Sustrans Design Manual Chapter 8
Bridges and other
structures (draft)
February 2015
September 2014
1
Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
About Sustrans
Contents
Sustrans makes smarter travel choices
possible, desirable and inevitable. We’re
a leading UK charity enabling people to
travel by foot, bike or public transport
for more of the journeys we make every
day. We work with families, communities,
policy-makers and partner organisations
so that people are able to choose
healthier, cleaner and cheaper journeys,
with better places and spaces to move
through and live in.
This chapter of the Sustrans Design Manual should be read in
conjunction with Chapter 1 “Principles and processes for cycle friendly
design.” That chapter includes key guidance on core design principles,
whether to integrate with or segregate from motor traffic, the space
required by cyclists and other road users as well as geometrical
considerations. Readers are also directed towards the “Handbook for
cycle-friendly design” which contains a concise illustrated compendium
of the technical guidance contained in the Design Manual. This chapter
has initially been issued as a draft and it is intended that it be reviewed
during 2015; feedback on the content is invited and should be made by
31 May 2015 to [email protected]
It’s time we all began making smarter
travel choices. Make your move and
support Sustrans today.
www.sustrans.org.uk
1. Key principles
Head Office
Sustrans
2 Cathedral Square
College Green
Bristol
BS1 5DD
4. Headroom, parapets and gradients
© Sustrans February 2015
Registered Charity No. 326550 (England and
Wales) SC039263 (Scotland)
2.Introduction
3. Underbridges and overbridges
5. Bridges: design
6. Bridges: utilising existing opportunities
7. Bridges: construction materials
8. Retaining structures
9.Boardwalks
10. Subways / underpasses
11. Wheeling ramps
Issue level: 01
Owned by: NCN Director
Contact:
[email protected]
Photography: Sustrans or CTC
Benchmarking unless noted otherwise
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February 2015
12.Tunnels
13. Key references
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
1. Key principles
• structures should enable creation of a continuous network
• derestricted roads carrying in excess of 10,000 vehicles per day are
difficult barriers to negotiate and it is often more practical to look at
using structures to resolve crossing issues
• modern structures should be as lightweight as possible and feel
spacious to the user even at busy times
• desire lines can be difficult to reflect, but any deviation from a direct
route should be limited
• minimise the effect of any approach gradients – 1 in 20 is preferred,
and appropriate for all user groups, however where necessary
steeper gradients are acceptable where achieving 1 in 20 would be
disproportionally expensive
• design widths should acknowledge suppressed demand and allow for
growth in user numbers
• deck width should allow for effect of parapets with a minimum
width of 3.5m
• avoid right angled turns on approach paths, which are difficult to
negotiate
• where desirable heights for parapets cannot be achieved on existing
structures this should not necessarily preclude their use as crossings
for cyclists
• consider maintenance and how it will be looked after
• when there is a requirement for a structure to be lit, lighting columns
should not restrict the usable width of the path. Lighting sources can
be imaginative and be an integral part of the structure, rather than an
“add on”
• not all usable structures are new. Farm accommodation crossings may
need to be adapted to create approach gradients that suit walking and
cycling, but these structures offer plenty of opportunity, especially in
rural areas
• road bridges can often be adapted to meet the requirements of a traffic
free route, particularly where carriageway space can be reallocated
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
2. Introduction
2.1
A traffic free path is likely to encounter any number of physical barriers
along its route. Railways, rivers, canals, small watercourses and roads
all create breaks in connectivity.
2.2
Dealing with the natural and artificial topography alongside routes
requires thought, as bridges and retaining structures, no matter how
small, are essential to solving some of the fundamental connectivity
issues that restrict non-motorised movement.
2.3
Structures on traffic free routes need to provide high quality continuous
routes that are fit for purpose. Particular attention needs to be paid to
their alignment and design, as well as construction details. Installing
structures can be complicated. Even on quieter urban roads a closure is
often necessary.
2.4
There may be particular constraints when re-using existing structures
including the presence of legally protected wildlife, such as bats and
birds.
3. Underbridges and overbridges
3.1
How a structure is defined depends upon how a traffic free route passes
across a barrier. The terms ‘underbridge’ and ‘overbridge’ date back to
how railway lines crossed obstacles, for which the structure is described
from the perspective of the train driver. This chapter describes the
structure from the perspective of the path user. Table 3.1 identifies
particular issues that may need to be considered in each case.
Table 3.1: Over and underbridges: items to consider
Overbridge (over line)
Underbridge (under line)
Traffic free routes pass under a road or
railway line
Traffic free routes pass over a road,
railway, river or canal
Likely to generate issues of
Likely to generate issues of
•headroom
• headroom (road/canal users’
requirements)
• personal security
•lighting
•drainage
• path width (if on canals /
riverside paths)
•visibility
• vandalism / broken glass
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• parapet heights
• visual impact
• approach gradients
• path alignment and
available land
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
4. Headroom, parapets and gradients
4.1
Headroom and parapet requirements will vary depending upon the type
of barrier encountered. Re-using old structures, especially railway and
canal infrastructure or ancient road bridges, can be an effective way
of creating continuity for a traffic free route. These do not necessarily
comply with the guidance around headroom or parapets for new
structures.
4.2
Similarly, whilst advice on preferred gradients should normally be
followed, there are other factors to consider in making a decision, such
as additional distance for users, aesthetics and the need for rest areas.
There may be particular constraints when re-using existing structures.
Headroom: underbridges
4.3
The amount of height gain required to achieve a minimum headroom
clearance will have a significant effect upon the length of any approach
ramps, and therefore upon land required, aesthetics and planning
requirements.
4.4
Clearances are measured from road, rail or water level, and these may
bear little relationship to the surrounding ground levels. It is therefore
important to establish the exact difference along the approaches to
ensure that any lengths of ramp are designed with the end user in mind.
