Clifton suspension bridge
The Clifton Suspension Bridge is currently an iconic bridge on the B3129 which spans the 214m wide Avon valley gorge from Clifton Down to Leigh Woods. A bridge is to be designed to replace the current solution. The new design will take into consideration the function of the bridge, the structure of the bridge and the bridges strength. Attention will also be made to the aesthetic properties and the cost of the bridge. The design includes making the bridge conform to building standards instructed in the Eurocodes, considering how the bridge is constructed and also taking into account the traffic crossing the bridge.
The design procedure will consist of many phases. To initiate the design procedure background research will be completed; this includes the history of the surrounding estates plus further research into the current solution. To initiate the design of the bridge, information is to be collected to aid the decision on which type of bridge is to be used. The conceptual design of the bridge requires the use of the collected bridge information; each type of bridge will be analysed by evaluating the bridges suitability with reference to a list of key issues. The key issues make up the fundamental design features of a bridge. (Chen, Wai-Fah. Duan, Lian. 1999).
Preliminary design employs the conclusion from the conceptual design phase and evolves a small set of bridge forms into plausible designs with approximate pricing. The final bridge form can then be chosen. (Chen, Wai-Fah. Duan, Lian. 1999). Functional design of the bridge will be completed next by evaluating how the bridge will be used; this will be followed then by the structural design of the major components. The structural design will comply to the EuroCodes and determine the appropriate loads, material strengths and various load combinations; then the partial factor method can be used to ensure the bridge conforms to both ultimate and serviceability limit state designs. (Eurocode, 2005, NA to BS EN 1990:2002+A1:2005)
Following the completion of the structural and functional design of the bridge, the procedure for constructing the bridge can be drafted along with the calculated life-cost of the structure. Technical drawings will also be completed.
2.0 History of Bristol
Pre 1066- The city of Bristol was first developed as a settlement in the Anglo-Saxon era, and was a popular trading town with Ireland and the south west of Wales. Its original name was Brigstowe and was situated between the river Avon and Frome.
1066- The castle commonly known now as Castle Park was built after the Norman Conquest. 14th century- Bristol was a popular trading settlement and had trading links with countries in Europe and Iceland.
1700-1800 Bristol was the second biggest slave trading port in Britain next to Liverpool. A combination of the total number of voyages made from Bristol and London were estimated to be 4000. Bristol was already a wealthy city due its trading but its economy boomed from the slave trade. The trading of slaves was banned in 1807. (Klein, S H. 1999)
1800- In this time the Clifton area of Bristol had become home to rich merchants, who chose to live a distance away from the docks. Brunel shaped the face of Bristol in a time where the trade industry was diminishing. His engineering expertise ranged from the S.S Great Britain to the Temple Meads Station, which are both on display today.
1831- The Bristol Riots which occurred due to the proposal of the reform bill by the government. The proposed principles directly contradicted the old ways of the Whig principles. A reign of terror was commenced for civil and religious liberty. The riots commenced for three days and ultimately caused the cease of the construction of the Clifton Suspension Bridge. (Eagles J, et al. 1832)
1900s The aerospace industry was based in Bristol along with the construction of the Concord. Currently Rolls Royce Engineers are designed at Filton. Bristol was also heavily bombed during the second world war, destroying a lot of the architecture. In the 1960s the invention of the concrete tower block was invented and was used as a quick cheap replacement. (Bristol Link)
Presently Today Bristol is the largest city in the south west of England. Some of its historic architecture is still visible amount the older parts of Bristol such as Clifton. The town has a very large shopping centre and in 2008 its newest edition Cabots Circus opened. Clifton is currently renowned for its Georgian architecture and the Clifton Suspension Bridge.
3.0 History of the Clifton Suspension Bridge
1753. Alderman Williams Vick of Bristol left 1000 in trust for a stone bridge to be build.
1829. The money had increased to the amount which Vick had estimated the price of the bridge would cost, but his estimate was short of the price of the masonry structure, so a competition was held for the best design with the leading engineer at the time Thomas Telford as the judge.
