Kumar-Deck Slab Continuity

Paper: Kumar

Paper

Deckslab continuity for composite bridges A. Kumar, BE, PhD, CEng,

MIStructE, FICE

Kumar Associates

Synopsis Precast beams acting compositely with .in situ concrete deckslabs are a popularform of bridge construction. In the last 40 years, the use of this form has beenextended to constructing multispan bridges on a span-by-spanbasis with movement joints ateach support. Such joints have not per$ormed satisfactorily because the penetration of road salts through them causes corrosion damage in these constricted areas of the bridge. Several methods of eliminating such joints have been evolved during the last 30 years; these are briefly described here. The deckslab continuity method, evolved by the authol; is then presented. Its concept, design and applications are described with a view to engineers extracting maximum advantage from this powequl, yet simple, method of achieving continuity in multispan bridges. It is also shown that this method generally results in large economies as compared with the other methods and some savings even as compared with the undesirable 'jointed' span-by-span construction. Introduction Largely owing to the rapid speed of construction and avoidanceof support falsework, precast concrete beams'.' or steel girders acting compositely with concrete deckslabs have becomea preeminent form of bridge construction of the road network and over the last40 years. The need for rapid expansion simplicity of designing and constructing such spans has resulted in thousands of multispan bridges having been built as series of simply supported spans, without any special attention being given to eliminating the joints which occur over the piers in such bridges. years Increased useof road salts during the winter months over the 40 last has resulted in the now well-known problem of salt water penetration through the joints. Attempts to waterproof such joints have been largely unsuccessful because live load, temperature, creep and shrinkage-related

in these conmovements, combined with generally poor workmanship stricted locations, tend to break down jointing systemsa period over of time. This has often led to rapid corrosionof reinforcement and deterioration of concrete in these areas of complicated details with limited access possibilities for repairs. Even with substantial expenditure, it is not always possible to rectify suc damage satisfactorily,and this results in significantly reduced durability, aesthetics and service life of such bridges. It is difficult to estimate accurately the cost of such damage but nationally it could beof the order of E20m p.a. in the form of repair bills and reduced life of such bridges, evenif the unquantifiable costs of traffic delays and disruptions to the public are disregarded. This paper is aimed at outlining the concept, design, construction and advantages of the deckslab continuity method towards eliminating joints in composite bridges, which may resultin some savings even compared with conventional, simply supported span construction. The concept can also be adapted to eliminate joints in some existing multispan bridges. The paper also briefly describes other methodsof achieving continuity witha view to comparing their features and probable costs in order to enable engineers to select appropriate forms of continuity for the particular circumstances of each bridge.

Outline of methods of achieving continuity The salient featuresof the main methods of achieving continuity are briefly as follows: Jointed tied-deck continuity(Maunsell 'S)method""'

This is essentially jointed, simply supported span construction except that the top decks of adjoining spans are tied together by placing heavy longitudinal bars which are debonded over short lengths over the pier supports as shown in Fig 1. Rotational movements are intended to be elastically accommodated in the debonding flexible sleeve of the tying bars. Except for

10 x 30mm chase filled with plastic of Grade 2 or similar

Maximum gradient 1:25

25mm dia hot-rolled high-yield welding-quality deformed bars

50mm dia polystyrenesleeve

Unformed concrete surfaces to be coated with a suitable primer

wrapping round the sleeve

Contact surfaces coated with three layers of bitumen paint

Drainage pipe through low end diaphragm of

Beams set on bedding mortar (min 5mm thick)

5Omm dia intermal beams

Pier reinforcement

-

' Laminated elastomeric bearings (max size 260 x 500mm) set level on insitu concrete plinth

Fig 1. Jointed tied-deck continuity (Maunsell's)method details

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-In

situ deck and diaphragm

,-Deflected

tendons

remforcement Bearings

(a) Twin row of bearings

(b)Single row of bearings Precast

-

,In

situ

Fig 3. Live load continuity rangeof design moments

(c)

shown in Fig 3. The depth of construction is similar to the simplysupported form of construction.

Bearing under alternate beams

*

Ends of precast beams

Reinforcing bars embedded in ends of precast angles

’Structural

An’gle

(d) Continuity of bottom flange projecting burs by welding Ends of precast beams

Diaphragm reinforcement

Reinforcing Larswith hook ends embedded in ends of precast beams

Diaphragm reinforcement

l

(e) Continuity of bottom flange projecting bars by interlocking Fig 2. Monolithic diaphragms(Mattock’s)method details

some local shrinkage crackingat this location, ‘tied-deck’ construction restrains longitudinal movement at the piersupports which are transferred to the ends of the bridge.Since the deckrotations at thepiers areunrestrained, these would be locally exerted on the surfacing and jointing materials;the long-term performance of such joints is therefore unlikely to bereliable. Monolithic diaphragms (Mattock’s) methods.’(’ In this method, also known as the ‘live loadcontinuity’method, the precast concrete beams are placed span-by-span as for simply supported construction. Heavy in situ concrete diaphragms, encasing theends of the beams,are constructed at the pier supports. Movement joints areprovided at theabutments only. Hogging moments at the precast beam ends generally require in situ deck slab deflected tendons and placing of heavy reinforcement in the which extends well into the spans. Sagging moment connection at the piers is achieved by joining bars projecting from the bottom flanges of the precast beamsprior to thecasting of the diaphragmconcrete. Someof the construction details are shown in Fig 2. Combinations of loading and other effects result ina large rangeof design momentsin the composite beams as

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Overhang diaphragms (Pritchard’s) method‘.’‘) This is an adaptation of Mattock’s method in that the precast beams are slightly shorter than the span lengths, requiring the beam ends to be supported on trestles during construction. This results in some reduction in the sagging moment at the midspan andin the hogging moment at the precast beam ends.Deflecting of tendons in the beams isstill required to copewith these latter moments. In situ concrete diaphragmsat the piersare longer and There could generally be a reduccarry more of the hogging moment field. tion of about S% in the depth of the precast beams as compared with Mattock’s method but the support diaphragms aregenerally deeper so as to properly encase the beam ends. The presence of large in situ concrete diaphragms facilitates accommodation of curvatures in bridge alignments. Transverse post-tensioningof diaphragms is oftenemployed,particularly if narrow piers are being used as supports. Someof the construction details6 are indicatedin Fig 4.