4.5
Table 4.1 below sets out various scenarios, and the minimum clearances
normally required to the underside of any new structure.
Table 4.1: Clearance / headroom requirements
Bridge Over
Min Headroom (to soffit of walking /
cycling bridge)
Watercourse / stream
Bridge soffit - 1 in 100 year flood +
600mm freeboard
Non navigable river
Bridge soffit - 1 in 100 year flood +
600mm freeboard
Navigable / Tidal river
Depends on location
Canal
2.7m from towpath preferred
Road
5.7m from top of kerb
Road - bus route
5.7m from top of kerb
Trunk Road
5.7m from top of kerb
Non electrified railway
4.78m from rail level
Electrified Railway
4.78m from rail level
Subway
2.4m (cycles)
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Headroom: overbridges
4.6
The recommended headroom for subways is 2.3m for pedestrians and
2.4m for cyclists, increasing to 2.6m and 2.7m for lengths in excess
of 23m. A headroom of 3.7m is required for mounted equestrians.
However, there are many examples of structures on public roads and
on traffic free routes with headroom well below 2.4m, with appropriate
warning signs, which operate without incident for cyclists, so low
headroom bridges should not prevent a route from being developed or
from being included for development with future funding streams. The
overall design of subways is covered in Section 10 below.
4.7
LTN 2/08 Section 10.10.2 takes a flexible approach to headroom around
subways and overbridges stating that “the headroom around existing
pedestrian subways is typically 2.3m, and routes under canal bridges
often have less clearance. The restricted height should not lead to
automatic rejection of a proposed permit to cycle. It may represent the
best available option if potential risk to users can be managed.”
4.8
On arched structures that have reduced headroom good through
visibility to the path beyond can ensure that users can use the section of
path with the greatest height, although this is not always possible.
Central Glasgow
Newton Abbot
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Northampton
Peak Forest Canal, Hyde, Manchester
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
Case Study:
Reach Lode, Cambridgeshire
In Cambridgeshire, the local fenland
environment has dropped following centuries
of peat removal, drainage and agricultural use.
The Lodes are a network of ancient waterways
linking the fens that now sit several metres
above the surrounding landscape. Several
are navigable, including Reach and Burwell
Lodes. Designing structures to cross these
needs to factor in the clearance required for
navigation, but in such a way that they still
have a positive aesthetic impact upon a very
flat landscape.
Case Study:
adapting an existing structure,
Rugby
Adapting existing structures can also lead to
designs for improved access requirements being
constrained by the surrounding topography. In
these images of Rugby, the main bridge structure
needed to clear overhead power lines for the West
Coast main line. The ground levels beyond the
Network Rail boundary were considerably lower
at this point, extending the length of access ramp
required.
A height gain of approximately 9m was necessary.
A single ramp at 1 in 20 would be 180m,
impractical in most locations. To create a solution
that worked the ramp gradient was steeper than
recommended, up to 1 in 15, and a “zig zag”
utilised to save space.
Land ownership constraints were a significant
factor in the design process, with a developer
unwilling to co-operate beyond what was
absolutely necessary under their planning remit –
sometimes third party involvement is all about the
commercial impact. It also explains the erection of
the palisade fencing, albeit until the adjacent land
is turned into housing.
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Parapet Height
Bridges for cyclists should ideally
have a parapet height of 1.4m (1.8m
if also providing for equestrian use).
On existing structures this cannot
always be achieved, but it should
not necessarily preclude their use as
crossings for cyclists. LTN 2/08 Section
10.8.2,
4.9
Parapet height for new bridges is normally 1.15m for pedestrians,
1.4m for cyclists, or 1.8m for equestrians. On existing structures being
converted to cycle use this parapet height cannot always be achieved,
but it should not necessarily preclude their use as crossings for cyclists;
advice is given in Sustrans Technical Information Note 30 Parapet
Heights on Cycle Routes.
4.10
Re-used old structures, especially railway infrastructure or ancient
road bridges, often do not comply with this guidance. There are many
examples of historic bridges on public roads with parapets below 1.4m
and no footway, which operate without incident for cyclists.
4.11
Where the 1.4m parapet height cannot be achieved cost effectively,
a risk assessment should be undertaken and there may be ways to
mitigate the main risks, for example:
• old railway structures can be adapted to improve existing parapet
heights. Where original ballast remains in situ, this is often at a greater
depth than is necessary to support walking and cycling loadings
Bristol (Bedminster)
• reducing the depth of the ballast layer can have a significant impact
upon the overall height of the parapet (Figure 4.1)
• take account of the thickness of the parapet walling; a thick wall
increases the effective height of the parapet
• where width permits, locate the paths in the middle of a structure and
discourage users from straying near the edges
Overall parapet height
using old railway track
bed level is below
1.40m
Topsham
Reduce
existing
railway
formation to
create extra
height to
parapet sides
Dished channel formed in
surfacing to remove surface
water away from parapets. Use
existing drainge system to carry
water away from structure if in
suitable condition
Alternatively
raise the existing
parapet height
by adding new
railings
Original track bed level
Check original waterproofing and add minimal depth
of construction to surface. Surface between parapet
walls to increase longevity of structure
Lambley Viaduct, Northumberland
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Fig 4.1 Indicative sketch for works to
the Nidd Viaduct, Harrogate
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
4.12
Adapting parapets is possible, but costs time and money to resolve,
although damaged stone parapets can be replaced with modern
installations that allow users better views of the local environment.