1830. Thomas Telford rejected all of the designs including the original proposed by Brunel, whom decided to design the bridge himself, but insufficient funds constrained the trustees, and the competition was reopening with Brunels updated Egyptian themed proposal, shown in figure 3.1, as one of competitors.
1831. Brunels proposal was excepted when Telfords was dubbed too costly, and Brunel was appointed chief engineer. Site preparation began three months later, but was disturbed by insufficient funds caused by the disturbed state of the country. The Bristol riots being evidence of this.
1836. Work finally commenced and suspension towers were built.
1843. Trustees funds became exhausted.
1864. The work was finally abandoned and the ironwork was sold to cover the 30,000 debt of the trustees. (Porter Goff, R.F.D. 1974)
1859. Thursday 15th September Brunel dies of either a heart attack or a stroke, shortly after he was aboard the SS Great Easters maiden voyage. (Byrne, Eugene; Gurr Simon. 2006)
1860. Members of the Institute of Civil Engineering (ICE) formed a company to complete the bridge as a monument to Brunel.
1865. The bridge was completed by engineers William Henry Barlow and Sir John Hawkshaw, using iron from another of Brunels bridges, the Hungerford Bridge which was demolished to make way for a railway bridge. (Porter Goff, R.F.D. 1974)
Brunels design differed from the final bridge which was completed after his death. One of the main differences is that three layers of chains were used instead of the two which Brunel predicted. Also the smooth stone towers were left as rough stoned towers and the Egyptian styled decoration which can be seen on top of the towers, in illustration figure 3.2, were not added. (Byrne, Eugene; Gurr Simon. 2006)
4.0 Types of Bridges
4.1 Beam Bridge
A beam bridge consists of a beam which is simply supported at each end. The beam bridge is the earliest form of bridge and was originally constructed by laying a log across the span. In modern times the beam is made using reinforced concrete which has an economical span of 8-12 (2.5m -3.5m). Alternatively a rolled I beam is used with an economical span of 20 50 (6m-15m), (Bangash, M, Y, H. 1999).
Beam bridges are used to carry a variety of loads including vehicles, pedestrians and on occasion are boats, and can easily be adapted for different loads by deepening the beam. They are commonly used to carry loads over motorways, by positioning a column in the central reservation as a support; this is a continuous span beam. An example of a single span and continuous span beam bridge is shown in figure 4.1.1.
The structure of the beam bridge is very simple; the beam transfers the applied load through bending and shearing to the adjacent columns, then through to the foundations.
As stated the beams are most efficient in relatively short spans. This is because as the span increases the depth of the beam has to also increase; hence large spans require very large depths of solid material increasing the price and weight.
The beam bridge is generally aesthetically unpleasing. They are generally inelegant with a large depth making them visually unappealing. The design can be altered to increase its appeal by designing the parapets to create shadows on the main structure, hence making the structure appear thinner. Also shown in figure 4.1.1 the appearance can be improved adding a haunch to reduce the volume to less structurally reliant areas, i.e. mid span; this also makes the structure more efficient (Gottermoeller, F. 1998).
4.2 Cantilever Bridge
A cantilever bridge consists of a beam which is either resisted by torsion at the support as demonstrated in figure 4.2.1 or cantilevered around a support with an opposing structure as shown in figure 4.2.2. The cantilever bridge has an economical span of 600 to 1575 (Bangash M. Y. H. 1999). Small cantilever bridges usually consists of a simple spanning beam however for large spans a trust structure or pre stressed concrete box girder is used to form fundamental structure; good examples of these are the Commondore Barry Bridge shown in figure 4.2.2 and the Pierre Pflimlin Bridge.
The bridge design can be very strong and stable, hence making it suitable for both roads and railways traffic. Ideally the bridge should be self counter weighting, using an anchoring span to counterweight the cantilevered span.
