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Design-Construction Feature

Byker Viaduct

Britain's First PrestressedSegmental Railway Bridge

W. J. R. SmythDirectorOve Arup & PartnersLondon, England

B yker Viaduct is an epoxy-gluedsegmental railway bridge on a

highly curved alignment which wasbuilt using very simple lifting proce-dures_ It was constructed partly as bal-anced free cantilevers and partly bycontinuous cantilevering.

The metropolitan county of Tyne &Wear in north-east England containsNewcastle and four other towns on therivers Tyne and Wear. Britain's newestrapid transit system, the Tyne & Wear

Metro, was opened during 1980. Whencurrent construction is complete theroute will be 54 km (34 miles) long in-cluding 41 km (26 miles) formerly oper-ated by British Rail and 13 krn (8 miles)of new construction. The Byker align-ment is one of the new sections and

contains the viaduct, two stations andtwo lengths of tunnel.

At its western end the viaduct crossesa valley which is about 30 m (100 ft)deep with steep side slopes coveredwith loose fill. At the bottom is astream, the Ouse Burn. The bedrockconsists of coal measures—sandstones,shales and coal seams which have beenmined; these are overlain by clays, andthere is a geological fault across thevalley. Access to the valley for con-struction was difficult and foundationsthere were also difficult.

The other half of the alignment runsacross a gently sloping hillside wherefoundations are straightforward, accessis easy, and the soffit has normal high-way clearance above the ground. The

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Describes the design considerations, constructionoptions and erection procedure for the BykerViaduct—a highly curved precast prestressedepoxy-glued segmental railway bridge recently builtin north-east England.

only restraints on the use of falseworkwere the three roads and a railway cut-ting which the viaduct crosses.

Design StudiesIt was clear that the economic spans

for the valley and hillside would bevery different. The valley needed con-struction methods which would dealwith its particular problems while thehillside was a straightforward site.However, there should he a cost ad-vantage in using the same plant andmethods for the whole structure andvisual unity is also important.

Other factors which affected the de-sign of the viaduct were:

1. The structure is an 5-bend in planwith minimum radii of 390 m (1280 ft).

2. The design loading is 60 kN/m (1.8tons/ft) on each track plus concentratedloads of 240 kN (24 tons) on each trackfor vertical loading. This is light byrailway standards but much more in-tense than typical highway loadings.The loading giving the worst cen-trifugal force is less, but the effects ofsideways forces on tall columns andtheir foundations are still considerable.

3, Conventional ballast is very heavy

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and, although concrete track supportsare more expensive, this can be morethan offset by the saving in the cost ofthe structure where the spans are large.The method of supporting the track offects the way in which expansion of thestructure has to be dealt with.

The first studies were made to com-pare the costs of different spans andmethods of construction for the sectioncrossing the valley. Spans ranging from44 to 92 m (144 to 302 ft) were consid-ered.

The 30-m (100 ft) height of the via-duct and the very steep valley slopeswith a considerable depth of loose fillin some places prevented falseworkfounded on the ground being used. Thecast-in-place concrete schemes whichwere considered were to be constructedby free cantilevering, or with falseworksupported off the pier pile caps, or sus-pended from launching girders.

The various schemes studied arebriefly outlined:

1. Twin concrete rib continuousbeam of 44-m (144 ft) span built offfalsework founded on the pier pilecaps.

2. Hammerhead columns of cast-in-place concrete with suspended spansconsisting of precast concrete U beams.The precast beams were 26 m (85 ft)long while the complete span was 52 m(170 ft). The hammerheads were to hebuilt off staging founded on the pilecaps, and the beams launched intoplace.

3. Similar to Scheme 4 but with pre-flex beams instead of concrete Ubeams.

4. Steel plate girders with concretecomposite top flange spanning 59 m(194 ft) as a continuous beam.

5. A continuous steel box girder ofconstant depth spanning 59 m (194 Ii).

6. A concrete box girder of constantdepth spanning 59 m (194 ft) built bycantilevering.

7. A continuous concrete box girder

of 59 m (194 ft) span, built by cantile-vering, with moment-stiff columns.