Integral bridges (Humbly’S) method 7.8“’ This is similar to Mattock’s method except that continuity of the deck isalso established with the abutments,resulting in an ‘integral’ bridgeeliminating all joints. The overall longitudinal movements of the bridge are cyclically accommodated by the sliding of the abutmentsor by the flexure of foundation piles. This inevitably causes large (generally unknown) structure-soil interaction forces in the precast beams for which these must be designed. This often requiresan empirical approachin overcoming designCode compliance difficulties. The range of moments in the deck would be even larger than in Mattock’s method, possibly requiring larger depth of construction than simply supported construction. Obviously, the longer the bridge, the larger wouldbe the damaging effect on the surfacing and backfill behind the ‘moving’ abutments of such bridges. Some of the construction details’ are indicated in Fig S. Deckslub continuity (author’s)method 1 ~ 4 . ’ 0 ~ ‘ 3 This is essentially unjointed simply supported spans constructionin which the deckslab is locally separatedfrom the topsof the precast beams over the piers. This allows thedeckslab tobe made structurallycontinuous over several spans.Bridges of up to about S00m length can be madecontinuous in this way. For one-or two-span bridges (say, up toabout 35m length), it may be possible toeliminate both the abutmentjoints. For longer bridges, at least one of the abutment joints must be retained. The depth of construction is similar to the simply supported.form of construction. Precast beams need not involve the expensive deflecting of tendons in their manufacture. The method retains the simplicity and economyof simply supported construction whilst obtaining the advantages of deckslab ~ontinuity’.~. The remaining paper dealswith this method of construction. Continuity in composite steel beam and concrete deck slab bridges is achieved by splicing and jointing (by HSFG bolts or welding) the flanges

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Paper: Kumar

45/20 in situ concrete crosshead post-tensioned transverselyby 24 10 x 0.76 tendons

G

support positions

strands to be debonded at both ends

permanent formwork

10 x 0.76 strand force anchorages Part section through deck

Part section through crossheads

Fig 4. Overhang diaphragms(Pritchard's)method details

r

lD

D

AA

Elevation

I

n

A-J

E - Epoxy coated reinforcement

Plan D -D

c-c

Fig 5. Integral bridges (ffarnbly'S) method details

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(a)M-beam construction Fig 7. Symmetric and asymmetric modes offlexure of transverse deckslab

( b ) U-beam construction

IXXXL

(c) Y-beam construction

(d)Inverted-T beam construction

1

Fig 8. Longitudinally connecting deckslab inflexure between spans

part of a pier being dislodged,e.g. beiause of vehicular impact. The useof diaphragms is, however, not essential for the basic workings of the system. The Highways Agency has recently published technical documents”“’ dealing with the durability aspectsof bridge design. These place particular emphasis on incorporating some form of continuity so as to eliminatejoints in such bridges. The various methods of achieving continuity, including this approach, are outlined in these documents, although, some preference is implied towards ‘integral’ bridges approach for up to 60m bridge lengths. This paper, itis hoped, will assist engineers toutilise this approach to continuity more beneficially and rationally in the elimination of joints in such bridges.

Analysis and design of the bridge deck

(e) Steel beam construction

An attractive feature of this approach is that the analysis and designof the bridge deck canstill be carriedout as each span being simply supported, i.e. Fig 6. Forms of composite bridge Construction typically by grillage analysis of each span separately. This is because the connecting deckslab provides a flexible connection between the adjoining and webs of steel beams at around the pointsof contraflexure, followed by spans as compared to the other methods. This avoids the build-up of large the casting of a continuous deckslab. This results in some reduction in the design moments indicated in Fig 3. Neglect of the limited moment restraint depth of the beams as compared to the simply supported form of construcprovided by the connecting deckslab would always result ainlower bound tion but requires several additional operations having to be carried out on (i.e. safe) design of the bridge. The connecting deckslabs would obviously temporary supports, increasing the period and cost of such construction. This need to be designed to accommodate the various movements and forces arisis why many multispan bridges hadin the past been constructed as jointed, ing in these elements of the bridge. simply supported spans, such as the many milesof the Midlands Link flyBecause of unusual configuration, for greater accuracy or for any other overs. The concepts described inthis paper are also generally applicable to reason, an engineer may elect to incorporate the connecting deckslabs and composite steel bridge construction, with possible construction simplificathe vertical stiffness of the bearings in his global analytical model repretion and cost savings. These are, however not explored further in view of senting several spans, as shown in Fig 13. Since the connecting deckslabs the rare occurrenceof such bridges. in the long term,it would would, in general, acquire some flexural cracking be appropriate (and safe) to consider only the ‘cracked’ section’ stiffness of

Concept of the deckslab continuity method Fig 6 indicates the typical formsof bridge decks for whichthis approach to ~ontinuity’.~ is generally applicable. Conventionally, thin deckslabs over differentially bending and twisting longitudinal beams undergo varying combinations of symmetric and asymmetric modesof flexure, as shown in Fig 7. In this approach the deckslab is continued over the pier supports to also elastically connect differentially moving adjoining spans, as shown in Fig 9. Deckslab continuity arrangement in multispan composite bridges Fig 8. The practical functioningof this approachis achieved by locally separating the deckslab from the topsof the beams, as shownin Fig 9. The bridge beams will generally deflect downwards under the traffic loads and the reverse(i.e. top cooler than the bottomof the deck) temperature distributions and upwards under the effect of creep due to prestress and positive (i.e. top warmer than the bottom) temperature distributions. The ‘connecting deckslabs’ will essentially flex in opposite curvatures to the beam spans, resultingin an elastically continuous deckslab, as shown in Figs Eig IO. Flexure of connecting deckslab with sagging of main spans 10 and 11. Transverse diaphragms, set a little distance from the endsof the beams, are very advantageous in forming discrete, longitudinally spanning ‘continuity strips’ of thedeckslaboverthepiers,asshown in Fig12. The diaphragms effectively frame the beams, minimise punching shears at the beam ends, reduce relative movements of the adjacent beam ends, contribute to the load distribution properties of the deck, permit easy jacking for bearings replacement and would support the edge beams in the event of Fig 11. Flexure of connecting deckslab with hogging of main spans