The construction of a self anchored cantilever bridge can be simple by expanding outwards from the bridge support. This makes it possible to construct over water where form work is not achievable. For a three span bridge two columns can be constructed at a quarter and three quarter the span, and then the two bridges can be extended to connect at midspan. Cantilever bridges can generally be quite expensive due to the amount of high amount material (Bangash, M, Y, H. 1999).
4.3 Arch Bridges
Arch Bridges rely on the compression of the structural materials; hence the use of concrete and masonry are commonly employed as they have much higher compression strengths in comparison to their tensile strength; a demonstrated arch bridge is shown in figure 4.3.1. Arch bridges were predominately used as pedestrian walkways and railway lines as they produce very stiff structures; ideal for rail lines.
The arch operates by transferring the vertical load into horizontal and vertical stresses to the supports; ultimately causing no bending moments through the beam. Most bridges span a small 5 -12 (Bangash, M, Y, H. 1999), and were commonly used in medieval times where composite iron technology had not yet flourished (Moore, Fuller. 1999). The height to span ratio of the arch will affect the stability of the structure; if the structure is too high the sides could be prone to buckling and if the arch is too shallow the bricks may fail in tension or shear. A setback with arches is failure under eccentric point loads thereby causing uneven distribution of the load.
An arch bridge is constructed with aid of false work; this makes an arch structure unsuitable for situations where the topography situated below the bridges span is inaccessible. In modern times the arch has been constructed using a truss arch structure; allowing longer spans to be reached with minimal materials.
Masonry arch bridges are generally aesthetically pleasing and are commonly decorated, they are a simple structure and high amounts of material mass is situated where observer expects; hence improving the bridges appearance (Gottermoeller, Frederick. 1998).
4.4 Suspension Bridges
Suspension bridges designs are generally used to span long distances where limited space or foundational stability disallows the use of false work. Primary suspension bridges were constructed of rope for humans to cross valleys.
A suspension bridge consists of a deck which is supported vertically in intervals by suspension hangers. The hangers are then attached to a suspension cable which is supported vertically by towers and horizontally by anchors; this is demonstrated in figure 4.4.1. The suspension should naturally form a catenary curve (Moore, Fuller. 1999). The structure is most efficient over approximately 1595 4200 (Bangash, M, Y, H. 1999).
Structural exertion can occur to the bridge when fluttering wind loading or oscillated imposed loading is applied (Simiu A. Et.al. 1978). A case study of extreme oscillation is the Tacoma Narrows Bridge (Galloping Gertie) which collapsed during a minor storm. The storm caused the bridge deck to rapidly displace laterally and produced oscillating vertical ripples along the bridges length imposing residual impulse loads in the rods, ultimately causing structural failure. Post Galloping Gertie solid girders have been used to oppose the loading from wind.
Similar to the cantilever bridge, the suspension bridge can be constructed without any form work (Moore, Fuller. 1999).
A longitudinal truss can be used longitudinally along the deck to restrict the distortion and distribute the concentrated stress on the cable; hence restricting the movement of the cable and deck. If there is no stiffening truss structure, the cable is free to achieve the position dictated by the applied load (Steinman, Dr D.B. 1929).
Recently steel box girders have been used to form the deck. When this was first introduced it caused an aerodynamic problem demonstrated by the forth bridge, the issue was resolved by streamlining the deck which was later introduced on the first Severn crossing (Roberts, Sir Gilbert. 1968-1969).
Analysis of the bridge has shown in figure 4.4.2 shows how the structure deforms under a uniformly distributed load and how it deforms under a point load.
A truss is a structure formed using triangular geometry. The triangle joints do not rotate when an induced force is applied; opposed to a rectangle where a shearing action can easily occur. In a general structure vertical elements are in compression and horizontal members are in tension as shown in figure 4.5.1. A good example of this is shown in a vultures wing which structure forms a tetrahedron shape. Truss structure forms a very rigid structure due to their inflexibility. This stiffness means it is very common for long span trains.