Where it is deemed appropriate to raise the parapets, the following
points should be considered:
• different parapet styles can be used within the same structure
depending upon the state of the original parapet walls
• lead-in rails on the approach to a structure will give a better aesthetic
when they are designed to the same style as the main structure
• timber post and rail fences are effective, but lack aesthetic quality and
can become a maintenance liability
Sub-standard parapets on cycle route, Bristol
• post and wire fencing has less visual impact and is a low cost solution,
but can also be a maintenance liability
• retain open views wherever possible, but restrict access to steep sides,
deep/fast flowing water or major roads
• does the structure contain legally protected wildlife,
such as bats and birds?
Hockley Viaduct, Winchester
Glen Ogle
On traffic free routes that
accommodate horses, but have
structures that are not designed to
permit mounted horse use, provision
of timber or stone mounting blocks
each side of a structure can aid
mounting, Kenilworth
Simple adaptation of existing structures, and the parapets, can
result in a visually appealing solution. Using parapets and bridge
decks as “art projects” could open up other funding streams
that would not necessarily be considered for route development.
Clydach, South Wales
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Gradients and approach paths
Table 4.2: Recommended gradients
3%
Preferred maximum
5%
Normal maximum - up to 100m
7%
Limited gradient - up to 30m
>7%
For short lengths
A maximum gradient of 3% is
recommended, but this can rise to 5%
for up to a maximum of 100m. Where
steeper slopes are unavoidable then the
limiting gradient is 7% over a distance
of up to 30m. Steeper gradients are
not recommended, except over short
distances. LTN 2/08 Section 8.7.2
The preferred maximum gradient for
off-carriageway routes is 3%, with an
acceptable maximum of 5%. Where new
routes are constructed alongside the
existing carriageway, the gradient will
need to reflect the conditions of the road.
As such where it is not practicable to
provide gradients not steeper than 5%,
steeper gradients may be considered
over shorter distances. DMRB TA90/05
Section 5.4
4.13
Approach paths designed to a maximum gradient of 1 in 20 will be
negotiable by all user groups, but depending upon the extent of height
gain required, this could create lengthy ramps. Recommended gradients
are included in Table 4.2.
4.14
Whilst advice on preferred gradients should normally be followed, there
are other factors to consider in making a decision, such as additional
distance for users, aesthetics, planning requirements and the need for
rest areas. There may be particular constraints when re-using existing
structures including the presence of legally protected wildlife, such as
bats and birds.
4.15
However, providing gradients that exceed the maximum recommended
should be the last resort, after exploring all other solutions, including
additional land purchase.
4.16
Approaches that are too steep or straight can generate a public
perception about speeding cyclists. Too flat and they will require
additional land, or become meandering zig zags, often caged in by
parapet sides resulting in expensive, aesthetically poor solutions.
4.17
In construction, ramped accesses into a new building or property are
required to provide level landing areas where they are at a gradient of
1 in 20 or greater. On a structure this is not necessarily appropriate
and a ramp that has several landing areas over its length can be
uncomfortable for cyclists. Each landing area adds extra length, so that
it is very easy for a 1 in 15 ramp to be similar in length to one of 1 in 20,
with no landing areas.
5. Bridges: design
5.1
Bridges can provide very useful connections along footpaths or cycle
tracks away from the road, avoiding conflicts at major roads and
taking routes across other barriers such as railways and waterways.
Where the topography is favourable the need for approach ramps
can be minimised and good natural surveillance improves personal
security. New bridges can be designed as features along a route and
may become attractors in their own right. New bridges are generally
considerably cheaper than new subways.
5.2
Particular benefits of bridges include:
• provides a conflict free crossing of a major barrier
• a new bridge may provide an opportunity for a landmark feature
• a bridge will often be cheaper than a subway
• good personal security
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Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
5.2
Key design features:
• bridges require considerable investment and should normally cater for
both pedestrians and cyclists
• where a bridge will attract high numbers of pedestrians and cyclists the
aim should be to provide effective segregation between them so that
each group can travel at their preferred speed
• where usage will be lower, an unsegregated bridge offers opportunities
for users to stop on either side to take in the view, and requires less
width than segregation
Killamarsh, Sheffield
• bridge approaches and decks should be straight or nearly straight.
Right angled turns are difficult for cyclists to negotiate
• gradients should be in accord with the maximum values given in Table
4.2, depending on slope length. Steeper gradients than 7% are not
recommended, except over very short distances
• where the topography is favourable the need for approach ramps can
be minimised
5.3
The key dimensions for bridge widths are summarised in Figures 5.1 and
5.2 and noted below:
• pedestrian only: minimum width of 2m, with additional width for busy
routes
Gateshead Millennium Bridge
• unsegregated pedestrian/cycle bridge: the width should reflect the
level and type of use forecast with a minimum of 4m width on main
cycle routes, or 3.5m on less busy secondary routes. On particularly
heavily trafficked routes it should be increased to 5m or more
• segregated pedestrian/cycle bridge:
• footway width should reflect the level and type of use forecast with
a minimum of 2m width, increasing to 3.5m width where there is
frequent use by groups
• cycle track width should be sufficient to accommodate the forecast
level of use with a minimum of 3m, preferably 4m or more
• a bridge parapet has little impact on pedestrians’ width but reduces
the usable width for cyclists by 500mm on each side
Diglis Bridge, Worcester
5.4
Parapet height for new bridges is normally 1.15m for pedestrians,
1.4m for cyclists, or 1.8m for equestrians. On existing structures being
converted to cycle use this parapet height cannot always be achieved,
but it should not necessarily preclude their use as crossings for cyclists;
advice is given in Sustrans Technical Information Note 30 Parapet
Heights on Cycle Routes. Parapet heights of existing and new structures
are discussed more fully in Section 4.
