Footbridge Design for the Dubai

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AbstractThe Dubai Metro Light Rail scheme is a flagship project in the United Arab

Emirates which is the longest fully automated rail system in the world. Thefirst line of the rail system was opened in September 2009 with a second linedue for completion in 2010. These first two lines include 35 elevated stations

along their combined 76 kilometre length. This paper describes the design andconstruction of the steel truss footbridges developed as part of the stationcontext planning work for all of the elevated stations. The footbridges are all

fully enclosed, air-conditioned corridors with the widths of many dictated bythe provision of automated walkways. A key aspect to the footbridge designconcept was to develop a modular system that could be rapidly designed

for scores of differing span arrangements to be suitable for each unique

location. Several of the footbridge truss elements comprised structural hollowsections, and the new rules in Eurocode 3 were adopted to design many of

the connections between such members. With simply supported spans up to45 metres long and external cladding creating irregular shaped cross sections,wind tunnel testing was required to demonstrate aerodynamic stability of the

footbridges. Several erection methods were also considered to minimise theinstallation time of completed footbridge spans over the major Dubai highways,and the pre-camber and deflection analysis associated with the methods

adopted for lifting were also important aspects considered in the design. Othercritical design issues resolved included the design of fillet welded connectionsin place of full strength full penetration butt welds and the design of several

special spans for connecting into non-standard stations and entrance structures.

Introduction

In July 2005, the government ofDubai Road and Transport Authority

(RTA) awarded a design and

build contract to the Dubai RapidLink (DURL) consortium for the

construction of the first stages of theDubai Metro Red and Green Lines.The DURL consortium comprised the

Japanese companies Mitsubishi HeavyIndustries, Mitsubishi Corporation,Obayashi Corporation and Kajima

Corporation together with YapiMerkezi of Turkey. Constructionof the infrastructure and stations

was the responsibility of a jointventure between Kajima, Obayashi

and Yapi Merkezi (Japan-Turkey-Metro joint venture or JTMjv).The JTMjv appointed Atkins astheir designer in 2006 and station

context planning design workcommenced in August 2007.

Senior Engineer

Engineer

Engineer

Tony Smith

Manuela

Chiarello

Alan Ho

Regional Head ofBridge Engineering

David A Smith

Highways & Transportation


Middle East & India


67Footbridge design for the DubaiMetro light rail project

Figure 1 - Red and Green Line Route maps


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Several of the footbridge trusselements comprised structuralhollow sections, and the new rules

in Eurocode 31 were adopted todesign many of the connectionsbetween such members. With simply

supported spans up to 45m long andexternal cladding creating irregularshaped cross sections, wind tunnel

testing was required to demonstrateaerodynamic stability of thefootbridges. Several erection methods

were also considered to minimisethe installation time of completedfootbridge spans over the major Dubai

highways, and the pre-camber and

deflection analysis associated with themethods adopted for lifting were also

important aspects considered. Othercritical issues resolved included thedesign of fillet welded connections in

place of full strength butt welds andthe design of several special spans forconnecting into non-standard stations.

This paper describes the design

and construction of the steeltruss footbridges connecting theelevated stations to their entrances.

Separate papers2,3 cover the designand construction of the viaduct

superstructure and substructure.

Footbridge form

and development of

preliminary design

The proposed form of the footbridges

was architecturally-led in appearance.Figure 3 gives an artists impressionof the proposed footbridge at the

conceptual design stage and muchof this form was retained in the finaldetailed design. The initial preliminary

structural design and architecturalperspective only included the 4.5mwide spans however, and was based

on the elevations being glazed overtheir full height. Hollow sectionswere chosen for the truss chords and

verticals with tie bars preferred bythe architect for the truss diagonals.

The total distance between entrancepods and station was up to 400m

at some stations and a decision wasmade at an early stage in the designprocess to include travelators when

travel distance exceeded about100m. Two 1.4m wide travelatorswere added to the cross section with

a normal walking zone between soas to provide the same deck areaif one travelator was out of use.

The Dubai Metro is a driverless,fully automated metro networkand is the longest fully automated

rail system in the world. Thefirst section of the Red Line wascompleted in September 2009, and

is due to be followed in 2010 withthe first section of Green Line.