S. A continuous concrete box girderof variable depth spanning 92 m (302ft), built by cantilevering, with mo-ment-stiff columns.

All of these schemes were designedto carry concrete track supports.Scheme 6 was also designed to carryconventional ballast.

The substructure on this section wasa large part of the total cost. The cen-trifugal forces on the curved track pro-duce large bending moments in thehigh piers, and grouting of the uppercoal seams makes the foundations moreexpensive.

The two steel schemes turned out tobe considerably more expensive thanany of the concrete schemes. The 92-m(302 ft) span in concrete was thecheapest and the similar 59-m (194 ft)span was next.

The ballasted viaduct was consid-erably more expensive than the equiv-alent scheme without ballast. Thismight have been because the 59 m (194ft) continuous structure was not par-ticularly suitable for the extra deadweight of the ballast, and a more de-tailed study was made of the costs ofballasted and non-ballasted viaducts.

The foundations on the hill sectionare not abnormally expensive, and theviaduct is fairly close to the ground, sothat the economic spans are much lessthan those over the valley. In an areawhere a highway interchange wasplanned, spans of about 50 m (164 ft)were needed. Generally, spans be-tween 28 and 50 m (92 to 164 ft) wereconsidered. Structural steel schemeswere not considered for the hill section.

Three types of deck were considered:

1. 26-m (85 ft) long precast concretetrough beams, supported on ham-merheads to give 52-m (170 ft) spansover the future interchange, and sup-ported on crossheads to give 28-m (92(t) spans elsewhere.

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2. A continuous prestressed concretedouble-tee beam.

3. A continuous prestressed concretebox beam.

For Types 2 and 3, various spanswere examined, and 36-m (118 ft) spanswere the cheapest that fitted the con-straints.

The preceding studies were used inexamining integrated schemes for theviaduct as a whole and two schemesstood out as being the cheapest andmost satisfactory.

1. A continuous prestressed concretebox structurally fixed to stiff columnsover the valley, and supported on sim-ple columns over Byker Hill. The spanswere approximately 36 m (118 ft) overthe eastern part, 50 m (160 ft) over theplanned interchange, and 70 m (230 ft)over the valley. This scheme was suit-able for a paved track.

2. A scheme based on the use of pre-cast and prestressed concrete troughbeams 26 m (85 fl) long. Over the east-ern end of the viaduct the beams weresupported on crossheads, to give a 28-m(92 ft) span, while over the interchangeand over the valley they were sup-ported on hammerhead piers to give a52-m (170 ft) span. This scheme wassuitable for either ballasted or non-bal-lasted track.

These two basic schemes werestudied in greater detail and estimatesof the cost of structure and track weremade. The continuous scheme withconcrete way beams for the track wasabout £100,000 cheaper than the simplysupported scheme with concrete waybeams, and some £230,000 cheaper thanthe simply supported structure withballast and sleepers (1975 prices). (Notethat the exchange rate at the end of1975 was about $2 to the Britishpound.) It was also a much betterlooking scheme. The continuous struc-ture was therefore recommended to theclient and accepted.

Final Design

The deck is 815 m (2674 ft) long and8.8 m (29 ft) wide. It is divided intothree sections; valley, interchange andhill, with expansion joints at each endand between succeeding sections asshown in Fig. 1.

The valley section consists of sevenspans as shown, including three of 69 m(226 ft), the interchange of three spansof about 52 m (170 11), and the hill sec-tion of seven spans of about 36 m (11Sft) with a 27-m (89 ft) end span. At theintermediate expansion joints, the in-terchange section rests on the other twosections through halved joints.

The deck is 4 m (13 ft) deep at thedouble valley piers and tapers to 2.25 m(7 ft 5 in.) over the 14-m (46 ft) longhaunch. The remainder of the deck is2.25 m deep.