The structure is very rigid due to the small length of each member and the geometry of the structure (Moore, Fuller. 1999). A truss bridge generally has an economical span of 100-300; as the span increases the truss height has to be increased which increases the cost (Bangash, M, Y, H. 1999).
A truss structure has to be constructed using false work or floated in if located over water, as shown in figure 4.5.2. This makes it difficult to construct in certain environments such as gorges or when the span is situated high above sea level.
Truss structures are generally aesthetically unpleased, this is because they are generally very deep complicated structures and often they are made using treated metal which does not normally fit in with the environment (Gottermoeller, Frederick. 1998).
4.6 Cable-stayed Bridges
Cable-stayed bridges decks are supported by cables which are directly attached to a tower. The three variations of cable stay bridges are shown in figure 4.6.2. The harp type cable-stayed bridge has all the cables running parallel to one another, and is deemed to be the most aesthetically pleasing. It also allows an early start to constructing the box girder deck since the cables initiate at a lower height; hence construction can commence before the completion of the towers. Large tensions are required to support the deck due to the lower cables running approximately perpendicular to the direction of span, causing inefficiency of the overall structure.
An example of a cable stay bridge is the cable stay bridge crossing the Rio Ebro in Spain. An illustration is shown in figure 4.6.1. The bridge spans 137.12m across the river and has a deck width of 28.90m. The column height is 58.80m and the bridge is a multi span harp type. (Bangash, M, Y, H. 1999).
The radial cable-stayed bridge cables are positions as vertical as practicable hence producing one of the most efficient cable stay forms. However the radial solution is rarely used in practice due to the complex arrangement of the cables within the saddle. The arrangement can look inept if the design is not successfully implemented.
The fan arrangement is most common due to its efficiency and ease of construction. Compared with the radial solution it is easier to design and still holds the aesthetic qualities of the harp. (Chen, W. Et al. 1999)
The cable stay bridge differs from the suspension bridge in two main ways. The first is that the deck is attached directly to the towers, and the second is that the hanger cables are not vertical. Cable stay bridges are the most efficient over very long spans when a repeated tower-deck unit is used. The cable stay has an advantage over very long distances as the cable stay bridge can be used over multiple spans where a suspension bridge ideally can only be a one or three span.
The deck of a cable stay bridge has to be relatively thick to withstand the horizontal load applied by the angle of the cables. The additional thickness of the deck and height of the towers decreases the affordability of cable-stayed bridges in urban areas. Another advantage of the cable-stayed bridge is its increased rigidity, hence allowing it to carry rail traffic, where a suspension bridge would not be rigid enough. (Walther, R. et al. 1999).
Cable stay bridges are commonly used for pedestrian walkways due to their aesthetic qualities. They are an elegant structure with a thin deck and very simple; they are also a continuous structure (Gottermoeller, Frederick. 1998).
5.0 The Clifton Suspension Bridge
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 5.1.1.
The towers are 26.2m high and the centre to centre width of the chain is 6.1m. There are two vehicle lanes and two pedestrian paths, which are frequently 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 funds support. 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.
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 Brunels 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 5.2.2 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 amount of chains, the structure becomes less reliant on each individual chain; hence failure of a chain should not lead to collapse. 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.
The dip to span ration of the chains contribute to the efficiency of the structure. High span to depth ratios cause larger chain tensions because the direction of the chain reacts approximately perpendicular to the load direction, creating an inefficient structure. Low span to depth ratios cause a more efficient chain curve but the heights of the towers are consequently increased which increases the total cost, also creating an inefficient structure. 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 maximum dip; this is done to allow the main deck to oscillate freely in the wind without putting excessive 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 5.2.2, 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 5.2.1, 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.)
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)
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.
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)
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.