers\. Together they carry the whole road\.

The Golden Gate has 250 pairs of hangers, about 15 metres apart\.

### 5. Follow the weight

Cars and trucks push down on the road\. Each hanger is pulled tight, which engineers call tension, and passes that pull up to the cables\. Send a truck across and watch the golden glow of tension rise above it\.

The dip of the road is exaggerated here so you can see it\.

### 6. Towers push down

The cables press down on the tops of the towers, so the towers are squeezed\. Engineers call that compression\. The blue glow shows the push travelling down through the towers into the piers and the ground\.

The cables pull and the towers push\. Neither could hold the road alone\.

### 7. Anchors hold the ends

The cables also pull inward, toward the middle of the bridge\. At each shore a huge concrete block called an anchorage holds the cable end back\. Without anchorages, the cables would drag the tower tops inward\.

Each Golden Gate anchorage weighs about 60,000 tons\.

### 8. Stiff in the wind

Wind pushes the long road sideways and can try to twist it\. The criss\-cross steel frame along the road, called a stiffening truss, keeps it firm\. Watch the road sway gently and keep its shape\.

In 1940 the Tacoma Narrows Bridge twisted in the wind and fell\. Engineers learned to build stiffer decks\.

### 9. Pulling and pushing in balance

Now see it all at once\. The road hangs from the hangers, the hangers pull on the cables, the towers carry the weight down and the anchorages hold the ends\. Together they let one road cross 1,280 metres of open water\.

Golden glow means pulling, tension\. Blue glow means pushing, compression\.