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  • The design of steel footbridges

    Corus Construction & Industrial

  • Steel bridgesthe gap

    Below:River Aire footbridge, Leeds, 1993Right:Lowry Footbridge, Manchester

  • The design of steel footbridges 3

    Contents1. Introduction

    2. Features and forms of construction

    for footbridges

    3. Conceptual design and detailing

    3.1 General arrangement

    3.2 Selection of type of construction

    3.3 Trusses and vierendeel girder bridges

    3.4 Steel beam bridges

    3.5 Composite beam bridges

    3.6 Cable stayed bridges

    3.7 Access ramps and stairs

    3.8 Bearings and expansion joints

    4. Design codes, standards and guidance

    4.1 British Standards

    4.2 Departmental standards

    4.3 Railway standards

    4.4 Design of hollow section joints

    4.5 Design of cable stayed and suspension bridges

    4.6 Design of steel and composite bridge beams

    4.7 Dynamic response

    4.8 Protective treatment

    4.9 Steel materials

    5. Flow charts

    6. References

    This guide has been prepared for Corus by:

    D C Iles MSc ACGI DIC CEng MICE Manager Bridges,

    The Steel Construction Institute.

    The author gratefully acknowledges the contributions

    made by Mr W Ramsay, Corus and Mr A C G Hayward,

    Cass Hayward and Partners, during the original

    preparation of the publication.

  • 4 The design of steel footbridges

    Introduction

    Footbridges are needed where a separate pathway has

    to be provided for people to cross traffic flows or some

    physical obstacle, such as a river. The loads they carry

    are, in relation to highway or railway bridges, quite

    modest, and in most circumstances a fairly light

    structure is required. They are, however, frequently

    required to give a long clear span, and stiffness then

    becomes an important consideration. The bridges are

    often very clearly on view to the public and therefore the

    appearance merits careful attention.

    Steel offers economic and attractive forms of

    construction which suit all the requirements demanded

    of a footbridge.

    A fully detailed design can be prepared with other

    contract documents for pricing by tenderers. However, it

    is common practice, particularly for smaller bridges, for

    the detailed design of a footbridge to be included as

    part of a design and construct package. Many

    fabricators are able to provide such a package, using

    methods and details of construction developed to suit

    their particular fabrication facilities and expertise.

    However, the engineer supervising the work still needs

    to be acquainted with the different forms of construction

    which might be used and to be aware of their

    advantages and limitations.

    Longer span bridges and those which form part of a

    larger scheme are likely to be designed in detail by a

    consultant or local authority. Within such an

    organisation the engineer carrying out the design needs

    to be familiar with the particular requirements for

    footbridges, their features and construction details.

    For the engineer in either of these situations, this

    publication presents guidance on the conceptual design

    of steel and composite footbridges, to aid the selection

    of an outline design.

    Typical key features are illustrated in section 3,

    references to codes and sources of further guidance are

    given in section 4. Simple flow charts showing the

    design steps are presented in section 5.

    1. Introduction

  • The design of steel footbridges 5

    Features and forms of construction for footbridges

    2. Features and forms of construction for footbridgesBasic requirements

    Footbridges, like any other bridge, must be long enough to

    clear the obstacle which is to be crossed and high enough

    not to interfere with whatever passes beneath the bridge.

    However, the access route onto the footbridge is often

    much different from what is familiar to the designer of a

    highway bridge: there is no necessity for a gentle horizontal

    alignment (indeed the preferred route may be sharply at

    right angles to the span). Structural continuity is therefore

    less common. The principle span is often a simply

    supported one.

    Provision of suitable access for wheelchairs and cyclists is

    often specified for footbridges. Access ramps must be

    provided and restricted to a maximum gradient. The

    consequent length of ramps where access is from the level

    of the road or rail track over which the bridge spans is

    generally much longer than the bridge itself. The form of

    construction suitable for the ramps may have a dominant

    influence on the final form of the bridge.

    The width of a footbridge is usually quite modest, just

    sufficient to permit free passage in both directions for

    pedestrians. Occasionally the bridge will have segregated

    provision for pedestrians and cyclists, in which case it will

    need to be wider.

    Parapets are provided for the safety of both the pedestrians

    and traffic flow. Footbridges over railway lines are required

    to have higher parapets and be provided with solid panels

    directly over the rail tracks.

    Truss and vierendeel girder beams

    Trusses offer a light and economical form of construction,

    particularly when the span is large. The members of the

    truss can be quite slender and this naturally leads to the

    use of structural hollow sections. Hollow sections have

    been used for footbridges for over 30 years and some

    fabricators have specialised in this form of construction,

    developing techniques and details which utilise them to the

    best advantage.

    Vierendeel girders using hollow section members offer an

    alternative but complementary structural form of similar

    proportion by substituting a rectangular form for the

    triangular arrangement used in trusses.

    Trusses and vierendeel girders are arranged with either

    half-through or through construction. Half-through

    construction is used for smaller spans, where the depth

    needed is relatively shallow. For larger spans, or where the

    truss is clad to provide a complete enclosure for the

    pedestrians, through trusses are used; the top chords are

    then braced together above head level.

    Steel beam bridges

    The simplest method of employing structural steel as the

    prime structural element of a footbridge is to use a pair of

    girders (fabricated or rolled sections), braced together for

    stability and acting as beams in bending, with a non-

    participating walkway surface on top. A typical small

    bridge deck might for example be formed by timbers

    placed transversely across the top of the beams. Precast

    slabs might also be used, without being shear connected

    to the steel and therefore not participating in global

    structural action.

    Left:Bells Bridge, GlasgowRight:Whatmans Field Bridge, Maidstone

  • 6 The design of steel footbridges

    Alternatively the floor might be formed by steel plate,

    suitably stiffened to carry the pedestrian loads, in which

    case the plate could also be made to act structurally as the

    top flange of the steel beams.

    Steel box girder bridges

    Another alternative is to use a small steel box girder. The

    top flange acts as the floor of the bridge, and there are

    usually short cantilevers either side of the box. This form

    has the benefits of good torsional stiffness which can

    simplify support arrangements and clean surfaces which

    minimise maintenance.

    Composite beam bridges

    Composite beams, steel girders with a concrete slab

    acting as both a walkway floor and participating as a

    top flange, are a practical solution for medium span

    footbridges. They are a lighter version of the form of

    composite construction frequently employed in

    highway bridges. Slabs may be cast insitu, though the

    lesser requirements for the shear connection and the

    lighter design loads on the slab allow greater

    opportunity to employ pre-cast slabs. The slab can also

    be cast on the beams in the works or other convenient

    site, since the weight and dimensions are often

    sufficiently modest to permit transport and erection of

    the complete superstructure.

    Although composite construction is usually associated

    with I section girders, a concrete slab can also be used

    with a steel box girder.

    Cable stayed bridges

    In seeking to provide a bridge of light appearance, the

    use of cable stays is found to be very successful. It

    often affords scope to create a visually striking structure

    which provides a landmark or a focus for the area in

    which it is located. Almost any form of construction can

    be used with stays, though when a cable stayed form is

    chosen, the structural requirements are often found to

    be of secondary consideration to the achievement of a

    pleasing appearance.

    Enclosed bridges

    Enclosure of the sides of a footbridge is often called for

    to discourage the throwing of objects from the bridge.

    This is a particular requirement for bridges over railway

    lines. Full enclosure, to the sides and the roof of the

    walkway, is called for in situations where the users are

    to be protected from the environment and where greater

    protection is required over railway lines. Such enclosure

    justifies the use of through truss or vierendeel

    construction. The form of construction will probably be

    dictated by consideration of appearance of the bridge

    and its relationship to adjacent structures. Whilst the

    general principles discussed in this guide are

    applicable, fully enclosed bridges are not specifically

    dealt with in detail in this guide.

  • Decorative features

    In addition to the basic impression made by the form of

    construction, the appearance can be greatly influenced

    by non-structural decorative features, such as parapets

    and handrails. Where particular effects are sought, the

    availability of different patterns for posts, rails, etc,

    should be investigated. Non-structural embellishments

    of supports can also contribute for example a cable

    stayed pylon can be extended to a spike or other feature

    above the level of the topmost stay connection.