The deck of the valley section is builtinto its four double piers, and there aresliding bearings on each single pier andon the abutment. The interchange sec-tion is pinned to Piers 8 and 9 and hassliding bearings at the halved joints.The hill section is pinned to Piers 13and 14 and has sliding bearings on theother supports.

The original studies were based oncast-in-place construction, but it ap-peared that it would be economical tobuild the deck from precast concretesegments with epoxy resin joints, if itcould he erected without an expensivelaunching girder. Over the hill andinterchange sections, ground level ac-cess to the construction head was avail-able and segments could he lifted by asimple device sitting on the deck. Overthe valley section there was no truckaccess to the construction head butsegments could he winched down thesteep side slopes. Two segment lengthswere chosen to keep the weights below50 t, 2.4 m (7 ft 10 in.) for haunch andsupport sections, 3.3 m (10 ft 10 in.)elsewhere.

The 69-m (226 ft) spans were de-

PCl JOURNAUMarch -April 1981 95

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signed to be built as balanced free can-tilevers from each double column; theinterchange section by cantileveringfrom Piers 8 and 9 with temporaryfalsework near the halved joints. Thehill section was designed to be con-structed by cantilevering continuouslyin two directions from Pier 14, usingthree temporary supports in each span.The design allowed for a lifting rigweighing 45 tons sitting on the nose ofthe cantilever and lifting a segmentweighing 45 tons. The junction of thedeck with the valley columns was de-signed so that a lifting rig could sit on itto lift up the first precast segments.

The deck is a single cell concrete boxgirder with cantilever wings (Fig. 2).The webs are normally 350 mm (13%in.) thick, but thicken to a maximum of700 mm (27 1/2 in.). The top slab onlyvaries in thickness transversely. Thebottom slab varies in thickness from160 to 550 mm (6 1/4 to 211/2 in.) Thereare diaphragms at all supports (except 8and 9 where the box walls and slab arevery thick) and at the outer ends of thehaunches in the 69 m (226 ft) spans.

The deck is prestressed by tendonsconsisting of twelve 13-mm (½ in.) di-

ameter strands housed in the webs, andby 32-mm (1 1/4 in.) bars housed in thetop and bottom slabs. The bars are inshort lengths coupled at each joint tohold the segments in place and applyclamping pressure to the glued joint.They are grouted in and act as part ofthe final prestress. The strand tendonswere threaded and stressed in laterstages. Some are anchored in the shearkeys on the end faces of the webs andothers inside the boxes in comer blis-te rs.

The typical hill pier shown in Fig. 3is 1-shaped, with end thickenings bigenough to take the bearings. The valleypiers have two leaves of the same basicshape, with larger end thickeningswhich flare outwards at their lowerends to take the large centrifugal forces.The double piers provide high momentfixity for the deck but can deform as itexpands and contracts. They also pro-vide a stiff support from which to can-tilever.

FoundationsThe valley piers have H-shaped pile

caps. Piers 3, 4, and 6 are founded on

PCI JOURNAL/March-April 1981 97

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Fig. 3. The two types of pier.

piles to rock and Pier 5 is founded di-rectly onto sandstone. At each of thesingle piers with mechanical bearingsthe deck can he raised and the bearingsre-levelled if the foundation settles.There appeared a need to provide alsofor settlement of the double columns,and the pile caps can be re-levelled; nospecial access for doing this has beenprovided as it is not likely to beneeded.

The single piers are founded on padson boulder clay except for Piers 7 and2. These are founded on piles goinginto the clay, to even out relative set-

dements between foundations in clayand rock.

Structural AnalysisTwo elements in the structural

analysis are particularly related to theconstruction methods, namely, the ef-fects of creep and of inaccuracy incasting.