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achieved by using smaller diameter barsat closer centres than larger diameter bars further apart. The following comments may assist in the design of the connecting deckslabs:

Effective span

Fig 12. Connecting deckslab and diaphragms detailat piers

t

Steel or elastomeric bearings

Fig 13. Grillage analysis of multispan bridges involving connecting deckslabs

the connecting deckslabs. If uncracked stiffnessis used, this would attract larger moments which, in turn, would cause cracking in the slab, thus degrading the stiffness to the ‘cracked’ value. As for simply supported span bridges, the interaction between the torsional stiffness of the diaphragms and the flexural stiffness of the beams would cause some hogging moments at the beam ends, which, incidentally, reduces the sagging moments in the span regions of the bridge. The restraint from the connecting deckslabs (if considered) would slightly increase these effectsbut the precast beams canstill be readily designed as ‘debonded’ tendon arrangements. For simplicity of design and construction, engineers should generally opt for the minimum size of diaphragms consistent with the ability tofulfil their various functions. It should generally be sufficiently accurate (and safe) to analyse and design such bridge decks as separate, simply supported spans with diaphragms located at the beam ends. For long lengths of bridges being made continuous the effectsof cumulative frictiodshearing forces in the bearings and the other coexistent forces should be considered in the design of the bridge decks and the connecting deckslabs in accordance with the design Codes”.”.

( 1 ) The sagging rotations (or moments) at the ends of the adjoining spans due to the adverse positioningof the live loads combined with the reverse temperature difference and any adverse differential settlement of the supports should be calculated. Since the worst rotations will be caused in the short term (i.e. immediately after the opening of the bridge to traffic), the hogging rotations caused by short-term creep (and shrinkage) due to prestress in the beams since the constructionof the connecting deckslab may be deducted from these rotations.A (stray) local wheel load placed directly on the connecting deckslab may accentuate the hogging moment at its edges, as shown in Fig14. Such consideration should essentially determine in the connecting the top longitudinal (hogging moment) reinforcement deckslab. (2) The hogging rotations (or moments) at the ends of the adjoining spans due to long-term creep (and shrinkage) causedby prestress (since the construction of the deckslab), combined with the positive temperature difference and any adverse differential settlementof the support, should be calculated. Live load neednot be consideredon these spans as this would havea relieving effect. Effects of local wheel loads placed directly on the connecting deckslab should be considered, as shown in Fig 15. Such consideration should essentially determine the bottom longitudinal (sagging moment) reinforcement in the connecting slab. (3) Critical conditionsof reverse curvature in the connecting deckslab could occur when only oneof the adjoining spansis heavily loaded with live loads. The rotations (or moments) for the short- and long-term creep and shrinkage and due to difference in deflection in the elastomeric bearings due to differing loads, as shown in Fig16, should be calculated. Temperature differences, differential settlements, local loads, etc., may all be combined to optimise the worst effects in the connecting deckslabs. If these arein excess of the values for (1) and/or (2), the respective reinforcement is increased accordingly. (4) The top and bottom transverse reinforcement is essentially designed for transversely distributing the local wheel loads and controlling crack widths in the connecting deckslab, which is restrained against early thermal shrinkage by the heavy diaphragm members. (5) The connecting deckslabs are broadly of similar span and thickness as the longitudinal deckslab panels between the bridge beams which flex dif-

Maximum hogging tension \ Wheel

Analysis and designof the connecting deckslab The clear and effective spans of the connecting deckslab are indicated in Fig 14. Consideration of maximum hogging tension in the connecting deckslab Fig 12. Apart from supporting the local wheel loads, the connecting deckslab accommodates the rotations at the piers due to the various loads (and strains) on the adjoining spans.If grillage analysis of single spans is being employed, the rotations outputby the computer can be combined with any other loadstrain rotations arising at the supports in accordance with the Codes”.”, leading to the design rotation values for these slabs. As a general basis, the rotations can be elastically distributed along the span of the connecting deckslab and the reinforcement determined to control cracking in the slab. The reinforcement required for supporting the local wheel loads can be calculated separately and added to this reinforcement, indicating the reinforcement for the connecting deckslab. Alternatively, the rotations may be converted into design forces and moments using the ‘cracked’ concrete slab stiffness and the relevant short- or long-term (or interpolated) modulus of elasticity values for concrete. These maythen be Maximum sagging combined with the wheel load effects as appropriate, resulting in the final tension reinforcement to satisfy the serviceability and ultimate limit state requireFig 15. Considerution of maximurn suggbzg tension in the corznecting deckslab ments. In general, for a given area of reinforcement better crack control is

I

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Paper: Kumar

b

/

Load

/ Steel slider or elastomeric bearing

Fig 16. Consideration of differential deck rotations and bearings deflection

Fig 20. Continuity with both abutments for short bridges

ferentially under live loads in the span regions, as shown in 7.Fig Obviously, the reversalof flexural effects would arise less frequently in the connecting deckslabs than reversals in the deckslab panels. Therefore, applying the same criteriaas for the restof the deck, there should be no special difficulty in complying with the Code ‘fatigue’ requirements in the design of the connecting deckslabs. (6) Although the behaviourof skewed decksis more complex, the calculatin the spanwise (shorted rotationsat the beam ends can be readily resolved est) and transverse directions of the connecting deckslab. This could marginin the spanwise direction ally reduce the flexural reinforcement requirement but may cause some increasein the transverse direction. Since there would be some increase in torsional moments, the spanwise requirement would probably be about the same asfor right bridge decks but somewhat larger in the transverse direction. (7) For longer lengthsof bridges the longitudinal forces on the deck, cumulative frictionhhearing forces in the bearings, horizontal bending due to wind and otherlateral forces, flexureof piers, centrifugal forces,etc., would progressively become more significant. These indicatea practical limit for conveniently reinforcing connecting deckslabs of about 350m bridge length. For longer bridges the build-up of bearing forces, inertial forces due to small longitudinal movements of the deck caused by span deflections due to the passageof loads over successive spans, and other secondary effects, may necessitate thicker, longer and more heavily reinforced connecting deckslabs for their proper functioning.