    Landmark structures

    It is an increasingly common requirement for footbridges

    in prominent or key locations to be landmark

    structures. Particular attention is given to the

    appearance of the structure and this may result in

    somewhat unusual forms of construction. Such

    structures can be allowed to be marginally less efficient

    (in terms of complexity of fabrication), but if the design

    is well executed the penalties should be small.

    There is more scope for innovative design when the

    structure is not over a road or railway, because the

    requirements for parapet details need not be so

    stringent. Parapets are often the most noticeable feature

    of a footbridge, and the freedom to use more attractive

    forms and more open post and rail arrangements can

    lead to a very pleasing appearance.

    The use of curved arch-type members is currently quite

    popular, as is the use of cable stays. Some recent

    examples are illustrated on this page.

    Since these landmark structures are generally innovative,

    it is inappropriate to try to include design guidance here,

    but the general requirements and design principles given

    in the following sections are largely still applicable.

    The design of steel footbridges 7

    Features and forms of construction for footbridges

    Left:Swansea Sail BridgeBelow:Halfpenny Bridge, SheffieldRight:Millennium Bridge, Gateshead

  • 3. Conceptual design and detailing

    3.1 General arrangementAs a first step, the basic requirements for access and

    safety should be determined. The width and form of

    access needed depends on the expected pedestrian

    traffic flow, though minimum dimensions are adequate in

    most cases.

    For a simple footway, a minimum clear width of 2.0m is

    required by the highways authorities. Railway station

    footbridges can be less wide. To the sides of this

    footway, parapets are required, which should be 1.15m

    high over roads or 1.5m high over railways, the height

    measured from the footway surface in both cases. In

    areas prone to vandalism, a height of 1.8m may be

    required over railways. The resulting minimum cross

    section to be provided is shown in Figure 1. An

    increased parapet height of 1.3m may be needed in

    areas of high prevailing wind and for bridges where the

    headroom under the bridge is more than 10m.

    Where pedestrians and cyclists share the pathway, the

    minimum width of 2.0m may be used for low traffic

    flows but a wider segregated pathway (1.5m + 1.5m

    minimum) may be required for higher traffic flows.

    Segregation can be achieved by a white line, colour

    contrast or difference in surface texture. At the same

    time the minimum parapet height is increased to 1.4m.

    The cross section for a combined pathway is also

    shown in Figure 1.

    Dimensional requirements for footbridges are given in

    Departmental Standard BD 29/03. That document refers

    to BS 7818 for minimum dimensions of parapets.

    The drainage requirements also affect the cross section,

    since kerbs will be needed to prevent run-off where the

    bridge is above a carriageway, a footpath or rail tracks.

    Typically an upstand of 50mm should be provided. This

    upstand can be provided by an edge beam, by the lower

    chord of a truss or by a flat welded to the floor plate.

    Figure 1: Basic sectional dimensions for bridges over highways

    8 The design of steel footbridges

    Conceptual design and detailing

    Footway Cycleway

    1.5m 1.5m

    1.4m

    Marked segregation

    Minimum footway

    2.0m

    1.15m

    Footway + cycleway

    2.0m

    1.4m

  • 4.5m

    5.7m

    Span

    Since there is usually no need to align the approaches

    to a footbridge, the span should normally be arranged

    square to the obstacle it has to cross.

    The minimum span required is that simply needed to

    clear the width of obstacle, carriageway or railway.

    However, the span may be increased in order that the

    supports are positioned far enough from a carriageway

    or rail track to avoid the risk of impact from an errant

    vehicle or derailed train. The supports of light structures

    such as footbridges are particularly prone to the effects

    of impact.

    For footbridges over highways, the span is determined

    by the dimensions of the carriageways, as given in the

    Departmental Standard TD 27/96.

    To avoid the imposition of impact loads the supports

    need to be set back 4.5m from the edge of the

    carriageway (see Figure 2). Where this can be arranged,

    perhaps additionally spanning a footway beside the

    road, the consequent savings in the cost of the

    substructure should be considered. Supports between

    carriageways should also be avoided if possible.

    The space needed for approach ramps and stairs will be

    significant in arranging the layout of a footbridge. This

    may influence the positioning of the bridge and its

    supports, and thus its span.

    Footbridges over railways are mostly required to cross

    two or four tracks, with resulting span of between 10

    and 25m. Where intermediate supports are placed

    closer than 4.5m to the nearest rail, Network Rail require

    the superstructure to be capable of supporting itself if

    one support were to be demolished in an accident.

    Clearance

    Over a highway, the clearance under new footbridges is

    required to be at least 5.7m (TD 27/96). With this

    clearance the superstructure need not be designed for

    impact loads (see Figure 2). If any relaxation on

    clearance were permitted in special cases it is likely that

    impact loads would have to be considered. This would

    be very onerous on the structural design. Clearance over

    railways is specified by Network Rail with a minimum of

    4.640m from rail level. The minimum clearance over

    electrified lines and over lines that might be electrified in

    the future is 4.780m. Greater clearances are required

    near level crossings and where there is free running

    (where the wires are not attached to the bridge).

    Clearly, where access to the bridge has to come from

    carriageway or track level, the rise needed for the stairs

    or ramps is the sum of the clearance plus the

    superstructure construction depth (walkway surface to

    structure soffit). This means that ramps will be long

    (about 120m at each end of the bridge over a road, for a

    1 in 20 grade). It also means that the depth of

    construction (for example the depth of a plate girder)

    can add significantly to the length of ramp, and thus to

    the cost of the whole structure. For this reason, half-

    through construction, with a very shallow construction

    depth, is usually preferred.

    Sufficient vertical camber is needed to ensure drainage

    of the footbridge to the ends, where the run-off can be

    carried to drains or a soakaway.

    Figure 2: Governing dimensions in elevation

    The design of steel footbridges 9

    Conceptual design and detailing

  • 10 The design of steel footbridges

    Conceptual design and detailing

    Stairs and ramps, ChristchurchSpiral ramp, Myton Footbridge, Hull

    Stairs and ramps

    Where access is required from a lower level, stairs and

    ramps must be provided. Stairs are only suitable for able

    pedestrians and it is general policy to provide ramps

    where possible. Such ramps should ideally be no steeper

    than 1 in 20, though gradients of up to 1 in 12 may be

    used for straight ramps where space is limited.

    A ramp can be either a series of straight sections or a

    spiral, depending on circumstances and space available

    (see Figure 3). The space occupied by a ramp is quite

    significant and may well influence the position of the

    bridge.

    A single straight ramp can be used where space and the

    desired access route permit. If the gradient is steeper

    than 1 in 20, the ramp should have intermediate landings

    (i.e. it should be a series of ramps with horizontal

    sections between). Ramps are often arranged in scissor

    fashion (i.e. with a 180 change of direction at an

    intermediate landing).

    Spiral ramps must have a minimum inside radius of

    5.5m (gradient measured 900mm from the inside edge).

    The same limits on gradient apply (i.e. a maximum of 1

    in 20 is desirable, up to 1 in 12 may be acceptable in

    some cases). Spiral ramps are unsuitable for a full 6m

    rise to a footbridge over a highway unless a large radius

    can be accommodated.

    Stepped ramps are sometimes used which, with a

    125mm step and a 1 in 12 slope between, can effectively

    achieve a 1 in 6 gradient. For spiral ramps this gives a

    rise of 6m in under 360 turn.

    Stairs are usually arranged in two or three flights with

    intermediate landings, depending on particular

    arrangements, to comply with normal safety

    requirements. They usually have semi-open risers, for

    lighter appearance. Handrails are provided on the inside

    faces of the parapets on stairs and ramps. Minimum

    widths must be maintained between these handrails.

    Services

    Occasionally the bridge may have to carry a service

    water pipes or electric cables, for example. It should

    normally be arranged that such pipes are supported out

    of sight, on brackets or cross-members between main

    beams for example. If a service is positioned inside a box

    girder, it is better to put it in a duct, so that any

    maintenance to the service does not require entry into the

    box girder. Gas or water pipes should not be sited inside

    a box girder, for safety reasons, unless placed in a steel

    sleeve which runs the length of the bridge.

  • The design of steel footbridges 11

    River Exe Suspension Bridge

    3.2 Selection of type of constructionAs mentioned previously, the depth of construction is

    very important to the overall extent of the footbridge

    where access is from the level of the road or railway

    being crossed. In those circumstances it is usually

    preferable to use a half-through form of construction.