1. Whenever the degree of redun-dancy of a concrete structure is changedduring construction the as-built selfweight and prestress moments will hemodified by creep of the concrete. Themoment diagram will tend to move to-

98

wards that which would have existed ifthe structure had been cast and stressedin its final redundant form. The selfweight and prestress moments aftercreep were used in calculating the finalworking stresses of the bridge.

2. Alignment errors in casting a stat-ically determinate structure such as acantilever will not produce locked-instresses. However, on the Byker Hillsection the structure is a continuousmatch cast beam with a number ofspans and it was considered desirableto calculate the moments which couldbe generated by inaccuracies in casting.These moments are reduced by creep.

Concrete FinishesThe surface finishes are of two kinds.

The precast segments have a smoothfinish with drips under the ends of thewings and a recessed soffit which pro-vide some resistance against waterruns. The surfaces of the cast-in-placeconcrete segments (half joints, can-tilever connections, tops of double col-umns) are also smooth and matched tothe precast segments as well as possi-ble.

The precast concrete parapets andthe cast-in-place concrete piers whichare the visible surfaces most exposed toNewcastle's wet and dirty atmospherehave a small scale vertically ribbedsurface with the outer surface of theribs tooled with a needle gun. Trials forthis were carried out by the Cementand Concrete Association at theirlaboratories and by the contractor onsite (Figs. 4a and 4b).

The size and shape of the valley piersmeant that horizontal constructionjoints were required and the ribs had tobe stopped on a groove. In order tohave a visually coherent arrangementwith the different lengths of pier, it hasbeen necessary to provide moregrooves than are needed for joints. Theeffects of water runs at such places asthe intersections of the valley piers

Fig. 4a. The finished viaduct showingconcrete finish on large piers.

with the deck have been consideredand an attempt has been made to dealwith them in the details.

BiddingContractors who had replied to ad-

vertisements were interviewed, mainlyto discuss how they would go aboutbuilding the viaduct. Bids were invitedfrom six of them in December 1975.The rules for bidding allowed for alter-natives to the official design, but nonewere offered.

The lowest hid for the whole contractwas £6,200,000 (January 1976 prices)

PCI JOURNAL/March-April 1981 99

and this included a price for the viaductof £2,200,000 or £300/m2 (f28/ft2). Thethree lowest bids were close togetherand the viaduct prices were ranged inthe same order as the total bids (Notethat the exchange rate in January 1976was ver y nearly $2 to the Britishpound.) The contractor whose bid forthe whole contract was lowest was alsocheapest for the viaduct.

The contract started on site in jinie1976.

ConstructionConsiderable planning, design, and

fabrication of plant was needed beforethe casting and erection of segments

Fig. 4b. Closeup of concrete finish onlarge piers.

could begin. The first segment was castabout 7 months after the start of thecontract. In the meantime the foun-dations and piers were being con-structed.

The contractor decided not to sub-contract the manufacture of segmentsbut to make them himself on site usingthe short line method. In this methodeach segment is cast on a pallet againsta fixed stop-end. The segment previ-ously cast, still on its pallet, is po-sitioned to act as the other stop-end.The design of the pallet allows fine an-gular adjustments to be made so thatthe segment acting as stop-end can bealigned relative to the mold to pro-duce the correct angle changes (verticaland horizontal) between the segments.The final alignment is the resultant ofall these small angle changes, and set-ting out for this method is analogous tonavigation by dead reckoning.

A fabrication shop with mobile over-head hoists was built to house themold and the reinforcing cage assem-bly jigs. Outside it, a semi-automatedconcrete batching plant capable of pro-ducing 20 m3/hr (706 ft3lhr) of concretewas set up. This plant was only used tomake segment concrete, although theaverage daily requirement was only 19in (67[ It3) as the consequences of thewrong mix in a segment were too greatto risk. Ready-mixed concrete was usedeverywhere else on the contract.

The mold was designed to producea segment every 24 hours. It consistedof one external shutter for box sides andwings, two pallets on rails and twocores, all fabricated from steel plate. Itcould be adapted to make haunch seg-ments of varying heights in addition toflat ones, and internally a number ofdifferent arrangements of anchorageblisters or none at all. To reduce com-plication, the diaphragms were not castwith the segments, and couplers wereused for the diaphragm reinforcement.