/

Fig 17. Continuity with one ofthe abutments

If required

Constructionjoint

7 I

Steel slider or elastomeric bearing

Fig 18. Continuity detail at the abutment

I

Steel rockerbearing

Fig 19. Continuity detail at the abutment with jlexible curtain wall

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Fig 21. Continuity detail for a halvingjoint

(8) The essential function of the connecting deckslab is to provide elastic continuity for the normal serviceability loadings on the bridge. Beyond this, inelastic behaviour may occur in the connecting deckslabs, as indeed it could in the beams and slabs of all bridges. Since the spans of such bridges are already designed as simply supported, the loadcarrying capacity would remain intact, regardless of any damage occurring in the connecting deckslabs. (9) Sufficient clearances below the connecting deckslabs and between the beam ends must exist to accommodate the movements corresponding to the ultimate limit stateso as to avoidany premature jammingof the system. Any ‘butting’ of the bottom flanges of the beams could cause local crushing or in spalling of the beams, as wellas causing large, undesirable tensile forces the connecting deckslab, potentially disruptingits normal functioning. ( 10) One (not both)of the abutment expansionjoints can also be eliminated by castinga connecting deckslab monolithic with an abutment. The deck would thus become effectively anchored to the topthis ofabutment with the maximum movement occurring at the other abutment where an expansion joint should be provided, as shown in Fig 17. If elastomeric bearings are used, there would be some rotations and forces due to creep increasing the deflection in the bearings over a period of time. A connecting deckslab length of about X of the pier deckslab length may therefore be provide, as shown in Fig 18. If steel bearings are used, these should be able to accommodate the small movements arising from the rotation of the span about the top of the abutment. Alternatively,a relatively flexible ‘curtain’ wallat the end of the abutmentmay be designed as shown in Fig 19, but this may need the installation of metal rocker bearings at this end to provide adequate restraint against horizontal forces. ( 1 1 ) In designing for continuityat the abutments, engineers should be aware of the potential need for the replacement of bearings at some point in the future. Becauseof the flexibility and creep in elastomeric bearings, the jacking-up required could be several millimetres. Connecting deckslabs should be designed to be able to tolerate such lifting without incurring excessive stresses during this operation. Adoption of steel bearings (with large vertical stiffness) at the abutments would simplify this operation, provided the retaining bolts and other arrangements had been properly adapted for ease of bearings replacement.If the bridge deckis at a gradient, it maybe preferend that any surface able to adopt such abutment continuity at the ‘lower’ so water run-off from the bridge deck may not enter the expansion joint. (1 2) For one- or two-span bridges of up to about 35m total length, the reversible (e.g. due to temperature) and irreversible (e.g. due to shrinkage and creep) movements of the deck would be quite small. By adopting the detail of Fig 18 at both ends, it may be possible to eliminateall joints from such bridges, as shown in Fig 20. This would, however, cause some ‘strutting’ forces (perhaps exceeding ‘at rest’ earth pressures) in the beams and connecting deckslabs of the bridge and some movement in the surfacing behind the abutments. In bridges constructed during the summer months, however, the shrinkage and creep shortening would counter some of the seasonal temperature expansions reducing these effects. Movements behind abutments can be minimised (i.e. kept within the top layers of the fill) by

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adopting the flexible 'curtain' wall detail of Fig 19 but with steel slider or elastomeric bearings, asshown in Fig 20. ( 1 3) The provision of deckslab continuity detail at halving joints, as shown in Fig21, could solve the durability problem but the inspection and replacement of bearings would still be virtually impossible; such joints should therefore not be used as faras possible. (14) For bridgedecks where concrete in-fill between the beams is used (see Fig 6(d)), the concrete topping may not besufficiently thick for forming connecting deckslabs. The ends of the beams may therefore be adapted and a detail similar toFig 22 adopted forcontinuity. (15) Additional dead load and joints and bearings capacities permitting, existing multispan, simply supported bridges, whetheror not of composite construction, canbe made continuousby casting continuous overslabs which are locally separated from the adjoining spans, as shown in Fig 23. Ifappropriate, this couldbe combined with the strengtheningof these bridges. (16) In certain existingbridges, it may be possible to locally excavate the deckslab concrete at the joints and install the connecting deckslabs to achieve continuity, but this may involve considerable alterationto details, bearings and expansionjoints of the bridge. ( 17) The diaphragms shouldbe designed for the slab edge reactions, global moments, shears and torsions, including any potentialjacking-up forces as appropriate. The links in the diaphragms should also be adequate for resisting 2.5% of the axial force in the connecting deckslabs to arrestany slab 'uplifting' tendency.

! k Spandispersalline

I

Y

I

'

1 Fig 24. Dispersal for notional effective length of parapet upstand in flexure

Treatment of the parapet upstands

Concrete upstandsfor parapets of around 300mm depthx 400mm widthare usually cast subsequent to the hardeningof the deckconcrete but their edge stiffening effect is often ignored in structural calculations.Such relatively small parapet upstands canbe continued uninterrupted over the connecting deckslabs forthe full length of the bridge, simplifyingmany construction details. Sufficient closely spaced longitudinal reinforcement on the exposed faces of the upstandshould, however, be provided to limit crack widths. For a more rational design, the increased edge stiffness should be allowed for in the analytical models. The presence of wide footpaths (say, about 2m wide) on the bridge could help to reducethe rotations (becauseof the distance between the upstands and the live loads) arisingat the stiffened edges of a connecting deckslab. In a differentway, the presenceof side cantilevers(say, about lm wide) on the deckcould reducethe rotations asthe imposedcurvatures would be disFig 25. Separation of parapet upstand from connecting deckslab tributed over longerlengths of the stiffened connecting deckslab,as shown in Fig 24. These conditionsmay justify the use of uninterrupted upstands on the bridge. lengths of the parapet upstands could be jointed and separated from the conIf the upstandsare substantial(e.g. for high containment parapets), short necting deckslab using flexible material,as shown in Fig 25, allowingit to act independently. The arrangementof Fig 25 would avoid the appearance of a horizontal joint in the upstand on the elevation of the bridge.

