Bristol Floating Harbour Landmark BridgeINTRODUCTION 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 BRITAINThe 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 ProblemIn 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 AFor 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 BFor 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 ResidentsThis 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 DevelopersThe 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 BritainUltimately 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 CouncilUltimately 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 UsersThis 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 GovernmentThe 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 STAKEHOLDERSWe 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

Bristol Floating Harbour Landmark Bridge

CONSTRUCTION SEQUENCE

The ability to build the eventual structure was a central aspect of the project and its feasibility was considered at every stage. Our proposed construction method involved the off-site pre assembly of key components which would then be brought to site using the river itself. One of the benefits of cable-stayed bridge construction is a reduced need for temporary load bearing structures during construction. The deck sections would be installed in segments beginning at the central mast and moving outwards (as shown below). The bridge would be assembled in the open position to allow for unobstructed river traffic flow during as much of the construction period as possible.

OPENING ARRANGEMENTS

To allow the passing of large vessels, the bridge operates on a swing opening mechanism, which rotates the deck through 90 degrees in the horizontal plane around a vertical axis. This affords unrestricted vertical clearance on either side of the structure.

Alluvial Drift

Redcliffe Sandstone in Keuper

Middle & Lower Coal Measures

Quartzitic Sandstone Group

AGV - Ashton Great Vein

ALV - Ashton Little Vein

Unconformity

500 ft

400 ft

300 ft

200 ft

100 ft

0

Floating Harbour

ENVIRONMENTAL CONSIDERATIONS

Under the current environmental impact assessment guidelines, the project did not fall under either schedule description. As such, we did not deem it necessary to undertake a project specific EIA for the proposed development. However, due to the location of the ongoing development on the north bank at Canon’s Marsh, and the recently completed Point development on the south bank, it was possible to view the respective projects environmental statements’ and gain an idea of the local existing ecology relevant to the site. It was concluded that there is little flora or fauna of special interest within the harbour and as such, no specialised mitigation techniques were required.

On both banks of the site, due to the nature of the former industries, the top fill was found to be heavily contaminated in parts. These contaminations are contained within localised ‘hot-spots’. The nature of the proposed project is such that the exposure of large portions of ground around the harbour banks will not be necessary. Care will have to be taken to ensure that the Floating Harbour water does not come into contact with any of the contaminated ground.

GEOLOGICAL OVERVIEW

The 1:10560 Geological Survey Sheet ST 57 SE shows the area of the site to be underlain by estuarine alluvium, which overlies the Mercia Mudstone Group of Triassic age, which in turn is underlain by mudstones, siltstones and sandstones of the Lower Coal Series of Carboniferous age. At depth, the Coal Measures are underlain by Quartzitic Sandstone of the Millstone Grit Series that outcrop at Brandon Hill to the northwest. A section through the site and surrounding area is shown below. The Estuarine Alluvium generally comprises of silty clays and clayey silts, giving way to sand and gravel towards the base. The strength varies from stiff near the surface, to very soft or loose at depth.

The pier foundations will be piled through the alluvium drift into the Redcliffe Sandstone. Due to the nature of the alluvium, the piles will have to be driven, in order to avoid collapsing of the soil. The possible decalcification of the sandstone, and its resulting degradation into sand in the top 3-5m of the stratum should be anticipated. Because of this, the piles must be driven to a suitable depth past this decalcification zone, which will be around 20m in depth.

ANALYSIS AND DETAILED DESIGN

Preliminary hand calculations had provided us with initial section parameters. Our GSA model then provided us with a more accurate analysis of loads throughout the structure. The sections selected, based on the GSA output, affected loading due to self-weight, making the design process iterative. With many elements to design, a system had to be produced that could accurately and reliably extract the GSA output and oversee the redesign of each element before this data could be re-entered in GSA. This was achieved through the implementation of a series of spreadsheets. The process is outlined in the flow chart below:

GSA

Output Spreadsheet

Properties Spreadsheet

Design Spreadsheet

Input Spreadsheet

1. The model gave outputs of stress and moments in each of the members based upon the load cases taken from the British Standards.

2. This information was exported into a spreadsheet with imbedded macros that extracted the maximum forces in each direction and ordered the elements according to groups and section properties.