    This usually leads to a selection of a truss or vierendeel

    girder bridge, though half-through plate girder forms such

    as that developed by Network Rail may also be used.

    However, not all bridges are subject to such constraints.

    Some simply cross, for example, a small river, or span

    across a deep cutting. In such cases the depth of

    construction is not so important and steel girders or steel

    composite construction may be employed. When the span

    is long, the dynamic response of the bridge becomes a

    significant consideration, particularly for the lighter all-

    steel bridge. The greater stiffness afforded by truss

    construction may well be advantageous. Alternatively,

    cable stayed construction can be employed.

    Cable stayed forms of construction can rarely be

    justified visually below about 40m. For spans up to

    100m a single pylon on one side of the main span is

    often appropriate, both visually and structurally. Beyond

    about 100m twin pylons should be considered.

    Suspension bridges are very rarely considered these

    days, but may still be chosen for appearance reasons

    when the span exceeds about 70m.

    A summary of approximate span ranges suitable for the

    various types is given in Table 1.

    Table 1

    Span ranges for different types of construction

    Construction type Span range (m)

    Truss 15 to 60

    Vierendeel girder 15 to 45

    Twin steel girders 10 to 25

    Steel girders + steel floor plate 10 to 30

    Steel box girder 20 to 60

    Composite beams 10 to 50

    Arches 25 upwards

    Cable stayed bridge 40 upwards

    Suspension bridge 70 upwards

    13 risers max

    1:20

    1:20

    2m

    Figure 3: Arrangement of typical stairs and ramp

  • 12 The design of steel footbridges

    Conceptual design and detailing

    3.3 Trusses and vierendeel girderbridgesAlthough trusses and vierendeel girders have a different

    structural action, there are many similar features when

    they are constructed of structural hollow section

    members, as used in footbridges. This section deals with

    both types of construction.

    Through and half-through construction

    Trusses and vierendeel girders for footbridges are

    normally arranged with the deck at the level of the

    bottom chord, in either through or half-through

    construction. Half-through construction is used for

    smaller spans, where the depth needed is less than the

    clearance height for people to walk through. For large

    spans, or where the bridge is clad to provide a

    complete enclosure for the pedestrians, through

    construction is used.

    The top chords can then be braced together above

    head level.

    Stability of the top compression chord in half-through

    construction is provided by the U-frame action of the

    side members and the cross-members of the deck. In

    through construction, lateral bracing between the two top

    chords offers a more direct means of stabilising them.

    Below and right:Through truss footbridge

  • The design of steel footbridges 13

    Conceptual design and detailing

    Configuration

    The type of truss usually employed is either a Warren

    truss or a modified Warren truss. Occasionally a Pratt

    truss may be used. The different types are illustrated in

    Figure 4.

    Warren trusses are the simplest form of truss, with all

    loads being carried principally as axial loads in the

    members and with the minimum of members meeting at

    joints. However, the loads which are carried to the

    bottom chords from the walkway floor can lead to

    significant bending in these members when the panels

    are large. A modified warren truss reduces the span of

    these chord members, though the additional vertical

    members add complexity to the fabrication. Pratt trusses

    are used where it is preferred that some members are

    vertical, for example to facilitate the fixing of cladding or

    decorative panels.

    Vierendeel girders have no diagonal members and rely

    on a combination of axial loading and bending to carry

    loads. The stiffness of the girder depends crucially on

    the bending stiffness of vertical and horizontal members

    and on the stiffness of the joints between the two. As a

    consequence they are much heavier, for a given span,

    than a Warren truss. However the appearance, which

    only shows vertical and horizontal lines, in harmony with

    the normal form of parapet (horizontal rails, vertical

    posts and infill), is often considered more pleasing.

    For the largest spans, the vierendeel girder will probably

    be too flexible, though they have been used successfully

    up to 45m span.

    Below:Half-through truss footbridge

    Below:Rutherglen station footbridge

    Figure 4: Types of truss and vierendeel girder

    Pratt truss

    Modified Warren truss

    Warren truss

    Vierendeel girder

  • 14 The design of steel footbridges

    Above:Large-span truss footbridgeLeft:Vierendeel footbridgeRight:Lower chord connection detailFar right:Large-span vierendeel footbridge, A27 Broadmarsh

    Proportions and appearance

    The familiar image of a truss is probably of a heavy-

    looking structure, relatively deep in proportion to span.

    Such trusses were often used for railway bridges.

    However, a truss footbridge can generally be of light

    appearance and of shallow depth/span proportion.

    With half-through construction, the minimum overall

    depth is determined by the parapet height; for a

    crossing over a highway the minimum is about 1.25m.

    For spans over about 30 metres the depth will need to

    be slightly greater, though span/depth ratios in excess

    of 30 can give a pleasing appearance.

    For spans over 50m full through construction will

    probably be necessary. Then the depth is determined by

    internal clearance, which is usually specified as 2.3m

    minimum. To reduce the tunnel effect and to keep the

    top bracing away from casual abuse a depth of about

    3m is needed. Such spans will have a deeper

    span/depth ratio, though the slender members will still

    give an impression of lightness.

    The arrangement of the bracing and the line of the

    parapets are the dominant features which are seen

    by road users. They therefore require careful attention

    and treatment.

    Where the depth of the vierendeel girder is determined

    by parapet height, the top chord can often be used as

    the parapet rail, with suitable infill bars fixed between

    the vertical members. For longer span vierendeel

    girders, where the depth is more than the parapet

    height, parapet panels complete with top rail can be

    fixed inside the rectangular panels of the girder. Where a

    truss is used, the parapet is usually fixed to the inner

    face of the diagonal members. The parapets are less

    conspicuous to road users than the truss members,

    though they are still evident in silhouette.

    Construction depth, from footway surface to underside

    of the truss or girder, is normally quite shallow, not more

    than the depth of the chord members. This contributes

    greatly to the light appearance.

    The top and bottom chords of a truss are usually made

    parallel, but for larger spans a less dominating

    appearance can be achieved by a hog-back

    configuration, with a gentle curve to the top chord

    reducing the depth at the ends of the span.

  • The design of steel footbridges 15

    Conceptual design and detailing

    Members and connections trusses

    Both circular and rectangular structural hollow sections

    are commonly used in trusses. The bottom chord is

    generally rectangular, to facilitate connection with deck

    and cross-members. Rolled sections or flats are

    sometimes used as cross-members or as stiffeners to

    steel floor plates. Chords and diagonals are usually

    arranged with centrelines intersecting where possible.

    Standard welding details have been developed for

    hollow section connections.

    For half-through trusses the connection with

    cross-members at the lower chord requires particular

    attention, since its stiffness and strength are

    fundamental to U-frame action.

    Where the bottom chords are of rectangular section,

    some designers specify plates slotted diagonally across

    the section at the position of the cross-members (Figure

    5) to prevent the chord lozenging or distorting.

    However, cutting slots in the hollow section and welding

    stiffeners adds to the fabrication cost. Research by the

    Steel Construction Institute for Corus (30) showed an

    un-stiffened connection designed to BS 5400: Part 3 to

    have a higher buckling resistance than that calculated

    even when a lower flexibility value is used.

    The failure loads calculated were relatively insensitive to

    the actual value of connection stiffness. This showed

    the use of diagonal stiffeners does not significantly add

    to the global strength of tubular U-frame footbridges.

    Where a steel floor plate is used it normally acts as the

    bracing to the bottom chords, to carry the lateral

    shear (mainly wind forces) back to the supports. If a

    non-participating form of floor is used, cross bracing in

    the plane of the bottom chord, to resist lateral forces,

    must be considered.

    Through trusses, used in longer spans, give lateral

    stability to the top compression chord by means

    of bracing in the plane of the top chord. Such bracing

    will also share in the carrying of any lateral forces,

    especially where the truss is clad on its sides and thus

    subject to significant wind loads. At the ends of the span

    these lateral forces have to be carried down to bearing

    level through portal action or through a braced frame.

  • Members and connections Vierendeel girders

    In footbridges, Vierendeel girders normally use

    rectangular hollow sections for greater stiffness

    and strength at the connections between verticals

    and chords.

    The nature of vierendeel action is that vertical shear is

    carried by shear/bending action of each length of chord,

    and the vertical members are subject to complementary

    horizontal shear and bending. Since shear is highest at

    the ends of the span, the fixed end moments are

    highest there also. The vertical members therefore need

    to be strongest at the ends of the span.