Detailed metro route maps are

illustrated in Figure 1. These firststages of Red and Green Linesincluded 35 elevated stations. Each

station needed to be connectedto entrance pod structures or carparks either side of busy, often

congested existing infrastructure,with a requirement for unique spanarrangements for a family of over

200 footbridges. Each footbridgecomprised a fully enclosed, air-conditioned corridor with the

widths of many dictated by theprovision of automated walkways.Additional structural steelwork

was also provided above pierswhere footbridge direction changeoccurred in plan. A key aspect to

the footbridge design concept was

to develop a modular system thatcould be rapidly designed for scores

of differing span arrangements to be

suitable for each unique location.

To give an overview of the extentof Red Line and Green Linefootbridges there is in excess of:

200 spans

70 spans with travelators(automated walkways)

9000 tonnes of structural steel

4.5km total length

Approximately 25,000

m2 of deck area.

All spans were based on an N-truss

design (Figure 2) with bracedfloor and roof planes. Three basicfootbridge types were designed:

Narrow Standard - 4.5mwidth between trusses,maximum 45m span

Travelator/Wide Standard -

7.7m width between trusses,maximum 45m span

Low Height Spans wherefootbridges passed below

the metro viaduct - 4.5mwidth between trusses,

maximum 26m span.

Footbridge design for the Dubai Metro light rail project67

Figuire 2 - N-truss type footbridge

Figure 3 - Initial architectural image


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Including the travelator structureand clearances, this increased theoverall width between truss centres

to 7.7m (as shown in Figure 4).

Deflection of the span under liveloading was critical to the successfulfunctioning of the travelator.

Preliminary assessment showed thatsingle tie bar diagonals would nothave sufficient axial stiffness to limit

this deflection. Twin tie bars wereconsidered but discounted becauseof concerns over load sharing and

the space required to accommodatethe end fixings. Hollow sectiondiagonals were therefore adopted.

Diagonals to the narrow spans werealso changed to hollow sectionsto give a similar appearance to all

footbridges throughout the project.

Another change from the initialpreliminary design was to move the

mechanical and electrical (M&E)services from below the floor to abovethe roof beams. The total weight

of services also increased. The M&Eservices included pipes supplyingchilled water to the stations and the

total load could be up to 10kN/m.

67Footbridge design for the Dubai Metro light rail project

Figure 4 - Typical footbridge cross sections (a) Narrow deck (b) Wide deck

a

b


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The piers generally comprised 1.75mdiameter columns founded on 2.2mdiameter monopiles, embedded

into the ground between 10mand 25m. A typical standard pieris shown in Figure 5. Using the

same design basis as used for theviaducts3, the pile vertical loadingresistance was provided entirely by

skin friction without any reliance onend bearing. The use of a monopilesolution minimised the excavation

requirements for the foundationconstruction and limited the durationof road closures or carriageway

possessions in the urban environment.

Design and detailing issues associatedwith monopile construction are

discussed further in Smith and Hendy3.

Due to the lack of availability oflarger drilling rigs in the early stages

of construction, 2.2m diameterpiles were the largest bored pilesavailable for use. At locations where

the 2.2m diameter monopiles couldnot be designed to give sufficientstrength (especially at several junction

locations where bending momentsat the base of the pier were highdue to the large eccentricity of

the bearings), twin 2.2m diameterpiles were used as a group.

Adopting twin pile groups was notalways easy however, due to the

many conflicts between the enlargedfoundation footprint and the existingutilities or highways. Several bespoke,

twin-pile foundation solutions weretherefore designed to solve specificconflicts. Such solutions included:

Designing an opening inthe middle of the pile cap(in elevation) to allow water

mains pipes to pass through

Offsetting the pile group from thepier centre to avoid gas mains

Rotating the orientation ofthe piles in plan to avoid

existing highways.

Later in the construction programme,larger piling rigs became available as

the Green Line viaduct substructurework was completed. 2.6mdiameter piles were introduced

for the footbridge foundations to

replace the twin pile groups andminimise the need for bespoke pile

group designs at many locations.

Placing these services at roof level

had significantly adverse effects onthe structural dynamics. Also, at therequest of the contractor, much of the

steelwork not visible in the completedwork was changed from hollowsections to rolled sections. Figure

4 illustrates the final cross sectionsadopted for the wide, travelator spansand the standard narrow spans.