3. The forces and moments were fed into the design spreadsheet. Here each element was checked for bending, axial and shear stress, and combined bending and axial stress.

4. Members experiencing excessive force/moments were flagged and the property group, of which they were a part, was strengthened. The design spreadsheet was then rerun.

5. The new section parameters were then reintroduced to GSA via a compatible output sheet. Likewise, a new table was generated for the new section weights and imported before a new run began.

PIER DEFENCE SYSTEM

A bespoke rubber fender system was designed to protect the bridge piers from low-speed minor collisions but a more substantial system was needed to protect the bridge against the possibility of a vessel loosing control and crashing directly into the structure. For this purpose a series of piles were designed to be located either side of the new bridge, guiding errantly directed ships into appropriate channels and also absorbing high-energy impacts.

The Modelling Tool

The structural analysis program, GSA, was used to model and test the bridge design. The program’s ability to model a complex range of load patterns enabled the bridge’s performance under the various load conditions specified by British Standards to be analysed (BS5400, parts 1&3). The program was able to isolate the worst-case conditions for each element allowing individual members to be sized, considering both the ultimate and serviceability limit states.

The bridge was modelled in both open and closed form, with the larger pedestrian generated loads not being applied to the open form.

Dynamic Analysis

Dynamic excitation of a bridge of this form is likely to occur due to either wind or pedestrian loading. GSA’s dynamic analysis mode allowed the natural frequencies of the bridge to be determined, and lateral excitation was found to be the biggest problem. This data was used to analyse the structure according to the Highways Agency Design Manual for Roads and Bridges Volume 1 Section 3 Part 3 (BD 49/01) – Design Rules for Aerodynamic Effects on Bridges. The pedestrian excitation analysis was achieved through studying academic research on the topic and comparisons with similar structures in the real world. It was recommended that damping be added to the structure to reduce the likelihood of excitation. The picture below shows the shape of one of the modes of excitation.

Bridge Piers

As links between the bridges’ super- and sub-structures, the bridge piers in the project’s design had great importance as individual elements within the overall structure. Particular focus within the project was given to the larger bridge’s pier as it was the critical load-carrying pier and several design complexities arose from the fact that it housed the opening mechanism.

Riverbed scour, an issue that can often be a controlling factor in the foundation design of many bridges, was discounted in this project due to the insignificant flow within the Floating Harbour.

The piers were required to transfer both the axial, shear and bending loads from the substructure into the load bearing ground (sandstone in keuper, 25 m below the mean water level). The piers were modelled as fixed points in the GSA analysis, and as such there was a requirement of the pier pile design to allow only minimal deflections.

The main pier was designed as a reinforced concrete shell attached to a CHS steel pile driven into the load bearing ground. The picture showing the pier detail depicts the concrete shell as semi-transparent, allowing the operating mechanism to be seen.

Opening Mechanism

The proposed opening mechanism was designed to function on a ‘lift and turn’ basis, a system becoming standard throughout Europe. It involved the bridge being raised from within the pier, before being rotated into the desired ‘open’ position. This movement would be achieved by firstly, the operation of four hydraulic jacks to lift the entire mechanism, and secondly, a suitable motor to drive a cog ring to the desired degree of rotation. It was decided that the power supply and hydraulic pumps would be housed in the ground within the bank in order to allow greater ease of access for maintenance. Whilst it was recognised that this system enabled the achievement of one of the main project objectives (affording the required vertical clearance for shipping within the harbour), the work carried out did not consider the detailed design of every element within the mechanism. The decision to consider only some of the elements in detail was taken due to the mainly mechanical nature of the system falling outside the group’s area of expertise. Whereas specialist subcontractors would normally be consulted, for the purposes of the project, the group designed a bespoke system to perform this operation. The exploded mechanism diagram (see below) shows the basic layout of the system, which is housed within the main bridge pier.

Mechanical Connections

The bridge to bridge and bridge to harbour wall connections were modelled as fully restrained so as to carry moments in the analysis. This was desirable to maintain the sleek nature of the design.

This would be achieved in practice through our locking mechanism (not detailed here) that would be activated when the bridge was closed.

Nigel de Grey, Adam Gait, Ben John, David Longhurst - 2005