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The Clifton Suspension Bridge

Function

The Clifton Suspension Bridge was designed by Isambard Kingdom Brunel and completed in 1864. The bridge spans 214m across the Avon Valley Gorge from Clifton to Leigh woods as shown in figure 3. The towers are 26.2m high and the bridge chain has a width of 6.1m centre to centre. There are two vehicle lanes and two pedestrian paths. The pedestrian walkway is additionally used as a viewing spot along the Avon gorge. There are approximately 10,000 cars crossing per day with a gross vehicle weight limit of four tons. The toll for the bridge is currently 50p (27/11/09). Being an historic bridge, the structure requires constant services which the tolls fund. The speed limit on the bridge is currently 15mph and the weight limit is controlled using a modern weight beam (Mitchell-Baker, D. Et al 1988). The bridge was designed to originally take the weight of horses and carriages. The site location was chosen because of its short and level span.

Structure

The bridge takes the form of a convention suspension bridge. This form was ambitious at the time of design and if it was completed on schedule it would have been the longest suspension bridge in the world. The suspension bridge was a suitable choice for two main reasons. The first reason is that the height between the deck and the river below is 75m and to construct a tower would be expensive. It also would have been expensive to construct any form work. A suspension bridge can be constructed without the aid of form work and Brunel's design did not require a tower compared with Telford proposal which included two gothic styled towers (Moore, Fuller. 1999). The second reason is that the strength of the surrounding topography allowed the existence of anchors. In situations where a suspension bridge would be suited, the condition of the ground is unsuitable for bearing the pressure from the cables. One of the disadvantage of using a suspension bridge on the site is that it can only be a single span. The anchors have to take the total tension of the chain compared with a three span bridge where the deck on the side span acts as a counter weight, relieving tension in the anchor.

The structure is formed mainly of a long chain, similar to a bike chain, two towers and the bridge deck. As it is shown in figure 5 the chain is formed of three sub-layers on each side, each layer consisting of an alternating 10 and 11 flat wrought iron bars. By increasing the number of chain layers the overall chance of collapse decreases. The flat wrought iron bars were designed to be as long as practicable. The long bars decrease the total weight of the chains by reducing the amount of heavy joints. . By increasing the amount of chains, the structure becomes less reliant on each individual chain; hence failure of a chain should not lead to collapse.

The dip to span ration of the chains contribute to the efficiency of the structure. High span to depth ratios cause high tension in the chain, this is because of the direction of the chain reacts approximately perpendicular to the weight, creating an inefficient structure. Low span to depth ratios cause a more efficient chain but the height of the towers have to increase hence increasing the cost. The most effective span to depth ratio is approximately 1:10 which is what was adopted by Brunel for the bridge. This is an improvement to the 1:13.5 ratio which Telford previously adopted on the Menai bridge. The main catenary chains are not attached directly to the deck at mid span; this is done to allow the main deck to oscillate freely in the wind without putting strain on the chains (Pugsley, Sir A. 1976), (Porter G. 1974).

Hanging at 8ft intervals are the wrought iron rods, which transfer the load onto the chain. The array of rods is designed to oppose collapse if a single rod should fail. The rods are attached loosely between the chain and longitudinal girder, see figure 5; this allows movement which decreases the chance of failure from bridge movement. This then imposes a vertical downward force onto the towers and tension along the chain and in the anchors.

The longitudinal girder as shown in figure 5, is a continuous stiffened I beam. The girder is 3ft deep and connects the cross girder to the rods. The original design proposed by Brunel used a timber lattice girder with a matching stiffness, but by the time of the bridges construction the timber lattice was replaced by an iron plated girder. The girder takes the stress applied to a single joint on the chain and traverses it along the longitudinal length of the chain.

As illustrated in figure 4, a truss structure is adopted for the cross girders. The truss structure is a very strong and efficient structure along the length. The shallow depth allows thin strips to form the trusses without buckling. The position of the cross girder is shown in figure 5.