Glasgow city centre
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
4m or more preferred
3.5m min
Parapet height (h)
• 1.4m preferred for cyclists,
but many existing bridges
operate well with lower
heights
• 1.8m for equestrian use
(mounted)
3m min
2m min
h
• effective width of bridge
reduced by 500mm at each
parapet
• for advice on substandard
parapet heights, refer
to Sustrans Technical
Information Note 30
Figure 5.1 Width requirements
for bridges: unsegregated
Figure 5.2 Width requirements for
bridges: segregated
5.5
Other design considerations that are important for bridges include:
Figure 5.3
Design of ramps
• design widths should take account of suppressed demand and allow
for growth in user numbers including potential new developments
along the route
Steps
• exposure of users to the weather should be considered - covered
bridges will be beneficial
• any new bridge over a road should also provide a good quality links to
that road
Gradient 5%
or less
(preferred
gradient 3%)
Guard rail
may be
appropriate
Appropriate
lead-in
barriers to
the bridge
parapet
should be
considered,
particularly if
the approach
is on an
incline
• designs can be as simple or as complex as the budget allows, but
aesthetics can be crucial. A bridge over a key road corridor can make
a statement; a simple design might be more appropriate in areas over
looked by housing
• clear signing for existing walkers and cyclists will give them key
destinations along the road and the traffic free route
• when designing a structure always ensure that the client, the designer
and the contractor know where utilities are located
• access onto structures should generally be barrier free
• where structures lift or rotate gated control is necessary. Locating
these controls requires careful thought so that the ability for all users to
access the bridge easily is not impaired
Pont y Weirin, Cardiff
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• the location of the bridge should take into account legally protected
wildlife either within existing embankments and structures or using the
feature being crossed, such as otters along a river
5.6
Similar criteria apply to the conversion of footways over road bridges to
shared use, and these are considered further in Section 6.
Millennium Bridge, Newcastle/Gateshead
Bradford
Peace Bridge, Derry/Londonderry
Hob Moat, York
New River, Cheshunt
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
6. Bridges: utilising existing opportunities
Introduction
6.1
When a new cycle bridge is required it may be possible to make use of
existing infrastructure to reduce the outlay. In some cases this may be at
a location where the existing embankments can be used when replacing
a demolished railway bridge, or where an existing busy road bridge can
be adapted to carry cyclists safely, or where use may be made of a farm
accommodation bridge. This section discusses issues that may need to
be considered when taking advantage of these opportunities.
Weymouth
Avebury to Chippenham
Kenilworth
Ecological considerations in existing structures
6.2
Where an existing structure is to be used, in particular structures made
from brick or stone such as old railway bridges or abutments, these
should be surveyed for wildlife at the earliest possible stage. The
presence of bats in a structure can significantly delay a construction
project and will lead to an increase in the complexity and cost of
proposals. It is unlikely that the presence of a protected species will
prevent construction but the sooner an issue is identified the easier it is
to deal with.
Cable Stay design, Scunthorpe
6.3
Similarly where existing embankments are to be used a survey should
be conducted for wildlife within the embankment or using the feature
that is being crossed (in the case of rivers or green space). In many
instances moving a development a few metres can substantially reduce
ecological impacts, allowing construction to take place whilst protecting
local wildlife.
Road bridges that retain the path level
6.4
In some locations, notably where a traffic free route uses an old
railway formation on which a bridge has been removed to allow a road
improvement, it may be feasible to construct a new bridge that needs
little or no change to the path level. In such situations the traffic free
route benefits from not having to drop down to the road to cross and
climb up again, with a better environment for walking and cycling. Traffic
on the road benefits from not being interrupted by pedestrians and
cyclists, particularly where a signalled crossing would be required. Such
a bridge will require a greater initial outlay than a controlled crossing,
but will need less regular maintenance. Convenient connections to the
road should be retained from both sides of the bridge.
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Adapting an existing road bridge
6.5
There are many locations across the UK that utilise existing structures
for walking and cycling, but the quality of provision is poor. Often cited
arguments are that shifting kerb lines and changing the dynamics of a
structure is impractical, that it is costly or that there are utilities affected.
However, where there is a desire to achieve a continuous and high
quality route many of these apparent obstacles can be overcome.
6.6
The scale of intervention may range from merely moving the kerb line
further into the carriageway to removal of a full lane of traffic in order
to provide a high quality route for cyclists that is segregated from both
traffic and pedestrians.
6.7
Shifting a kerb line to create better space requires consideration of the
following factors:
A127 Hall Lane, Upminster (before)
• the benefits that cohesive routes built to a high standard can give
• the amount of space that can be taken from the carriageway, bearing
in mind the level and type of traffic using the road
• political will to take space from the motorist for pedestrian
and cycle use
• minimising the costs associated with shifting or protecting
existing utilities
• structural assessments of both bridge and parapets
A127 Hall Lane, Upminster (after)
• timescales, times of day when works are restricted
Figure 6:1 Footway on existing bridge:
improving for shared use
Unsegregated
cycle track/footway
3.0m min two way
2.0m min one way
0.5m
Margin
0.5m where practical
(widen into
carriageway
if needed)
h
Redcliffe Bridge, Bristol (before)
Not to scale
Parapet height (h)
• 1.4m preferred for cyclists, but many existing bridges
operate well with lower heights
• 1.8m for equestrian use (mounted)
• effective width of bridge reduced by 500mm at each
parapet
• for advice on substandard parapet heights, refer to
Sustrans Technical Information Note 30
Redcliffe Bridge, Bristol (after)
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Adapting a farm accommodation bridge
6.8
Re-using farm accommodation crossings can be key to retaining route
continuity in rural areas. Existing approach gradients are often steep
and will require extensive works to create new embankments with better
gradients, and land requirements will always need to be considered with
any purchase costs factored in.
