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 popular form of bridge construction. In the last 40 years, the use of this form has been extended to constructing multispan bridges on a span-by-span basis with movement joints at each 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 avoidance of support falsework, precast concrete beams'.' or steel girders acting compositely with concrete deckslabs have become a preeminent form of bridge construction over the last 40 years. The need for rapid expansion of the road network and simplicity of designing and constructing such spans has resulted in thou- sands 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.

Increased use of road salts during the winter months over the last 40 years 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

movements, combined with generally poor workmanship in these constricted locations, tend to break down jointing systems over a period of time. This has often led to rapid corrosion of 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 such damage satisfactorily, and this results in significantly reduced durability, aes- thetics and service life of such bridges. It is difficult to estimate accurate- ly the cost of such damage but nationally it could be of the order of E20m p.a. in the form of repair bills and reduced life of such bridges, even if the unquantifiable costs of traffic delays and disruptions to the public are dis- regarded.

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 result in 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 methods of achieving continuity with a 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 features of 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

high

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

Maximum gradient 1 :25

25mm dia hot-rolled -yield welding-quality

deformed bars 50mm dia polystyrene sleeve

Unformed concrete surfaces to be coated with a

suitable primer wrapping round the sleeve

Contact surfaces coated with three layers of bitumen

paint

Beams set on bedding mortar

(min 5mm thick)

Drainage pipe through low end diaphragm of

5Omm dia intermal beams

Pier reinforcement -

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

The Structural Engineer Volume 76/Nos 23 & 24 8 December 1998

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

447

Paper: Kumar

-In situ deck and diaphragm ,-Deflected tendons

remforcement Precast

Bearings

(a) Twin row of bearings

(b) Single row of bearings

Precast - ,In situ

(c) Bearing under alternate beams

Ends of precast beams * Reinforcing bars

embedded in ends of precast

An’gle ’Structural angles

(d) Continuity of bottom flange projecting burs by welding

Diaphragm reinforcement

Ends of precast beams

Reinforcing Lars with 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 cracking at this location, ‘tied-deck’ construction re- strains longitudinal movement at the pier supports which are transferred to the ends of the bridge. Since the deck rotations at the piers are unrestrained, these would be locally exerted on the surfacing and jointing materials; the long-term performance of such joints is therefore unlikely to be reliable.

Monolithic diaphragms (Mattock’s) methods.’(’ In this method, also known as the ‘live load continuity’ method, the precast concrete beams are placed span-by-span as for simply supported construction. Heavy in situ concrete diaphragms, encasing the ends of the beams, are constructed at the pier supports. Movement joints are provided at the abutments only. Hogging moments at the precast beam ends generally require deflected tendons and placing of heavy reinforcement in the in situ deck slab 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 beams prior to the casting of the diaphragm concrete. Some of the construction details are shown in Fig 2. Combinations of loading and other effects result in a large range of design moments in the composite beams as

Fig 3. Live load continuity range of design moments

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

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 and in the hogging moment at the precast beam ends. Deflecting of tendons in the beams is still required to cope with these latter moments. In situ concrete diaphragms at the piers are longer and carry more of the hogging moment field. There could generally be a reduction of about S% in the depth of the precast beams as compared with Mattock’s method but the support diaphragms are generally 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-tensioning of diaphragms is often employed, particularly if narrow piers are being used as supports. Some of the construction details6 are indicated in Fig 4.

Integral bridges (Humbly ’S) method 7.8“’

This is similar to Mattock’s method except that continuity of the deck is also established with the abutments, resulting in an ‘integral’ bridge eliminating all joints. The overall longitudinal movements of the bridge are cyclically accommodated by the sliding of the abutments or by the flexure of founda- tion piles. This inevitably causes large (generally unknown) structure-soil interaction forces in the precast beams for which these must be designed. This often requires an empirical approach in overcoming design Code com- pliance 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 would be 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 construction in which the deckslab is locally separated from the tops of the precast beams over the piers. This allows the deckslab to be made structurally continuous over several spans. Bridges of up to about S00m length can be made continuous in this way. For one-or two-span bridges (say, up to about 35m length), it may be possible to eliminate both the abutment joints. 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 economy of simply supported construction whilst obtaining the advantages of deckslab ~ontinuity’.~. The remaining paper deals with 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

448 The Structural Engineer Volume 76/Nos 23 & 24 8 December 1998

Paper: Kumar

45/20 in situ concrete crosshead post-tensioned transversely by 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

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

Part section through crossheads

r D l

D

Elevation AA

I

E - Epoxy coated reinforcement

c - c

n

Plan D -D

A-J

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

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Paper: Kumar

(a) M-beam construction

(b) U-beam construction

IXXXL (c) Y-beam construction

(d) Inverted-T beam construction

(e) Steel beam construction

Fig 6. Forms of composite bridge Construction

1

and webs of steel beams at around the points of contraflexure, followed by the casting of a continuous deckslab. This results in some reduction in the depth of the beams as compared to the simply supported form of construction but requires several additional operations having to be carried out on temporary supports, increasing the period and cost of such construction. This is why many multispan bridges had in the past been constructed as jointed, simply supported spans, such as the many miles of the Midlands Link fly- overs. The concepts described in this paper are also generally applicable to composite steel bridge construction, with possible construction simplifica- tion and cost savings. These are, however not explored further in view of the rare occurrence of such bridges.

