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The design of steel footbridgesCorus Construction & IndustrialSteel bridgesthe gapBelow:River Aire footbridge, Leeds, 1993Right:Lowry Footbridge, ManchesterThe design of steel footbridges 3Contents1. Introduction2. Features and forms of constructionfor footbridges3. Conceptual design and detailing3.1 General arrangement3.2 Selection of type of construction3.3 Trusses and vierendeel girder bridges3.4 Steel beam bridges3.5 Composite beam bridges3.6 Cable stayed bridges3.7 Access ramps and stairs3.8 Bearings and expansion joints4. Design codes, standards and guidance

4.1 British Standards4.2 Departmental standards4.3 Railway standards4.4 Design of hollow section joints4.5 Design of cable stayed and suspension bridges4.6 Design of steel and composite bridge beams4.7 Dynamic response4.8 Protective treatment4.9 Steel materials5. Flow charts6. ReferencesThis 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 contributionsmade by Mr W Ramsay, Corus and Mr A C G Hayward,Cass Hayward and Partners, during the originalpreparation of the publication.4 The design of steel footbridgesIntroductionFootbridges are needed where a separate pathway hasto be provided for people to cross traffic flows or somephysical obstacle, such as a river. The loads they carryare, in relation to highway or railway bridges, quitemodest, and in most circumstances a fairly lightstructure is required. They are, however, frequently

required to give a long clear span, and stiffness thenbecomes an important consideration. The bridges areoften very clearly on view to the public and therefore theappearance merits careful attention.Steel offers economic and attractive forms ofconstruction which suit all the requirements demandedof a footbridge.A fully detailed design can be prepared with othercontract documents for pricing by tenderers. However, itis common practice, particularly for smaller bridges, for


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the detailed design of a footbridge to be included aspart of a design and construct package. Manyfabricators are able to provide such a package, usingmethods and details of construction developed to suittheir particular fabrication facilities and expertise.However, the engineer supervising the work still needsto be acquainted with the different forms of constructionwhich might be used and to be aware of theiradvantages and limitations.Longer span bridges and those which form part of alarger scheme are likely to be designed in detail by aconsultant or local authority. Within such anorganisation the engineer carrying out the design needsto be familiar with the particular requirements forfootbridges, their features and construction details.For the engineer in either of these situations, thispublication presents guidance on the conceptual designof steel and composite footbridges, to aid the selectionof an outline design.Typical key features are illustrated in section 3,references to codes and sources of further guidance aregiven in section 4. Simple flow charts showing thedesign steps are presented in section 5.1. Introduction

The design of steel footbridges 5Features and forms of construction for footbridges2. Features and forms of construction for footbridgesBasic requirementsFootbridges, like any other bridge, must be long enough toclear the obstacle which is to be crossed and high enoughnot to interfere with whatever passes beneath the bridge.However, the access route onto the footbridge is oftenmuch different from what is familiar to the designer of ahighway bridge: there is no necessity for a gentle horizontalalignment (indeed the preferred route may be sharply atright angles to the span). Structural continuity is thereforeless common. The principle span is often a simply

supported one.Provision of suitable access for wheelchairs and cyclists isoften specified for footbridges. Access ramps must beprovided and restricted to a maximum gradient. Theconsequent length of ramps where access is from the levelof the road or rail track over which the bridge spans isgenerally much longer than the bridge itself. The form ofconstruction suitable for the ramps may have a dominantinfluence on the final form of the bridge.The width of a footbridge is usually quite modest, justsufficient to permit free passage in both directions forpedestrians. Occasionally the bridge will have segregatedprovision for pedestrians and cyclists, in which case it will

need to be wider.Parapets are provided for the safety of both the pedestriansand traffic flow. Footbridges over railway lines are requiredto have higher parapets and be provided with solid panelsdirectly over the rail tracks.Truss and vierendeel girder beamsTrusses offer a light and economical form of construction,particularly when the span is large. The members of thetruss can be quite slender and this naturally leads to theuse of structural hollow sections. Hollow sections have


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been used for footbridges for over 30 years and somefabricators have specialised in this form of construction,developing techniques and details which utilise them to thebest advantage.Vierendeel girders using hollow section members offer analternative but complementary structural form of similarproportion by substituting a rectangular form for thetriangular arrangement used in trusses.Trusses and vierendeel girders are arranged with eitherhalf-through or through construction. Half-throughconstruction is used for smaller spans, where the depthneeded is relatively shallow. For larger spans, or where thetruss is clad to provide a complete enclosure for thepedestrians, through trusses are used; the top chords arethen braced together above head level.Steel beam bridgesThe simplest method of employing structural steel as theprime structural element of a footbridge is to use a pair ofgirders (fabricated or rolled sections), braced together forstability and acting as beams in bending, with a non-participating walkway surface on top. A typical smallbridge deck might for example be formed by timbersplaced transversely across the top of the beams. Precastslabs might also be used, without being shear connected

to the steel and therefore not participating in globalstructural action.Left:Bell's Bridge, GlasgowRight:Whatman's Field Bridge, Maidstone6 The design of steel footbridgesAlternatively the floor might be formed by steel plate,suitably stiffened to carry the pedestrian loads, in whichcase the plate could also be made to act structurally as thetop flange of the steel beams.Steel box girder bridgesAnother alternative is to use a small steel box girder. The

top flange acts as the floor of the bridge, and there areusually short cantilevers either side of the box. This formhas the benefits of good torsional stiffness which cansimplify support arrangements and clean surfaces whichminimise maintenance.Composite beam bridgesComposite beams, steel girders with a concrete slabacting as both a walkway floor and participating as atop flange, are a practical solution for medium spanfootbridges. They are a lighter version of the form ofcomposite construction frequently employed inhighway bridges. Slabs may be cast insitu, though thelesser requirements for the shear connection and the

lighter design loads on the slab allow greateropportunity to employ pre-cast slabs. The slab can alsobe cast on the beams in the works or other convenientsite, since the weight and dimensions are oftensufficiently modest to permit transport and erection ofthe complete superstructure.Although composite construction is usually associatedwith I section girders, a concrete slab can also be usedwith a steel box girder.Cable stayed bridges


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In seeking to provide a bridge of light appearance, theuse of cable stays is found to be very successful. Itoften affords scope to create a visually striking structurewhich provides a landmark or a focus for the area inwhich it is located. Almost any form of construction canbe used with stays, though when a cable stayed form ischosen, the structural requirements are often found tobe of secondary consideration to the achievement of apleasing appearance.Enclosed bridgesEnclosure of the sides of a footbridge is often called forto discourage the throwing of objects from the bridge.This is a particular requirement for bridges over railwaylines. Full enclosure, to the sides and the roof of thewalkway, is called for in situations where the users areto be protected from the environment and where greaterprotection is required over railway lines. Such enclosurejustifies the use of through truss or vierendeelconstruction. The form of construction will probably bedictated by consideration of appearance of the bridgeand its relationship to adjacent structures. Whilst thegeneral principles discussed in this guide areapplicable, fully enclosed bridges are not specificallydealt with in detail in this guide.