The reinforcing cages were prefabri-cated complete with stressing ducts,

Fig. 5. A segment on its pallet with the mold behind. The shear keys can be seen andholes in the web for prestressing strands, and the holes and recesses in the flanges forthe completed bars. Two blisters for anchorages are inside at the junctions of web andbottom flange.

using jigs which provided all combina-tions of duct positions. The internalform was placed in the cage before itwas inserted into the external mold.Internal vibrators were used and exter-nal form vibrators for the bottom an-chorage zone where there was notenough room for pokers.

Each segment being used as stop-endhad to he set out relative to the mouldwith the correct angular relationships inplan and elevation. The small errorswhich were cast in had to be measuredeach time and corrected for in castingsubsequent segments. A precise leveland theodolite were set up on a steelH-pile driven into boulder clay, sight-ing through the shop onto another steelpile on the opposite side. These pileswere insulated with polystyrene tominimize thermal effects. Fig. 5 showsa completed segment and the mold.

The segments were lifted off the

casting pallet and placed on the bed ofa low Ioader which was fitted with adevice for jacking up the segment,transferring it to either side, and low-ering it on to concrete stools in the stor-age yard. It was also used to pick thesegments up and take them to the placeof erection (Fig, 6).

The matched joint surfaces werelightly sandblasted to etch the surfaceand to remove any contamination fromthe parting agent used in casting.

Low-Level ErectionThe low-level erection methods

applied to 25 segments at the westernend of the valley section, all of the in-terchange section, and all of the hillsection.

At the start of the erection sequence,temporary supports at the pier provideda stable platform from which to start.

PCI JOURNALIMarch-April 1981 101

Fig. 6. A segment on the low loader used for transport. Prestressing anchorage can beseen in the shear keys.

Three segments were placed by amobile crane and joined together. Thelifting rig, a simple triangular steelframe with two 5-ton diesel electricwinches, was then placed on top. Dur-ing hoisting, the frame was anchoreddown to the lifting eyes cast into thesegments. It was mounted on a lightduty rail track incorporating a turn-tableenabling it to be moved to the ends ofthe structure and to be rotated through180 deg.

As the erection proceeded away froma pier, temporary supports were used,typically three sets in a span (Fig. 7).Each set consisted of two 610-mm (24in.) steel tubes founded on a grillage.The deck was sensitive to settlement ofthe supports, so jacks were used be-tween the supports and the segments,coupled to hydraulicinitrogen ac-cumulators to maintain a constant force.

The lifting rig was not designed totwist and turn, so as it approached eachpier, the pier obstructed it from liftingthe pier segment. The segment waslifted in advance and placed over the

pier on a scaffold where the rig couldreach it.

Two formulations of resin and hard-ener were used depending on the tem-perature. Various methods of applyingthe resin were tried and the most satis-factory tool was a gloved hand. On oneoccasion, due to difficulty with a cou-pler, the segments had to be separatedand the resin hosed off.

At several joints, 100-mm (4 in.) coreswere taken so that the glue line withinthe joint could he inspected. Somewere used for splitting tests. (Note thatthe split always occurred in the con-crete.) At the later stages of erection,cores were mainly used to check thegrouting of starved joints.

Error CorrectionThe specification called for several

trials to be carried out, including trialsof a means of making minor angularadjustments to alignment by differentialpacking of the joints with layers ofwoven glass fiber. The trials were suc-cessful and the method was used from

102

Fig. 7. Low level erection. Continuous cantilevering using temporary props. The first setof falsework in this span is in place. When the next segment is erected another set offalsework will be inserted underneath and a third set will be inserted just before thedeck reaches the next pier.

time to time during erection to steer theunits. The opening up of the counter-cast surface produced a connectionbetween adjacent ducts, which causedproblems when grouting. The methodcan only correct "bending" and not"twisting" misalignment. It is a craftmethod, giving a small and impreciseadjustment.