Concept of the deckslab continuity method Fig 6 indicates the typical forms of bridge decks for which this approach to ~ontinuity’.~ is generally applicable. Conventionally, thin deckslabs over differentially bending and twisting longitudinal beams undergo varying combinations of symmetric and asymmetric modes of 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 8. The practical functioning of this approach is achieved by locally sep- arating the deckslab from the tops of the beams, as shown in Fig 9.

The bridge beams will generally deflect downwards under the traffic loads and the reverse (i.e. top cooler than the bottom of 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, resulting in an elastically continuous deckslab, as shown in Figs 10 and 11.

Transverse diaphragms, set a little distance from the ends of the beams, are very advantageous in forming discrete, longitudinally spanning ‘continuity strips’ of the deckslab over the piers, as shown in Fig 12. The diaphragms effectively frame the beams, minimise punching shears at the beam ends, reduce relative movements of the adjacent beam ends, con- tribute 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 7. Symmetric and asymmetric modes offlexure of transverse deckslab

Fig 8. Longitudinally connecting deckslab inflexure between spans

part of a pier being dislodged, e.g. beiause of vehicular impact. The use of diaphragms is, however, not essential for the basic workings of the system.

The Highways Agency has recently published technical documents”“’ dealing with the durability aspects of bridge design. These place particular emphasis on incorporating some form of continuity so as to eliminate joints 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, it is hoped, will assist engineers to utilise this approach to continuity more beneficially and rationally in the elimination of joints in such bridges.

Analysis and design of the bridge deck An attractive feature of this approach is that the analysis and design of the bridge deck can still be carried out as each span being simply supported, i.e. typically by grillage analysis of each span separately. This is because the connecting deckslab provides a flexible connection between the adjoining spans as compared to the other methods. This avoids the build-up of large design moments indicated in Fig 3. Neglect of the limited moment restraint provided by the connecting deckslab would always result in a lower bound (i.e. safe) design of the bridge. The connecting deckslabs would obviously need to be designed to accommodate the various movements and forces arising in these elements of the bridge.

Because of unusual configuration, for greater accuracy or for any other reason, an engineer may elect to incorporate the connecting deckslabs and the vertical stiffness of the bearings in his global analytical model repre- senting several spans, as shown in Fig 13. Since the connecting deckslabs would, in general, acquire some flexural cracking in the long term, it would be appropriate (and safe) to consider only the ‘cracked’ section’ stiffness of

Fig 9. Deckslab continuity arrangement in multispan composite bridges

Eig IO. Flexure of connecting deckslab with sagging of main spans

Fig 11. Flexure of connecting deckslab with hogging of main spans


Paper: Kumar

Effective span

Fig 12. Connecting deckslab and diaphragms detail at piers

t Steel or elastomeric

bearings

Fig 13. Grillage analysis of multispan bridges involving connecting deckslabs

the connecting deckslabs. If uncracked stiffness is 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, incidental- ly, reduces the sagging moments in the span regions of the bridge. The restraint from the connecting deckslabs (if considered) would slightly increase these effects but the precast beams can still be readily designed as ‘debonded’ tendon arrangements. For simplicity of design and construction, engineers should generally opt for the minimum size of diaphragms con- sistent with the ability to fulfil 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 effects of 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”.”.

Analysis and design of the connecting deckslab The clear and effective spans of the connecting deckslab are indicated in 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 output by 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. Alterna- tively, 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 may then be combined with the wheel load effects as appropriate, resulting in the final reinforcement to satisfy the serviceability and ultimate limit state requirements. In general, for a given area of reinforcement better crack control is


achieved by using smaller diameter bars at closer centres than larger diameter bars further apart.