Decorative featuresIn addition to the basic impression made by the form ofconstruction, the appearance can be greatly influencedby non-structural decorative features, such as parapetsand handrails. Where particular effects are sought, theavailability of different patterns for posts, rails, etc,should be investigated. Non-structural embellishmentsof supports can also contribute for example a cablestayed pylon can be extended to a spike or other featureabove the level of the topmost stay connection.Landmark structuresIt is an increasingly common requirement for footbridgesin prominent or key locations to be `landmark

structures'. Particular attention is given to theappearance of the structure and this may result insomewhat unusual forms of construction. Suchstructures can be allowed to be marginally less efficient(in terms of complexity of fabrication), but if the designis well executed the penalties should be small.There is more scope for innovative design when thestructure is not over a road or railway, because therequirements for parapet details need not be sostringent. Parapets are often the most noticeable featureof a footbridge, and the freedom to use more attractiveforms and more open post and rail arrangements canlead to a very pleasing appearance.

The use of curved arch-type members is currently quitepopular, as is the use of cable stays. Some recentexamples 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 givenin the following sections are largely still applicable.The design of steel footbridges 7Features and forms of construction for footbridgesLeft:


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Swansea Sail BridgeBelow:Halfpenny Bridge, SheffieldRight:Millennium Bridge, Gateshead3. Conceptual design and detailing3.1 General arrangementAs a first step, the basic requirements for access andsafety should be determined. The width and form ofaccess needed depends on the expected pedestriantraffic flow, though minimum dimensions are adequate inmost cases.For a simple footway, a minimum clear width of 2.0m isrequired by the highways authorities. Railway stationfootbridges can be less wide. To the sides of thisfootway, parapets are required, which should be 1.15mhigh over roads or 1.5m high over railways, the heightmeasured from the footway surface in both cases. Inareas prone to vandalism, a height of 1.8m may berequired over railways. The resulting minimum crosssection to be provided is shown in Figure 1. Anincreased parapet height of 1.3m may be needed inareas of high prevailing wind and for bridges where theheadroom under the bridge is more than 10m.

Where pedestrians and cyclists share the pathway, theminimum width of 2.0m may be used for low trafficflows but a wider segregated pathway (1.5m + 1.5mminimum) may be required for higher traffic flows.Segregation can be achieved by a white line, colourcontrast or difference in surface texture. At the sametime the minimum parapet height is increased to 1.4m.The cross section for a combined pathway is alsoshown in Figure 1.Dimensional requirements for footbridges are given inDepartmental Standard BD 29/03. That document refersto 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 thebridge is above a carriageway, a footpath or rail tracks.Typically an upstand of 50mm should be provided. Thisupstand can be provided by an edge beam, by the lowerchord of a truss or by a flat welded to the floor plate.Figure 1: Basic sectional dimensions for bridges over highways8 The design of steel footbridgesConceptual design and detailingFootway Cycleway1.5m 1.5m1.4

mMarked segregationMinimum footway2.0m1.15mFootway + cycleway


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2.0m1.4m4.5m5.7mSpanSince there is usually no need to align the approachesto a footbridge, the span should normally be arrangedsquare to the obstacle it has to cross.The minimum span required is that simply needed toclear the width of obstacle, carriageway or railway.However, the span may be increased in order that thesupports are positioned far enough from a carriagewayor rail track to avoid the risk of impact from an errantvehicle or derailed train. The supports of light structuressuch as footbridges are particularly prone to the effectsof impact.For footbridges over highways, the span is determinedby the dimensions of the carriageways, as given in theDepartmental Standard TD 27/96.To avoid the imposition of impact loads the supportsneed 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 theroad, the consequent savings in the cost of thesubstructure should be considered. Supports betweencarriageways should also be avoided if possible.The space needed for approach ramps and stairs will besignificant in arranging the layout of a footbridge. Thismay influence the positioning of the bridge and itssupports, and thus its span.Footbridges over railways are mostly required to crosstwo or four tracks, with resulting span of between 10and 25m. Where intermediate supports are placedcloser than 4.5m to the nearest rail, Network Rail require

the superstructure to be capable of supporting itself ifone support were to be demolished in an accident.ClearanceOver a highway, the clearance under new footbridges isrequired to be at least 5.7m (TD 27/96). With thisclearance the superstructure need not be designed forimpact loads (see Figure 2). If any relaxation onclearance were permitted in special cases it is likely thatimpact loads would have to be considered. This wouldbe very onerous on the structural design. Clearance overrailways is specified by Network Rail with a minimum of4.640m from rail level. The minimum clearance overelectrified lines and over lines that might be electrified in

the future is 4.780m. Greater clearances are requirednear 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 fromcarriageway or track level, the rise needed for the stairsor ramps is the sum of the clearance plus thesuperstructure construction depth (walkway surface tostructure soffit). This means that ramps will be long(about 120m at each end of the bridge over a road, for a1 in 20 grade). It also means that the depth of


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construction (for example the depth of a plate girder)can add significantly to the length of ramp, and thus tothe cost of the whole structure. For this reason, half-through construction, with a very shallow constructiondepth, is usually preferred.Sufficient vertical camber is needed to ensure drainageof the footbridge to the ends, where the run-off can becarried to drains or a soakaway.Figure 2: Governing dimensions in elevationThe design of steel footbridges 9Conceptual design and detailing10 The design of steel footbridgesConceptual design and detailingStairs and ramps, Christchurch Spiral ramp, Myton Footbridge, HullStairs and rampsWhere access is required from a lower level, stairs andramps must be provided. Stairs are only suitable for ablepedestrians and it is general policy to provide rampswhere possible. Such ramps should ideally be no steeperthan 1 in 20, though gradients of up to 1 in 12 may beused for straight ramps where space is limited.A ramp can be either a series of straight sections or aspiral, 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 thebridge.A single straight ramp can be used where space and thedesired access route permit. If the gradient is steeperthan 1 in 20, the ramp should have intermediate landings(i.e. it should be a series of ramps with horizontalsections between). Ramps are often arranged in scissorfashion (i.e. with a 180 change of direction at anintermediate landing).Spiral ramps must have a minimum inside radius of5.5m (gradient measured 900mm from the inside edge).The same limits on gradient apply (i.e. a maximum of 1in 20 is desirable, up to 1 in 12 may be acceptable in

some cases). Spiral ramps are unsuitable for a full 6mrise to a footbridge over a highway unless a large radiuscan be accommodated.Stepped ramps are sometimes used which, with a125mm step and a 1 in 12 slope between, can effectivelyachieve a 1 in 6 gradient. For spiral ramps this gives arise of 6m in under 360 turn.Stairs are usually arranged in two or three flights withintermediate landings, depending on particulararrangements, to comply with normal safetyrequirements. They usually have semi-open risers, forlighter appearance. Handrails are provided on the insidefaces of the parapets on stairs and ramps. Minimum

widths must be maintained between these handrails.ServicesOccasionally the bridge may have to carry a service water pipes or electric cables, for example. It shouldnormally be arranged that such pipes are supported outof sight, on brackets or cross-members between mainbeams for example. If a service is positioned inside a boxgirder, it is better to put it in a duct, so that anymaintenance to the service does not require entry into thebox girder. Gas or water pipes should not be sited inside


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a box girder, for safety reasons, unless placed in a steelsleeve which runs the length of the bridge.The design of steel footbridges 11River Exe Suspension Bridge3.2 Selection of type of constructionAs mentioned previously, the depth of construction isvery important to the overall extent of the footbridgewhere access is from the level of the road or railwaybeing crossed. In those circumstances it is usuallypreferable to use a half-through form of construction.This usually leads to a selection of a truss or vierendeelgirder bridge, though half-through plate girder forms suchas 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 spanacross a deep cutting. In such cases the depth ofconstruction is not so important and steel girders or steelcomposite construction may be employed. When the spanis long, the dynamic response of the bridge becomes asignificant consideration, particularly for the lighter all-steel bridge. The greater stiffness afforded by trussconstruction 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 to100m a single pylon on one side of the main span isoften appropriate, both visually and structurally. Beyondabout 100m twin pylons should be considered.Suspension bridges are very rarely considered thesedays, but may still be chosen for appearance reasonswhen the span exceeds about 70m.A summary of approximate span ranges suitable for thevarious types is given in Table 1.Table 1Span ranges for different types of constructionConstruction type Span range (m)Truss 15 to 60

