Design of Steel Footbridges 2005

Corus Construction & Industrial

The design of steel footbridges

Steel bridges the gap Below: River Aire footbridge, Leeds, 1993 Right: Lowry Footbridge, Manchester

Contents

This guide has been prepared for Corus by:

1. Introduction

D C Iles MSc ACGI DIC CEng MICE Manager – Bridges,

2. Features and forms of construction

The Steel Construction Institute.

for footbridges 3. Conceptual design and detailing

The author gratefully acknowledges the contributions

3.1 General arrangement

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

3.2 Selection of type of construction

Cass Hayward and Partners, during the original

3.3 Trusses and vierendeel girder bridges

preparation of the publication.

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


3

Introduction

1. Introduction Footbridges are needed where a separate pathway has

Longer span bridges and those which form part of a

to be provided for people to cross traffic flows or some

larger scheme are likely to be designed in detail by a

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

consultant or local authority. Within such an

are, in relation to highway or railway bridges, quite

organisation the engineer carrying out the design needs

modest, and in most circumstances a fairly light

to be familiar with the particular requirements for

structure is required. They are, however, frequently

footbridges, their features and construction details.

required to give a long clear span, and stiffness then becomes an important consideration. The bridges are

For the engineer in either of these situations, this

often very clearly on view to the public and therefore the

publication presents guidance on the conceptual design

appearance merits careful attention.

of steel and composite footbridges, to aid the selection of an outline design.

Steel offers economic and attractive forms of construction which suit all the requirements demanded

Typical key features are illustrated in section 3,

of a footbridge.

references to codes and sources of further guidance are given in section 4. Simple flow charts showing the

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.

4


design steps are presented in section 5.

Features and forms of construction for footbridges

2. Features and forms of construction for footbridges Basic requirements

Truss and vierendeel girder beams

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

Trusses offer a light and economical form of construction,

clear the obstacle which is to be crossed and high enough

particularly when the span is large. The members of the

not to interfere with whatever passes beneath the bridge.

truss can be quite slender and this naturally leads to the

However, the access route onto the footbridge is often

use of structural hollow sections. Hollow sections have

much different from what is familiar to the designer of a

been used for footbridges for over 30 years and some

highway bridge: there is no necessity for a gentle horizontal

fabricators have specialised in this form of construction,

alignment (indeed the preferred route may be sharply at

developing techniques and details which utilise them to the

right angles to the span). Structural continuity is therefore

best advantage.

less common. The principle span is often a simply supported one.

Vierendeel girders using hollow section members offer an alternative but complementary structural form of similar

Provision of suitable access for wheelchairs and cyclists is

proportion by substituting a rectangular form for the

often specified for footbridges. Access ramps must be

triangular arrangement used in trusses.

provided and restricted to a maximum gradient. The consequent length of ramps where access is from the level

Trusses and vierendeel girders are arranged with either

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

half-through or through construction. Half-through

generally much longer than the bridge itself. The form of

construction is used for smaller spans, where the depth

construction suitable for the ramps may have a dominant

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

influence on the final form of the bridge.

truss is clad to provide a complete enclosure for the pedestrians, through trusses are used; the top chords are

The width of a footbridge is usually quite modest, just

then braced together above head level.

sufficient to permit free passage in both directions for pedestrians. Occasionally the bridge will have segregated

Steel beam bridges

provision for pedestrians and cyclists, in which case it will

The simplest method of employing structural steel as the

need to be wider.

prime structural element of a footbridge is to use a pair of girders (fabricated or rolled sections), braced together for

Parapets are provided for the safety of both the pedestrians

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

and traffic flow. Footbridges over railway lines are required

participating walkway surface on top. A typical small

to have higher parapets and be provided with solid panels

bridge deck might for example be formed by timbers

directly over the rail tracks.

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: Bell’s Bridge, Glasgow Right: Whatman’s Field Bridge, Maidstone


5

Alternatively the floor might be formed by steel plate,

Although composite construction is usually associated

suitably stiffened to carry the pedestrian loads, in which

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

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

with a steel box girder.

top flange of the steel beams. Cable stayed bridges Steel box girder bridges

In seeking to provide a bridge of light appearance, the

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

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

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

often affords scope to create a visually striking structure

usually short cantilevers either side of the box. This form

which provides a landmark or a focus for the area in

has the benefits of good torsional stiffness which can

which it is located. Almost any form of construction can

simplify support arrangements and clean surfaces which

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

minimise maintenance.

chosen, the structural requirements are often found to be of secondary consideration to the achievement of a

Composite beam bridges

pleasing appearance.

Composite beams, steel girders with a concrete slab acting as both a walkway floor and participating as a

Enclosed bridges

top flange, are a practical solution for medium span

Enclosure of the sides of a footbridge is often called for

footbridges. They are a lighter version of the form of

to discourage the throwing of objects from the bridge.

composite construction frequently employed in

This is a particular requirement for bridges over railway

highway bridges. Slabs may be cast insitu, though the

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

lesser requirements for the shear connection and the

walkway, is called for in situations where the users are

lighter design loads on the slab allow greater

to be protected from the environment and where greater

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

protection is required over railway lines. Such enclosure

be cast on the beams in the works or other convenient

justifies the use of through truss or vierendeel

site, since the weight and dimensions are often

construction. The form of construction will probably be

sufficiently modest to permit transport and erection of

dictated by consideration of appearance of the bridge

the complete superstructure.