The following comments may assist in the design of the connecting deckslabs:

( 1 ) The sagging rotations (or moments) at the ends of the adjoining spans due to the adverse positioning of 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 construction of 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 Fig 14. Such consideration should essentially determine the top longitudinal (hogging moment) reinforcement in the connecting deckslab. (2) The hogging rotations (or moments) at the ends of the adjoining spans due to long-term creep (and shrinkage) caused by prestress (since the construction of the deckslab), combined with the positive temperature difference and any adverse differential settlement of the support, should be calculated. Live load need not be considered on these spans as this would have a reliev- ing 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 conditions of reverse curvature in the connecting deckslab could occur when only one of the adjoining spans is 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 Fig 16, 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 are in 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

Fig 14. Consideration of maximum hogging tension in the connecting deckslab

I l Maximum sagging

tension

Fig 15. Considerution of maximurn suggbzg tension in the corznecting deckslab

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Paper: Kumar

Load / b /

Fig 16. Consideration of differential deck rotations and bearings deflection

Fig 20. ferentially under live loads in the span regions, as shown in Fig 7. Obviously, the reversal of flexural effects would arise less frequently in the connecting deckslabs than reversals in the deckslab panels. Therefore, applying the same criteria as for the rest of the deck, there should be no special difficul- ty in complying with the Code ‘fatigue’ requirements in the design of the connecting deckslabs. (6) Although the behaviour of skewed decks is more complex, the calculated rotations at the beam ends can be readily resolved in the spanwise (short- est) and transverse directions of the connecting deckslab. This could margin- ally reduce the flexural reinforcement requirement in the spanwise direction but may cause some increase in the transverse direction. Since there would be some increase in torsional moments, the spanwise requirement would probably be about the same as for right bridge decks but somewhat larger in the transverse direction. (7) For longer lengths of bridges the longitudinal forces on the deck, cumulative frictionhhearing forces in the bearings, horizontal bending due to wind and other lateral forces, flexure of piers, centrifugal forces, etc., would progressively become more significant. These indicate a 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 passage of 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

I Steel slider or

elastomeric bearing

Fig 18. Continuity detail at the abutment

If required

7 Construction joint

I Steel rocker bearing

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

Steel slider or elastomeric bearing

Continuity with both abutments for short bridges

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 state so as to avoid any premature jamming of the system. Any ‘butting’ of the bottom flanges of the beams could cause local crushing or spalling of the beams, as well as causing large, undesirable tensile forces in the connecting deckslab, potentially disrupting its normal functioning. ( 10) One (not both) of the abutment expansion joints can also be eliminated by casting a connecting deckslab monolithic with an abutment. The deck would thus become effectively anchored to the top of this abutment 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’ wall at the end of the abutment may 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 continuity at the abutments, engineers should be aware of the potential need for the replacement of bearings at some point in the future. Because of 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 deck is at a gradient, it may be prefer- able to adopt such abutment continuity at the ‘lower’ end so that any surface 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 eliminate all joints from such bridges, as shown in Fig 20. This would, however, cause some ‘strut- ting’ 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 sea- sonal temperature expansions reducing these effects. Movements behind abutments can be minimised (i.e. kept within the top layers of the f i l l ) by


Paper: Kumar

adopting the flexible 'curtain' wall detail of Fig 19 but with steel slider or elastomeric bearings, as shown in Fig 20. ( 1 3) The provision of deckslab continuity detail at halving joints, as shown in Fig 21, 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 far as possible. (14) For bridge decks where concrete in-fill between the beams is used (see Fig 6(d)), the concrete topping may not be sufficiently thick for forming connecting deckslabs. The ends of the beams may therefore be adapted and a detail similar to Fig 22 adopted for continuity. (15) Additional dead load and joints and bearings capacities permitting, existing multispan, simply supported bridges, whether or not of composite construction, can be made continuous by casting continuous overslabs which are locally separated from the adjoining spans, as shown in Fig 23. If appropriate, this could be combined with the strengthening of these bridges. (16) In certain existing bridges, 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 alteration to details, bearings and expansion joints of the bridge. ( 17) The diaphragms should be designed for the slab edge reactions, global moments, shears and torsions, including any potential jacking-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 arrest any slab 'uplifting' tendency.

Treatment of the parapet upstands Concrete upstands for parapets of around 300mm depth x 400mm width are usually cast subsequent to the hardening of the deck concrete but their edge stiffening effect is often ignored in structural calculations. Such relatively small parapet upstands can be continued uninterrupted over the connecting deckslabs for the full length of the bridge, simplifying many construction details. Sufficient closely spaced longitudinal reinforcement on the exposed faces of the upstand should, 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 reduce the rotations (because of the distance between the upstands and the live loads) arising at the stiffened edges of a connecting deckslab. In a different way, the presence of side cantilevers (say, about lm wide) on the deck could reduce the rotations as the imposed curvatures would be distributed over longer lengths of the stiffened connecting deckslab, as shown in Fig 24. These conditions may justify the use of uninterrupted upstands on the bridge.