/'---c------

\

Fig 22. Continuity detailfor inverted T-beam decks

Compressible material

Fig 23. Continuity detailfor existing simply supported multispanbridges

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Practical dimensions and reinforcing details

The thickness of the deckslab spanning over longitudinal beams spacedat Im to 1.5m is typically of the orderof 160 to 180mm. When concrete beams are used at spacings larger than conventional andfor steel beams, deckslab thicknesses of up to about225mm are adopted.It is anestablished factthat such deckslabs supportedon longitudinal beams behave sensiblyelastically under serviceability loadings and that flexible surfacings laid on such decks behave perfectly satisfactorily. Since the connecting deckslabs are also designed using the same Code criteria, it follows that these slabs and surfacings above will behave satisfactorily. For thefunctioning of the connecting deckslabonly, the thinner theconnecting deckslab, the shorter could be its length (i.e. itsspan). Thus,several combinations of thickness and length could be acceptable for accommodating the full regime of rotations and other load effects. However, excessively thin and short slabswould result in large rates of rotation (i.e. curvatures) which could potentially cause disintegrationof the surfacing at these locations. A practical minimum slab thickness of 160 mm should allow for adequate reinforcing at the top and bottom faces of such slabs. The clear length of the connecting deckslabbetween thediaphragms should be about 1 m. Interestingly, the reinforcing requirements are likely to be fairly insensitive to small changes in the chosen lengthof a connecting deckslab because of the compensating natures of the local and global requirements, as indicated in Fig 26. Since the principal action of the connecting deckslabis in the longitudinal direction of the bridge, the main reinforcement shouldbe orientated in this direction and placed at the largest lever arms with distribution (trans-

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Paper: Kumar

.-

Moments

-

1

2 Span (m)

Fig 26. Interaction of local and global effects in connectingdeckslabs

verse) reinforcement placed inside it. Although differing amounts of reinforcement may be calculated by the design process, it would be advisable to place the ‘larger’amount on bothfaces of the connecting deckslabs. This would be simpler to fix on the site and may also compensate for any minor inaccuracies in the design process. Subject to detailed calculations, the preliminary indications for reinforcing the connecting deckslabs forbridges up to 500m continuous length in Table 1 may prove adequate:

TABLE I Bridne length upto 50m 125m 225m 350m 500m

I I

Longitudinal T I 6 @ 150mm

I I

I I

Faces

TI2 @ 125mm

top and bottom

TI6 @ lOOmm

T12 @ 125mm

top and bottom

T20 @ 75mm

I

T12 @ IOOmm

topand bottom

T12 @ IOOmm

I top and bottom

It is probable that Table 1 reinforcements would work satisfactorily for bridge skews of up to about2 0 and for minor angular changes at pier positions (such as those due to horizontal or vertical curvatures of the bridge alignment). For larger angles of skew,although detailed consideration would be required, a clear square distanceof about l m between the diaphragms should still be provided for the satisfactory functioning of the connecting deckslab. Since the rotations at the beam ends are essentially along the beams, reinforcements as described in the last paragraph, placed at right angles and parallel to the diaphragms, may still prove adequate. For skew angles up to about 45’ there should not be a large increase in the main reinforcement in the connecting deckslabs. There would, however, be a progressive increase in the distribution reinforcement requirement as an increasing component of the rotation will arise in this direction, possibly requiring up to 50% additional reinforcement. If reinforcements are placed at different orientations than suggested in the last paragraph, there may be further increases in the requirements because of the decreased efficiency of such reinforcement. The connecting deckslab must not be connected to the tops of the precast beams via projecting shear connectors orby any other means, ensuring its completely independent action from the beam ends. The links in the ends of the precast beams should therefore be detailed to be closed within the beam section itself and designed to be capable of resisting theshear, bursting, reaction and all other forces which may arise in the beam section at these locations. Thiswould generally require a simplereshaping of the precast beam ends as shown in Fig 27. The permanent forms and the layer of compressible material are placed on the beam ends as shown in Figs 27 and 28. The layer of compressible material (such as polyethylene) of about 20mm thickness may be glued to the tops of the beams to prevent it blowing away in the wind or being dislodged during construction. The thickness of the compressible material should not become less than 15mm under the weights of the forms, reinforcement, and wet concrete. The requirement for the compressible material layer could be avoided if the top of the beam ends can be cast slightly

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Fig 27. Simple reshaping oftops of precast beam endsfor River Frome Bridge