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.

6


Features and forms of construction for footbridges

Left: Swansea Sail Bridge Below: Halfpenny Bridge, Sheffield Right: Millennium Bridge, Gateshead

Decorative features

The use of curved arch-type members is currently quite

In addition to the basic impression made by the form of

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

construction, the appearance can be greatly influenced

examples are illustrated on this page.

by non-structural decorative features, such as parapets and handrails. Where particular effects are sought, the

Since these landmark structures are generally innovative,

availability of different patterns for posts, rails, etc,

it is inappropriate to try to include design guidance here,

should be investigated. Non-structural embellishments

but the general requirements and design principles given

of supports can also contribute – for example a cable

in the following sections are largely still applicable.

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.


7

Conceptual design and detailing

3. Conceptual design and detailing 3.1 General arrangement

Minimum footway

As a first step, the basic requirements for access and safety should be determined. The width and form of

2.0m

access needed depends on the expected pedestrian traffic flow, though minimum dimensions are adequate in 1.15m

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 Footway + cycleway

required over railways. The resulting minimum cross section to be provided is shown in Figure 1. An

2.0m

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.

1.4m

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. Marked segregation

Dimensional requirements for footbridges are given in Departmental Standard BD 29/03. That document refers

1.5m

1.5m

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 Typically an upstand of 50mm should be provided. This

Footway

Cycleway

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


1.4m

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


5.7m 4.5m

Figure 2: Governing dimensions in elevation

Span

the superstructure to be capable of supporting itself if

Since there is usually no need to align the approaches

one support were to be demolished in an accident.

to a footbridge, the span should normally be arranged square to the obstacle it has to cross.

Clearance Over a highway, the clearance under new footbridges is

The minimum span required is that simply needed to

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

clear the width of obstacle, carriageway or railway.

clearance the superstructure need not be designed for

However, the span may be increased in order that the

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

supports are positioned far enough from a carriageway

clearance were permitted in special cases it is likely that

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

impact loads would have to be considered. This would

vehicle or derailed train. The supports of light structures

be very onerous on the structural design. Clearance over

such as footbridges are particularly prone to the effects

railways is specified by Network Rail with a minimum of

of impact.

4.640m from rail level. The minimum clearance over electrified lines and over lines that might be electrified in

For footbridges over highways, the span is determined

the future is 4.780m. Greater clearances are required

by the dimensions of the carriageways, as given in the

near level crossings and where there is ‘free running’

Departmental Standard TD 27/96.

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

To avoid the imposition of impact loads the supports

Clearly, where access to the bridge has to come from

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

carriageway or track level, the rise needed for the stairs

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

or ramps is the sum of the clearance plus the

perhaps additionally spanning a footway beside the

superstructure construction depth (walkway surface to

road, the consequent savings in the cost of the

structure soffit). This means that ramps will be long

substructure should be considered. Supports between

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

carriageways should also be avoided if possible.

1 in 20 grade). It also means that the depth of construction (for example the depth of a plate girder)

The space needed for approach ramps and stairs will be

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

significant in arranging the layout of a footbridge. This

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

may influence the positioning of the bridge and its

through construction, with a very shallow construction

supports, and thus its span.

depth, is usually preferred.

Footbridges over railways are mostly required to cross

Sufficient vertical camber is needed to ensure drainage

two or four tracks, with resulting span of between 10

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

and 25m. Where intermediate supports are placed

carried to drains or a soakaway.

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


9


Spiral ramp, Myton Footbridge, Hull

Stairs and ramps, Christchurch

Stairs and ramps

Stairs are usually arranged in two or three flights with

Where access is required from a lower level, stairs and

intermediate landings, depending on particular

ramps must be provided. Stairs are only suitable for able

arrangements, to comply with normal safety

pedestrians and it is general policy to provide ramps

requirements. They usually have semi-open risers, for

where possible. Such ramps should ideally be no steeper

lighter appearance. Handrails are provided on the inside

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

faces of the parapets on stairs and ramps. Minimum

used for straight ramps where space is limited.

widths must be maintained between these handrails.

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

Services

spiral, depending on circumstances and space available

Occasionally the bridge may have to carry a service –

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

water pipes or electric cables, for example. It should

significant and may well influence the position of the

normally be arranged that such pipes are supported out

bridge.

of sight, on brackets or cross-members between main beams for example. If a service is positioned inside a box

A single straight ramp can be used where space and the

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

desired access route permit. If the gradient is steeper

maintenance to the service does not require entry into the

than 1 in 20, the ramp should have intermediate landings

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

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

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

sections between). Ramps are often arranged in scissor

sleeve which runs the length of the bridge.

fashion (i.e. with a 180º 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 360º turn.

10 The design of steel footbridges

River Exe Suspension Bridge

3.2 Selection of type of construction

often appropriate, both visually and structurally. Beyond

As mentioned previously, the depth of construction is

about 100m twin pylons should be considered.

very important to the overall extent of the footbridge where access is from the level of the road or railway

Suspension bridges are very rarely considered these

being crossed. In those circumstances it is usually

days, but may still be chosen for appearance reasons

preferable to use a half-through form of construction.

when the span exceeds about 70m.