The machinery end pits for thetravelators also needed to be locatedwithin the footbridges. These pits

extended to approximately 1100mmbelow the floor level and occupied thespace vacated by the M&E services.

Their presence impacted on the

local design of truss elements inthis location as well as affectingprecamber calculation, dictating

travelator setting out and influencingthe erection methodology.

Substructure designTo meet the tight footbridgeerection programme, it wasnecessary to complete the design

and construction of the footbridgesubstructures within a 12 monthperiod. To accelerate the design

process, the footbridge substructuredesigns were developed from theviaduct substructure designs already

completed and mostly constructed.

Figure 5 - Typical standard pier arrangement during construction

Figure 6 - Special pier head

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Throughout the project, thecontractor encountered considerable

difficulties in arranging majorutility diversions (permanently ortemporarily) in a timely manner

to meet the overall constructionprogramme. The exact locationof many of these utilities was not

discovered until trial pits werecompleted once access had beenprovided. Thus the decision on

final piling locations was alwaysa difficult compromise betweenavoiding clashes, minimising utility

diversions and finding a suitableposition for structural support suchthat as many standard footbridge

designs as possible could be used.

The footbridge pier heads whichtransfer the vertical load from thefootbridge truss to the pier columns

were designed with differentshapes to suit the orientationsof the footbridge spans in plan,

according to the design alignment.

The most commonly used pier headswere modified from the form of thoseused for the viaduct pier heads which

comprised prestressed concrete beamsformed from precast concrete shells

with in-situ concrete infill. Due tothe lower overall loads however, thefootbridge pier heads did not requirepresetressing and were constructed

from reinforced concrete alone.

To further shorten the constructionperiod, the need for shear linksbetween the outer shells and the

inner concrete infill used in theoriginal design was eliminated, bytreating the precast shell as a non-

structural, permanent formworkcomponent. To save further time in

construction, the standard footbridgepier heads were constructed fromthe same outer curved profilesused for the viaduct pier heads.

At locations where the footbridgetrusses required a wider angular

change in plan, or at locations where

more than two footbridges combineat a junction above the same pier, the

bearing locations from the standardtrusses could not be accommodatedwithin the extent of the standard

pier head. Bespoke in-situ reinforcedconcrete pier heads were designedfor such junctions. The appearance

of these bespoke pier heads wasdeveloped in conjunction with thearchitects to ensure their style was

consistent with the standard pierheads however. The complex curved

profiles of the standard pier headswere replaced by a combination offlat surfaces to reduce the complexityof the formwork on site (Figure 6).

The footbridge trusses are supported

on the pier heads along their edgesand a steel junction frame wasuniquely designed on top of the

junction pierheads to fill in the gapbetween the trusses. Such a structuralarrangement significantly simplified

the design and construction of thefootbridge trusses themselves, whichled to swift design output whenever

design changes were required.

Standard deck design

The superstructure steelworkwas designed to BS 5400:Part 34

including the BD 135 modifications.Connections involving hollow sectionswere designed to BS EN 1993-1-81

with partial factors adjusted for usewith BS 5400 load combinations.

BS EN 1993-1-8 includes rules andformula for design of connections

between hollow section componentsand also between hollow sections andI-beams and plates. These rules are

generally those developed in variousCIDECT (Comit International pourle Dveloppement et lEtude de la

Construction Tubulaire) publicationsand given in CORUS/British Steeldesign guides, but include several

arrangements not in the designguides. Even with the additionaldetails covered by BS EN 1993-1-8

some connection details were not fullycovered. For example, floor beamswere eccentric to the centre line of

the chord and some connections hadboth I-beam and hollow sectionsframing in to a further hollow

section. Where these occurred theformulae had to be used with care.

Figure 7 - U-shaped stiffeners



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Design automation

for specific locations

Span lengths were dependent onwhere piers could be located whichin turn depended on ground level

and sub-surface obstructions. Asa result nearly every span was adifferent length. The basic truss

bay length between verticals wasset at 4.8m to align with glazing/cladding joints at 1.2m centres and

external feature ribs at 2.4m centres.Transverse roof beams were alignedwith the vertical truss members;

floor beams also aligned with theverticals with additional beams athalf spacing (2.4m nominal) but

floor beams had to be excludedat travelator pit locations.