    On the other hand the central portions of the chords

    sustain predominantly axial load, whilst the ends sustain

    predominantly bending load. There is less need to vary

    the size of the chord members, and usually only

    thickness is varied, if at all.

    The consequences are that the vertical members are

    often wider (in the plane of the girder) at the ends of the

    span and are sometimes closer together, variations

    which are clearly visible in silhouette.

    The strength of the joint between chord and vertical

    members must be adequate to transmit the fixed end

    moments. To do this both should have the same width

    (normal to the plane of the girder). Under the higher

    moments on the joints toward the ends of the span a

    simple square joint may have inadequate strength, and

    either triangular fillets (cut from the same section as the

    vertical) or reinforcing plates may need to be added to

    increase stiffness and strength (see Figure 6). The

    appearance of these additions may not always be

    acceptable and heavier sections may be preferred.

    Stability of the compression chord again requires

    U-frame action of the cross section and this again

    requires adequate stiffness and strength of the

    cross-member to vertical connection at the bottom

    chord. Even with the heavier sections usually required

    for a vierendeel girder, it may be necessary to insert

    diagonal plates, as mentioned previously.

    Figure 6: Detail of a haunched joint in a vierendeel girder

    10 thickinsert plateslotted intochord

    100 x 100 10 RHS

    Weldgroundflush

    Figure 5: Detail of diagonal plate through bottom chord

    16 The design of steel footbridges

    Conceptual design and detailing

    Right:Stiffened plate floor constructionFar right:Typical floor construction

  • Floor construction

    The floor of a truss or vierendeel girder footbridge will

    usually be of steel plate, though precast planks have

    been used with trusses. The lighter steel deck is now

    generally preferred.

    The plate, typically 6mm or 8mm thick, is supported on

    and welded to steel cross-members between the

    chords. These cross-members form part of the U-frames

    which stabilise the top chord and are themselves usually

    hollow sections. The plate panels between chords

    and cross-members are divided transversely and

    sometimes longitudinally by stiffeners (usually flats) to

    give added support.

    On top of this plate a waterproof layer is required for

    corrosion protection, and to give a non-slip surface for

    safety. This is usually achieved with a thin membrane

    (which acts both as waterproofing and as a binder) and a

    surface dressing of fine aggregate. The total thickness is

    about 4mm. This surface is often applied in the works

    and does not add significantly to erection weights.

    When precast planks are used it is necessary to provide

    a shelf angle on the inner face of the chords on which

    the planks can sit. It is very important that the joint

    between concrete and steel is properly sealed or it could

    become a moisture and corrosion trap.

    Where drainage over the edges of the bridge is not

    permitted, arrangements must be made to carry

    rainwater to the ends of the bridge and then to drains or

    a soakaway. A vertical curve or longitudinal camber

    should be provided on a bridge which otherwise would

    be level.

    Where rainwater can be allowed to run off the side of the

    bridge (for example over a river), the floor may be slightly

    cambered transversely to facilitate drainage. With

    stiffened thin steel plate decks, care also needs to be

    exercised that panels do not dish between stiffeners and

    allow ponding of water the spacing of stiffeners is

    usually limited for this reason. Weld sizes should be kept

    to a minimum, to reduce distortion from welding.

    (see GN 2.10 (31))

    The design of steel footbridges 17

    Conceptual design and detailing

  • 18 The design of steel footbridges

    Conceptual design and detailing

    Parapets

    Parapets are normally designed to comply with a

    DMRB standard (see section 4.2). The parapet may be

    either a separate item or may be combined with

    structural members.

    For trusses, the parapet is provided as separate units

    fixed to the inside faces of the truss diagonals. The

    diagonals must then be designed to carry lateral loads

    from the parapet, and the parapet rails must be

    designed to span between the diagonals which support

    them. Parapet posts can alternatively be fixed to the

    footway deck, though the attachment would need to be

    strong enough to withstand the overturning moment

    arising from lateral forces on the top rail.

    Where vierendeel girders are used it is convenient to fix

    parapet panels in the rectangular panels of the girders,

    effectively using the vertical members as parapet posts.

    This achieves an integrated appearance and produces a

    slightly lesser overall width of bridge than with separate

    parapets on the inner faces of the girder. The top chord

    of the girder may also function as the top parapet rail, or,

    if it is higher than the required parapet height, a separate

    rail can be provided in addition to the top chord.

    Cladding

    Over rail tracks, the highway and rail authorities require

    that solid non-climbable cladding be provided on the

    inside face of the truss or vierendeel girder. This is

    usually achieved by profiled steel sheeting, rigidised

    aluminium, GRP panels or even flat sheets. Fine mesh

    (maximum 50mm apertures) may be used over non-

    electrified lines. Although the cladding is only required

    over the tracks, a better appearance is often achieved

    by providing the cladding over the full length of the

    span. Great care needs to be exercised in detailing the

    cladding, to avoid the creation of small inaccessible

    sheltered ledges on the top of the lower chord where

    moss and debris can accumulate or which may be used

    for handholds or footholds.

    Left:Parapets in vierendeel girder, HoramRight:In-line splice detailFar right:Erection of Christchurch footpath

  • The design of steel footbridges 19

    Conceptual design and detailing

    Supports

    Trusses and vierendeel girders are supported either on

    bearings (if they span between concrete abutments, for

    example) or directly on top of a simple steel

    substructure without any bearings.

    At abutments the point of support is normally directly

    below the end vertical or diagonal members and thus

    does not give rise to local bending of the chord section.

    Other supports should also preferably be arranged

    similarly. Where it is not convenient to do so, for

    instance when a top landing cantilevers a short distance

    beyond the support columns and the support is midway

    between bracing connections, the bottom chord is

    subjected to bending. It is then common to use a

    heavier chord section over the last one or two panels of

    the truss (see photograph below right).

    Fabrication of trusses

    Fabricators who specialise in hollow section fabrication

    are familiar with all the types of detail needed for truss

    footbridges and have appropriate equipment, such as

    profile cutting equipment for tubulars etc.

    A wide range of sizes of hollow sections is available

    from the rolling mills, but it must be remembered that

    the fabricator has to purchase material for each job,

    either from the mill or from a stockist, and his orders

    may be subject to minimum quantities and premiums for

    small quantities. The designer should therefore try as far

    as possible to standardise his choice of section size and

    material grade.

    Erection

    Fortunately, most footbridges can be fabricated as a

    complete length of the span and then transported, with

    spans up to about 45m. Although fabrications over 27m

    in length require special permission to travel on the public

    highway, most fabricators prefer to complete fabrication

    in the works wherever possible and are familiar with

    arrangements for the movement of long lengths.

    Bolted hollow section flanged joint details can be used

    for site splices, though it may be felt that flange plate

    end connections are somewhat cumbersome in

    appearance. In-line splice details are much less

    obtrusive, but require more effort in design and

    fabrication (see photograph below left). In most cases,

    spans must be complete before lifting, because closure

    or possession periods will be very short.

  • 20 The design of steel footbridges

    Conceptual design and detailing

    3.4 Steel beam bridgesTypes of construction

    Four types of construction are considered in this

    section:

    a pair of steel beams with a non-structural floor on top

    (e.g. timber)

    a pair of steel beams with a structurally participating

    steel floor plate

    a steel box girder

    a half-through plate girder bridge as developed by

    British Rail

    The first three are appropriate where depth of

    construction is not important. The fourth is appropriate

    where minimum construction depth is critical.

    Proportions and appearance

    For the relatively light loading on a footbridge, the depth

    of beam in all cases can be arranged to be about 1/30

    of the span. A typical bridge over a river or canal might

    then have a span of 30m and a beam depth of 1m.

    A simple I-beam bridge with non-structural floor might

    comprise two girders about 1.5m apart on which is fixed

    a floor of, in some instances, timber planks. Parapet

    posts would be fixed to the top flange or the outer face

    of the steel beams.

    Steel girders with a structural participating steel floor

    plate would be of similar overall proportions. Parapets

    would be fixed on top of the floor plate.