Vierendeel girder 15 to 45Twin steel girders 10 to 25Steel girders + steel floor plate 10 to 30Steel box girder 20 to 60Composite beams 10 to 50Arches 25 upwardsCable stayed bridge 40 upwardsSuspension bridge 70 upwards13 risers max1:201:20 2mFigure 3: Arrangement of typical stairs and ramp

12 The design of steel footbridgesConceptual design and detailing3.3 Trusses and vierendeel girderbridgesAlthough trusses and vierendeel girders have a differentstructural action, there are many similar features whenthey are constructed of structural hollow sectionmembers, as used in footbridges. This section deals withboth types of construction.Through and half-through construction


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Trusses and vierendeel girders for footbridges arenormally arranged with the deck at the level of thebottom chord, in either through or half-throughconstruction. Half-through construction is used forsmaller spans, where the depth needed is less than theclearance height for people to walk through. For largespans, or where the bridge is clad to provide acomplete enclosure for the pedestrians, throughconstruction is used.The top chords can then be braced together abovehead level.Stability of the top compression chord in half-throughconstruction is provided by the U-frame action of theside members and the cross-members of the deck. Inthrough construction, lateral bracing between the two topchords offers a more direct means of stabilising them.Below and right:Through truss footbridgeThe design of steel footbridges 13Conceptual design and detailingConfigurationThe type of truss usually employed is either a Warrentruss or a modified Warren truss. Occasionally a Pratttruss may be used. The different types are illustrated in

Figure 4.Warren trusses are the simplest form of truss, with allloads being carried principally as axial loads in themembers and with the minimum of members meeting atjoints. However, the loads which are carried to thebottom chords from the walkway floor can lead tosignificant bending in these members when the panelsare large. A modified warren truss reduces the span ofthese chord members, though the additional verticalmembers add complexity to the fabrication. Pratt trussesare used where it is preferred that some members arevertical, for example to facilitate the fixing of cladding ordecorative panels.

Vierendeel girders have no diagonal members and relyon a combination of axial loading and bending to carryloads. The stiffness of the girder depends crucially onthe bending stiffness of vertical and horizontal membersand on the stiffness of the joints between the two. As aconsequence they are much heavier, for a given span,than a Warren truss. However the appearance, whichonly shows vertical and horizontal lines, in harmony withthe normal form of parapet (horizontal rails, verticalposts and infill), is often considered more pleasing.For the largest spans, the vierendeel girder will probablybe too flexible, though they have been used successfullyup to 45m span.

Below:Half-through truss footbridgeBelow:Rutherglen station footbridgeFigure 4: Types of truss and vierendeel girderPratt trussModified Warren trussWarren trussVierendeel girder14 The design of steel footbridges


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Above:Large-span truss footbridgeLeft:Vierendeel footbridgeRight:Lower chord connection detailFar right:Large-span vierendeel footbridge, A27 BroadmarshProportions and appearanceThe 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 lightappearance and of shallow depth/span proportion.With half-through construction, the minimum overalldepth is determined by the parapet height; for acrossing over a highway the minimum is about 1.25m.For spans over about 30 metres the depth will need tobe slightly greater, though span/depth ratios in excessof 30 can give a pleasing appearance.For spans over 50m full through construction willprobably be necessary. Then the depth is determined byinternal clearance, which is usually specified as 2.3mminimum. To reduce the tunnel effect and to keep the

top bracing away from casual abuse a depth of about3m is needed. Such spans will have a deeperspan/depth ratio, though the slender members will stillgive an impression of lightness.The arrangement of the bracing and the line of theparapets are the dominant features which are seenby road users. They therefore require careful attentionand treatment.Where the depth of the vierendeel girder is determinedby parapet height, the top chord can often be used asthe parapet rail, with suitable infill bars fixed betweenthe vertical members. For longer span vierendeelgirders, where the depth is more than the parapet

height, parapet panels complete with top rail can befixed inside the rectangular panels of the girder. Where atruss is used, the parapet is usually fixed to the innerface of the diagonal members. The parapets are lessconspicuous to road users than the truss members,though they are still evident in silhouette.Construction depth, from footway surface to undersideof the truss or girder, is normally quite shallow, not morethan the depth of the chord members. This contributesgreatly to the light appearance.The top and bottom chords of a truss are usually madeparallel, but for larger spans a less dominatingappearance can be achieved by a hog-back

configuration, with a gentle curve to the top chordreducing the depth at the ends of the span.The design of steel footbridges 15Conceptual design and detailingMembers and connections trussesBoth circular and rectangular structural hollow sectionsare commonly used in trusses. The bottom chord isgenerally rectangular, to facilitate connection with deckand cross-members. Rolled sections or flats aresometimes used as cross-members or as stiffeners to


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steel floor plates. Chords and diagonals are usuallyarranged with centrelines intersecting where possible.Standard welding details have been developed forhollow section connections.For half-through trusses the connection withcross-members at the lower chord requires particularattention, since its stiffness and strength arefundamental to U-frame action.Where the bottom chords are of rectangular section,some designers specify plates slotted diagonally acrossthe section at the position of the cross-members (Figure5) to prevent the chord lozenging or distorting.However, cutting slots in the hollow section and weldingstiffeners adds to the fabrication cost. Research by theSteel Construction Institute for Corus(30)showed anun-stiffened connection designed to BS 5400: Part 3 tohave a higher buckling resistance than that calculatedeven when a lower flexibility value is used.The failure loads calculated were relatively insensitive tothe actual value of connection stiffness. This showedthe use of diagonal stiffeners does not significantly addto the global strength of tubular U-frame footbridges.

Where a steel floor plate is used it normally acts as thebracing to the bottom chords, to carry the lateralshear (mainly wind forces) back to the supports. If anon-participating form of floor is used, cross bracing inthe plane of the bottom chord, to resist lateral forces,must be considered.Through trusses, used in longer spans, give lateralstability to the top compression chord by meansof bracing in the plane of the top chord. Such bracingwill also share in the carrying of any lateral forces,especially where the truss is clad on its sides and thussubject to significant wind loads. At the ends of the spanthese lateral forces have to be carried down to bearing

level through portal action or through a braced frame.Members and connections Vierendeel girdersIn footbridges, Vierendeel girders normally userectangular hollow sections for greater stiffnessand strength at the connections between verticalsand chords.The nature of vierendeel action is that vertical shear iscarried by shear/bending action of each length of chord,and the vertical members are subject to complementaryhorizontal shear and bending. Since shear is highest atthe ends of the span, the fixed end moments arehighest there also. The vertical members therefore needto be strongest at the ends of the span.