Clearly most spans would notbe an exact multiple of 4.8m so

several options were considered fordealing with this length variation.The final solution adopted was

based on the following logic:

Span symmetric evennumber of bays

No bays greater than 4.8m long

End bay not shorter than

2.4m (if possible)

The wide spans had hollow section

verticals and diagonals framing into I-beam chords; these connectionscan be inefficient since much of

the axial loads from the verticals/diagonals need to pass into theI-beam web. To enhance the capacity,

U-shaped web stiffeners cut fromhollow sections were provided tocontinue the verticals and inclined

web plates were added to continuethe line of the diagonal as shown in

Figure 7. The U-shaped stiffeners alsoprovided a connection point for thetransverse floor and roof beams.

Several inconsistencies anduncertainties were found in using

EN 1993-1-8 and these wereraised with the Standard draftingcommittee which resulted in a

(then) unpublished corrigenda beingmade available. The corrigendawere quite extensive including some

significant changes that hadntbeen anticipated. Fortunately these

changes did not have an adverseeffect on the large number ofconnections already designed.

The structural analysis was basedon simple 3D space-frame models

without joint eccentricity. For standardspans only the longest span wasanalysed but most special spans

were analysed individually. Lateralbracing diagonals were considered astension only following a contractor

requested change from hollowsection to angles. Destabilisingforces (FR and FS) were considered

separately on a plane frame model of

the end ring frames and combinedwith those from the 3D model for

design of sections and connections.

For the travelator spans, two deckconfigurations were considered travelators running the full length of

the span and no travelators within the(wider) span. The latter gave the worstdesign load for most elements since

live load was carried over a muchgreater area. Most real spans hadsome lengths with full-width deck.

Figure 8 - Typical automated schematic CAD drawing

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Allow 2nd, and in extreme cases

3rd, bay from end to be 3.6m

End bay should be shorterthan 2nd bay.

Whilst the contractor/fabricator

was responsible for producing the

fabrication drawings for individualspans there was a requirement for

the designers to provide data onbay length and travelator positions.Travelator spans also contained 5

different floor beam designs whoseposition depended on exact travelatorlocation. Initially it was planned to

provide this information in tabularform only on drawings but it soonbecame clear that some drawn

information was required to supportthe tables. Faced with having todraw, and in many cases revise several

times, the layouts for over 200 spans,it became obvious that this requiredautomated drawing production.

Span data was already supplied bythe substructure team in spreadsheetform and further spreadsheets were

used by the superstructure designteam to determine bay lengths

and position travelator supports.These spreadsheets were used withminimal amendment to provide

data for an AutoCAD macro.

Typically a layout drawing covered arun of up to 5 spans at one station

and provided elevation and floorlevel plans for each span togetherwith a diagrammatic elevation of

the travelator. Plans and elevationswere aligned in terms of chainagegiving a visual appreciation of their

relative positioning. Travelatorspan lengths were included on thetravelator elevation but all other

dimensions were covered by a tablelinked to the spreadsheet. The floorplan also included the floor beam

type references. Figure 8 showsextracts of a typical automateddrawing. With the spreadsheet data

available, production of the drawingitself only took a few minutes - ittook longer to add revision clouds.

Gradients and changes in directionin plan were not shown, but stationsand junction types were indicated.

As the resulting drawings werestandard AutoCAD drawings, finalfinishing adjustments such as adding

special end details could easily beincorporated. The drawings alsoincluded information on pre-camber,

lifting point position and crossreferences to other drawings. Bearingtype details were also included

initially but this information was latertransferred to a separate drawing.

As noted previously, travelators were

deployed for walk distances greaterthan approximately 100m. Duringthe design, consideration was given

to using travelators up to 100m longand also to containing travelators

wholly within a single span. Theformer was rejected as it would stillhave left long walking distances inthe event of breakdowns; the latter

whilst removing travelators crossingmovement joints resulted in lots of

Figure 9 - Footbridge idealisation for dynamic analysis



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short travelator lengths and was alsorejected. The final solution involvedtravelators up to 50m long, continuing

from one span to the next if needed.

The travelators have their own sidetrusses which are supported of themain footbridge floor beams at a

maximum of 9.6m centres usingsimple bearings. End pit spans werelimited to 7.2m but had to be at

least 5.2m long. Where travelatorspassed over joints between spans itwas necessary to keep the travelator

support point as far from the jointas possible in each span to minimisethe change in grade of the travelator.