This usually leads to a selection of a truss or vierendeel girder bridge, though half-through plate girder forms such

A summary of approximate span ranges suitable for the

as that developed by Network Rail may also be used.

various types is given in Table 1.

However, not all bridges are subject to such constraints.

Table 1

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

Span ranges for different types of construction

across a deep cutting. In such cases the depth of

Construction type

Span range (m)

construction is not so important and steel girders or steel

Truss

15 to 60

composite construction may be employed. When the span

Vierendeel girder

15 to 45

Twin steel girders

10 to 25

Steel girders + steel floor plate

10 to 30

construction may well be advantageous. Alternatively,

Steel box girder

20 to 60

cable stayed construction can be employed.

Composite beams

10 to 50

Arches

25 upwards

Cable stayed forms of construction can rarely be

Cable stayed bridge

40 upwards

justified visually below about 40m. For spans up to

Suspension bridge

70 upwards

is long, the dynamic response of the bridge becomes a significant consideration, particularly for the lighter allsteel bridge. The greater stiffness afforded by truss

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

1:20

13 risers max

≥ 2m

1:20

Figure 3: Arrangement of typical stairs and ramp

The design of steel footbridges 11


3.3 Trusses and vierendeel girder bridges Although 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



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 truss

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

Modified Warren truss

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. Pratt truss

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

Vierendeel girder

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.

Figure 4: Types of truss and vierendeel girder

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


Proportions and appearance

by road users. They therefore require careful attention

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

and treatment.

looking structure, relatively deep in proportion to span. Such trusses were often used for railway bridges.

Where the depth of the vierendeel girder is determined

However, a truss footbridge can generally be of light

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

appearance and of shallow depth/span proportion.

the parapet rail, with suitable infill bars fixed between the vertical members. For longer span vierendeel

With half-through construction, the minimum overall

girders, where the depth is more than the parapet

depth is determined by the parapet height; for a

height, parapet panels complete with top rail can be

crossing over a highway the minimum is about 1.25m.

fixed inside the rectangular panels of the girder. Where a

For spans over about 30 metres the depth will need to

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

be slightly greater, though span/depth ratios in excess

face of the diagonal members. The parapets are less

of 30 can give a pleasing appearance.

conspicuous to road users than the truss members, though they are still evident in silhouette.

For spans over 50m full through construction will probably be necessary. Then the depth is determined by

Construction depth, from footway surface to underside

internal clearance, which is usually specified as 2.3m

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

minimum. To reduce the tunnel effect and to keep the

than the depth of the chord members. This contributes

top bracing away from casual abuse a depth of about

greatly to the light appearance.

3m is needed. Such spans will have a deeper span/depth ratio, though the slender members will still

The top and bottom chords of a truss are usually made

give an impression of lightness.

parallel, but for larger spans a less dominating appearance can be achieved by a hog-back

The arrangement of the bracing and the line of the

configuration, with a gentle curve to the top chord

parapets are the dominant features which are seen

reducing the depth at the ends of the span.

Above: Large-span truss footbridge Left: Vierendeel footbridge Right: Lower chord connection detail Far right: Large-span vierendeel footbridge, A27 Broadmarsh



Members and connections – trusses

have a higher buckling resistance than that calculated

Both circular and rectangular structural hollow sections

even when a lower flexibility value is used.

are commonly used in trusses. The bottom chord is generally rectangular, to facilitate connection with deck

The failure loads calculated were relatively insensitive to

and cross-members. Rolled sections or flats are

the actual value of connection stiffness. This showed

sometimes used as cross-members or as stiffeners to

the use of diagonal stiffeners does not significantly add

steel floor plates. Chords and diagonals are usually

to the global strength of tubular U-frame footbridges.

arranged with centrelines intersecting where possible. Standard welding details have been developed for

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

hollow section connections.

“bracing” to the bottom chords, to carry the lateral shear (mainly wind forces) back to the supports. If a

For half-through trusses the connection with

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

cross-members at the lower chord requires particular

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

attention, since its stiffness and strength are

must be considered.

fundamental to U-frame action. Through trusses, used in longer spans, give lateral Where the bottom chords are of rectangular section,

stability to the top compression chord by means

some designers specify plates slotted diagonally across

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

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

will also share in the carrying of any lateral forces,

5) to prevent the chord lozenging or distorting.

especially where the truss is clad on its sides and thus subject to significant wind loads. At the ends of the span

However, cutting slots in the hollow section and welding

these lateral forces have to be carried down to bearing

stiffeners adds to the fabrication cost. Research by the

level through portal action or through a braced frame.

Steel Construction Institute for Corus (30) showed an un-stiffened connection designed to BS 5400: Part 3 to



Members and connections – Vierendeel girders

Stability of the compression chord again requires

In footbridges, Vierendeel girders normally use

U-frame action of the cross section and this again

rectangular hollow sections for greater stiffness

requires adequate stiffness and strength of the

and strength at the connections between verticals

cross-member to vertical connection at the bottom

and chords.

chord. Even with the heavier sections usually required for a vierendeel girder, it may be necessary to insert

The nature of vierendeel action is that vertical shear is

diagonal plates, as mentioned previously.