topand bottom

T I 6 @ 125mm T16 @ 75mm

I

Transverse T I 2 @ 150mm

Fig 28. Formworkfor the connecting deckslab.for River Frome Bridge

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tapered or lowered, as shown inthe arrangements of Fig 29. The ends of the beams at the piers should be at least 40mm apart, ensuring that no possible contact could occur with the adjoining span beams under service or ultimate loadings on the bridge. Regarding the sizing of the diaphragms, these should be as deepas possible and of adequate widths to fulfil the required structural functions and the practicalities of the location. In long bridges, construction joints in the deckslab could be located within the widths of the diaphragm, minimising the possibility of water penetration in the event of waterproofing failure in this area. In standard precast concrete beam decks, diaphragm widths of 300mm and 900mm can be obtained by passing reinforcing bars throughone and two standard web holes, respectively. Link reinforcement enclosing these and deck reinforcing bars should beprovided to resist various forces and control any flexural or torsional cracking. The narrow diaphragms (300 or 400mm wide) will offer little torsional stiffness and would be of negligible value towards load distribution between the beams, but it may still be possible to reinforce these adequately for providing the variousother practical advantages of framing the beams. For steel beam composite construction, since dead loads are usually to beminimised, relatively narrow width concrete or steel section diaphragms may be appropriate. As for the rest of the deckslab, concrete of 40N/mm2 characteristic strength with 30mm cover to reinforcement should be appropriate for the connecting deckslabs. Larger covers do not necessarily result in better protection to reinforcement, as the ensuing cracking could become less controlled. Long-term durability is improved by good workmanship, combined with the avoidance of laitence and adequacy of cement content and proper curing of concrete. Drainage arrangements must be provided at the abutment shelves where expansion joints are being provided, as these may leak in the long term. Since space is less restricted here thanat the piers, it is often easier tomaintain bearings, expansion joints and drainage arrangements at these locations. Typical reinforcing details for an end diaphragm are shown in Fig 30. A ‘drip’can be cast in.thein situconcrete which should prevent water running along the soffit as aresult of wind or slope of the bridge. If desired, the gap between the beams at the pier can be masked by short walls erected at the ends of the pier or by suitably designed copings, as appropriate.

Fig 31. Beams being lifed into position for River Fmme Bridge

Fig 32. Finished view of River Frome Bridge

Tapered or straight

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Fig 29. Alternative shaping of precast beam endsfor avoiding compressible material layer

The concepts of this method of achieving continuity in composite bridges have to date been applied in eight bridges in the UK. The earliest application was onthe five-span 105m-long X 1 lm-wide River Frome Bridge supporting the Bristol ring road, constructed in 1989 and still performing satisfactorily.Standard precast concrete M-beams withUM beams at the edges were all simply supported on individual metal bearings on 800mm-wide piers. Expansionjoints which usuallyoccur at the tops of all piers were eliminated using the connecting deckslabs and diaphragms, as generally shown in Fig 12. The bridge has rocker bearings at one abutment and is free to expand and contract atthe other with the cumulative movement capacities in the bearings. Thus a rotational joint occurs at oneof the abutments and full expansion joint at theother. The parapet upstands are locally separated from the top of the connecting deckslabs as shown in Fig 25. Figs 27,28, 31 and 32 show the bridge through its construction. The tendered cost of construction was about ;E750 000 at 1988 prices. The contractor was reported to have experienced greater easeand faster completion than expected for conventionally jointed construction. For a two-span 45m-long x 17m-wide bridge the reinforcement detailing for the connecting deckslab, diaphragms and the uninterrupted parapet upstand details at the central pier are shown in Figs 33 to 35. Figs 36 and 37 show the deck reinforcement and elevation of the bridge, respectively. The construction was completed in the summer of 1997.

Financial case

e Fig 30. Detailing of a typical end diaphragmat an expansionjoint

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As mentioned earlier there are now large numbers of simply supported, composite bridges, built usingthe conventional ‘jointed’method of construction, which are generally deteriorating at the jointlocations. Apart for thelargely unquantifiable costs of traffic delays and disruption to the public, the direct costs of repair and shortening of bridge lifecould be of the orderof E20m p.a. In order to minimise this cost and public inconvenience arising from future bridges, it is desirable that improved methods of construction are adopted. Multispan composite bridges vary greatly in shapes and sizes, the small-

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B

c G Fig 33. Reinforcement detailing of the connecting deckslab and diaphragms

T I 6-150%

T I 2-150%

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TABLE 2

Initial cost saving as Alternative methods of construction compared to the alternative methds Jointed simply supported spans (conventional) method*

E1 Smlyear

Jointed tied-deck (Maunsell’s) method* Monolithic diaphragms (Mattock’s) method Section A-A

Fig 34. Reinforcement detailing of the parapet upstand and deckslab

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Fig 35. Reinforcement detailing of the parapet upstand and deckslab at the connecting deckslab

E3mlyear E4.5dyear E6mlyear

Overhang diaphragms (Pritchard’s) method Integral bridges (Hambly’s) method*

&9m/year

compared with the the live load continuity and integral bridges methods, respectively. The application of this method may result in small savings of the order of E1.Sm in initial costs even as compared with the undesirable, jointed simply supported spans (conventional) method of construction. in enormous Global utilisationof this approach to continuity should result savings, with improved constructions and minimisation of maintenance costs for such bridges. It may also be possible to incorporate this form of continuity in some existing multispan bridges, minimising maintenance costs for their remaininglife, something which does not appear to be reada comily feasible with other methods. The Appendix to the paper delineates parative view of the continuity methods.

Conclusions This simple and rational method of achieving structural continuity of the deckslab should be used as widely as possible towards solving the joints deterioration problemin new composite bridge construction, whilst making significant economies in construction and maintenance costs, particularly as compared with the other ‘equivalent’ methods. Possibilities exist forits application in some existing multispan bridges and during the strengthening of a few others.

est being (say) two no. 20m spansX 12m wide and the largest (say) 12 no. 30m spans X 30m wide. The typical costs of construction using the deckslab continuity method would probably be in the range E0.5m and &8m, respectively. The incorporation of live load continuity (Mattock’s or Pritchard’s method) may arguably cost an additional E100 000 and E1 Sm, respectively. The incorporation of ‘integral’ bridges (Hambly’s method) References may arguably costan additional El 50 000 and E2m, respectively. l . Kumar,A.: ‘Composite concrete bridgesuperstuctures’,Wexham During the heady days of road construction in the 197Os, some 800 Springs, British Cement Association, PublicationNo. 46.505, 1988 bridges were built annuallyin the country. Even during the recent times of 2. Kumar, A.: ‘Detailed design of composite concrete bridge superstrucmuch reduced activityin this sector,it is thought that some 150 bridges connected with new roads and replacement of old bridges are probably being No. tures’, Wexham Springs, British Cement Association, Publication constructed each year.It is further estimated that abouta fifth of these (i.e. 46.506, 1988 some 30 bridges p.a.) could directly benefit from the application of this 3. Kumar,A:‘Locallyseparated deckslab continuity in composite in such bridges. Remaining method of continuity towards eliminating joints bridges’, Proc. IABSE Henderson Colloquium: ‘Towards joint-free bridges could be of other forms of construction or may utilise other existbridge decks’, July 1993, Cambridge,E & F N Spon ing methods of continuity, as appropriate. 4. Kumar,A.:‘Connecting slabsfor multispan composite bridges’, Elm to be the average cost of a typConsidered nationally and assuming London, The Patent Office, PatentNo. 2 183 700, 1988 ical multispan bridge, the savings in construction costs shown in Table 2 5. Mattock, A. H., Kaar,P. H., Hanson, N. W., Hognested, E., and Kriz, as compared could accrueby the adoptionof the deckslab continuity method L. B.: ‘Precast concrete bridges’ (a series of seven papers publishedin with alternative formsof construction: the journal of research and development laboratories of the Portland (*) methods involve disadvantages and/or mainCement Association, USA, 1960-61) In addition, the asterisked tenance costs,traffic disruption costs and difficulties to the public which are 6. Pritchard, B. P.: ‘The use of continuous precast beam decks for the unquantifiable. These apart, from the application of this methodof continuM1 I Woodford Interchange viaducts’, The Structural Engineer, 54, ity, there could be initial cost savings of the order of E4.5m to &9mp.a. as No. 10, October 1976