On the other hand the central portions of the chordssustain predominantly axial load, whilst the ends sustainpredominantly bending load. There is less need to varythe size of the chord members, and usually onlythickness is varied, if at all.The consequences are that the vertical members areoften wider (in the plane of the girder) at the ends of thespan and are sometimes closer together, variationswhich are clearly visible in silhouette.The strength of the joint between chord and vertical


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members must be adequate to transmit the fixed endmoments. To do this both should have the same width(normal to the plane of the girder). Under the highermoments on the joints toward the ends of the span asimple square joint may have inadequate strength, andeither triangular fillets (cut from the same section as thevertical) or reinforcing plates may need to be added toincrease stiffness and strength (see Figure 6). Theappearance of these additions may not always beacceptable and heavier sections may be preferred.Stability of the compression chord again requiresU-frame action of the cross section and this againrequires adequate stiffness and strength of thecross-member to vertical connection at the bottomchord. Even with the heavier sections usually requiredfor a vierendeel girder, it may be necessary to insertdiagonal plates, as mentioned previously.Figure 6: Detail of a haunched joint in a vierendeel girder10 thickinsert plateslotted intochord100 x 100 10 RHSWeld

groundflushFigure 5: Detail of diagonal plate through bottom chord16 The design of steel footbridgesConceptual design and detailingRight:Stiffened plate floor constructionFar right:Typical floor constructionFloor constructionThe floor of a truss or vierendeel girder footbridge willusually be of steel plate, though precast planks havebeen used with trusses. The lighter steel deck is now

generally preferred.The plate, typically 6mm or 8mm thick, is supported onand welded to steel cross-members between thechords. These cross-members form part of the U-frameswhich stabilise the top chord and are themselves usuallyhollow sections. The plate panels between chordsand cross-members are divided transversely andsometimes longitudinally by stiffeners (usually flats) togive added support.On top of this plate a waterproof layer is required forcorrosion protection, and to give a non-slip surface forsafety. 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 isabout 4mm. This surface is often applied in the worksand does not add significantly to erection weights.When precast planks are used it is necessary to providea shelf angle on the inner face of the chords on whichthe planks can sit. It is very important that the jointbetween concrete and steel is properly sealed or it couldbecome a moisture and corrosion trap.Where drainage over the edges of the bridge is notpermitted, arrangements must be made to carry


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rainwater to the ends of the bridge and then to drains ora soakaway. A vertical curve or longitudinal cambershould be provided on a bridge which otherwise wouldbe level.Where rainwater can be allowed to run off the side of thebridge (for example over a river), the floor may be slightlycambered transversely to facilitate drainage. Withstiffened thin steel plate decks, care also needs to beexercised that panels do not dish between stiffeners andallow ponding of water the spacing of stiffeners isusually limited for this reason. Weld sizes should be keptto a minimum, to reduce distortion from welding.(see GN 2.10(31))The design of steel footbridges 17Conceptual design and detailing18 The design of steel footbridgesConceptual design and detailingParapetsParapets are normally designed to comply with aDMRB standard (see section 4.2). The parapet may beeither a separate item or may be combined withstructural members.

For trusses, the parapet is provided as separate unitsfixed to the inside faces of the truss diagonals. Thediagonals must then be designed to carry lateral loadsfrom the parapet, and the parapet rails must bedesigned to span between the diagonals which supportthem. Parapet posts can alternatively be fixed to thefootway deck, though the attachment would need to bestrong enough to withstand the overturning momentarising from lateral forces on the top rail.Where vierendeel girders are used it is convenient to fixparapet 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 separateparapets on the inner faces of the girder. The top chordof the girder may also function as the top parapet rail, or,if it is higher than the required parapet height, a separaterail can be provided in addition to the top chord.CladdingOver rail tracks, the highway and rail authorities requirethat solid non-climbable cladding be provided on theinside face of the truss or vierendeel girder. This isusually achieved by profiled steel sheeting, rigidisedaluminium, 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 achievedby providing the cladding over the full length of thespan. Great care needs to be exercised in detailing thecladding, to avoid the creation of small inaccessiblesheltered ledges on the top of the lower chord wheremoss and debris can accumulate or which may be usedfor handholds or footholds.Left:Parapets in vierendeel girder, HoramRight:


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In-line splice detailFar right:Erection of Christchurch footpathThe design of steel footbridges 19Conceptual design and detailingSupportsTrusses and vierendeel girders are supported either onbearings (if they span between concrete abutments, forexample) or directly on top of a simple steelsubstructure without any bearings.At abutments the point of support is normally directlybelow the end vertical or diagonal members and thusdoes not give rise to local bending of the chord section.Other supports should also preferably be arrangedsimilarly. Where it is not convenient to do so, forinstance when a top landing cantilevers a short distancebeyond the support columns and the support is midwaybetween bracing connections, the bottom chord issubjected to bending. It is then common to use aheavier chord section over the last one or two panels ofthe truss (see photograph below right).Fabrication of trussesFabricators who specialise in hollow section fabricationare familiar with all the types of detail needed for truss

footbridges and have appropriate equipment, such asprofile cutting equipment for tubulars etc.A wide range of sizes of hollow sections is availablefrom the rolling mills, but it must be remembered thatthe fabricator has to purchase material for each job,either from the mill or from a stockist, and his ordersmay be subject to minimum quantities and premiums forsmall quantities. The designer should therefore try as faras possible to standardise his choice of section size andmaterial grade.ErectionFortunately, most footbridges can be fabricated as acomplete length of the span and then transported, with

spans up to about 45m. Although fabrications over 27min length require special permission to travel on the publichighway, most fabricators prefer to complete fabricationin the works wherever possible and are familiar witharrangements for the movement of long lengths.Bolted hollow section flanged joint details can be usedfor site splices, though it may be felt that flange plateend connections are somewhat cumbersome inappearance. In-line splice details are much lessobtrusive, but require more effort in design andfabrication (see photograph below left). In most cases,spans must be complete before lifting, because closureor possession periods will be very short.

20 The design of steel footbridgesConceptual design and detailing3.4 Steel beam bridgesTypes of constructionFour types of construction are considered in thissection: a pair of steel beams with a non-structural floor on top(e.g. timber) a pair of steel beams with a structurally participatingsteel floor plate


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a steel box girder a half-through plate girder bridge as developed byBritish RailThe first three are appropriate where depth ofconstruction is not important. The fourth is appropriatewhere minimum construction depth is critical.Proportions and appearanceFor the relatively light loading on a footbridge, the depthof beam in all cases can be arranged to be about 1/30of the span. A typical bridge over a river or canal mightthen have a span of 30m and a beam depth of 1m.A simple I-beam bridge with non-structural floor mightcomprise two girders about 1.5m apart on which is fixeda floor of, in some instances, timber planks. Parapetposts would be fixed to the top flange or the outer faceof the steel beams.Steel girders with a structural participating steel floorplate would be of similar overall proportions. Parapetswould be fixed on top of the floor plate.With both forms, the girders can have a clean web overtheir full length, as web stiffeners are needed only atsupports and on the inner faces for attachment ofbracing. The structural element therefore looks cleanand simple. The appearance will be influenced strongly

by the treatment of the parapet rails, posts and anyother feature added to the bridge. The use of simpleparapet details will contribute to a good non-fussyoverall appearance.In some circumstances a distinct curvature in elevation(more than would suffice just to aid drainage to theends) will add character to the appearance.The use of a steel box girder extends the clean lines tothe soffit of the bridge. It can be complemented by asimple basic parapet or can be contrasted byembellishment with ornate fixtures and fittings. Typicallythe box would be about 1.0m wide, with short steelcantilevers either side to provide the necessary width.