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

100 x 100 10 RHS

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

10 thick insert plate slotted into chord

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

Weld ground flush

often wider (in the plane of the girder) at the ends of the span and are sometimes closer together, variations

Figure 5: Detail of diagonal plate through bottom chord

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.

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

Right: Stiffened plate floor construction Far right: Typical floor construction



Floor construction

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

The floor of a truss or vierendeel girder footbridge will

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

usually be of steel plate, though precast planks have

cambered transversely to facilitate drainage. With

been used with trusses. The lighter steel deck is now

stiffened thin steel plate decks, care also needs to be

generally preferred.

exercised that panels do not dish between stiffeners and allow ponding of water – the spacing of stiffeners is

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

usually limited for this reason. Weld sizes should be kept

and welded to steel cross-members between the

to a minimum, to reduce distortion from welding.

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

(see GN 2.10 (31))

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.



Parapets

Cladding

Parapets are normally designed to comply with a

Over rail tracks, the highway and rail authorities require

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

that solid non-climbable cladding be provided on the

either a separate item or may be combined with

inside face of the truss or vierendeel girder. This is

structural members.

usually achieved by profiled steel sheeting, rigidised aluminium, GRP panels or even flat sheets. Fine mesh

For trusses, the parapet is provided as separate units

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

fixed to the inside faces of the truss diagonals. The

electrified lines. Although the cladding is only required

diagonals must then be designed to carry lateral loads

over the tracks, a better appearance is often achieved

from the parapet, and the parapet rails must be

by providing the cladding over the full length of the

designed to span between the diagonals which support

span. Great care needs to be exercised in detailing the

them. Parapet posts can alternatively be fixed to the

cladding, to avoid the creation of small inaccessible

footway deck, though the attachment would need to be

sheltered ledges on the top of the lower chord where

strong enough to withstand the overturning moment

moss and debris can accumulate or which may be used

arising from lateral forces on the top rail.

for handholds or footholds.

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.

Left: Parapets in vierendeel girder, Horam Right: In-line splice detail Far right: Erection of Christchurch footpath



Supports

Erection

Trusses and vierendeel girders are supported either on

Fortunately, most footbridges can be fabricated as a

bearings (if they span between concrete abutments, for

complete length of the span and then transported, with

example) or directly on top of a simple steel

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

substructure without any bearings.

in length require special permission to travel on the public highway, most fabricators prefer to complete fabrication

At abutments the point of support is normally directly

in the works wherever possible and are familiar with

below the end vertical or diagonal members and thus

arrangements for the movement of long lengths.

does not give rise to local bending of the chord section. Other supports should also preferably be arranged

Bolted hollow section flanged joint details can be used

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

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

instance when a top landing cantilevers a short distance

end connections are somewhat cumbersome in

beyond the support columns and the support is midway

appearance. In-line splice details are much less

between bracing connections, the bottom chord is

obtrusive, but require more effort in design and

subjected to bending. It is then common to use a

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

heavier chord section over the last one or two panels of

spans must be complete before lifting, because closure

the truss (see photograph below right).

or possession periods will be very short.

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.



Left: Footbridge using rolled sections, Swale Right: Footbridge with timber deck and parapets Far right: Box girder footbridge and cycleway, Gablecross

3.4 Steel beam bridges

by the treatment of the parapet rails, posts and any

Types of construction

other feature added to the bridge. The use of simple

Four types of construction are considered in this

parapet details will contribute to a good non-fussy

section:

overall appearance.

•

•

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

In some circumstances a distinct curvature in elevation

(e.g. timber)

(more than would suffice just to aid drainage to the

a pair of steel beams with a structurally participating

ends) will add character to the appearance.

steel floor plate •

a steel box girder

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

•

a half-through plate girder bridge as developed by

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

British Rail

simple basic parapet or can be contrasted by embellishment with ornate fixtures and fittings. Typically

The first three are appropriate where depth of

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

construction is not important. The fourth is appropriate

cantilevers either side to provide the necessary width.

where minimum construction depth is critical. Half-through plate girder bridges will usually have their Proportions and appearance

U-frame stiffeners on the outside faces and generally

For the relatively light loading on a footbridge, the depth

look more heavy. Nevertheless, the half-through plate

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

girder bridge developed by British Rail (see page 22)

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

achieves a pleasing appearance.

then have a span of 30m and a beam depth of 1m. A simple I-beam bridge with non-structural floor might

Members and connections – I-beams/girders

comprise two girders about 1.5m apart on which is fixed

For economical design, the pair of beams need to be

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

braced together to stabilise them against lateral

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

torsional buckling. Bracing at several positions in the

of the steel beams.

span will be necessary, roughly at 15 to 20 times the top flange width to achieve reasonable limiting stress levels.

Steel girders with a structural participating steel floor

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

plate would be of similar overall proportions. Parapets

position, bolted to stiffeners on the inside faces of the

would be fixed on top of the floor plate.

webs. For the main girders, fabricated I-sections are likely to be lighter and more economic than Universal

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

Beams. Castellated beams can provide a weight saving

their full length, as web stiffeners are needed only at

in some circumstances whilst offering an interesting and

supports and on the inner faces for attachment of

different appearance.

bracing. The structural element therefore looks clean and simple. The appearance will be influenced strongly



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

To improve appearance it is common to use slightly

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

sloping webs, creating a trapezoidal cross section.