Erection of the Byker Hill sectionstarted from Column 14 and after thefirst balanced pair of half-spans, wasproceeding westwards. While the spanfrom Pier 13 to Pier 12 was beingerected, the deck started to develop atwist. On the 18th unit from the start, acrossfalI of 0.6 percent had developedwhich it turned out was increasinglogarithmically.

Erection was stopped and, after 5weeks of investigation, it was discov-ered that a fit with the error, measuredon the deck already erected, was ob-tained from one of the secondary cor-rections used in the casting procedure.

Assuming that this was the cause, theerror predicted at the west end of the

hill section gave a 10 percent crosslal][over 760 mm (30 in.) betweenparapets], which was not acceptable.The complete run of segments for thehill section had already been cast.Going the other way, the predictederror was just acceptable and so erec-tion was started eastwards to gain timeto correct the error in the westerlyspans. The erection was carefully mon-itored and the actual twist was veryclose to the predicted twist.

Meanwhile, six special "untwisting"segments were being cast. Only oneend of each could be countercast and itwas necessary to use a 100-mm (4 in.)cast-in-place concrete joint at the otherend and wait for it to develop strengthbefore continuing.

The hill section was completed withan acceptable tolerance. Fig. 8 showshow close the actual twist was to thepredictions.

The precast parapet units had to beslightly modified to get a satisfactoryalignment of the edge of the viaductover the twisted section.

PCI JOURNAUMarch-April 1981 103

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High-Level ErectionA second lift rig was used for the

section over the valley. The highest liftwas some 30 m (100 ft) and this rig hada 10-ton diesel hydraulic capstanwinch with a total capacity of about 250m (820 ft) of rope.

The segments were transported to thetop of the valley and then lowereddown the slope on a trolley running onrails (Fig. 9). The rails passed throughthe holes in the large columns, to avoidhaving to adjust the lifting rig.

The high level section consisted ofthe four double leaf piers at 69-m (226ft) centers from which 33.9-m (111 ft)balanced cantilevers were erected, witha 1.2-m (4 ft) cast-in-place stitch be-tween them at midspan. The leaves ofthe three highest piers had to be bracedtogether during construction and thiswas done by means of blocks of cast-in-place concrete and stressed rodswhich can be seen on Fig, 9. At the topof each double pier a 4-m (13 ft) longin-place segment was cast monolithicwith the pier legs, and a lifting rig waserected on it.

The segments on each side of thecast-in-place segment were lifted andpositioned on adjustable jigs. They hadto be aligned very carefully because ofthe large lever arm to the end of thecantilever. A 100-mm (4 in.) wide con-crete stitch was cast between the in-place and precast segments. When thestitches had gained sufficient strength,erection proceeded with not more thanone segment out of balance, the liftingframe moving from one end to the otherafter each two segments (see Figs. 10and 11).

The lining tip of meeting ends ofcantilevers (usually a critical factor incantilever construction) was achievedwith less than 25 mm (I in.) out of linevertically or horizontally. The in-placeconcrete connections were cast in sim-ple suspended formwork. Some of thecontinuity tendons passing through

Fig. 9. A segment being lowered into thevalley. The groups of dots halfway up thenearest column are the rods whichtemporarily connect the two leaves duringconstruction.

these joints were in position beforethey were concreted. Concreting wasnormally carried out in the afternoonand a partial stress applied early thefollowing morning. This procedure wasnot followed during the construction ofthe stitch between the cantilevers onthe interchange section and a crack ap-peared between the new concrete andan adjacent segment, which closed onstressing.