6.9
In developing proposals to use an accommodation bridge the following
points should be considered:
• parapet heights may need to be raised, or existing systems replaced
• it is worth stripping the existing surfacing back to bridge deck and
waterproofing the whole structure
• kerblines are not always necessary, but will prevent cyclists from
getting too close to the parapets, and will allow space for pedestrians
to stand if they want to stop. Always retain a clear route with a
minimum usable width of 2.5m
• it may not always be possible to achieve a 1 in 20 gradient, but this
should not necessarily deter use of the structure
• if more land is required talk to adjacent land owners, but always go
prepared with drawings showing what is actually required – and have
them in a format that farmers and land owners understand
Sleaford, Lincolnshire (before)
Sleaford, Lincolnshire (after)
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7. Bridges: construction materials
7.1
Bridge design should be delivered to BS5400, the code of practice
for steel, concrete and composite bridges; in part some of this is
superseded by Euro codes. Designers should be encouraged to start
with this, but then allow a flexible approach to provide for pedestrian
and cycle movements.
7.2
Larger structures will give more scope for something creative, especially
if the client wants an iconic design. Smaller structures, perhaps up to
20m in length, can be “bought” from supplier catalogues, at a unit cost.
Some of the UK’s most iconic structures provide striking solutions,
space for pedestrians and cycles to mix freely and have been the
catalyst to the expansion of a wider walkway and cycle network.
7.3
Often bridge designers will choose materials that they know will best do
the functional job required, but that shouldn’t necessarily preclude other
options from being considered. A bridge built from concrete and steel
will be significantly heavier than one built entirely from steel, or timber,
with obvious implications for delivering to site, or the size of crane
required to lift it into place.
7.4
The simplest structures are often the easiest to transport from workshop
to site location, and will require very little effort to install. Considering
that most walking and cycling barriers are minor watercourses, a simple
lightweight timber, fibre reinforced plastic or steel structure will often
suffice. Materials and products should be sympathetic to the location.
Fig 7.1: Typical construction detail, lightweight timber structure
Rails: 45x120
redwood, fixed
to posts using
coach bolts
Deck planks
45x145 redwood
Hi-Grip Excel
410
410
1400
1928
410
Posts:
70x120
redwood
fixed to rails
using coach
bolts
All dimensions in mm
Post
packers
(hardwood)
Bearers: 50x150
hardwood fixed @
600 centres
Main beams
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Reinforced plastic
7.5
Several small structures around Watermead Park in Leicester use a
design consisting of green oak timber handrails and a glass reinforced
plastic decking material more commonly found in marinas. Using
a simple steel lattice frame as a basis these materials produce an
aesthetic yet extremely lightweight structure, ideal for the type of
loadings associated with walking and cycling.
7.6
Structures designed at 2.5m wide may suit some locations where usage
is likely to be very low, but designers should always seek to use the
maximum width available. Increasing to 3.5m for this style would require
longer transverse beams.
Welded on bolt
heads to recieve
timber parapets
1400
2500
Fig 7.2: Cross section
through one of the
Watermead Park
lightweight bridges
Mesh to
BS 7818
“Marina
Deck”
decking
February 2015
75 x 38mm
timber rail
38 x 225mm
timber rail
Transverse beam
600 x 300 fabricated
box stringer
18
75 x 225mm
timber rail
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
Steel
7.7
Pure steel structures can be aesthetically pleasing, or simply functional
as the images below show. A simple warren truss design may lack visual
beauty but fabrication costs are considerably less. Iconic structures will
have a visual impact, but these come with a hefty price tag.
Northwich - River Dane
Bath - Two Tunnels
Northampton - River Nene & Grand Union Canal
Omagh - River Strule
Newton Abbot - River Teign
Workington - River Derwent
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Concrete
7.8
Concrete, or a composite of concrete and steel, produces heavy
structures often partially, or wholly, built in situ, creating huge
construction sites, but the structures that they produce are often iconic
and blend into the landscape or urban environment with relative ease.
Such structures are often part of a much wider series of enhancements.
Parapets
7.9
The choice of parapet style and deck surface treatment is important.
A sympathetic approach to both can leave a lasting impression of a
structure that has a good aesthetic quality.
7.10
Long structures over rivers and tidal areas that have used glass or
open sides, will allow pedestrians and cyclists to stop and enjoy
the environment through which they are passing. Even in urban
environments, structures over major roads need to be designed to allow
the user to feel that they remain connected to the outside world.
Glasgow
Shoreham
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February 2015
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
8. Retaining Structures
8.1
Retaining structures are often needed to support a new path or the
adjacent embankment, and the different options available produce
differing aesthetics and have a wide range of costs. As Table 8.1
illustrates, there are a variety of options available and each type can
work in a variety of situations. Some are easier to construct (gabions),
and others easier to maintain (brickwork). Brickwork can attract graffiti
and become unsightly, but the ability to plant “crib walling” can soften
the aesthetics of the structure.
Table 8.1 Wall construction types and approximate costs
Hastings
Type of retaining structure
Approximate
cost per linear
metre
Gabion boxes (1m x 1m x 1m + class 6G stone fill)
£150
Steel I beam & timbers (175mm x 175mm I / 2.4m timber)
£120
Brick retaining wall
£300-400
Criblock wall
£300-400
Exmouth
8.2
Banks up to 1 in 2 do not generally require retaining structures,
providing that they are planted; the root systems will bind together the
sub soils creating a natural stability.
Gabion box
8.3
The most common solution is a wire mesh and stone filled gabion
box. These are relatively cheap and easy to install, and have been
successfully used in a wide variety of situations. Aesthetically they are
harsh, perhaps more suited to an urban environment, but functional.
Killamarsh, Sheffield
Original
ground profile
Timber post
and rail fence
Cycle path
1100
Figure 8.1 Typical
gabion box layering
Layer 4 1x1x1
Powderham, Devon
Layer 3 1.5 x1x1
Temporary
works batter
Layer 2x1x1
1
2.5
Layer 1.5x1x1 &1x1x1
Possible
continuous line of
fence sheeting
6°
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Timber wall
8.4
The simplest solution may still be a series of timber sleepers or steel
I beams set vertically, approximately 1700mm apart with horizontal
timbers, roughly 150 x 200 in size. The adjacent bank can be re-graded
so that it suits multiples of the timber size.