Particular attention should be paid to detailing, to minimise crevices where dirt and moisture can

The use of steel box girders has the advantage of

accumulate.

torsional strength and stiffness. They can be used in continuous construction to simplify supports or to curve

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

the bridge in plan when desired for appearance. In a

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

straight bridge, torsional restraint (usually by means of

usually welded to the main girders and can therefore be

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

assumed to act structurally with them. Cross-members

bearing will suffice at intermediate supports, thus

will be required to carry the floor loading to the main

allowing the use of a single slender column.

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

Figure 7: Cross section through a typical box girder footbridge

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.



Members and connections – half through girders

Fabrication

Half through plate girder footbridges are often used over

Whether using rolled I-beams or fabricated I-section

railways. The solid web provides the required screening

girders, the processes of drilling holes, adding stiffeners

without the need for any non-structural additions. This

etc. poses no difficulty to the fabricator. The fabricated

form has developed from the half-through plate girder

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

concept often seen in railway bridges. A particular form

automatic welding or by a T and I automatic welding

developed by the former Midland Region of British Rail

machine. Curvature in elevation is easily achieved with

is illustrated in photographs shown above. Two features

fabricated girders, and universal beams can readily be

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

curved by specialist bending companies prior to

turned through 45° it forms a steeple cope, which

fabrication. Fabrication of box sections requires more

discourages walking along the flange; the absence of

traditional methods, and the completion of the closed

any projection of the bottom flange prevents climbing

box makes it almost essential for manual work internally.

along the outer face.

Details should be arranged for ease of access for work and inspection.

U-frame action is provided by the flat intermediate stiffeners to web and bottom flange. Typically they are

Splices

provided about every 1.5m.

For spans up to around 40m, it is quite likely that the beams would be transported full length and splices

Parapets

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

Where there are no cantilevers the parapet can either be

into at least two lengths; site connections would

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

normally be bolted.

girder. The attachment positions should coincide with bracing or cross-members, to provide restraint against

Bolted splices are quite conventional, with few problems.

rotation under lateral loads on the parapet rail.

If a completely clean face is sought,it will be necessary to have a site welded joint.

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.


3.5 Composite beam bridges

Members and connections


Composite construction produces a much heavier

Composite construction is seen in footbridges in two

structure than an all-steel footbridge; the dead

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

load accounts for over half of the total load in most

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

cases. The extra weight and consequent stiffness of this

open steel box form with slab which is sometimes used

form of construction has the advantage of being less

in highway bridges is not normally seen in footbridges

responsive to dynamic excitation.

Slabs may be cast insitu, though the relatively modest

Where transverse joints between precast units are not

extent of the shear connection and lighter design loads

designed to carry transverse shear, plan bracing will

on the slab allow greater opportunity to employ pre-cast

also be needed.

slabs. Such slabs are provided with open pockets to fit over the shear connectors. The pockets and the joints

Floor construction

between slab sections are filled with concrete to create

Reinforced concrete slabs for footbridges are typically

the necessary structural continuity.

about 150mm thick. They can be constructed insitu on falsework or by using precast slabs.

Proportions and appearance Composite footbridges typically have a span/depth ratio

Sometimes they can be cast in the fabrication yard, and

of about 20 (depth measured from top of slab to

the complete composite structure transported to site

underside of girder).

and erected.

Short cantilevers outside the lines of the webs will give

A waterproofing membrane is required, plus some form

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

of durable wearing surface. A combined membrane and

highway bridges. A small upstand is needed at the

wearing course with aggregate dressing, similar to that

edges to provide a mounting for the parapets and to act

used on steel decks, can be used.

as a drainage upstand. A thick edge beam would create a rather heavy appearance.

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 Rail Above: Composite curved ‘I’ beam footbridge, Washington



3.6 Cable stayed bridges

A single backstay is usually sufficient, anchored to the

Footbridges carry only relatively light loading. However,

girder at the abutment which supports the end of the

when the main span is long, the requirements of

backspan. Further backstays are only needed if the

supporting its own dead load and of providing a

backspan is long and requires intermediate support. The

sufficiently stiff structure lead toward a much more

stays are normally anchored at floor level to longitudinal

substantial structure than would seem appropriate for a

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

“mere” footbridge. As a result, an increasingly popular

span between anchor points and they may need to be

solution for longer spans is the use of a cable stayed

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

arrangement. This effectively divides the span into shorter

depth, might be achieved by using a vierendeel girder and

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

half-through construction. Footbridge pylons are usually

for these bridges also add a strong visual feature which is

steel box or circular sections, for slender appearance,

often welcomed.

ease of construction and economy.


Members and connections

Cable stays can be used with any of the forms of

The cable stays will normally be made from wire rope or

construction previously described, though to complement

spiral strand. Strands are made by winding together, or

the light appearance, a slim form of deck construction is

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

likely to be more appropriate for all except the largest

made up of a number of small strands wound together.

spans. Supports can be provided to the main beams at

Ropes and spiral strands have a lower effective modulus

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

than solid steel. Parallel wire strands are also available.

slender deck.

Advice should be sought from specialist manufacturers on the selection of strands.

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.



In the dead load condition the stays are effectively

For very long spans, the deflection under load changes

prestressed. It is important to calculate accurately the

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

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

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

the correct geometry of the structure is achieved.

effects should be taken into account in the analysis.