Temperature movements were ex-

PCI JOURNALJMarch-April 1981 105

pected to cause other difficulties duringerection but they did not happen, un-less the twisting error in casting thesegments was due to diflerential tem-perature. The rest of the continuitytendons were then threaded andstressed. It was more difficult to threadtendons than is usual because of thelarge number of joints in the ducts andthe slight steps which can occur at eachone, as well as interconnection whengrouting. There was also some diffi-culty in getting the right extensions.The prestressed halved joints were

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Fig. 10. High-level erection. Balancedcantilever construction under way. Asegment is being lifted on one of thecantilevers from Pier 4, The rail mountedtrolley used to transport the segmentdown the valley is in front of the hole inthe pier.

constructed in-place, similarly to theconnections between the cantilevers.

Trackwork Constructionon Viaduct

The track on Byker Viaduct wasoriginally intended to consist of railssupported by concrete way beams witha continuous flexible pad between waybeam and rail. A trial section of thistrack was not successful and it was de-cided to use track slabs constructed bythe British Rail/McGregor slipformpaver with the rails clipped to baseplates bolted to the slabs.

The horizontal alignment over theviaduct is a reverse curve of 390-m(1280 it) radius with 60-m (197 ft) longtransitions and a maximum cant of 110m (4% in.). The slab under each trackis 2200 mm (7 ft 3 in.) wide, with aminimum thickness under the rail seatof 165 mm (6' in.). The slabs are bro-ken at roughly 18-m (59 ft) intervals, toprovide drainage channels and ducts fortendons to impedance bonds, and toprevent the track slab acting as part ofthe deck structure.

The paving machine tractor is atracked machine, and the concreteplacing unit runs on rails, The two areconnected by a gantry, carrying a con-veyor for concrete, and the reinforce-ment is set up underneath. Concrete isfed by the conveyor into a small hopperat the front of the placing unit. Thereare poker vibrators inside it, and im-mediately behind is the conformingplate, which gives the profile to theslab. There are two plate vibrators be-hind this again.

The workability of the concrete iscrucial to the paving operation, and theslump was designed to he between 5and 25 mm (is and 1 in.). The start ofpaving was on a transition, with in-creasing cant. The conforming platestarted to ride up on the concrete, someof the wheels lifting off the guide rails,and the concrete was not being laid

106

Fig. 11. Erection in the valley. The cantilevers from Pier 6 (foreground) and Pier 4are complete, and the cantilevers from Pier 5 are being erected.

properly on the high side of the slab.The first slabs were broken out and themachine was modified by increasingthe weight on the conforming plate anddividing the hopper into left and righthalves. The slump of the concrete wasincreased to between 25 and 35 mm (1and 1% in.). After these alterations, theperformance improved.

The whole paving operation took 5weeks including the time taken to re-turn the machine for its second run anda week for the abortive start. The con-crete was transported by dumper fromone end, along the track on which themachine was working. The restrictedaccess and the breaks in the slab madethe operation much slower than a nor-mal paving operation at grade.

The geometry of the deck surface iscomplicated by the concrete slabs forthe track and the cross channels fordrainage. A trowelled membrane pro-tected by no fines concrete was used towaterproof it.

Closing Remarks

Byker Viaduct was the first bridgeusing epoxy joints to be built by can-tilever construction in the UnitedKingdom, although only by a shorthead, as a number of others are nowbeing built. This is rather surprising.Epoxy jointing of precast prestressedconcrete has been around for a longtime; our firm used it 20 years ago inCoventry Cathedral, and a few yearslater for the Sydney Opera House..

Byker Viaduct was the first bridgedesigned by the design engineerswhere it looked like the project wouldbe an economical method of constnie-tion. However, there must have beenmany other bridges built in Britain inthe meantime which were large enoughto have justified the cost of the plant,the risks and the Iearning curve. Onecan only conclude that things happenwhen the time is ripe, or that engineersare just as subject to fashions as ar-

PCi JOURNAL,March-April 1981 107

chiteets are. At any rate, both the de-signers and the contractor in this casewould do it again.