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Fig 36. Deck reinforcement for GrifinLane Bridge

Fig 37. Finished view of Gnfin Lane Bridge

7. Hambly, E. C., Nicholson, B. A.: Prestressed beam integral bridges, Leicester, Prestressed Concrete Association, 1995 8. Hambly, E. C., Nicholson, B. A.: ‘Prestressed beam integral bridges’, The Structural Engineer,68, No. 23,4 December 1990 9. Department of Transport: ‘Design for durability’, Departmental Standard BD 57/95, London, HMSO, August 1995 10. Department of Transport: ‘Design fordurability’,Departmental Advice Note BA 57/95, London, HMSO, August 1995 1 I . BS 5400 Steel, concrete and composite bridges: Part 4 ‘Code of practicefordesign of concrete bridges’, London, BritishStandards Institution, 1990 12. Department of Transport: ‘Loads for highway bridges’, Departmental Standard BD 37/88, London, HMSO, 1989 13. Kumar, A.: ‘Continuity in composite concrete bridge construction’, Proceedings of the second international conference on short and medium span bridges, August 1986, Ottawa, Canada

(7) Risk of corrosion of reinforcement potentially reducing the life of the structure. (8) Poor riding qualityof the deck due to joints. (9) Damage to suflacing due tothe concentrated rotations and movements at the joints. (IO) Long-term maintenance commitmentincluding disruption and delays to trafJic during repairs.

APPENDIX Comparative viewof continuity methods In view of the diverse range of applications, techniques, parameters and requirements, such a comparison has to be broadly based, largely subjective in nature, and mainly indicative of trends. Cost indications can be notoriously variable as they would also depend on market forces, location, ground conditions, contractor expertise, programming, etc. The author’s view of the probable cost of initial construction using the various alternative forms of construction for atypical medium-sized bridge are indicated in the brackets below. In this context, ithas been assumed that the cost of constructing the River Frome Bridge would probably be around Elm at current (1998)prices. The comparisons below list the various relevant features of design and construction specific to each form of construction. Regarding costs, the important factors are the number, complexity and specialist natures of operations required in the construction process; these affect the labour content, the construction period and ultimately the costof the bridge. The materials content isunlikely to be significantly different for the various forms. In addition, there are alsomany qualitative and maintenance-related factors pertaining to each method of construction. These largely unquantifiable and generally detrimental aspects arenoted in italics at the end of each list and are excluded from probable cost estimates for the bridge.

Jointed, simply supported spans (conventional) method (bridge cost: El.05m) ( l ) Diaphragms detailing and construction are complicated by the restricted space. (2) Decks require casting nosings at the location of joints. (3) Forming and filling expansion joints atall supports aretime consuming and costly. (4) Footpaths and parapet upstands complicate joint details, potentially causing failures. ( 5 ) Waterproofing layers have to be shaped around joints, nosings, etc. (6) Deck drainage iscomplicated by nosings, and piers also require drainage arrangements as waterproofing and drainage often fail.

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Jointed tied-deck (Maunsell’s) method (bridge cost: approx. El. 1m) ( I ) Diaphragm detailing and construction are more complicated in very restricted space. (2) Requires accurate shaping of concrete and placement of part debonded tying bars. (3) Filling and forming pier joints aretime-consuming and costly. (4) Bearings are designed to accommodate cumulative longitudinal movements. ( 5 ) Expansion joints of adequate capacity are provided at the abutments. (6) Potential for joint failure due to local shrinkage and cyclic concentrated rotations, requiring provision for pier drainage tocover for the possibility of water penetration. (7) Risk of corrosion of reinforcement potentially reducing the life of the structure. (8)Improved but still inferior riding quality of the deck due to suflacing joint. (9) Potentialfor damage to suflacing due the to concentrated rotations at the joints. (1 0 ) Potential long-term maintenance commitment including disruption and delays to trafJic during repairs.

Monolithic diaphragms (Mattock’s) method (bridge cost: approx. f. I . 15m) ( l ) The design for live load continuity is complex and time-consuming. (2) Precast beams would generally require expensive deflected tendons because of the large hogging moments which would arise atthe beam ends. (3) Precast beams would generally require reinforcement projecting from the endsof the bottom flanges (which are difficult to accommodate in the beam manufacturing process) because of the large saggingmoments which could arise atthe beam ends. (4) Precast beams may require temporary bearings under each beam end at piers during construction which could be replaced by a singlerow of bearings afterwards. ( 5 ) Substantial quantitiesof longitudinal reinforcement, stretching well into the adjoining spans is required in the top slab to copewith the hogging moments above the piers. (6) Establishing sagging moment connection between the bottom flange projecting bars in the constricted space is difficult and time-consuming. Inadequate provision could result in cracks developing here which may reciprocate with the daily temperature differences in the deck. (7) Diaphragm reinforcing details and construction for combinations of longitudinal and transverse flexural, shearand torsion effects arecomplex and may incorporate transverse post-tensioning and grouting. (8) Support reactions are increased owing to continuity effects and adverse combinations of secondary effects, potentially requiring larger foundations and substructures.