Provision should be made for length adjustment in the

Computer programs are available which automatically

stays, to accommodate tolerances and errors.

take account of the non-linear effects of varying geometry under load.

Stays must obviously be sufficiently strong to support the beams, but often more significant for small bridges

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

is the need to provide sufficiently stiff supports to the

experience has shown that they should be

beams and to avoid slack stays which will be easily

inspected from time to time to check for corrosion and

vibrated.

fatigue, particularly at the lower ends. The stay anchorages should be accessible for such inspection

With twin planes of stays, the natural arrangement for

and maintenance. The design should also be such that

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

any one stay can be removed and replaced.

which the stays are attached. The floor then spans transversely between the beams. A single plane of stays

Dynamic response

can only be used where a torsionally stiff box girder is

Cable stayed bridges are relatively flexible and are more

provided; the stays would be attached on the centreline

prone to oscillation under wind or under deliberate

of the bridge. This is not normally convenient for a

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

single footway.

very low level of structural damping, which can allow the oscillations to grow significantly. The dynamic response

As well as provision for adjustment in length during

of the bridge should therefore be checked carefully.

installation, attachment details should also be arranged

Artificial damping, such as tuned mass dampers, can be

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

provided if necessary.

practice to make sure that the anchorages are as strong at ULS as the breaking load of the stays.

Floor construction Deck construction is usually of stiffened steel plate,

Under the action of live load the stays provide stiff

though timber or reinforced concrete are sometimes

support to the main beams and they thus behave

used instead.

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.

Far left: Cable stayed ‘I’ beam footbridge, Cumbernauld Left: Royal Victoria Dock Bridge, London Right: Cable stay anchorage



3.7 Access ramps and stairs

Handrails must be provided on the inside faces of

Where approach ramps or stairs are needed they are

parapets on stairs and ramps, for safety reasons. A

usually structurally independent, except for the need to

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

be supported at the top end either on the footbridge

and any adjacent members.

superstructure or on a common substructure support. They can therefore be of a structurally different form.

Stairs normally have semi-open risers. Fully open risers

However, it is generally preferable to achieve harmony

are not permitted by BD 29/03.

of appearance between the two and to use a similar construction form.

At the bottom of flights of stairs, details should be chosen which avoid acute corners, since they can trap

Stairs usually require, at most, one intermediate support

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

beneath the landing at mid-flight. Ramps require more

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

supports and indeed are small bridges themselves. Even

between the underside of the stringers and ground level.

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.

Below: Stairs showing open treads and handrails Right: Scissor ramp



3.8 Bearings and expansion joints

Consideration should be given to fixing long ramps at

The provisions for restraint or the accommodation of

the bottom end. Maximum longitudinal movement at the

movement due to expansion or other reasons depends

far end therefore occurs where the columns are tallest

very much on the general arrangement of the bridge,

and most able to accommodate it.

ramps and stairs. Stairs should preferably be fixed at the bottom and When the bridge spans between bankseats or

bolted to column supports. This effectively provides a

abutments, expansion joints are needed, and the

restraint for any ramp or bridge connected to the top of

structure will sit on bearings. At one end the bearings

a straight flight.

may be fixed longitudinally, but if laminated bearings are used, both ends can be 'free', as long as the bearings

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

can transmit any longitudinal forces.

direct connections to columns, should be designed to resist at least a nominal uplift.

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.

Below: Expansion joint leaf plate Right: End bearing box girder


Design codes, standards and guidance

4. Design codes, standards and guidance 4.1 British Standards

4.5m of the edge of the carriageway and to

In most circumstances, the British Standard BS 5400 (1)

superstructures which have less than 5.7m clearance

will apply to the design and construction of footbridges.

above the surface of the carriageway.

In some cases, possibly where the bridge is connected to a building, BS 5950 (2) might be called for.

Other standards and advice notes also relate to the design of footbridges. Design criteria for footbridges are

For design of steel and composite structures, the

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

following Parts of BS 5400 are applicable

are given in TD 27 (6). Selected information from these two documents is included in section 3. Standard TD 27

Part 2

Specification for loads

specifies a minimum clearance for footbridges of 5.7m.

Part 3

Code of practice for design of steel bridges

This avoids the necessity of applying the impact

Part 4

Code of practice for design of concrete bridges

requirements of BD 37 on the superstructure, which

Part 5

Code of practice for design of composite bridges

would be particularly onerous on a light structure such

Part 6

Specification for materials and workmanship, steel

as a footbridge.

These codes cover all aspects of design for footbridges

Where supports need to be close to the edge of the

of beam and truss construction. Design of tubular joints

carriageway, they are required to be provided with

is not covered in detail within Part 3 – see section 4.4

protective plinths and designed for impact loads. Where

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

they can be kept back from the carriageway, perhaps to

the strands and their anchorages, are not covered by

span a footway beside the road, the consequent savings

these codes – refer to section 4.5 for guidance.

in the cost of the substructure should be considered. Supports between carriageways should also be avoided

Dimensional and safety requirements for stairs are given

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

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

road, which is not usually feasible).

the departmental standard for footbridges. The design of parapets on footbridges is referred by

4.2 Departmental standards

BD 29 to the Interim Rules for Road Restraint Systems

The requirements of the four UK highways authority (the

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

Highways Agency, the Scottish Executive, the Welsh

not currently part of the DMRB, although it does state

Assembly Government and the Department for Regional

that it supersedes a number of DMRB documents, such

Development Northern Ireland) are set out in the Design

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

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

which gives dimensional requirements, design

collection of individual standards (BD documents) and

requirements and a specification for construction of

advice notes (BA documents).

metal parapets, and it specifies the design loading classes for rails, posts and infill.