Exmouth
Conwy
Crib walling
8.5
For structures greater than 2.0m in height, or where there is a need to
provide something more aesthetically pleasing, using a system called
“criblock walling” may be an option. This is a wall set roughly at 15
degrees from the vertical and manufactured by several UK companies.
It is generally a timber solution, but the system can also be concrete.
Planting softens the aesthetics, creating a green wall over time.
8.6
Crib walling gives a flexible construction that can withstand some
degree of settlement and movement without detriment to the stability of
the wall. It is best applied in areas where dry subsoils exist.
Padiham, New criblock walling ramp supports the main path and enables access
ramp construction
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February 2015
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
Paving slabs
8.7
Low earth embankments may be effectively retained by use of paving
slabs set no steeper than 7 degrees from the vertical.
Reinforced earth
8.8
Reinforced earth, where embankments are constructed in layers, and
the ground strengthened by installing primary and secondary layers of a
geogrid is another approach that has been proven to be successful and
will allow slopes of up to 35°. It can be extremely useful when wanting
to re-work existing railway embankments, especially those that were cut
back following removal of an original structure many years ago.
Paving slabs retaining a low embankment, Bedford
Reinforced earth approach to reconstructing original embankment, Bath
Wildlife enhancements
8.9
Retaining structures and enhancements offer opportunities for creating
habitats for wildlife along a route. The use of subsoil to create low
nutrient conditions on an embankment will encourage wild flowers and
butterflies. Gabion boxes and criblock walling create small holes and
crevices that can be used by small mammals, reptiles and amphibians
to hibernate. Retaining structures can also be used as the basis for a
‘green wall’ where vegetation is encouraged through the introduction of
soil, retained by a barrier, to a vertical surface.
Green wall, NCN 71 Whitehaven to Rowrah in Cumbria
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
9. Boardwalks
9.1
Boardwalks and similar elevated structures are often viable solutions in,
or through, areas of ecological and environmental importance, or within
floodplains. Hardwood timbers and recycled plastics can work equally
well.
9.2
Boardwalks often require parapets. Where the risk of injury from falling
is minimal parapets are not necessary, however there is a risk that users
will step off a path if it is busy.
9.3
The minimum width for a boardwalk is normally 3.5m, but greater
width may be needed if it is expected to be busy. Many locations for
boardwalks are in coastal environments which can be popular during
summer months.
9.4
Boardwalks are not cheap to install, and a path that meanders through
sensitive areas may still be more practical.
9.5
Verges left between new boardwalks and existing structures or fences
require maintenance.
9.6
Boardwalks may be built at ground level where walking, cycling and
mobility access would otherwise have been difficult.
Southampton
Between Newport and Caerleon, with River Usk in flood
24
February 2015
Exe Estuary Trail
Burton Point, North Wales
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
10. Subways / underpasses
10.1
Subways/underpasses can provide very useful connections for
footpaths or cycle tracks away from the road, avoiding conflicts at
major roads and taking routes across other barriers such as railways.
Where the topography is favourable the need for approach ramps can
be minimised and good natural surveillance is essential for personal
security. Often this option will involve the conversion of an existing
pedestrian subway or an underpass provided for private access.
10.2
Poorly designed subways in particular can be intimidating places;
those with tight blind corners have a higher perceived safety concern
than those constructed higher and wider than the minimum guidance
suggests.
10.3
Particular benefits of subways include:
• provides a conflict free crossing of a major barrier
• avoids exposure to the weather
• the longitudinal profile of an underpass (down then up) is more
comfortable for cyclists than bridges with approach ramps
New at-grade crossings provided with existing subway retained, Croydon
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
10.4
Key design features:
• underpasses require considerable investment and should normally
cater for both pedestrians and cyclists
• underpasses can attract high numbers of pedestrians and cyclists and
the aim should be to provide effective segregation between them so
that each group can travel at their preferred speed
• underpass approaches and the crossings themselves should be
straight or nearly straight. Right angled turns are difficult for cyclists to
negotiate
• gradients should be in accord with the maximum values given in Table
4.2, depending on slope length. Steeper gradients than 7% are not
recommended, except over very short distances
Bridge with sub-standard headroom on cycle route,
Nottingham
• where the topography is favourable the need for approach ramps can
be minimised
10.5
The key minimum dimensions for new subways are summarised in
Figures 10.1 and 10.2, and noted below:
• subways for pedestrians only require headroom of at least 2.3m (2.6m
for lengths over 23m) and a width of 3.0m (2.3m for light use)
• subways for use by cyclists require headroom of 2.4m (2.7m for
lengths over 23m) and width of at least 4.0m (3m for light use) if
unsegregated
Artwork in subway, Southampton
• segregated: the width for pedestrians should be at least 2m, the cycle
track 2.5m and the margin strip 0.5m
• headroom for cyclists and pedestrians as above
• a headroom of 3.7m is required for mounted equestrians
Figure 10.1 Minimum requirements for
new shared use subways: segregated
2.4m (2.7m)
0.5m
margin
2.3m (2.6m)
2.5m cycle
track
Note: dimensions in
brackets apply to subway
lengths>23m
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February 2015
2.0m
footpath
Figure 10.2 Minimum requirements for
new shared use subways: unsegregated
2.4m
(2.7m)
4.0m (3.0m with light usage)
Note: dimensions in
brackets apply to subway
lengths>23m
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
10.6
The headroom in existing pedestrian subways is typically 2.3m; the
restricted height or width available should not lead to automatic
rejection of a proposal to permit cycling. There are many examples of
structures on public roads and on traffic free routes with headroom well
below 2.4m, which operate without incident for cyclists. Any restricted
headroom should be clearly signed. The Cyclists Dismount sign should
not be used.
