Bristol Floating Harbour Landmark Bridge INTRODUCTION TO THE PROJECT The city of Bristol’s maritime tradition is renowned and has had a substantial influence in the creation of the city that we see before us today. The Floating Harbour, at the heart of the city, is a triumph of 19th century engineering and an icon of the Bristol dock’s proud history. The area has become the focus of a substantial redevelopment programme aiming to create a centre of tourism, culture and leisure for Bristol and the South West. Poor pedestrian crossing facilities across the harbour restrict the access to the many tourist attractions in the area, and with the construction of new residential developments, the need for a new crossing point is apparent. The brief was to design a landmark bridge spanning the Floating Harbour at a location near to the SS Great Britain, the focal point for the area’s tourism and redevelopment. Existing and Planned Harbourside Development Canon’s Marsh, a sixteen-acre area of land at the heart of the Floating Harbour, is the site of one of the last major developments of the Harbourside. The overall development is regarded as “the critical piece of the jigsaw that will ensure commercial success for the whole of the Harbourside” (John Savage, Chair of Bristol Harbourside Sponsors Group). Explanation for chosen Location The above image shows the location chosen following an extensive options analysis. Ultimately we decided that it tied in best with the needs of the users and with the current Harbourside developments. It also provides very good access to the SS Great Britain and links it with other tourism attractions in the vicinity of Millennium Square. This location, slightly east of Brunel’s masterpiece, was selected to site our bridge as it was able to accommodate a more contemporary structure allowing it to integrate more easily with its surroundings. This avoided the potential for aesthetic conflict with the SS Great Britain’s redevelopment scheme. Cathedral Walk “a wide green place that stretches down to the dockside with tree-lined paths, and looks northwards to the Cathedral, rising above its medieval and modern foreground buildings. Cathedral Walk is deliberately set out to preserve and accentuate this view” Edward Cullinan, Cannon’s Marsh Architect Our selection of site was also influenced by the proposed Cathedral Walk. This is a promenade being constructed as part of the Cannon’s Marsh development and one that we believe is well suited to the positioning of a bridge where it meets the harbour. Gateshead Millennium Bridge Bristol Floating Harbour Bridge It was required that the bridge be: It is required that the bridge be: A fitting symbol of Tyneside at the start of the new millennium A fitting symbol of Bristol, and in particular, the regenerated Floating Harbour Complementary to the river’s existing bridges Considerate to the history of the Floating Harbour, whilst complementary to its revival Of an engineering standard to match its neighbours A structure of an engineering quality to echo the city’s proud engineering past As innovative as some of its neighbours were when they were built An enhancement of the views of the Tyne from the existing bridges A sympathetic backdrop to the SS Great Britain A magnificent site from which to view the existing bridges (and be photographed against them) A magnificent site from which to view the SS Great Britain Able to find its way quickly into the affections of the Tyneside public Designed in such a way as to not excessively hinder the day to day running of the harbour A LANDMARK STRUCTURE The Bridge needed to be a landmark structure that would be an attraction in its own right. As the Gateshead Millennium Bridge is the best example of a structure meeting with this criterion we compared our objectives with those proposed for it: INITIAL DESIGN Our initial brainstorming and conceptual ideas had given rise to a design that we believed was most fitting for the project. The basic premise is a large cantilevered bridge with two walkways supported by a central mast. This was selected due to the innovative design, which formed part of the brief, and its ship-like form, which would complement the nearby S.S. Great Britain – a major feature of our chosen location. A swing bridge opening mechanism was decided upon as it allows for a smaller opening mechanism due to the efficiency of rotating the dead weight of the deck instead of lifting it against gravity. This design was then developed further to produce a practical solution. From Concept to Viable Design As our aesthetic preference for a bridge with a single pier seemed impractical due to the lengthy cantilevered decks and the interference with the central shipping channel. It was decided to consider other forms of design. This pointed us towards the possibility of two spans of cantilevered decks as reviewed in the table below. 