Bristol Floating Harbour Landmark Bridge

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Bristol Floating Harbour Landmark Bridge
INTRODUCTION TO THE PROJECT The city of
Bristols 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 docks 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 areas tourism and redevelopment.
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.
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.
Existing and Planned Harbourside
Development Canons 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).
Courtesy of Edward Cullinan
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.
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.
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 Cannons Marsh Architect Our
selection of site was also influenced by the
proposed Cathedral Walk. This is a promenade
being constructed as part of the Cannons Marsh
development and one that we believe is well
suited to the positioning of a bridge where it
meets the harbour.
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.
Courtesy of Edward Cullinan
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.
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 harbours illustrious shipping history.
Bristol City Council Ultimately it is the extent
of the Councils 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.
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.
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.
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 Brunels 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 Britains
redevelopment scheme.
SS Great Britain Ultimately the bridge would be
hugely beneficial to the SS Great Britain. Our
design compliments the ships silhouette and the
location provides good access without interfering
with the traditional views of the ship or with
its planned future development.
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.
Nigel de Grey Adam Gait Ben John David
Longhurst - 2005
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Bristol Floating Harbour Landmark Bridge
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 groups 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.
GEOLOGICAL OVERVIEW The 110560 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
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.
The Modelling Tool The structural analysis
program GSA was used to model and test the
bridge design. The programs ability to model a
complex range of load patterns enabled the
bridges performance under the various load
conditions specified by British Standards to be
analysed (BS5400 parts 13). 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.
Bridge Piers As links between the bridges
super- and sub-structures the bridge piers in
the projects design had great importance as
individual elements within the overall structure.
Particular focus within the project was given to
the larger bridges 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.
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.
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.
Dynamic Analysis Dynamic excitation of a bridge
of this form is likely to occur due to either
wind or pedestrian loading. GSAs 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.
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 Canons 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.
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.
Nigel de Grey Adam Gait Ben John David
Longhurst - 2005