Changing the environment to
permit cycling can be valuable, with
clear demarcation that cycling is
permissible
10.7
Other design considerations that are important for subways include:
• lighting should be vandal proof
• no corners/recesses
• exit must be visible on entering the subway
• where segregation is required a shallow, or 45º kerb face, is normally
sufficient. A full height, or 90º, kerb presents a barrier to cycles and a
trip hazard to pedestrians
Dover before
• generous headroom and width will be highly beneficial in terms of
subjective safety, natural surveillance and personal security
• a greater width or walls diverging towards the top increases
natural light
• light wells are desirable to maximise natural illumination
10.8
Where an existing subway is being adapted for use by cyclists
consideration needs to be given to wildlife that might use the feature.
In particular where a subway is not currently well lit it may act as a roost
site or commuting route for local wildlife that would be disturbed or
prevented from using the feature should it become well lit throughout
the night.
Dover after
10.9
Where it is proposed to permit cyclists to use a pedestrian-only subway
this takes time and is likely to generate strong opinions on both sides.
Any such change needs to fully assess the suitability of alternative
options for providing connectivity for cyclists, to demonstrate that the
value of permitting cycling outweighs the disadvantages.
10.10
Maintenance can be a big concern; graffiti covered walls, broken glass,
broken lights, blind corners and litter give out a very different message
to the public when compared to something that has good quality
lighting, both naturally and artificially, has artwork rather than graffiti, is
accessible by street sweepers, and retains a straight through route.
New subway under the main railway line, Royston
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
11. Wheeling ramps
11.1
Where cycle routes are introduced onto routes originally designed for
pedestrian use only, such as canal towpaths or railway footbridges,
flights of steps are sometimes unavoidable, at least in the short term. To
assist cyclists, wheeling ramps should be added to one or both sides of
the flights using steel sections or by forming them in concrete.
11.2
This though is by no means the best solution; it still forces cyclists to
dismount and many are poorly located, making them difficult to use.
Steps are still a barrier to wheelchair users and non-standard cycles,
and difficult to negotiate for parents with small children and buggies.
11.3
A design for a retrofit steel ramp is included as Figure 11.1, with a new
build concrete ramp in Figure 11.2.
11.4
Key design features:
• locating the wheeling ramp close to the wall minimises the trip hazard
for pedestrians
• the distance between the ramp and the wall should be enough to
ensure that the pedals and handlebars do not clash with the wall or
handrail while the bike is being held reasonably vertically
• the wheeling channel needs to extend beyond the top and bottom
steps to provide a smooth transition
• steel sections should have a nonslip surface so that the tyres grip the
ramp on descent
• in most cases the ramp is fitted to one side, usually on the right for
people climbing, but on well used routes a ramp on each side should
be considered
200
Handrail
Elevation
See detail for
top and bottom
ends
Fig 11.1 Steel
wheeling ramp,
retrofit
100 x 50 steel
channel bolted to
existing steps
Channel end
rounded off
100 x 50 steel channel
fixed to existing steps
100mm flat end
for fixing to the
ground
steps
Section AA
Bottom end detail
100 x 50 steel
channel bolted to
existing steps
Channel
end
flattened
off
200
Section A A
28
February 2015
100
Top end detail
100mm flat end
for fixing to the
ground
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
11.5
Other considerations that are important when considering wheeling
ramps include:
• wheeling ramps should not obstruct convenient access to the handrail
nor be located in the centre of the steps where they might form a trip
hazard
200
Elevation
50mm dia semi-circular
channel formed in
concrete
Handrail
• where a ramp is constructed in metal, a continuous piece is preferred
• joints in the metal can damage tyres if they are left un-maintained, and
all fixings should be counter sunk and left flush, or ideally fixed from
below
• in some instances timber and stone surfaces blend better with the
original construction
Steps
Section AA
• the considerable effort required from cyclists, especially with luggage
• they are of no benefit to many non-standard cycles such as tricycles,
cargo bikes and cycles with trailers
• signing can be a great benefit, especially if route users can be advised
in advance and given alternative routes as an option
• ramps need regular checking; loose fixings, collections of broken
glass, and damaged sections can all be problematic for cyclists
50mm dia semi-circular
channel formed in
concrete
200
100
Edge of
steps
50
Section A - A
Fig 11.2 Concrete wheeling
ramp, new build
Ramp highlighted with colour,
Ladygrove
Cast concrete strip sited away from
the handrail, Hamilton
Aesthetically pleasing facility is set far enough
away from the wall to enable a cycle to be moved
upright, Nottingham February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
12. Tunnels
12.1
In a similar way that re-opening old railway viaducts and bridges
provides a connection for walking and cycling routes above ground,
railway tunnels provide a similar, subterranean advantage. Where the
bridge or viaduct allows a path to continue across a valley without
significant level changes, the tunnel can provide the connection beneath
hillsides.
Fencing prevents access onto the tunnel entrance, Bath
A cage protects path users from falling rocks, Peak District
Anti-vandal lighting, Dartford
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February 2015
Concreted refuges, Bath
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
12.2
Victorian railway engineers have already undertaken the hardest part,
and in many cases there is no technical reason why today’s engineers
should shy away from re-opening structures that are structurally sound
enough for walking and cycling infrastructure.
12.3
Understanding the challenges faced in developing a potential idea into a
coherent and spectacular route is only a small part of the process, and
the levels of risk that often get associated with such a project can be
widely off the mark. In order for a project to be successful the delivery
team will need to identify the key risks from a technical (engineering &
construction), ecological (bats) and end user perspective.