Half-through plate girder bridges will usually have theirU-frame stiffeners on the outside faces and generallylook more heavy. Nevertheless, the half-through plategirder bridge developed by British Rail (see page 22)achieves a pleasing appearance.Members and connections I-beams/girdersFor economical design, the pair of beams need to bebraced together to stabilise them against lateraltorsional buckling. Bracing at several positions in thespan will be necessary, roughly at 15 to 20 times the topflange width to achieve reasonable limiting stress levels.Bracing can simply be an X brace with single tie at eachposition, bolted to stiffeners on the inside faces of the

webs. For the main girders, fabricated I-sections arelikely to be lighter and more economic than UniversalBeams. Castellated beams can provide a weight savingin some circumstances whilst offering an interesting anddifferent appearance.Left:Footbridge using rolled sections, SwaleRight:Footbridge with timber deck and parapetsFar right:


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Box girder footbridge and cycleway, GablecrossThe design of steel footbridges 21Conceptual design and detailingA non-structural deck, such as timber planking, can besimply bolted down to the top flange of the I-beams.Particular attention should be paid to detailing, tominimise crevices where dirt and moisture canaccumulate.In many instances steel plate is used for the floor of thebridge. The plate, typically about 6mm or 8mm thick, isusually welded to the main girders and can therefore beassumed to act structurally with them. Cross-memberswill be required to carry the floor loading to the mainbeams and these are sometimes extended by short steelcantilevers outside the beam web, in which case anedge beam is provided to give a neat face and to givesupport to the parapet. A thin waterproof wearingsurface is normally specified, dressed with fineaggregate for grip and durability. The surface is oftenapplied in the works.Members and connections box girdersBox girders are essentially similar to the paired plategirders with steel deck, as described above, except thatthe bottom flange joins the two webs and encloses the

space between. They are usually considered only forspans over about 30m. The thickness of the top flangewhich also forms the floor plate will be determined byoverall bending strength rather than local floor loading.The plate is typically supported by transverse stiffenerswhich cantilever to edge beams. Two or threelongitudinal stiffeners may be provided to stiffen the floorplate when acting as the compression flange of the box.Diaphragms are needed at supports and are oftenprovided at several positions along the length of thegirder (typically the third points) to control distortion.Large holes will be required in the diaphragms if accessis required during fabrication or maintenance.

To improve appearance it is common to use slightlysloping webs, creating a trapezoidal cross section.The use of steel box girders has the advantage oftorsional strength and stiffness. They can be used incontinuous construction to simplify supports or to curvethe bridge in plan when desired for appearance. In astraight bridge, torsional restraint (usually by means oftwin bearings) is needed only at the ends: a singlebearing will suffice at intermediate supports, thusallowing the use of a single slender column.Figure 7: Cross section through a typical box girder footbridge22 The design of steel footbridgesConceptual design and detailing

Members and connections half through girdersHalf through plate girder footbridges are often used overrailways. The solid web provides the required screeningwithout the need for any non-structural additions. Thisform has developed from the half-through plate girderconcept often seen in railway bridges. A particular formdeveloped by the former Midland Region of British Railis illustrated in photographs shown above. Two featuresto note are: the use of a hollow section as top flange,turned through 45 it forms a steeple cope, which


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discourages walking along the flange; the absence ofany projection of the bottom flange prevents climbingalong the outer face.U-frame action is provided by the flat intermediatestiffeners to web and bottom flange. Typically they areprovided about every 1.5m.ParapetsWhere there are no cantilevers the parapet can either befixed to the top flange of the box or to the web of thegirder. The attachment positions should coincide withbracing or cross-members, to provide restraint againstrotation under lateral loads on the parapet rail.Where there are cantilevers, either the posts shouldcoincide with the cantilever positions or they should bemounted on a torsionally stiff hollow section edge beam.FabricationWhether using rolled I-beams or fabricated I-sectiongirders, the processes of drilling holes, adding stiffenersetc. poses no difficulty to the fabricator. The fabricatedI-section can either be made using jigs and semi-automatic welding or by a T and I automatic weldingmachine. Curvature in elevation is easily achieved withfabricated girders, and universal beams can readily becurved by specialist bending companies prior to

fabrication. Fabrication of box sections requires moretraditional methods, and the completion of the closedbox makes it almost essential for manual work internally.Details should be arranged for ease of access for workand inspection.SplicesFor spans up to around 40m, it is quite likely that thebeams would be transported full length and spliceswould not be needed. Over 40m they would be splitinto at least two lengths; site connections wouldnormally 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 233.5 Composite beam bridgesTypes of constructionComposite construction is seen in footbridges in twoforms a concrete slab on top of two I-girders or aconcrete slab on top of a closed steel box girder. Theopen steel box form with slab which is sometimes usedin highway bridges is not normally seen in footbridgesSlabs may be cast insitu, though the relatively modestextent of the shear connection and lighter design loadson the slab allow greater opportunity to employ pre-castslabs. Such slabs are provided with open pockets to fit

over the shear connectors. The pockets and the jointsbetween slab sections are filled with concrete to createthe necessary structural continuity.Proportions and appearanceComposite footbridges typically have a span/depth ratioof about 20 (depth measured from top of slab tounderside of girder).Short cantilevers outside the lines of the webs will givea better appearance, in the same way as they do forhighway bridges. A small upstand is needed at the


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edges to provide a mounting for the parapets and to actas a drainage upstand. A thick edge beam would createa rather heavy appearance.Members and connectionsComposite construction produces a much heavierstructure than an all-steel footbridge; the deadload accounts for over half of the total load in mostcases. The extra weight and consequent stiffness of thisform of construction has the advantage of being lessresponsive to dynamic excitation.Where transverse joints between precast units are notdesigned to carry transverse shear, plan bracing willalso be needed.Floor constructionReinforced concrete slabs for footbridges are typicallyabout 150mm thick. They can be constructed insitu onfalsework or by using precast slabs.Sometimes they can be cast in the fabrication yard, andthe complete composite structure transported to siteand erected.A waterproofing membrane is required, plus some formof durable wearing surface. A combined membrane andwearing course with aggregate dressing, similar to thatused on steel decks, can be used.

ParapetsAs for other forms of construction, parapets mustcomply with DMRB or Network Rail requirements.The parapet posts are fixed to the concrete slab or edgebeam with conventional holding down bolts.Opposite page:Half through plate girder footbridge, Network RailAbove:Composite curved I' beam footbridge, Washington24 The design of steel footbridgesConceptual design and detailing3.6 Cable stayed bridgesFootbridges carry only relatively light loading. However,

when the main span is long, the requirements ofsupporting its own dead load and of providing asufficiently stiff structure lead toward a much moresubstantial structure than would seem appropriate for amere footbridge. As a result, an increasingly popularsolution for longer spans is the use of a cable stayedarrangement. This effectively divides the span into shorterlengths, for which lighter beams can be used. The pylonsfor these bridges also add a strong visual feature which isoften welcomed.Types of constructionCable stays can be used with any of the forms ofconstruction previously described, though to complement

the light appearance, a slim form of deck construction islikely to be more appropriate for all except the largestspans. Supports can be provided to the main beams atabout 10m to 15m spacing, which facilitates the use of aslender deck.For most footbridges, twin planes of cable stays willnormally be used, one to each side of the bridge deck. Apylon at one end of the main span will suffice up to about100m span. Very long spans may require the use of pylonsat both ends. A frame pylons are popular, with the two


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stay planes inclined. Alternatively, individual pylon legs foreach cable plane can be arranged, or a goal-postarrangement can be used; the stays can then lie in avertical plane.Usually, at least two forestays should be provided in eachplane a single stay is hard to justify on economic orappearance grounds. The minimum span for a cablestayed bridge with two forestays is thus around 35m.A single backstay is usually sufficient, anchored to thegirder at the abutment which supports the end of thebackspan. Further backstays are only needed if thebackspan is long and requires intermediate support. Thestays are normally anchored at floor level to longitudinalbeams. The beams need to be stiff and strong enough tospan between anchor points and they may need to befairly deep. A lighter appearance, with shallow beam/floordepth, might be achieved by using a vierendeel girder andhalf-through construction. Footbridge pylons are usuallysteel box or circular sections, for slender appearance,ease of construction and economy.Members and connectionsThe cable stays will normally be made from wire rope orspiral strand. Strands are made by winding together, orlaying 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 modulusthan solid steel. Parallel wire strands are also available.Advice should be sought from specialist manufacturers onthe selection of strands.The design of steel footbridges 25Conceptual design and detailingIn the dead load condition the stays are effectivelyprestressed. It is important to calculate accurately thestretch of the stays in the dead load condition, so thatthe correct geometry of the structure is achieved.Provision should be made for length adjustment in thestays, to accommodate tolerances and errors.