The travelator was never supported atthe footbridge truss end ring frame.

Aerodynamic stability

In-keeping with the basic standard

designs, simply-supported spansup to 45 metres in length werechecked for susceptibility to

aerodynamic excitation againstthe simplified rules in BD 496.

Natural frequency analyses were

undertaken to obtain the primaryvertical, lateral and torsional modesof vibration. Finite element models

were developed using LUSAS7 with3D thick beam elements used tomodel the primary structural members

of the steel truss, concrete pierhead, pier column and foundationas illustrated in Figure 9. Joint

elements of appropriate stiffness andfreedoms were used to representthe articulation of the bearings;

joint elements with an appropriateeccentricity were also used aslumped masses at specific locations

to represent the weight of cladding,services, walls and floor and rooffinishes. Exact locations of the lumped

masses were refined in steps froman initially crude, global applicationto a more refined distribution

in order to avoid local bucklingmodes which had the potential tomodify the overall response of the

structure. Eccentricities betweenthe pier head and the footbridgefloor levels were also taken into

account using rigid link elements.The soil-structure interaction of thepiled foundations was modelled

using equivalent cantilevers.

Figure 10 (a) - Natural frequencies - Torsional mode (1.41 Hz)

Figure 10 (b) - Natural frequencies - Lateral mode (1.59 Hz)

Figure 10 (c) - Natural frequencies - Vertical mode (1.97 Hz)



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The fundamental torsional (combinedwith transverse), lateral (longitudinal)and vertical modes obtained for

the footbridge are illustrated inFigure 10, with frequencies ofapproximately 1.41Hz, 1.59Hz

and 1.97Hz respectively. Thesedominant modes were identifiedon the basis of an analysis of total

mass participation over a range offrequencies for each excitation vector.

The fully-enclosed nature of thefootbridges together with the

irregular-shaped cross sections due tothe geometry of the external claddingmeant that the footbridges fell outside

the scope of BD 49/01. Nevertheless,using the rules for an initial calculationsuggested that, whilst the footbridges

satisfied the limits for vortexexcitation, they didnt comply withthe divergent amplitude response

checks for galloping and stall flutter.

It was therefore necessary to

demonstrate by other means thatthe wind speed required to induce

the onset of galloping was in excessof the critical wind speed limit.

A wind tunnel test was subsequentlycommissioned to determine this andalso to calculate an equivalent drag

coefficient for the specific sectiongeometry. The wind tunnel tests werecarried out in BMT Fluid Mechanics

Limiteds aeronautical wind tunnelusing a 2 dimensional, 1:25 scalesection model of the bridge deck,

illustrated in Figures 11 and 12.

Static load coefficients were measuredin smooth flow for wind angles in the

range of 5. Divergent responseswere investigated in smooth flow fora series of configurations (with and

without a ground plane) for a rangeof wind angle and structural damping.

The footbridge was found to

be stable up to wind speeds ofapproximately 70m/s (sufficiently

in excess of the maximum designwind speed of 54m/s) based on

the first bending frequency of thebridge at approximately 2.0Hz.

For a logarithmic decrement due

to structural damping of 0.03,considered appropriate for structureslike the Dubai Metro footbridge,

the vortex shedding responsesfrom the testing were negligible.

The footbridge was also checked forvibration serviceability to BD 378, toensure that the structure was not

excessively excited by pedestrian use.

Footbridge erection

Most primary fabrication took placeoutside Dubai. Originally it wasconsidered that large sections of

side truss could be fabricated anddelivered to site laid on their sidewith a minimum of site welding. In

practice, delivery was in much smallercomponents with increased sitewelding. Many of the metro stations

are situated along Sheikh ZayedRoad (SZR), a major dual carriagewayroad with up to 6 lanes in each

direction (as indicated in Figure 13).

Carriageway possessions forfootbridge erection and construction

were clearly very limited and so thesespans were constructed includingcladding and finishes to the side of

the road on temporary trestles andthen positioned on the piers duringovernight possessions using self-

propelled modular trailers (SPMTs).Figure 14 shows a typical footbridge

span being erected using a SPMT.