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(9) Considerable increase in number of operations, complexity and construction period. (10) Bearings are designed to accommodate cumulative longitudinal movements. ( l 1) Expansion joints of adequate capacity are providedat the abutments. (1 2) Drainage arrangements for the leaking waterat the piers are avoided. (1 3) Waterproofingof the deck is simplified. (14) Excellent riding quality of the deck. (15) No risk of corrosion of reinforcement from the leakingjoints at the piers. (16) No long term-maintenance commitment or disruption totraffic during repairs.

Overhang diaphragms (Pritchard’s) method (bridge cost: approx. &l.2m) ( l ) The design for live load continuity with significant length diaphragms is complex and time-consuming. (2) Precast beams may generally require expensive deflected tendons because of the significant hogging moments which could arise at the beam ends. (3) Precast beams would generally incorporate reinforcement projecting from the endsof the bottom flanges (which are difficult to accommodate in the beam manufacturing process) because of the large sagging moments which could ariseat the beam ends. (4) Precast beams would require temporarytrestle support under each beam end during construction which could be replacedby a single row of bearings afterwards. ( 5 ) Substantial quantities of longitudinal reinforcement, stretching well into the adjoining spans, is required in the top slab to cope with the hogging moments above the piers. (6) Although space is not so restricted, the reinforcing details and construction of diaphragms surrounding the beam ends are complex and time consuming and may involve transverse post-tensioning and grouting, particularly for narrow pier supports. (7) Support reactions are increased owing to continuity effects and adverse combinations of secondary effects potentially requiring larger foundations and substructures. (8) Possibility exists for small reduction (say, about5%) in the depth of beams required. (9) Considerable increase in numberof operations, complexity and construction period. (10) Bearings are designed to accommodate cumulative longitudinal movements. (1 1) Expansionjoints of adequate capacity are providedat the abutments. ( 13) Drainage arrangements for the leaking water at the piers are avoided. ( 1 3) Waterproofing of the deck is simplified. (14) Excellent riding qualityof the deck. (1 5 ) No risk of corrosion of reinforcement from the leakingjoints at the piers. (1 6) No long-term maintenance commitment or disruption totraffic during repairs.

Integral bridges (Hambly’s) method (bridge cost: approx.& l .3m) ( l ) The design for live load continuity, also involving the abutments with their largely indeterminate structure-soil interaction forces,is complex and time-consuming. (2) Precast beams would generally require expensive deflected tendons because of the large hogging moments which could arise at the beam ends, particularly at the abutment supports. (3) Precast beams would generally require reinforcement projecting from the ends of the bottom flanges (which are difficult to accommodate in the beam manufacturing process) becauseof the large sagging moments which could ariseat the beam ends, particularlyat the abutment supports. (4) Larger precast beams and construction depth may be required to resist large, but generally indeterminate, earth pressures behind abutments. ( 5 ) Precast beams are provided with dowelled elastomeric bearings to accommodate rotations at the piers, and the foundations are designed to accommodate longitudinal movements sometimes designed on vertical piles. (6) Substantial quantities of longitudinal reinforcement, stretching well into the adjoining spans, is requiredin the top slab to cope with the hogging moments above the piers. (7) Shorter abutments and/or vertical pilesmay need to be designed to

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reduce the abutment sliding and earth forces to manageable proportions, causing moments and forces in the deck elements. The need for shorter abutments may lengthen endspans or require additional spans. (8) Establishing sagging moment connection between the bottom flange projecting bars in the constricted spaceis difficult and time-consuming. (9) Diaphragm reinforcing details and construction for combinationsof longitudinal and transverse flexural, shear and torsional effects are complex. ( 10) Support reactions are increased owing to continuity effects and adverse combinations of secondary effects potentially requiring larger foundations and substructures. ( l I ) Considerable increase in numberof operations, complexity and construction period. (12) The backfill behind abutments and surfacing undergo cyclic disturbances due to expansion-contractionof the deck; therefore, to avoid excessive damage, a run-on slab and some formof compressible joint is recommended if concrete pavements are present. (13) Bearings are providedat piers for rotations only. (14) All expansionjoints are eliminated. (15 ) Drainage arrangements for the leaking waterat the piers and abutments are avoided. (16) Waterproofing of the deck is simplified. (1 7) No risk of corrosion of reinforcement from leaking pier and abutment joints. (l 8) Excellent riding qualityof the deck but approaches couldbe adversely affected. (1 9) Lack of knowledge about the state offorce distribution within the various partsof the bridge as theyare dependent upon the unknown structure-soil interaction. (20)Long-term maintenance commitmentto the backfill, run-on slab and its joints and sugacing behind moving abutments, including potential disruption and delays to trafic during repairs.

Deckslab continuity (author’s) method (bridge cost: approx. Elm) (1) The design of simply supported spans is simple. The connecting deckslabs also require designing for loads and movements but the resulting details are essentially simple. (2) The ends of the beams require minor adjustment and links require to be closed within the beam section. (3) Construction of the diaphragms, being away from the beam ends, is greatly simplified. (4) Local permanent forms on compressible layers are required over piers to construct connecting deckslabstrips. ( 5 ) The connecting deckslabs are moderately reinforced, providing elastic continuity. (6) Bearings are designed to accommodate cumulative longitudinal movements. (7) Expansion joints of adequate capacity are providedat the abutments. Potential exists for the eliminationof one of the abutment expansion joint. For short bridge lengths it may be possible to eliminate both abutment expansion joints. (8) Drainage arrangements at the piers are avoided. (9) Waterproofing of the deck is simplified. (10) Possibilities exist for installing continuity in existing multispan bridges. ( l l )Excellent riding qualityof the deck. ( 12) No risk of corrosion of reinforcement from the leakingjoints at the piers. ( 13) No long-term maintenance commitment or disruption to traffic during repairs.

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