Stays must obviously be sufficiently strong to supportthe beams, but often more significant for small bridgesis the need to provide sufficiently stiff supports to thebeams and to avoid slack stays which will be easilyvibrated.With twin planes of stays, the natural arrangement forthe deck structure is with main beams at either edge, towhich the stays are attached. The floor then spanstransversely between the beams. A single plane of stayscan only be used where a torsionally stiff box girder isprovided; the stays would be attached on the centrelineof the bridge. This is not normally convenient for asingle footway.

As well as provision for adjustment in length duringinstallation, attachment details should also be arrangedsuch that any stay can be replaced if need be. It is goodpractice to make sure that the anchorages are as strongat ULS as the breaking load of the stays.Under the action of live load the stays provide stiffsupport to the main beams and they thus behaveessentially as continuous beams. Axial load is alsotransmitted to the beams by the stays, so the beamsmust be designed for the combined load effects.


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For very long spans, the deflection under load changesthe geometry of the structure. If the sag of the stays issignificant they will act as non-linear springs. Both theseeffects should be taken into account in the analysis.Computer programs are available which automaticallytake account of the non-linear effects of varyinggeometry under load.Whilst ropes and strand can last the life of the bridge,experience has shown that they should beinspected from time to time to check for corrosion andfatigue, particularly at the lower ends. The stayanchorages should be accessible for such inspectionand maintenance. The design should also be such thatany one stay can be removed and replaced.Dynamic responseCable stayed bridges are relatively flexible and are moreprone to oscillation under wind or under deliberateexcitation by users. An all-steel construction results in avery low level of structural damping, which can allow theoscillations to grow significantly. The dynamic responseof the bridge should therefore be checked carefully.Artificial damping, such as tuned mass dampers, can beprovided if necessary.Floor construction

Deck construction is usually of stiffened steel plate,though timber or reinforced concrete are sometimesused instead.Far left:Cable stayed I' beam footbridge, CumbernauldLeft:Royal Victoria Dock Bridge, LondonRight:Cable stay anchorage3.7 Access ramps and stairsWhere approach ramps or stairs are needed they areusually structurally independent, except for the need tobe 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 harmonyof appearance between the two and to use a similarconstruction form.Stairs usually require, at most, one intermediate supportbeneath the landing at mid-flight. Ramps require moresupports and indeed are small bridges themselves. Evenfor ramps, the number of intermediate supports shouldbe kept as small as possible, with spans of at least 10m.Supports should also be as simple as possible aT-shaped column and crosshead should be sufficientin most cases (provided that resistance to impact is

not necessary).Where supports may be subject to impact loads, theywill need to be significantly more substantial. Thefoundations will also have to be larger. In thesecircumstances the designer can choose eitherreinforced concrete columns or a robust steel structure.Since landings are nominally level, care needs to beexercised to avoid ponding of water and accumulationof debris. Extra drain holes in these areas together witha small fall will suffice.


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Handrails must be provided on the inside faces ofparapets on stairs and ramps, for safety reasons. Aclear gap of at least 40mm is desirable between the railsand any adjacent members.Stairs normally have semi-open risers. Fully open risersare not permitted by BD 29/03.At the bottom of flights of stairs, details should bechosen which avoid acute corners, since they can trapdebris. To avoid this, stairs can be supported just abovethe bottom of the flight, so that there is a clear gapbetween the underside of the stringers and ground level.26 The design of steel footbridgesConceptual design and detailingBelow:Stairs showing open treads and handrailsRight:Scissor ramp3.8 Bearings and expansion jointsThe provisions for restraint or the accommodation ofmovement due to expansion or other reasons dependsvery much on the general arrangement of the bridge,ramps and stairs.When the bridge spans between bankseats orabutments, expansion joints are needed, and the

structure will sit on bearings. At one end the bearingsmay be fixed longitudinally, but if laminated bearings areused, both ends can be free , as long as the bearingscan transmit any longitudinal forces.Expansion joints need to accommodate movementranges of about 20mm, depending on span. Even atends which are longitudinally restrained there has to besome provision for movement at deck level, owing torotational movements under live load.For footbridge expansion joints, a simple detail shouldbe chosen, one which does not collect dirt or debris andwhich can be dismantled for maintenance if required. Asimple leaf plate fixed to the bridge on one side and

sliding on a second plate on the fixed side can usuallybe arranged in most circumstances. Particular attentionshould always be given to the avoidance of steps facinguphill, even as little as 5mm, since they always tend toaccumulate material washed down by run-off.Where the bridge spans between steel column supports,no bearings are needed. The bridge is simply bolteddown to the tops of the columns. Expansion isaccommodated by flexing of the columns and noexpansion joints are needed.Consideration should be given to fixing long ramps atthe bottom end. Maximum longitudinal movement at thefar end therefore occurs where the columns are tallest

and most able to accommodate it.Stairs should preferably be fixed at the bottom andbolted to column supports. This effectively provides arestraint for any ramp or bridge connected to the top ofa straight flight.For light all-steel bridges, all support details, bearings ordirect connections to columns, should be designed toresist at least a nominal uplift.The design of steel footbridges 27Conceptual design and detailing


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Below:Expansion joint leaf plateRight:End bearing box girder28 The design of steel footbridgesDesign codes, standards and guidance4. Design codes, standards and guidance4.1 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 connectedto a building, BS 5950(2)might be called for.For design of steel and composite structures, thefollowing Parts of BS 5400 are applicablePart 2 Specification for loadsPart 3 Code of practice for design of steel bridgesPart 4 Code of practice for design of concrete bridgesPart 5 Code of practice for design of composite bridgesPart 6 Specification for materials and workmanship, steelThese codes cover all aspects of design for footbridgesof beam and truss construction. Design of tubular joints

is not covered in detail within Part 3 see section 4.4for further guidance. Similarly, the design of cable stays,the strands and their anchorages, are not covered bythese codes refer to section 4.5 for guidance.Dimensional and safety requirements for stairs are givenin BS 5395(3). These requirements are amended slightly bythe departmental standard for footbridges.4.2 Departmental standardsThe requirements of the four UK highways authority (theHighways Agency, the Scottish Executive, the WelshAssembly Government and the Department for Regional