With most of the permanent load

in place when lifting the spans withthe SPMT, lifting points needed tobe under a truss vertical and end

clearances meant these were often2 bays in from each end. The needto brace the compression chord

above the lifting point meant thatlifting points could not be withinthe length of the travelator end

pits - where transverse beamsand bracing had to be omitted.

Positioning of travelators wastherefore inter-dependent on methodof erection and span geometry.

Figure 11 - Footbridge model in wind tunnel

Figure 12 - Footbridge model in wind tunnel



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the station structure itself. At theinterface the station roof is curvedin both plan and section. Since plan

curvature was small, the bearingswere offset slightly from the stationbeam web to give a square end to

the footbridge in plan. The curve insection however, meant that the topchords and end ring frame needed to

be set back from the bearing locationwith short bottom chord stubs.

Two spans, one at each of MarinaStation and Jameira Lake Towers

Station, were to be built over stationsfor the Al Sufouh tram and neededpedestrian interface with the tram

access. The exact tram alignmenthad not been established however,and so a footbridge pier could not

be positioned within the station. AnN-truss arrangement does not allowfor side access within the span and

it was necessary to modify thesespans to include a Vierendeel truss

section. This required larger chordsand verticals; the bottom chord alsohad to be lowered to permit thefootbridge deck to pass over it.

At Nakheel Station the adjacentfootbridge spans had to be higherthan the station concourse level

to give clearance to the adjacenthighways. A flight of stairs withinthe footbridge was introduced

at the station end together witha lift for disabled access. Furthercomplications for these spans were

that the overall depth had to bereduced above the stairs and lift tofit below the metro viaduct and a

station roof brace member had topass through the bottom chord.

Also, for one span, the width hadto reduce from being wide at the

remote end to narrow at the stationend with the width change takingplace over a short distance within

the span. Clearance between theroof brace and footbridge chord hadto be sufficient to accommodate

the independent flexing of bothstructures under transient loads. Theexact location of the opening in the

footbridge chord could not be finallyestablished until the roof was erected.

A span with width change but

without the additional complicationsof stairs and station roof bracewas also required at Al Qiyadah

Station on the Green Line.

This also included an end detail fordirect connection to a Type T2 station.

At Al Karama station a span with35o end skew was required meaningit needed one more truss bay on

one side. Although the longestside of this span was close to themaximum standard span design

length, accommodating the skewwas mainly a detailing problem.

About one-third of the stationsinvolved footbridge runs with change

of direction or bifurcations; manyof these were of simple 90o L or T

form with most combinations ofwide to narrow spans covered. Thesewere designed as structural steel

extensions mounted directly on the

pier-head concrete and independentof the footbridge span steelwork.

Figures 16 and 17 illustrate a typicalexample of this. Small angularchanges were accommodated by

tapered cantilever extensions tothe span steelwork with largerangular changes using independent

steelwork as for T and L junctions.

Design and construction

problems

Primary section design was

straightforward but connection designdetails were a critical issue, particularlythe connection between the floor

beam and vertical of the end frames.In normal working conditions themoment on this connection was not

too problematic but future bearingreplacement requires jacking underthe floor beam remote from the

main truss planes, generating verylarge moments in the connection.

Figure 14 - Footbridge supported on SPMT

Figure 15 - Footbridge erection using mobile crane



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Data from these spans was then used

to generate algorithms from whichwelds for further spans could berapidly assessed. Butt welds were used

on all remaining spans not fabricatedat the time the error was discovered.

The floor construction consistedof a reinforced concrete slab with

permanent formwork topped bytiles with mortar bedding and thestructural design specified this to be

of constant thickness over the wholespan, following the deck pre-camber.

For the narrow spans there wassufficient space between floorlevel and top of pier to deepen

the beam but the floor beams ontravelator spans had to be lowerfor the travelator to pass over and

could not be deepened sufficiently.

Hollow section verticals hadstrengthening plates addedbut it was still necessary to

haunch the beam ends withinthe short distance between thetravelator and end vertical.

Trial pits were required at most

pier positions before they could befinalised. Inevitably some services

were not where expected and piershad to be re-located or deletedand this led to continual needs toupdate the span layouts. The use of

the automated drawing productionprocess was essential to ensure timelydelivery of these designs ahead of

the tight construction programme.