    With both forms, the girders can have a clean web over

    their full length, as web stiffeners are needed only at

    supports and on the inner faces for attachment of

    bracing. The structural element therefore looks clean

    and simple. The appearance will be influenced strongly

    by the treatment of the parapet rails, posts and any

    other feature added to the bridge. The use of simple

    parapet details will contribute to a good non-fussy

    overall appearance.

    In some circumstances a distinct curvature in elevation

    (more than would suffice just to aid drainage to the

    ends) will add character to the appearance.

    The use of a steel box girder extends the clean lines to

    the soffit of the bridge. It can be complemented by a

    simple basic parapet or can be contrasted by

    embellishment with ornate fixtures and fittings. Typically

    the box would be about 1.0m wide, with short steel

    cantilevers either side to provide the necessary width.

    Half-through plate girder bridges will usually have their

    U-frame stiffeners on the outside faces and generally

    look more heavy. Nevertheless, the half-through plate

    girder bridge developed by British Rail (see page 22)

    achieves a pleasing appearance.

    Members and connections I-beams/girders

    For economical design, the pair of beams need to be

    braced together to stabilise them against lateral

    torsional buckling. Bracing at several positions in the

    span will be necessary, roughly at 15 to 20 times the top

    flange width to achieve reasonable limiting stress levels.

    Bracing can simply be an X brace with single tie at each

    position, bolted to stiffeners on the inside faces of the

    webs. For the main girders, fabricated I-sections are

    likely to be lighter and more economic than Universal

    Beams. Castellated beams can provide a weight saving

    in some circumstances whilst offering an interesting and

    different appearance.

    Left:Footbridge using rolled sections, SwaleRight:Footbridge with timber deck and parapetsFar right:Box girder footbridge and cycleway, Gablecross

  • The design of steel footbridges 21

    Conceptual design and detailing

    A non-structural deck, such as timber planking, can be

    simply bolted down to the top flange of the I-beams.

    Particular attention should be paid to detailing, to

    minimise crevices where dirt and moisture can

    accumulate.

    In many instances steel plate is used for the floor of the

    bridge. The plate, typically about 6mm or 8mm thick, is

    usually welded to the main girders and can therefore be

    assumed to act structurally with them. Cross-members

    will be required to carry the floor loading to the main

    beams and these are sometimes extended by short steel

    cantilevers outside the beam web, in which case an

    edge beam is provided to give a neat face and to give

    support to the parapet. A thin waterproof wearing

    surface is normally specified, dressed with fine

    aggregate for grip and durability. The surface is often

    applied in the works.

    Members and connections box girders

    Box girders are essentially similar to the paired plate

    girders with steel deck, as described above, except that

    the bottom flange joins the two webs and encloses the

    space between. They are usually considered only for

    spans over about 30m. The thickness of the top flange

    which also forms the floor plate will be determined by

    overall bending strength rather than local floor loading.

    The plate is typically supported by transverse stiffeners

    which cantilever to edge beams. Two or three

    longitudinal stiffeners may be provided to stiffen the floor

    plate when acting as the compression flange of the box.

    Diaphragms are needed at supports and are often

    provided at several positions along the length of the

    girder (typically the third points) to control distortion.

    Large holes will be required in the diaphragms if access

    is required during fabrication or maintenance.

    To improve appearance it is common to use slightly

    sloping webs, creating a trapezoidal cross section.

    The use of steel box girders has the advantage of

    torsional strength and stiffness. They can be used in

    continuous construction to simplify supports or to curve

    the bridge in plan when desired for appearance. In a

    straight bridge, torsional restraint (usually by means of

    twin bearings) is needed only at the ends: a single

    bearing will suffice at intermediate supports, thus

    allowing the use of a single slender column.

    Figure 7: Cross section through a typical box girder footbridge

  • 22 The design of steel footbridges

    Conceptual design and detailing

    Members and connections half through girders

    Half through plate girder footbridges are often used over

    railways. The solid web provides the required screening

    without the need for any non-structural additions. This

    form has developed from the half-through plate girder

    concept often seen in railway bridges. A particular form

    developed by the former Midland Region of British Rail

    is illustrated in photographs shown above. Two features

    to note are: the use of a hollow section as top flange,

    turned through 45 it forms a steeple cope, which

    discourages walking along the flange; the absence of

    any projection of the bottom flange prevents climbing

    along the outer face.

    U-frame action is provided by the flat intermediate

    stiffeners to web and bottom flange. Typically they are

    provided about every 1.5m.

    Parapets

    Where there are no cantilevers the parapet can either be

    fixed to the top flange of the box or to the web of the

    girder. The attachment positions should coincide with

    bracing or cross-members, to provide restraint against

    rotation under lateral loads on the parapet rail.

    Where there are cantilevers, either the posts should

    coincide with the cantilever positions or they should be

    mounted on a torsionally stiff hollow section edge beam.

    Fabrication

    Whether using rolled I-beams or fabricated I-section

    girders, the processes of drilling holes, adding stiffeners

    etc. poses no difficulty to the fabricator. The fabricated

    I-section can either be made using jigs and semi-

    automatic welding or by a T and I automatic welding

    machine. Curvature in elevation is easily achieved with

    fabricated girders, and universal beams can readily be

    curved by specialist bending companies prior to

    fabrication. Fabrication of box sections requires more

    traditional methods, and the completion of the closed

    box makes it almost essential for manual work internally.

    Details should be arranged for ease of access for work

    and inspection.

    Splices

    For spans up to around 40m, it is quite likely that the

    beams would be transported full length and splices

    would not be needed. Over 40m they would be split

    into at least two lengths; site connections would

    normally be bolted.

    Bolted splices are quite conventional, with few problems.

    If a completely clean face is sought,it will be necessary

    to have a site welded joint.

  • The design of steel footbridges 23

    3.5 Composite beam bridgesTypes of construction

    Composite construction is seen in footbridges in two

    forms a concrete slab on top of two I-girders or a

    concrete slab on top of a closed steel box girder. The

    open steel box form with slab which is sometimes used

    in highway bridges is not normally seen in footbridges

    Slabs may be cast insitu, though the relatively modest

    extent of the shear connection and lighter design loads

    on the slab allow greater opportunity to employ pre-cast

    slabs. Such slabs are provided with open pockets to fit

    over the shear connectors. The pockets and the joints

    between slab sections are filled with concrete to create

    the necessary structural continuity.

    Proportions and appearance

    Composite footbridges typically have a span/depth ratio

    of about 20 (depth measured from top of slab to

    underside of girder).

    Short cantilevers outside the lines of the webs will give

    a better appearance, in the same way as they do for

    highway bridges. A small upstand is needed at the

    edges to provide a mounting for the parapets and to act

    as a drainage upstand. A thick edge beam would create

    a rather heavy appearance.

    Members and connections

    Composite construction produces a much heavier

    structure than an all-steel footbridge; the dead

    load accounts for over half of the total load in most

    cases. The extra weight and consequent stiffness of this

    form of construction has the advantage of being less

    responsive to dynamic excitation.

    Where transverse joints between precast units are not

    designed to carry transverse shear, plan bracing will

    also be needed.

    Floor construction

    Reinforced concrete slabs for footbridges are typically

    about 150mm thick. They can be constructed insitu on

    falsework or by using precast slabs.

    Sometimes they can be cast in the fabrication yard, and

    the complete composite structure transported to site

    and erected.

    A waterproofing membrane is required, plus some form

    of durable wearing surface. A combined membrane and

    wearing course with aggregate dressing, similar to that

    used on steel decks, can be used.

    Parapets

    As for other forms of construction, parapets must

    comply with DMRB or Network Rail requirements.

    The parapet posts are fixed to the concrete slab or edge

    beam with conventional holding down bolts.

    Opposite page:Half through plate girder footbridge, Network RailAbove:Composite curved I beam footbridge, Washington

  • 24 The design of steel footbridges

    Conceptual design and detailing

    3.6 Cable stayed bridgesFootbridges carry only relatively light loading. However,

    when the main span is long, the requirements of

    supporting its own dead load and of providing a

    sufficiently stiff structure lead toward a much more

    substantial structure than would seem appropriate for a

    mere footbridge. As a result, an increasingly popular

    solution for longer spans is the use of a cable stayed

    arrangement. This effectively divides the span into shorter

    lengths, for which lighter beams can be used. The pylons

    for these bridges also add a strong visual feature which is

    often welcomed.