In fact, the designers embarked onthe method with a certain anxiety. Nomatter how well a consulting engineerexamines the costs of alternativemethods, the information he has avail-able is much less good than a contrac-tor's. Several of the bidders said theyintended to put in alternatives in steelor concrete. In the end, none of themdid and the lowest bid was about 20percent below the designers' estimate.There was a shortage of work at thetime which no doubt helped.

The concrete slab supports for thetrackwork turned out to he much moreexpensive than the estimates, not quiteenough to falsify the comparison withballast but the difference was muchmore marginal than was estimated.

It is interesting to compare BykerViaduct with the Islington AvenueBridge described in an excellent paperby Lovell in the PCI JOURNAL.* Inthat case the glued segmental methodusing a launching girder was viable be-

cause the site, crossing a dense web ofrailway tracks, was so difficult.

However, in the case of Byker, a spe-cial launching girder which could havecoped with the curvature would havebeen far too expensive and the viabilitydepended on using a simple plant fortransport and erection. It was fortunatethat the contractor came to almost thesame conclusions as the designers as tohow to go about the construction, (Inci-dentally, the contractor put in a consid-erable amount of thought and time indesigning the methods and plant usedon site and using them.)

The viaduct is in a location where itcan be seen from all sorts of viewpointsand appearance was one of the impor-tant considerations. The consulting ar-chitect was involved from the start ofthe design and the various engineeringpossibilities were also discussed interms of their aesthetic possibilities andproblems. The appearance of a bridge

*Lovell, J.A.B., "The Islington Avenue Bridge,"PCI JOURNAL. V. 25. No. 3. May-June 1980. pp.32.80.

Fig. 12. Aerial shot of viaduct. Byker wall and housing is at bottom left.

has to come out of the structure, but notas a passive acceptance of what the cal-culations say. The boxes look a bit deepwhere they run over Byker Hill quiteclose to the ground. The long spansover the valley have to work hardwhere the haunch ends, but longerhaunches do not look attractive, Thedepth of the boxes over the hill and inthe central part of' the valley spansshould be the same—both for unity ofappearance and for simplicity of con-struction. (Figs. 12 and 13).

The actual depth used comes out of areconciliation of the various factorswhich did not happen by dictate of oneparty, but because, after argument, theteam always reached agreement onwhat was the best overall solution, Ta-pered sides on the boxes would havereduced the visual height somewhatand would have been a reasonablething to do if it had not been for thehaunches.

There was more argument about howto deal with the tops of the double col-umns than anything else. How do youfinish off the columns so that they don'tcollide visually with the deck can-tilever? Should you? Should you ex-press what is happening? How do you?Should the deck segment which is castin place as part of the column have thesame finish as the column? Or as thedeck? What was actually clone looks in-evitable now, but it didn't seem so atthe time.

In the end the contractor built it verywell and various nice things have beensaid about the design. The Royal FineArts Commission commended it, andthe Concrete Society gave the Viaducttheir Award for 1980.

AcknowledgmentThe author wishes to express his gratitude

to Mr. C. J. Hancock of John Mowlem andCompany Limited for providing informationon construction of the viaduct. The illustra-tions were furnished by Ore Arup andPartners and John Mowlem and Company.

Fig. 13. The finished viaduct. Pier 10 is inforeground and a halved joint in the deckjust beyond. The deck in the foregroundhas been corrected for twist (see alsoFig. 8).

CreditsClient: Tyne and Wear Passenger Transport

Executive (Director of Engineering:D. F. Howard; Chief Civil Engineer:P. Layfield).

Consulting Engineer: Ove Arup andPartners.

Consulting Architect for Structure and Land-scape Architect; Renton Howard WoodLevin Partnership.

Main Contractor, John Mowlem and Com-pany Limited.

Consultant to Contractor for Glued Seg-mental Construction: Bouvy, Van DerVlugt and Van Der Niet (BVN).

Segment Mould: Stelmo Limited.Trackslab on Viaduct: McGregor Paving

Limited.

PCI JOURNAL/March-April 1981 109