Three separate steelwork fabricatorswere employed on the footbridges.Each had their own supply line and

sought to use alternative sectionsmore readily available to them. This

was mainly for angle sections andhollow sections, angles in particularwere often offered in unusual sizesor in steel other than BS EN 100259.Alternative sizes could be easily

checked for adequacy but steelgrades needed further investigationto ensure adequate yield strength

and ductility. Based on feedbackfrom the outline design a plate girderalternative was included for the

bottom chord (1016305 UKB), withplate thicknesses proposed by onefabricator; a second fabricator was

unable to source the plate thicknessproposed by the first so a furtheralternative needed to be developed.

End welds to hollow section verticals,

diagonals and chords were designedto be full penetration full strengthbutt welds but several spans were

fabricated and erected with only filletwelds of inadequate capacity. Themajor problem this created was how

to enhance the capacity given thatmany of the affected spans were overSZR with at least external cladding

in place. Cutting out the welds andpreparing for and providing buttwelds was not an option with spans

over SZR, since too much furtherstrength would be lost and temporarypropping could not be installed.

Discussions on site between designerslocal engineers, welding specialists

and welding staff determined thatsufficient access space existed tothe external cladding to allow fillet

welds to be augmented. To limittemporary local loss of weld strengthduring over-welding, heat input

needed to be controlled meaningsmall weld beads and multiple passeswere required. All spans had been

designed on a generic basis giving,in many cases, conservative values

of member forces. In an effort tominimise the amount of re-weldingseveral spans were analysed indetail considering their actual bay

lengths and travelator positions.

Figure 16 - Steel extensions above pier heads at footbridge junctions during construction

Figure 17 - Steel extensions above pier heads at footbridge junctions during construction



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It was recognised early on thattravelators needed to be linearbetween their ends but this should

have been able to be accommodatedby varying the show on the travelatorparapet/hand-rail and adjusting

packing below the travelatorbearings. This requirement was notadhered to on a number of spans

leading to excess deck slab thickness.Fortunately most local floor beamcomponents had been designed for

a thicker deck (as the deck thicknesswas later reduced in an effort toreduce dynamic mass) and actual

span configurations gave a marginfor global increase allowing mostincreases to be accepted. Two spans

where the increase had been excessive

did however need to be re-profiled.

Conclusions

The design of over 200 footbridgespans as part of the Dubai Metrostation context planning was

challenging. Great use of automationand a modular construction approachallowed for the frequently changing

span requirements to be rapidlydesigned to a tight design andconstruction programme. All Red

Line footbridges were completedin time for the opening of theirrespective stations in 2009 (Figures

18 and 19), with the Green Linefootbridges due to follow in 2010.

Acknowledgements

This paper is published with thepermission of the Dubai RTA andthe JTMjv. The authors acknowledge

Genta Tabe and his JTMjv team,particularly Naohito Nishikawa. Theywould also like to acknowledge

Atkins Project Director, John Newby,and his management team led byRoss McKenzie for their excellent

co-ordination of the numerousdesign interfaces. Regular, closeliaison with Atkins team of

architects, led by Adrian Lindon,was also vital to the successfuldelivery of the footbridge designs.

Figure 18 - Completed footbridge (interior)

Figure 19 - Completed footbridges (exterior)



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References

BRITISH STANDARDS INSTITUTION. Eurocode 3: Design of Steel Structures Part1.

3-1-8: Design of joints. BSI, London, 2005, BS EN 1993-1-8.SMITH D.A., HEWSON N.R. & HENDY C.R. Design of the Dubai Metro Light Rail Viaducts Superstructure.2.Proceedings of the Institution of Civil Engineers Bridge Engineering, 2009, 162, No. 2, 55-62.

SMITH D.A. & HENDY C.R. Design of the Dubai Metro Light Rail Viaducts Substructure. Proceedings3.of the Institution of Civil Engineers Bridge Engineering, 2009, 162, No. 2, 63-74.

BRITISH STANDARDS INSTITUTION. Steel, Concrete and Composite Bridges, Part 3 4.Code of practice for design of steel bridges. BSI, London, 2000, BS5400.

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HIGHWAYS AGENCY. Design Manual for Roads and Bridges, BD49 Design Rules6.for Aerodynamic Effects on Bridges. Highways Agency, London, 2001.

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Footbridge Design for the Dubai

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