    Types of construction

    Cable stays can be used with any of the forms of

    construction previously described, though to complement

    the light appearance, a slim form of deck construction is

    likely to be more appropriate for all except the largest

    spans. Supports can be provided to the main beams at

    about 10m to 15m spacing, which facilitates the use of a

    slender deck.

    For most footbridges, twin planes of cable stays will

    normally be used, one to each side of the bridge deck. A

    pylon at one end of the main span will suffice up to about

    100m span. Very long spans may require the use of pylons

    at both ends. 'A' frame pylons are popular, with the two

    stay planes inclined. Alternatively, individual pylon legs for

    each cable plane can be arranged, or a goal-post

    arrangement can be used; the stays can then lie in a

    vertical plane.

    Usually, at least two forestays should be provided in each

    plane a single stay is hard to justify on economic or

    appearance grounds. The minimum span for a cable

    stayed bridge with two forestays is thus around 35m.

    A single backstay is usually sufficient, anchored to the

    girder at the abutment which supports the end of the

    backspan. Further backstays are only needed if the

    backspan is long and requires intermediate support. The

    stays are normally anchored at floor level to longitudinal

    beams. The beams need to be stiff and strong enough to

    span between anchor points and they may need to be

    fairly deep. A lighter appearance, with shallow beam/floor

    depth, might be achieved by using a vierendeel girder and

    half-through construction. Footbridge pylons are usually

    steel box or circular sections, for slender appearance,

    ease of construction and economy.

    Members and connections

    The cable stays will normally be made from wire rope or

    spiral strand. Strands are made by winding together, or

    laying up, a number of galvanised steel wires. Ropes are

    made up of a number of small strands wound together.

    Ropes and spiral strands have a lower effective modulus

    than solid steel. Parallel wire strands are also available.

    Advice should be sought from specialist manufacturers on

    the selection of strands.

  • The design of steel footbridges 25

    Conceptual design and detailing

    In the dead load condition the stays are effectively

    prestressed. It is important to calculate accurately the

    stretch of the stays in the dead load condition, so that

    the correct geometry of the structure is achieved.

    Provision should be made for length adjustment in the

    stays, to accommodate tolerances and errors.

    Stays must obviously be sufficiently strong to support

    the beams, but often more significant for small bridges

    is the need to provide sufficiently stiff supports to the

    beams and to avoid slack stays which will be easily

    vibrated.

    With twin planes of stays, the natural arrangement for

    the deck structure is with main beams at either edge, to

    which the stays are attached. The floor then spans

    transversely between the beams. A single plane of stays

    can only be used where a torsionally stiff box girder is

    provided; the stays would be attached on the centreline

    of the bridge. This is not normally convenient for a

    single footway.

    As well as provision for adjustment in length during

    installation, attachment details should also be arranged

    such that any stay can be replaced if need be. It is good

    practice to make sure that the anchorages are as strong

    at ULS as the breaking load of the stays.

    Under the action of live load the stays provide stiff

    support to the main beams and they thus behave

    essentially as continuous beams. Axial load is also

    transmitted to the beams by the stays, so the beams

    must be designed for the combined load effects.

    For very long spans, the deflection under load changes

    the geometry of the structure. If the sag of the stays is

    significant they will act as non-linear springs. Both these

    effects should be taken into account in the analysis.

    Computer programs are available which automatically

    take account of the non-linear effects of varying

    geometry under load.

    Whilst ropes and strand can last the life of the bridge,

    experience has shown that they should be

    inspected from time to time to check for corrosion and

    fatigue, particularly at the lower ends. The stay

    anchorages should be accessible for such inspection

    and maintenance. The design should also be such that

    any one stay can be removed and replaced.

    Dynamic response

    Cable stayed bridges are relatively flexible and are more

    prone to oscillation under wind or under deliberate

    excitation by users. An all-steel construction results in a

    very low level of structural damping, which can allow the

    oscillations to grow significantly. The dynamic response

    of the bridge should therefore be checked carefully.

    Artificial damping, such as tuned mass dampers, can be

    provided if necessary.

    Floor construction

    Deck construction is usually of stiffened steel plate,

    though timber or reinforced concrete are sometimes

    used instead.

    Far left:Cable stayed I beam footbridge, CumbernauldLeft:Royal Victoria Dock Bridge, LondonRight:Cable stay anchorage

  • 3.7 Access ramps and stairsWhere approach ramps or stairs are needed they are

    usually structurally independent, except for the need to

    be supported at the top end either on the footbridge

    superstructure or on a common substructure support.

    They can therefore be of a structurally different form.

    However, it is generally preferable to achieve harmony

    of appearance between the two and to use a similar

    construction form.

    Stairs usually require, at most, one intermediate support

    beneath the landing at mid-flight. Ramps require more

    supports and indeed are small bridges themselves. Even

    for ramps, the number of intermediate supports should

    be kept as small as possible, with spans of at least 10m.

    Supports should also be as simple as possible a

    T-shaped column and crosshead should be sufficient

    in most cases (provided that resistance to impact is

    not necessary).

    Where supports may be subject to impact loads, they

    will need to be significantly more substantial. The

    foundations will also have to be larger. In these

    circumstances the designer can choose either

    reinforced concrete columns or a robust steel structure.

    Since landings are nominally level, care needs to be

    exercised to avoid ponding of water and accumulation

    of debris. Extra drain holes in these areas together with

    a small fall will suffice.

    Handrails must be provided on the inside faces of

    parapets on stairs and ramps, for safety reasons. A

    clear gap of at least 40mm is desirable between the rails

    and any adjacent members.

    Stairs normally have semi-open risers. Fully open risers

    are not permitted by BD 29/03.

    At the bottom of flights of stairs, details should be

    chosen which avoid acute corners, since they can trap

    debris. To avoid this, stairs can be supported just above

    the bottom of the flight, so that there is a clear gap

    between the underside of the stringers and ground level.

    26 The design of steel footbridges

    Conceptual design and detailing

    Below:Stairs showing open treads and handrailsRight:Scissor ramp

  • 3.8 Bearings and expansion jointsThe provisions for restraint or the accommodation of

    movement due to expansion or other reasons depends

    very much on the general arrangement of the bridge,

    ramps and stairs.

    When the bridge spans between bankseats or

    abutments, expansion joints are needed, and the

    structure will sit on bearings. At one end the bearings

    may be fixed longitudinally, but if laminated bearings are

    used, both ends can be 'free', as long as the bearings

    can transmit any longitudinal forces.

    Expansion joints need to accommodate movement

    ranges of about 20mm, depending on span. Even at

    ends which are longitudinally restrained there has to be

    some provision for movement at deck level, owing to

    rotational movements under live load.

    For footbridge expansion joints, a simple detail should

    be chosen, one which does not collect dirt or debris and

    which can be dismantled for maintenance if required. A

    simple leaf plate fixed to the bridge on one side and

    sliding on a second plate on the fixed side can usually

    be arranged in most circumstances. Particular attention

    should always be given to the avoidance of steps facing

    uphill, even as little as 5mm, since they always tend to

    accumulate material washed down by run-off.

    Where the bridge spans between steel column supports,

    no bearings are needed. The bridge is simply bolted

    down to the tops of the columns. Expansion is

    accommodated by flexing of the columns and no

    expansion joints are needed.

    Consideration should be given to fixing long ramps at

    the bottom end. Maximum longitudinal movement at the

    far end therefore occurs where the columns are tallest

    and most able to accommodate it.

    Stairs should preferably be fixed at the bottom and

    bolted to column supports. This effectively provides a

    restraint for any ramp or bridge connected to the top of

    a straight flight.

    For light all-steel bridges, all support details, bearings or

    direct connections to columns, should be designed to

    resist at least a nominal uplift.

    The design of steel footbridges 27

    Conceptual design and detailing

    Below:Expansion joint leaf plateRight:End bearing box girder

  • 28 The design of steel footbridges

    Design codes, standards and guidance

    4. Design codes, standards and guidance

    4.1 British StandardsIn most circumstances, the British Standard BS 5400 (1)

    will apply to the design and construction of footbridges.

    In some cases, possibly where the bridge is connected

    to a building, BS 5950 (2) might be called for.