Development Northern Ireland) are set out in the DesignManual for Roads and Bridges (DMRB). This manual is acollection of individual standards (BD documents) andadvice notes (BA documents).Each of the design code parts of BS 5400 isimplemented by a BD standard(4), and some ofthese standards vary certain aspects of the part thatthey implement (notably BD 37 for Part 2 and BD 16 forPart 5). For footbridges, a particular point to note is thatthe requirements in relation to loads resulting fromcollision of vehicles with the structure have been

significantly modified. The impact loads and thecircumstances in which they should be applied arespecified in BD 60 & BD 37 (the DMRB version of BS5400 Part 2) and an amendment to it. The provisionsrelate to the impact loads on supports located within4.5m of the edge of the carriageway and tosuperstructures which have less than 5.7m clearanceabove the surface of the carriageway.Other standards and advice notes also relate to thedesign of footbridges. Design criteria for footbridges are


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given in BD 29(5). Highway cross sections and headroomare given in TD 27(6). Selected information from thesetwo documents is included in section 3. Standard TD 27specifies a minimum clearance for footbridges of 5.7m.This avoids the necessity of applying the impactrequirements of BD 37 on the superstructure, whichwould be particularly onerous on a light structure suchas a footbridge.Where supports need to be close to the edge of thecarriageway, they are required to be provided withprotective plinths and designed for impact loads. Wherethey can be kept back from the carriageway, perhaps tospan a footway beside the road, the consequent savingsin 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 theroad, which is not usually feasible).The design of parapets on footbridges is referred byBD 29 to the Interim Rules for Road Restraint SystemsIRRRS). The IRRRS

(7)is a Highways Agency document,not currently part of the DMRB, although it does statethat it supersedes a number of DMRB documents, suchas the earlier BD 52/93. The IRRRS refers to BS 7818(8),which gives dimensional requirements, designrequirements and a specification for construction ofmetal parapets, and it specifies the design loadingclasses for rails, posts and infill.4.3 Railway standardsNetwork Rail are particularly concerned with prevention

of unauthorised access and are legally obliged to fenceits boundaries. Network Rail and the Railway Safety andStandards Board also have more stringent requirementsin relation to collision loads. Reference should be madeto GC/RC5510: Recommendations for the Design ofBridges(27). The following comments are based on advicegiven in recent projects.The design of steel footbridges 29Design codes, standards and guidanceIn considering the prevention of unauthorised access,not only must the pedestrian face of the bridge be

designed to be non-climbable, it must also beimpossible to climb along the outer face from the endsof the bridge this usually means that trusses are cladeither side of the diagonals at the ends. The top flanges,chords or parapets must be arranged so that they areimpossible to walk along.The zone within 4.5m of the outermost running rail isconsidered a danger zone; if any support is locatedwithin that zone, collision effects must be considered.Any substructure column must be able to withstand an


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impact load, and the superstructure must be able tocontinue to carry some live load without support fromthe column. Design recommendations are given inGC/RC5510.4.4 Design of hollow section jointsThe design of hollow section joints is not fully coveredby the requirements of BS 5400: Part 3. There ishowever extensive background research into thebehaviour of tubular joints and various documents havebeen published which provide guidance.For triangulated structures, where the joints transmitessentially axial loads from one member to another, thedesign of the joint involves checks on (a) the adequacyof the welds at the end of the member and (b) thebending of the walls of the hollow sections (which aresubjected to out of plane forces).Guidance literature is available both for circular sectionsand for rectangular sections. General guidance is givenin CIDECT publications(9), (10) & (11)and guidance in relationto 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. Similarrules will be included in EN 1993-1-8(13).The extent of guidance on the design of joints for themoments associated with vierendeel action (or withU-frame action) is more limited, though there has alsobeen research on this topic. A stiffer and more efficientjoint is achieved when the bracing member is the samewidth (normal to the moment plane) as the chordmember. Design guidance for this type of joint can alsobe found in a Corus publication(12)

. Adequacy of boththe bracing member and the chord member must bechecked. If necessary, reinforcement of the joint canbe designed.4.5 Design of cable stayed andsuspension bridgesFor general guidance on the design of cable stayedbridges, reference should be made to standard texts,such as Walther(14)or Troitsky(15). These are

comprehensive books, but they do include specificcomment on footbridges with illustrated examples.The provisions of BS 5400 do not cover in detail thedesign of wire ropes or similar elements, nor is there anyother appropriate national code. The designer thereforeneeds to base his detailed design on an empiricalapproach, based on load effects calculated in the usualmanner according to BS 5400 and adopting the generalobjectives of the code.Details of the specification of wire ropes and strands


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can be found by reference to BS 302(16), and of thesockets by reference to BS 463(17). The cold drawn wireused for ropes and strands does not have a linearstress/strain relationship, with a definite yield plateau,as does structural steel. The relationship is generallysmooth, with decreasing tangent modulus as loadincreases. Design of stays has therefore been basedtraditionally on permissible stresses calculated bydividing the ultimate or breaking strength by a suitablylarge factor (i.e. a working stress philosophy). In theabsence of formal codes on a limit state basis, divisionof this strength by a partial factor gm of about 2.0 atULS, in conjunction with normal values of g1and g3

ives results consistent with the traditional approach.Guidance on the desi n of suspension brid es can befound in texts such as Pu sley(18)

. The tensile elementsmay be wire rope or strand, as for cable stayed brid es,thou h hi h tensile steel rods may be used for the maintension members.30 The desi n of steel footbrid esDesi n codes, Standards and Guidance4.6 Desi n of steel and compositebrid e beamsGuidance on the desi n of composite hi hway brid esis iven in a series of publications by The SteelConstruction Institute(19). These can be used as eneral

uidance in the desi

n of footbrid

es in accordancewith BS 5400, both for composite beam and all-steelbeam desi ns.Guidance on a wide ran e of practical aspects related tosteel brid e construction is iven in a series of GuidanceNotes produced by the Steel Brid e Group(31).4.7 Dynamic responseLimitations on the dynamic response of footbrid es are

iven in HA standard BD 37. The vertical naturalfrequency of many footbrid es will be below 5Hz andthe response must be checked. If the horizontal natural

frequency is less than 1.5Hz, checks must be made forpossible lateral excitation.The susceptibility of a footbrid e to aerodynamicexcitation has to be checked in accordance withBD 49(20). Brid es under 30m span are unlikely to besusceptible. Detailed rules are iven in BD 49 forbrid es that are susceptible.4.8 Protective treatment


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For brid es subject to hi hways authority requirements,the protective treatment specifications should beselected from those listed in the uidance notes to theSpecifications for Hi hway Works (SHW)(21), (22). Whenusin those notes, access conditions should normallybe taken as difficult, which will result in use of metalspray for the first coat. Galvanisin may be suitable forsmall components, such as parapets.For Network Rail owned brid es, the protectivetreatment and walkway surfacin must comply withNetwork Rail line standard RT/CE/S/039(28). Advice is

iven in RT/CE/C/002(29).For other brid es, the HA specifications, or alternatives,may be used, with the clients a reement.In some circumstances, Weather Resistant Steels mi htbe used, provided that environmental constraints can bemet.(23), (24)

4.9 Steel materialsSteel material for plates, rolled sections and structuralhollow sections is covered by British StandardsEN 10025, EN 10210(25). Information about the productsavailable from Corus(26)can be obtained from the CorusConstruction Centre. Contact details are on the back ofthis brochure.The desi n of steel footbrid es 31Flow charts

(Fi

ure 5.2) (Fi

ure 5.3) (Fi

ure 5.4) (Fi

ure 5.5)Trusses andvierendeel irdersSteel beams Composite beamsChoosestructural formDetermine eometricconstraintsScheme-specific detailsCable stayed brid es Ramps and stairsFi ure 5.1: Flow dia ram for the desi n of footbrid es5. Flow charts