1 Bridge 2 Equal Bridges 2 Bridges: Span Ratio 3:1 Aesthetics Most natural and simplistic form. Aesthetics of Symmetry. Sail-like arrangement of bridges. Opening One large mobile structure. Would require two opening bridges. Only the larger of the two bridges would need to open. Interference With Channel Splits the harbour into two separate channels. Both large but not in the centre where the channel is deepest. One large central channel and two smaller channels either side. One large central channel including the central, deepest section. Impact Protection Just one pier to defend but to protect it when open the defences would need to extend 40m either side of the bridge. Two piers to defend with two defence systems protruding 20m perpendicularly each side of the bridge. Two piers to defend (though one immobile). Also need to consider the protection of the exposed cantilever of the smaller bridge when the other is open. Ultimately the decision was made to pursue the third option as it required only one opening bridge and did not restrict the centre of the channel. Fitting the Design to the Site The plan of the bridge was formed using ellipses to produce a neat, flowing finish. Each deck was made 2.5m wide to allow for the minimum allowed (2m wheelchair clearance) and room for other services such as handrails. The main constraint was the need to cross the central channel with a clearance of 4.1m. This was coupled with restrictions on maximum gradient (1/12) and lack of space on either side of the channel (the bridge needed to have a level gradient where it met the two banks – a failure of the initial blue quadratic profile). This enforced a limited solution upon us. A cubic equation (pink) was derived to ensure the compatibility of the structure with the site. This profile allowed us to bridge the 80m span achieving permanent vertical clearances of 3.3m at either bank and a 26m central channel of vertical clearance no less than 4.2m in the ‘closed’ position. A LOCATION IN THE VICINITY OF THE SS GREAT BRITAIN The bridge was required to be located near the SS Great Britain in order to serve its visitors’ needs. However, the SS Great Britain Trust is currently implementing a substantial regeneration scheme in the area immediately surrounding the ship aiming to recreate the character of the Victorian Industrialist buildings destroyed during WWII. Any bridge sited directly within this area would have had to have been sympathetic with the new development. -50 -40 -30 -20 -10 0 10 20 30 40 50 Supporting the Deck Since the bridge is only meant for pedestrians and cyclists the deck itself did not need to be too bulky. Indeed, a lighter deck would also reduce the load on the piers and on the moving parts of the bridge. For this reason a three-dimensional triangular truss was devised to give it the axial stiffness it required and yet remain lightweight. The main issue was the lateral stiffness of the deck system. This is explored below with the solutions applied. The Problem In the original concept each deck was only supported on one side. A solution needed to be found to balance the load otherwise the decks would twist. Two solutions were devised which were implemented in different areas. Solution A For the sections of deck near the piers, struts were introduced connecting the outsides of the walkway to the top of the piers. This would have the ideal effect of balancing the decks on each of their sides. As the decks moved away from the piers the struts would become less effective and would impinge on the clearance of the structure. A second solution was required. Solution B For sections of the bridge away from the pier a different solution was implemented. A cross beam was designed to bridge the gap between the two decks effectively forming a single beam supported on both sides with the decks acting as cantilevers. A fairly sizeable central member is required in order to allow for the large bending moments experienced. Local Residents This group will ultimately benefit from the project with a convenient extra crossing. The location was chosen in order to link key residential areas. The design aims to enhance the look of the area, and remain true to the harbour’s illustrious shipping history. Local Developers The value of developments in the area could greatly increase. New residential properties on the south of the harbour would gain a useful link to the city centre. This group could be a prospective source of income for the project and so the design compliments their modern constructions. SS Great Britain Ultimately the bridge would be hugely beneficial to the SS Great Britain. Our design compliments the ship’s silhouette and the location provides good access without interfering with the traditional views of the ship or with its planned future development. Bristol City Council Ultimately it is the extent of the Council’s support that could push the construction through or render the project redundant. The bridge needed to demonstrate its worth as an investment. Consideration was given to the benefit that local communities and tourism could gain from this project. Bristol Harbour Master and Harbour Users This is the group that will be the most adversely affected. In order to appease them we have kept the central channel open and located the bridge upstream of the marina and much of the channel in order to reduce its opening frequency. The bridge also has one moving part to lessen the workload. National Government The Government represent a potential funding source, possibly through the National Lottery. Whilst the bridge design was aesthetically led, costs had to be considered to ensure the financial viability of the project in order to attract investment. KEY STAKEHOLDERS We analysed the needs of the potential users of the proposed bridge along with other stakeholders in order to understand their requirements, allowing us to add value to the project from the beginning of the design stage. Nigel de Grey, Adam Gait, Ben John, David Longhurst - 2005 Courtesy of Edward Cullinan Courtesy of Edward Cullinan