    For design of steel and composite structures, the

    following Parts of BS 5400 are applicable

    Part 2 Specification for loads

    Part 3 Code of practice for design of steel bridges

    Part 4 Code of practice for design of concrete bridges

    Part 5 Code of practice for design of composite bridges

    Part 6 Specification for materials and workmanship, steel

    These codes cover all aspects of design for footbridges

    of beam and truss construction. Design of tubular joints

    is not covered in detail within Part 3 see section 4.4

    for further guidance. Similarly, the design of cable stays,

    the strands and their anchorages, are not covered by

    these codes refer to section 4.5 for guidance.

    Dimensional and safety requirements for stairs are given

    in BS 5395 (3). These requirements are amended slightly by

    the departmental standard for footbridges.

    4.2 Departmental standardsThe requirements of the four UK highways authority (the

    Highways Agency, the Scottish Executive, the Welsh

    Assembly Government and the Department for Regional

    Development Northern Ireland) are set out in the Design

    Manual for Roads and Bridges (DMRB). This manual is a

    collection of individual standards (BD documents) and

    advice notes (BA documents).

    Each of the design code parts of BS 5400 is

    implemented by a BD standard (4), and some of

    these standards vary certain aspects of the part that

    they implement (notably BD 37 for Part 2 and BD 16 for

    Part 5). For footbridges, a particular point to note is that

    the requirements in relation to loads resulting from

    collision of vehicles with the structure have been

    significantly modified. The impact loads and the

    circumstances in which they should be applied are

    specified in BD 60 & BD 37 (the DMRB version of BS

    5400 Part 2) and an amendment to it. The provisions

    relate to the impact loads on supports located within

    4.5m of the edge of the carriageway and to

    superstructures which have less than 5.7m clearance

    above the surface of the carriageway.

    Other standards and advice notes also relate to the

    design of footbridges. Design criteria for footbridges are

    given in BD 29 (5). Highway cross sections and headroom

    are given in TD 27 (6). Selected information from these

    two documents is included in section 3. Standard TD 27

    specifies a minimum clearance for footbridges of 5.7m.

    This avoids the necessity of applying the impact

    requirements of BD 37 on the superstructure, which

    would be particularly onerous on a light structure such

    as a footbridge.

    Where supports need to be close to the edge of the

    carriageway, they are required to be provided with

    protective plinths and designed for impact loads. Where

    they can be kept back from the carriageway, perhaps to

    span a footway beside the road, the consequent savings

    in the cost of the substructure should be considered.

    Supports between carriageways should also be avoided

    (unless they can be located more than 4.5m from the

    road, which is not usually feasible).

    The design of parapets on footbridges is referred by

    BD 29 to the Interim Rules for Road Restraint Systems

    IRRRS). The IRRRS (7) is a Highways Agency document,

    not currently part of the DMRB, although it does state

    that it supersedes a number of DMRB documents, such

    as the earlier BD 52/93. The IRRRS refers to BS 7818 (8),

    which gives dimensional requirements, design

    requirements and a specification for construction of

    metal parapets, and it specifies the design loading

    classes for rails, posts and infill.

    4.3 Railway standardsNetwork Rail are particularly concerned with prevention

    of unauthorised access and are legally obliged to fence

    its boundaries. Network Rail and the Railway Safety and

    Standards Board also have more stringent requirements

    in relation to collision loads. Reference should be made

    to GC/RC5510: Recommendations for the Design of

    Bridges (27). The following comments are based on advice

    given in recent projects.

  • The design of steel footbridges 29

    Design codes, standards and guidance

    In considering the prevention of unauthorised access,

    not only must the pedestrian face of the bridge be

    designed to be non-climbable, it must also be

    impossible to climb along the outer face from the ends

    of the bridge this usually means that trusses are clad

    either side of the diagonals at the ends. The top flanges,

    chords or parapets must be arranged so that they are

    impossible to walk along.

    The zone within 4.5m of the outermost running rail is

    considered a danger zone; if any support is located

    within that zone, collision effects must be considered.

    Any substructure column must be able to withstand an

    impact load, and the superstructure must be able to

    continue to carry some live load without support from

    the column. Design recommendations are given in

    GC/RC5510.

    4.4 Design of hollow section jointsThe design of hollow section joints is not fully covered

    by the requirements of BS 5400: Part 3. There is

    however extensive background research into the

    behaviour of tubular joints and various documents have

    been published which provide guidance.

    For triangulated structures, where the joints transmit

    essentially axial loads from one member to another, the

    design of the joint involves checks on (a) the adequacy

    of the welds at the end of the member and (b) the

    bending of the walls of the hollow sections (which are

    subjected to out of plane forces).

    Guidance literature is available both for circular sections

    and for rectangular sections. General guidance is given

    in CIDECT publications (9), (10) & (11) and guidance in relation

    to BS 5950: Part 1 is given in a Corus publication. (12)

    Design rules in both of these documents may be applied

    using partial factors appropriate to BS 5400. Similar

    rules will be included in EN 1993-1-8 (13).

    The extent of guidance on the design of joints for the

    moments associated with vierendeel action (or with

    U-frame action) is more limited, though there has also

    been research on this topic. A stiffer and more efficient

    joint is achieved when the bracing member is the same

    width (normal to the moment plane) as the chord

    member. Design guidance for this type of joint can also

    be found in a Corus publication (12). Adequacy of both

    the bracing member and the chord member must be

    checked. If necessary, reinforcement of the joint can

    be designed.

    4.5 Design of cable stayed andsuspension bridgesFor general guidance on the design of cable stayed

    bridges, reference should be made to standard texts,

    such as Walther (14) or Troitsky (15). These are

    comprehensive books, but they do include specific

    comment on footbridges with illustrated examples.

    The provisions of BS 5400 do not cover in detail the

    design of wire ropes or similar elements, nor is there any

    other appropriate national code. The designer therefore

    needs to base his detailed design on an empirical

    approach, based on load effects calculated in the usual

    manner according to BS 5400 and adopting the general

    objectives of the code.

    Details of the specification of wire ropes and strands

    can be found by reference to BS 302 (16), and of the

    sockets by reference to BS 463 (17). The cold drawn wire

    used for ropes and strands does not have a linear

    stress/strain relationship, with a definite yield plateau,

    as does structural steel. The relationship is generally

    smooth, with decreasing tangent modulus as load

    increases. Design of stays has therefore been based

    traditionally on permissible stresses calculated by

    dividing the ultimate or breaking strength by a suitably

    large factor (i.e. a working stress philosophy). In the

    absence of formal codes on a limit state basis, division

    of this strength by a partial factor m of about 2.0 atULS, in conjunction with normal values of

    1and

    3

    gives results consistent with the traditional approach.

    Guidance on the design of suspension bridges can be

    found in texts such as Pugsley (18). The tensile elements

    may be wire rope or strand, as for cable stayed bridges,

    though high tensile steel rods may be used for the main

    tension members.

  • 30 The design of steel footbridges

    Design codes, Standards and Guidance

    4.6 Design of steel and compositebridge beamsGuidance on the design of composite highway bridges

    is given in a series of publications by The Steel

    Construction Institute (19). These can be used as general

    guidance in the design of footbridges in accordance

    with BS 5400, both for composite beam and all-steel

    beam designs.

    Guidance on a wide range of practical aspects related to

    steel bridge construction is given in a series of Guidance

    Notes produced by the Steel Bridge Group (31).

    4.7 Dynamic responseLimitations on the dynamic response of footbridges are

    given in HA standard BD 37. The vertical natural

    frequency of many footbridges will be below 5Hz and

    the response must be checked. If the horizontal natural

    frequency is less than 1.5Hz, checks must be made for

    possible lateral excitation.

    The susceptibility of a footbridge to aerodynamic

    excitation has to be checked in accordance with

    BD 49 (20). Bridges under 30m span are unlikely to be

    susceptible. Detailed rules are given in BD 49 for

    bridges that are susceptible.

    4.8 Protective treatmentFor bridges subject to highways authority requirements,

    the protective treatment specifications should be

    selected from those listed in the guidance notes to the

    Specifications for Highway Works (SHW) (21), (22). When

    using those notes, access conditions should normally

    be taken as difficult, which will result in use of metal

    spray for the first coat. Galvanising may be suitable for

    small components, such as parapets.

    For Network Rail owned bridges, the protective

    treatment and walkway surfacing must comply with

    Network Rail line standard RT/CE/S/039 (28). Advice is

    given in RT/CE/C/002 (29).