Electric box in secure cage, Bath
Notice for users, Bath
Fencing to prevent boulders from falling onto path, Peak District
Bat box, Bath
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Technical Risks: Checklist
Understanding the tunnel
•previous inspections/examinations – have they
highlighted any areas of concern?
•have remedial works been
carried out as a result?
•who owns it? - private landowners still have a
responsibility to undertake regular inspections of
any tunnels that they own, and act accordingly
when remedial works are necessary
Understanding the tunnel entrance and
the immediate approaches
• are wingwalls (side walls at the tunnel
entrance) free from vegetation?
• do wingwalls have visible signs of cracking, which
may indicate settlement or movement?
Are tunnel portals free of vegetation or will
remedial works be required to clear away self-seeded
vegetation?
• is the ground above the tunnel entrance suitably
stable or will remedial works be necessary to remove
any loose material, including rocks / boulders?
•what does the tunnel go through?
• bored through rock
• brick lined structure
• are approaches free draining or is
there any sign of standing water?
•single or double bore (i.e. did it have
• has the tunnel been blocked off with access gates,
•does it have ventilation shafts or side adits?
• what materials were used to fill in a tunnel entrance
one or two railway lines through it?)
•historical records
• local interest groups
• local libraries
• old land plans
• national archives
• the presence of statutory designations such as
Sites of Special Scientific Interest (SSSIs)
•are there legally protected species, especially
bats? are there any historic
records of bats in the local area?
32
February 2015
partially or wholly filled in?
– inert or contaminated?
(Mining areas may have utilised colliery waste leading
to methane or coal gas build ups)
• noxious gases from rotting vegetation/timbers?
• asbestos based materials?
• if a tunnel has been filled in around the
entrance is this because of structural issues with the
tunnel mouth (this may not be noted in recent reports)?
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
Inside the tunnel
Construction
• is the tunnel floor dry? if not,
• identify essential works to make a tunnel safe for opening (List A)
identify where water has entered
from and assess implications if left
alone.
• does the tunnel have a drainage
system - can it be traced, repaired
and re-used?
• refuges (niches in the brickwork
that allowed railwaymen to step
away from trains) can be places
for people to hide - can they be
blocked up or made inaccessible?
• are there ventilation shafts? if so
do they have timber / metal sheets
attached that would have deflected
incoming rainwater away from the
shaft - are they securely fixed or
can they be removed?
• identify works that are required, but not essential (List B)
• use organisations that understand what it takes to re-open a tunnel.
Local Authorities’ approach to undertaking such projects is based on
using in-house term contractors and engineering consultancies and can
be very risk averse. Sustrans’ approach uses ex-railway engineers, who
understand railway structures
• understand the difference between essential and non-essential works
– it can greatly reduce the amount of work that you think you need to do,
with an obvious impact on costs
• understand who the client is (it may or may not be the tunnel owner)
• ensure that all known information, including historical records,
is given to the designer and contractor
• ensure that the design team understand the hazards including access,
• does the air flow through
operational plant, materials
the tunnel?
• where there is spalling to brickwork or the mortar face is damaged/
• are any previous
missing, it may not be necessary to repair or replace every little defect
repairs obvious?
• wedged brickwork
• colliery arches
• timber beams
• RSJs
• concrete beams
• is it obvious what defects exist?
• spalled brickwork
• loose masonry
• bulging brickwork
• failure of rock or brick lining
• is the tunnel used by bats
to roost, breed, swarm and/or
hibernate at different times of year?
• if a tunnel is wet when first opened good ventilation during construction
can help to dry it out
• ensure that everyone working in a tunnel understands
that they are working in a confined space environment
• use natural falls where possible to
ensure that paths within tunnels remain dry
• railway tunnels are likely to be lined with soot; a light
clean up to 3m above path level is all that is required, it is
not necessary to clean every last piece of brickwork
• can the former trackbed material be re-used as a path sub base?
• where bats are present can mitigation be put in place to avoid harming
them and do we need to apply for a licence from Natural England?
February 2015
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Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)
Maintenance
Public perception and concerns
• regular inspections are important, with
• how are you going to communicate to the public the length
annual routine inspections and a 6 yearly
principal inspection
• general visual inspections should be
carried out to assess the following:
• path surface
• drainage concerns (especially after
heavy rainfall, or in winter for snow/ice)
• lighting - faults should be reported
immediately
• vegetation - remove any that appears
around wingwalls, abutments and tunnel
portals
• signing
• items identified as non-essential
should be programmed when determined
appropriate
of the tunnel and time it takes to walk/cycle through it?
• how will the public communicate an emergency?
• how will anti-social behaviour be managed
(piped classical music played on a continuous loop has been
used to deter loitering, for example)?
• is personal security an issue?
• is the tunnel lit? all day or timed on/off?
• is there CCTV provision?
• are there contact numbers clearly visible – landlines where
mobile signals are poor?
• emergency access paths are kept free
and usable at all times, ensure that any
locked gates have keys!
• engage emergency services/power
suppliers/local authorities to ensure
that everyone knows what to do if an
emergency arises
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February 2015
• how is a route signed within the local area? good signing will
encourage more use, so that a route becomes self-policing
Sustrans Design Manual • Chapter 14: Bridges and other structures (2014, draft)
13. Key References
Technical Information Note 29: Lighting of Cycle Paths, Sustrans 2012
Technical Information Note 30: Parapet Heights on Cycle Routes,
Sustrans 2012
Bats in Bridges, Bat Conservation Trust (undated)
Bats and Lighting in the UK, Bat Conservation Trust, 2009
Forestry Bridges, Forestry Commission
Path Bridges, Paths for All, 2006
Subways for Pedestrians and Cyclists Layout and Dimensions, TD36/93,
Highways Agency 1993
Design Criteria for Footbridges, BD 29/04, Highways Agency 2004
Inclusive Mobility, DfT, 2002
February 2015
35