DMRB Standardsfor footbrid esDMRB Standardsfor hi hwaycross sectionand headroomFar left:Renaissance Brid e, BedfordLeft:Smithkline Beecham, Marlow


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32 The desi n of steel footbrid esFlow chartsCheck adequacyat ULSCheck asa `truss'Global analysisGlobal analysisDetermine effectivelen thsCheck adequacyat ULSDetermine effectivelen thsCheck U-Frame actionCheck adequacyof lateral bracin TensionmembersCompressionmembersLon itudinal effects Lateral effectsTensionmembers

CompressionmembersTrian ulatedtruss?Stren thadequate?Stren thadequate?Stren thadequate?Stren thadequate?Stren th

adequate?Slender orcompact?Check adequacyat ULSSatisfactoryCheck adequacyat SLSYes* For in-plane bucklin , use the len th between intersections (a); for out of plane bucklin use (a) if there are effective lateral restraints or use 12.5.1otherwise.12.2.3

10.6.210.6.312.2.3NoYes10.6.112.412.511.5.29.9


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I=a*12.5.1Yes 10.6.210.6.39.9YesYes12.211.5.112.1 12.612.5YesYes No12.3Check adequacyat ULSFi ure 5.2: Flow chart for trusses and vierendeel irdersCheck combinedbendin and axialeffectsThe desi n of steel footbrid es 33Flow chartsCheck ULS momentand shear capacities

SatisfactoryYes NoYes9.149.9.89.99.69.79.89.49.109.119.16

9.17YesNo YesFi ure 5.3: Flow chart for steel beamsCheck adequacyat SLSCheck bearin stiffenersUnsymmetriccompactsection?Check diaphra msand crossframes

All stren

thsadequate?All stren thsadequate?Determine limitin stresses for LTBDetermine effectivesectionDetermine limitin stresses and check


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capacitiesBox

irder?Global analysis34 The desi n of steel footbrid esFlow chartsSatisfactory Satisfactory9.9.89.9.5.2Yes9.145/5.2.4.25/5.2.64/4.1.1.1NoYes5/6.1.24/4.8.39.9Fi ure 5.4: Flow chart for composite beamsAll stren thsadequate?Check slabadequacy at ULS

Checkbearin stiffenersUnsymmetriccompactI-beam?Check slabadequacy at ULSCheck beamadequacy at ULSGlobal analysisCheck beamadequacy at SLSYes

Fi

ure 5.5: Flow chart for cable stayed brid

esDetermine dead loadprestress in staysCheck adequacyof cable staysCheck localeffects at cableanchora esCheck adequacyof pylonCheck adequacyof members astrusses or beams

Global analysisNon-linear analysis ifdeflections or DL sa of stays are si nificantAll stren thsadequate?Include effectsdurin replacementof each stayThe desi n of steel footbrid es 35


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References6 References1. British Standards InstitutionBS 5400: Steel, concrete and composite brid es Parts 1 to 10,BSI, London (various dates)2. British Standards InstitutionBS 5950, Structural use of steelwork in buildin , BSI, London3. British Standards InstitutionBS 5395, Stairs, ladders and walkways, BSI, London4. Hi hways A encyDesi n manual for roads and brid es, Volume 1 Section 3:BD 13, Desi n of steel brid es: use of BS 5400 Part 3;BD 16, Desi n of composite brid es:use of BS 5400: Part 5;BD 37; Loads for hi hway brid es,BD 60; The desi n of hi hway brid es for vehicle collision loads,The Stationery Office5. Hi hways A encyDesi n manual for roads and brid es, Volume 2, Section 2, BD 29Desi n criteria for footbrid es, The Stationery Office6. Hi hways A encyDesi n manual for roads and brid es, Volume 6 Section 1, TD 27Cross-sections and headroom, The Stationery Office7. Hi hways A encyInterim Requirements for Road Restraint Systems (IRRRS), The

Hi

hways A

ency, 2002 (contact the Hi

hways A

ency for copies)8. British Standards InstitutionBS 7818:1995 Specification for pedestrian restraint systems inmetal9. CIDECTDesi n uide for circular hollow sections (RHS) underpredominantly static loadin , Verla TV, Colo ne, 199110. CIDECTDesi n uide for rectan ular hollow sections (RHS) joints underpredominantly static loadin , TV, Colo ne, 199211. CIDECTStructural stability of hollow sections, Verla TV, Colo ne, 199212. Corus Tubes

Desi

n of SHS welded joints, CT16, Corus Tubes, Corby 200113. British Standards InstitutionprEN 1993-1-8, Desi n of Steel Structures, Desi n of Joints,December 200314. Walther, R. et al,Cable stayed brid es, Thomas Telford, London, 198815. Troitsky, M. S.,Cable-stayed brid es, BSP, Oxford, 198816. British Standards InstitutionBS 302, Stranded steel wire ropes, BSI, London17. British Standards InstitutionBS 463: Part 2:1970 Specification for sockets for wire ropes(metric units), BSI, London

18. Pu

sley, A.The theory of suspension brid es, Edward Arnold, London, 195719. Iles, D. C.Desi n uide for composite hi hway brid es (P289)Desi n uide for composite hi hway brid es: Worked examples(P290)The Steel Construction Institute, 200120. Hi hways A encyDesi n manual for roads and brid es, Volume 1, Section 3, BD 49,Desi n rules for aerodynamic effects on brid es, The Stationery


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Office21. Hi hways A encyManual of contract documents for hi hway works, The StationeryOffice; Volume 1: Specifications for hi hway works series 1900,Protection of steel a ainst corrosionVolume 2: Notes for uidance on the specification for hi hwayworks,Series NG1900, Protection of steelwork a ainst corrosion22. CorusCorrosion Protection of Steel Brid es, 200223. Hi hways A encyDesi n manual for roads and brid es, Volume 2, Section 3, BD 7,Weatherin steel for hi hway structures, The Stationery Office24. CorusWeatherin Steel Brid es, 200225. British Standards InstitutionBS EN 10025: 2004, Hot rolled products of structural steels.BS EN 10210, Hot finished structural hollow sections of non-alloyand fine rain structural steels, Part 1: 1994 Technical deliveryrequirements.26. CorusProduct & Technical brochuresStructural sectionsStructural plates

Structural hollow sections27. Railway Safety and Standards BoardGroup StandardGC/RC5510: Recommendations for the Desi n of Brid es28. Network RailLine StandardRT/CE/S/039; Specification RT98 - Protective Treatment forRailtrack Infrastructure29. Network RailLine StandardRT/CE/C/002: Application and Reapplication of protectivetreatment to Railtrack Infrastructure30. Corus Tubes

Connection flexibility in tubular U frame footbrid

es RT 451,December 199431. Evans, J. E. and Iles, D. C.Steel Brid e Group: Guidance notes on best practice in steel brid econstruction (P185), The Steel Construction Institute, 2002Care has been taken to ensure that thisinformation is accurate, but Corus Group plc,includin its subsidiaries, does not acceptresponsibility or liability for errors orinformation which is found to be misleadin .Copyri ht 2005CorusDesi ned and produced by

Orchard Corporate Ltd.www.corus roup.comCorus Construction & IndustrialPO Box 1Bri RoadScunthorpeNorth LincolnshireDN16 1BPT +44 (0) 1724 405060F +44 (0) 1724 404224


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