Each of the design code parts of BS 5400 is implemented by a BD standard (4), and some of

4.3 Railway standards

these standards vary certain aspects of the part that

Network Rail are particularly concerned with prevention

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

of unauthorised access and are legally obliged to fence

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

its boundaries. Network Rail and the Railway Safety and

the requirements in relation to loads resulting from

Standards Board also have more stringent requirements

collision of vehicles with the structure have been

in relation to collision loads. Reference should be made

significantly modified. The impact loads and the

to GC/RC5510: Recommendations for the Design of

circumstances in which they should be applied are

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

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

given in recent projects.

5400 Part 2) and an amendment to it. The provisions relate to the impact loads on supports located within


Design codes, standards and guidance

In considering the prevention of unauthorised access,

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

not only must the pedestrian face of the bridge be

the bracing member and the chord member must be

designed to be non-climbable, it must also be

checked. If necessary, reinforcement of the joint can

impossible to climb along the outer face from the ends

be designed.

of the bridge – this usually means that trusses are clad chords or parapets must be arranged so that they are

4.5 Design of cable stayed and suspension bridges

impossible to walk along.

For general guidance on the design of cable stayed

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

bridges, reference should be made to standard texts, The zone within 4.5m of the outermost running rail is

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

considered a danger zone; if any support is located

comprehensive books, but they do include specific

within that zone, collision effects must be considered.

comment on footbridges with illustrated examples.

Any substructure column must be able to withstand an impact load, and the superstructure must be able to

The provisions of BS 5400 do not cover in detail the

continue to carry some live load without support from

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

the column. Design recommendations are given in

other appropriate national code. The designer therefore

GC/RC5510.

needs to base his detailed design on an empirical approach, based on load effects calculated in the usual

4.4 Design of hollow section joints

manner according to BS 5400 and adopting the general

The design of hollow section joints is not fully covered

objectives of the code.

by the requirements of BS 5400: Part 3. There is however extensive background research into the

Details of the specification of wire ropes and strands

behaviour of tubular joints and various documents have

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

been published which provide guidance.

sockets by reference to BS 463 (17). The cold drawn wire used for ropes and strands does not have a linear

For triangulated structures, where the joints transmit

stress/strain relationship, with a definite yield plateau,

essentially axial loads from one member to another, the

as does structural steel. The relationship is generally

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

smooth, with decreasing tangent modulus as load

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

increases. Design of stays has therefore been based

bending of the walls of the hollow sections (which are

traditionally on permissible stresses calculated by

subjected to out of plane forces).

dividing the ultimate or breaking strength by a suitably large factor (i.e. a working stress philosophy). In the

Guidance literature is available both for circular sections and for rectangular sections. General guidance is given

absence of formal codes on a limit state basis, division of this strength by a partial factor γm of about 2.0 at

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

ULS, in conjunction with normal values of γƒ1 and γƒ3

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

gives results consistent with the traditional approach.

Design rules in both of these documents may be applied using partial factors appropriate to BS 5400. Similar

Guidance on the design of suspension bridges can be

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

found in texts such as Pugsley (18). The tensile elements may be wire rope or strand, as for cable stayed bridges,

The extent of guidance on the design of joints for the

though high tensile steel rods may be used for the main

moments associated with vierendeel action (or with

tension members.

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


Design codes, Standards and Guidance

4.6 Design of steel and composite bridge beams

For Network Rail owned bridges, the protective

Guidance on the design of composite highway bridges

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

is given in a series of publications by The Steel

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

treatment and walkway surfacing must comply with

Construction Institute (19). These can be used as general guidance in the design of footbridges in accordance

For other bridges, the HA specifications, or alternatives,

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

may be used, with the clients agreement.

beam designs. In some circumstances, Weather Resistant Steels might Guidance on a wide range of practical aspects related to

be used, provided that environmental constraints can be

steel bridge construction is given in a series of Guidance

met. (23), (24)

Notes produced by the Steel Bridge Group (31).

4.9 Steel materials 4.7 Dynamic response

Steel material for plates, rolled sections and structural

Limitations on the dynamic response of footbridges are

hollow sections is covered by British Standards

given in HA standard BD 37. The vertical natural

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

frequency of many footbridges will be below 5Hz and

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

the response must be checked. If the horizontal natural

Construction Centre. Contact details are on the back of

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

this brochure.

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 treatment For 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.


Flow charts

5. Flow charts Figure 5.1: Flow diagram for the design of footbridges

DMRB Standards for highway cross section and headroom

Schemespecific details

DMRB Standards for footbridges

Determine geometric constraints

Choose structural form

Trusses and vierendeel girders

Steel beams

Composite beams

Cable stayed bridges

(Figure 5.2)

(Figure 5.3)

(Figure 5.4)

(Figure 5.5)

Ramps and stairs

Far left: Renaissance Bridge, Bedford Left: Smithkline Beecham, Marlow


Flow charts

Figure 5.2: Flow chart for trusses and vierendeel girders

Global analysis Global analysis 12.3

Longitudinal effects

Yes

Lateral effects

No Triangulated truss?