    For other bridges, the HA specifications, or alternatives,

    may be used, with the clients agreement.

    In some circumstances, Weather Resistant Steels might

    be used, provided that environmental constraints can be

    met. (23), (24)

    4.9 Steel materialsSteel material for plates, rolled sections and structural

    hollow sections is covered by British Standards

    EN 10025, EN 10210 (25). Information about the products

    available from Corus (26) can be obtained from the Corus

    Construction Centre. Contact details are on the back of

    this brochure.

  • The design of steel footbridges 31

    Flow charts

    (Figure 5.2) (Figure 5.3) (Figure 5.4) (Figure 5.5)

    Trusses and vierendeel girders Steel beams Composite beams

    Choose structural form

    Determine geometricconstraints

    Scheme-specific details

    Cable stayed bridges Ramps and stairs

    Figure 5.1: Flow diagram for the design of footbridges

    5. Flow charts

    DMRB Standards for footbridges

    DMRB Standards for highway

    cross section and headroom

    Far left:Renaissance Bridge, BedfordLeft:Smithkline Beecham, Marlow

  • 32 The design of steel footbridges

    Flow charts

    Check adequacy at ULS

    Check as a truss

    Global analysis

    Global analysis

    Determine effectivelengths

    Check adequacy at ULS

    Determine effectivelengths Check U-Frame action

    Check adequacy of lateral bracing

    Tension members

    Compressionmembers

    Longitudinal effects Lateral effects

    Tension members

    Compressionmembers

    Triangulatedtruss?

    Strength adequate?

    Strength adequate?

    Strength adequate?

    Strength adequate?

    Strength adequate?

    Slender orcompact?

    Check adequacy at ULS

    Satisfactory

    Check adequacy at SLS

    Yes

    * For in-plane buckling, use the length between intersections (a); for out of plane buckling use (a) if there are effective lateral restraints or use 12.5.1 otherwise.

    12.2.310.6.210.6.3

    12.2.3No

    Yes

    10.6.1

    12.412.5

    11.5.29.9

    I=a*12.5.1

    Yes 10.6.210.6.39.9

    Yes

    Yes

    12.211.5.1

    12.1 12.6

    12.5

    Yes

    Yes No

    12.3

    Check adequacy at ULS

    Figure 5.2: Flow chart for trusses and vierendeel girders

    Check combinedbending and axial

    effects

  • The design of steel footbridges 33

    Flow charts

    Check ULS momentand shear capacities

    Satisfactory

    YesNo

    Yes

    9.14

    9.9.8

    9.9

    9.69.79.8

    9.4

    9.109.11

    9.169.17

    Yes

    No Yes

    Figure 5.3: Flow chart for steel beams

    Check adequacy at SLS

    Check bearingstiffeners

    Unsymmetriccompactsection?

    Check diaphragms and crossframes

    All strengthsadequate?

    All strengthsadequate?

    Determine limitingstresses for LTB

    Determine effectivesection

    Determine limitingstresses and check

    capacities

    Box girder?

    Global analysis

  • 34 The design of steel footbridges

    Flow charts

    Satisfactory Satisfactory

    9.9.89.9.5.2

    Yes

    9.14

    5/5.2.4.25/5.2.64/4.1.1.1

    No

    Yes

    5/6.1.24/4.8.3

    9.9

    Figure 5.4: Flow chart for composite beams

    All strengthsadequate?

    Check slab adequacy at ULS

    Check bearing stiffeners

    Unsymmetriccompact I-beam?

    Check slab adequacy at ULS

    Check beam adequacy at ULS

    Global analysis

    Check beam adequacy at SLS

    Yes

    Figure 5.5: Flow chart for cable stayed bridges

    Determine dead loadprestress in stays

    Check adequacy of cable stays

    Check local effects at cable

    anchorages

    Check adequacy of pylon

    Check adequacy of members as

    trusses or beams

    Global analysisNon-linear analysis ifdeflections or DL sag of stays are significant

    All strengthsadequate?

    Include effects during replacement

    of each stay

  • The design of steel footbridges 35

    References

    6 References1. British Standards Institution

    BS 5400: Steel, concrete and composite bridges Parts 1 to 10,BSI, London (various dates)

    2. British Standards InstitutionBS 5950, Structural use of steelwork in building, BSI, London

    3. British Standards InstitutionBS 5395, Stairs, ladders and walkways, BSI, London

    4. Highways AgencyDesign manual for roads and bridges, Volume 1 Section 3:BD 13, Design of steel bridges: use of BS 5400 Part 3;BD 16, Design of composite bridges:use of BS 5400: Part 5;BD 37; Loads for highway bridges,BD 60; The design of highway bridges for vehicle collision loads,The Stationery Office

    5. Highways AgencyDesign manual for roads and bridges, Volume 2, Section 2, BD 29Design criteria for footbridges, The Stationery Office

    6. Highways AgencyDesign manual for roads and bridges, Volume 6 Section 1, TD 27Cross-sections and headroom, The Stationery Office

    7. Highways AgencyInterim Requirements for Road Restraint Systems (IRRRS), TheHighways Agency, 2002 (contact the Highways Agency for copies)

    8. British Standards InstitutionBS 7818:1995 Specification for pedestrian restraint systems inmetal

    9. CIDECTDesign guide for circular hollow sections (RHS) underpredominantly static loading, Verlag TV, Cologne, 1991

    10. CIDECTDesign guide for rectangular hollow sections (RHS) joints underpredominantly static loading, TV, Cologne, 1992

    11. CIDECTStructural stability of hollow sections, Verlag TV, Cologne, 1992

    12. Corus TubesDesign of SHS welded joints, CT16, Corus Tubes, Corby 2001

    13. British Standards InstitutionprEN 1993-1-8, Design of Steel Structures, Design of Joints,December 2003

    14. Walther, R. et al,Cable stayed bridges, Thomas Telford, London, 1988

    15. Troitsky, M. S.,Cable-stayed bridges, BSP, Oxford, 1988

    16. British Standards InstitutionBS 302, Stranded steel wire ropes, BSI, London

    17. British Standards InstitutionBS 463: Part 2:1970 Specification for sockets for wire ropes (metric units), BSI, London

    18. Pugsley, A.The theory of suspension bridges, Edward Arnold, London, 1957

    19. Iles, D. C.Design guide for composite highway bridges (P289)Design guide for composite highway bridges: Worked examples (P290) The Steel Construction Institute, 2001

    20. Highways AgencyDesign manual for roads and bridges, Volume 1, Section 3, BD 49,Design rules for aerodynamic effects on bridges, The StationeryOffice

    21. Highways AgencyManual of contract documents for highway works, The StationeryOffice; Volume 1: Specifications for highway works series 1900,Protection of steel against corrosionVolume 2: Notes for guidance on the specification for highwayworks,Series NG1900, Protection of steelwork against corrosion

    22. CorusCorrosion Protection of Steel Bridges, 2002

    23. Highways AgencyDesign manual for roads and bridges, Volume 2, Section 3, BD 7,Weathering steel for highway structures, The Stationery Office

    24. CorusWeathering Steel Bridges, 2002

    25. British Standards InstitutionBS EN 10025: 2004, Hot rolled products of structural steels. BS EN 10210, Hot finished structural hollow sections of non-alloyand fine grain structural steels, Part 1: 1994 Technical deliveryrequirements.

    26. CorusProduct & Technical brochuresStructural sectionsStructural platesStructural hollow sections

    27. Railway Safety and Standards BoardGroup StandardGC/RC5510: Recommendations for the Design of Bridges

    28. Network RailLine StandardRT/CE/S/039; Specification RT98 - Protective Treatment forRailtrack Infrastructure

    29. Network RailLine StandardRT/CE/C/002: Application and Reapplication of protectivetreatment to Railtrack Infrastructure

    30. Corus TubesConnection flexibility in tubular U frame footbridges RT 451, December 1994

    31. Evans, J. E. and Iles, D. C.Steel Bridge Group: Guidance notes on best practice in steel bridgeconstruction (P185), The Steel Construction Institute, 2002

  • Care has been taken to ensure that thisinformation is accurate, but Corus Group plc,including its subsidiaries, does not acceptresponsibility or liability for errors orinformation which is found to be misleading.

    Copyright 2005Corus

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    English language version CD:3000:UK:01/2005