The cross girders are braced horizontally using thin iron strips to react against wind loading. The quadrangle shape deforms under load by rotation at the joints. The bracing forms a triangular shaped structure; deformation in the triangular shaped structure occurs by bending of each member, hence increasing the elements stiffness.

Live loads are transferred onto the timber beams which span between each cross girder; applying a vertical load and causing a bending moment in the girder. The timber decking was chosen because it was light and appropriately strong; it also allows easy accessible repairs. A wrought iron decking frame was added by Barlow and Hawkshaw to increase rigidity.

The saddles are situated on top of the towers. The saddles allow lateral movement of the chains. Lateral movement occurs due to temperature changes or unsymmetrical loading. The saddles also allow the vertical stress to be reduced in the stone tower by increasing the contact area, minimising the risk of crushing the stone.

The anchors are 17m below the ground and the chains are spread into a chamber to create a stable foundation. The spreading of the chain allows the tension to be opposed by the compression of the cliff, not solely relying on the friction. The advantage of this is that the strength of the anchors increases; it also takes advantage of the strong limestone foundation. (Pugsley, A. 1976.)

Strength

The structures weight limit for vehicles is currently four tons. This weight limit was introduced approximately fifty years ago when engineers decided that the large loads and fatigue could lead to collapse. The amount of vehicles on the bridge at one time is controlled by the toll booths. This is done to reduce the total load on the bridge at any moment.

Brunel designed the bridge to withstand an adventitious load of 100lbf/sq.ft which is approximately equal to 4.7 KPa of pressure (Porter G, R.F.D. 1974). . This is simular to the live loading which is adopted on modern bridges with similar spans (Bangash, M, Y, H. 1999)

Construction

After designing the bridge, the foundations and abutments were the first elements to be constructed. Then the towers were the next thing to be constructed. The chains were pulled across by rope and then the deck was attached to the chain.

Maintenance

Between 1864 and 1953 the timber decking had been replaced three times and the ironwork had been treated twice and at the end of this period was still in good condition. Two suspender rods had failed in a severe storm in 1877 and three more had failed in 1887. Both occasions the exact cause of the failure is still not convincingly known.

In 1861 a 6 tonne vehicle weight limit was appointed to the bridge. The size and weight of vehicles was continuously increasing and engineers were concerned with the bridges structure. It was based on the theory that repeated loading on metal causes its fibrous structure to become crystallite.

In 1918 one every ten bolts from the rods were removed for testing, and they were all found to be of adequate strength though some crack appeared; these cracks were likely to occur from forging.

The drainage of the bridge had caused serious corrosion to ground level chains. In 1925 an extra link was added to the chains but there were still concerns for the anchors condition. To overcome this concern concrete was poured to a depth of above 9ft above the anchor.

In 1953 the responsibility of the bridge was passed to the trusses and the national heritage. The point loading of wheels was known to be more damaging to the deck than to the structure as a whole, causing the deck to be again in a severe condition. The weight limit was then changed from 6 ton limit to 2.5 ton axle weight and 4 ton vehicle weight. (Mitchell-Baker, D. . Cullimore, M. S. G. 1988)

In 2009 a pedestrian noticed a serious crack in one of the suspension rods and the closure of the bridge followed. Temporary supports were put in place while the rod was replaced. A year before work to improve the waterproofing, drainage and new road surface was completed. This would decrease the amount of corrosion of the iron work. (BBC News. 2009)

Aesthetics

The bridge is very aesthetically pleasing, it has few individual elements, and each element is similar in function. The girders are very thin and it has a continuous span which also makes it more appealing to people, most people would agree the bridge is beautiful. The shape of the structure also reflects the force applied to it, being thinner in the middle and thicker as it gets towards the edges where the greater moments would occur. The bridge also has a bold and striking outline when viewed from along the valley (Gottermoeller, F. 1998). The towers are curved so that they appear tall from below, forming a bold structure. The towers are also in good proportions compared with the immediate surrounding and harmonious in three dimensions. The bridge is constructed using locally sourced materials, integrating the structure into the environment. (Chen,W et al. 1999). The bridge is 3ft higher on the Clifton side. This is done to stop the illusion that the bridge deck is falling towards the cliff.