Check combined bending and axial effects

Check as a ‘truss’

Check adequacy of lateral bracing

12.1

12.6

Tension members

Compression members

Tension members

Compression members

Check adequacy at ULS

Determine effective lengths


Determine effective lengths

12.4 12.5

12.2 11.5.1


Strength adequate? Yes

10.6.1

11.5.2 9.9



Slender or compact? Yes

I=a* 12.5.1

10.6.2 10.6.3 9.9

Check U-Frame action

12.5


Strength adequate? 12.2.3 No Yes

Check adequacy at SLS 12.2.3 10.6.2 10.6.3 Strength adequate?

Yes

Satisfactory

* 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.


Flow charts

Figure 5.3: Flow chart for steel beams Global analysis

No

Yes Box girder?

Determine limiting stresses for LTB

Determine effective section

9.6 9.7 9.8

9.4

Determine limiting stresses and check capacities

Check ULS moment and shear capacities

9.10 9.11

9.9

Check diaphragms and crossframes

Unsymmetric compact section? No

Yes

9.16 9.17

Check adequacy at SLS

9.9.8

Check bearing stiffeners 9.14

All strengths adequate?

All strengths adequate? Yes

Yes

Satisfactory


Flow charts

Figure 5.5: Flow chart for cable stayed bridges

Figure 5.4: Flow chart for composite beams

Global analysis

Global analysis

Check beam adequacy at ULS

Check adequacy of members as trusses or beams

Non-linear analysis if deflections or DL sag of stays are significant

9.9 Include effects during replacement of each stay Check slab adequacy at ULS

Check adequacy of cable stays

5/6.1.2 4/4.8.3

Unsymmetric compact I-beam?

Check local effects at cable anchorages

Yes

No

Check beam adequacy at SLS

Check adequacy of pylon

9.9.8 9.9.5.2

Check slab adequacy at ULS

Determine dead load prestress in stays

5/5.2.4.2 5/5.2.6 4/4.1.1.1

Check bearing stiffeners All strengths adequate? 9.14 Yes

All strengths adequate? Yes

Satisfactory


Satisfactory

References

6 References 1.

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

2.

British Standards Institution BS 5950, Structural use of steelwork in building, BSI, London

3.

British Standards Institution BS 5395, Stairs, ladders and walkways, BSI, London

4.

Highways Agency Design 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.

6.

Highways Agency Design manual for roads and bridges, Volume 2, Section 2, BD 29 Design criteria for footbridges, The Stationery Office Highways Agency Design manual for roads and bridges, Volume 6 Section 1, TD 27 Cross-sections and headroom, The Stationery Office

20. Highways Agency Design manual for roads and bridges, Volume 1, Section 3, BD 49, Design rules for aerodynamic effects on bridges, The Stationery Office 21. Highways Agency Manual of contract documents for highway works, The Stationery Office; Volume 1: Specifications for highway works series 1900, Protection of steel against corrosion Volume 2: Notes for guidance on the specification for highway works, Series NG1900, Protection of steelwork against corrosion 22. Corus Corrosion Protection of Steel Bridges, 2002 23. Highways Agency Design manual for roads and bridges, Volume 2, Section 3, BD 7, Weathering steel for highway structures, The Stationery Office 24. Corus Weathering Steel Bridges, 2002 25. British Standards Institution BS EN 10025: 2004, Hot rolled products of structural steels. BS EN 10210, Hot finished structural hollow sections of non-alloy and fine grain structural steels, Part 1: 1994 Technical delivery requirements.

7.

Highways Agency Interim Requirements for Road Restraint Systems (IRRRS), The Highways Agency, 2002 (contact the Highways Agency for copies)

8.

British Standards Institution BS 7818:1995 Specification for pedestrian restraint systems in metal

26. Corus Product & Technical brochures Structural sections Structural plates Structural hollow sections

9.

CIDECT Design guide for circular hollow sections (RHS) under predominantly static loading, Verlag TÜV, Cologne, 1991

27.

10. CIDECT Design guide for rectangular hollow sections (RHS) joints under predominantly static loading, TÜV, Cologne, 1992 11.

CIDECT Structural stability of hollow sections, Verlag TÜV, Cologne, 1992

12. Corus Tubes Design of SHS welded joints, CT16, Corus Tubes, Corby 2001 13. British Standards Institution prEN 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 Institution BS 302, Stranded steel wire ropes, BSI, London

17.

British Standards Institution BS 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

Railway Safety and Standards Board Group Standard GC/RC5510: Recommendations for the Design of Bridges

28. Network Rail Line Standard RT/CE/S/039; Specification RT98 - Protective Treatment for Railtrack Infrastructure 29. Network Rail Line Standard RT/CE/C/002: Application and Reapplication of protective treatment to Railtrack Infrastructure 30. Corus Tubes Connection 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 bridge construction (P185), The Steel Construction Institute, 2002

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


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CD:3000:UK:01/2005

Design of Steel Footbridges 2005

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