Suspension bridges are designed to be flexible. They will move vertically (up and down) and laterally (side to side, like a swing) to accommodate wind, change in temperature or load (the weights placed upon them). A suspension bridge that was not flexible would break very quickly! The challenge for bridge engineers like Brunel is to find a way to allow the bridge to move safely.
What might cause a suspension bridge to move a lot?
High or gusty winds might cause a bridge to sway from side to side or twist. At the Clifton Suspension Bridge, we have weather stations which monitor wind speed and direction as well as monitoring equipment which tells us how much the bridge is moving. We have only had to close the bridge a few times due to high winds in living memory – and that’s because it only causes a problem when it comes from a certain direction and at a certain angle.
Rhythmically moving traffic or people walking in step might also cause a problem. If these movements are in harmony with the natural frequency (rate of vibration) of the bridge, they will become bigger and bigger, causing the bridge to twist and roll. You can see a famous film of this happening online if you search for ‘Tacoma Narrows Bridge’ or ‘Galloping Gertie’. During the Second World War, soldiers who crossed the bridge were asked to break step to stop this from happening.
How do we reduce bridge movement?
The Clifton Suspension Bridge has a wooden deck which absorbs vibrations from traffic and pedestrians. There are two layers of timber planks. For strength, one layer runs along the length of the bridge and the other runs across the width. This is topped with mastic asphalt, which provides the road and pavement surface.
The deck is supported by wrought iron girders. A pair of longitudinal girders run the length of the bridge from Clifton to Leigh Woods, whilst cross girders (widthways) make a rigid frame to steady the bridge against strong winds. Additional diagonal bars (plan bracing) provide strength and stability. At each side of the deck are the parapet girders which add to the strength and stiffness of the bridge as well as providing a handrail.
Underneath the bridge, where the road deck joins the abutments, are sway guides which prevent too much movement from side to side. You can see it in the picture below.
How can we see or feel the bridge move?
There are lots of ways to see the movement in the bridge. Here are a couple which will be suitable if you visit as a group.
- Stand at the base of a tower and look up at the hangers – the long rods which connect the road deck to the suspension chains all the way across the bridge. You’ll be able to see the longest hangers vibrating in the wind or as traffic crosses the bridge.
- Take it in turns to stand on the abutment (the paved area beneath the tower) looking down the length of the bridge. Look for the place where the stone walls of the abutment end and the iron parapet (the handrail along the side of the bridge) begins. Keep your eyes on this spot and when a car leaves or enters on to the deck, you’ll see the parapet rise and fall.
- Stand in a line along the footway at the centre of the bridge deck. Look across the roadway and along the gorge until you find a building or other stationary object in the distance. Line the building up so that it sits on top of the bridge handrail or one of the safety wires and wait. You should see it rise into the air or sink below the wire as the bridge sways!
- If you are visiting with younger children, ask them to line up along the footway and gently place their hands on the wire railings. Stand on tiptoe and close your eyes and you may feel the movement of the bridge…