If the upstands are substantial (e.g. for high containment parapets), short

\ /'---c------ - Fig 22. Continuity detail for inverted T-beam decks

Compressible material

Fig 23. Continuity detail for existing simply supported multispan bridges

!k Span dispersal line I

I '

Y

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

Fig 25. Separation of parapet upstand from connecting deckslab

lengths of the parapet upstands could be jointed and separated from the connecting deckslab using flexible material, as shown in Fig 25, allowing it to act independently. The arrangement of Fig 25 would avoid the appearance of a horizontal joint in the upstand on the elevation of the bridge.

Practical dimensions and reinforcing details The thickness of the deckslab spanning over longitudinal beams spaced at Im to 1.5m is typically of the order of 160 to 180mm. When concrete beams are used at spacings larger than conventional and for steel beams, deckslab thicknesses of up to about 225mm are adopted. It is an established fact that such deckslabs supported on longitudinal beams behave sensibly elastically 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 the functioning of the connecting deckslab only, the thinner the connecting deckslab, the shorter could be its length (i.e. its span). Thus, several combinations of thickness and length could be acceptable for accom- modating the full regime of rotations and other load effects. However, excessively thin and short slabs would result in large rates of rotation (i.e. curvatures) which could potentially cause disintegration of 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 deckslab between the diaphragms should be about 1 m. Interest- ingly, the reinforcing requirements are likely to be fairly insensitive to small changes in the chosen length of a connecting deckslab because of the com- pensating natures of the local and global requirements, as indicated in Fig 26.

Since the principal action of the connecting deckslab is in the longitudinal direction of the bridge, the main reinforcement should be orientated in this direction and placed at the largest lever arms with distribution (trans-

The Structural Engineer Volume 76/Nos 23 & 24 8 December 1998 453

Paper: Kumar

Moments

1 2 Span (m)

Fig 26. Interaction of local and global effects in connecting deckslabs

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 both faces 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 for bridges up to 500m continuous length in Table 1 may prove adequate:

TABLE I Bridne length I Longitudinal I Transverse I Faces upto 50m I TI6 @ 150mm I TI2 @ 150mm I topand bottom

125m

topand bottom T12 @ IOOmm T16 @ 75mm 350m top and bottom T12 @ 125mm TI6 @ lOOmm 225m top and bottom TI2 @ 125mm TI6 @ 125mm

500m I T20 @ 75mm I T12 @ IOOmm I top and bottom

It is probable that Table 1 reinforcements would work satisfactorily for bridge skews of up to about 2 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 distance of about lm 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 effi- ciency of such reinforcement.

The connecting deckslab must not be connected to the tops of the precast beams via projecting shear connectors or by 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 the shear, burst- ing, reaction and all other forces which may arise in the beam section at these locations. This would generally require a simple reshaping 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

.- -

Fig 27. Simple reshaping of tops of precast beam ends for River Frome Bridge

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

n r


Paper: Kumar

tapered or lowered, as shown in the 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 deep as 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 through one 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 various other practical advantages of framing the beams. For steel beam composite construction, since dead loads are usually to be minimised, 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 pro- tection to reinforcement, as the ensuing cracking could become less con- trolled. 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 than at the piers, it is often easier to main- tain 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.the in situ concrete which should prevent water running along the soffit as a result 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.

Tapered or Permanent straight shutter

I I 1 \, /l \

Void Hilti

l l Fig 29. Alternative shaping of precast beam ends for avoiding compressible material layer

e Fig 30. Detailing of a typical end diaphragm at an expansion joint

The Structural Engineer Volume 76/NOS 23 & 24 8 December 1998

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

Fig 32. Finished view of River Frome Bridge

Applications 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 on the 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 with UM beams at the edges were all simply supported on individual metal bearings on 800mm-wide piers. Expansion joints which usually occur 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 at the other with the cumulative movement capacities in the bearings. Thus a rotational joint occurs at one of the abutments and full expansion joint at the other. 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 report- ed to have experienced greater ease and 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 As mentioned earlier there are now large numbers of simply supported, composite bridges, built using the conventional ‘jointed’ method of construction, which are generally deteriorating at the joint locations. Apart for the largely unquantifiable costs of traffic delays and disruption to the public, the direct costs of repair and shortening of bridge life could be of the order of 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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c

B

G

Fig 33. Reinforcement detailing of the connecting deckslab and diaphragms

T I 6-1 50% 7 T I 2-1 50% 7 m, Section A-A

Fig 34. Reinforcement detailing of the parapet upstand and deckslab

T i 6-1 50‘1~ 7 m n ( , LPLrmanent

I

Section B-B material shutter

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

est being (say) two no. 20m spans X 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 Prit- chard’s method) may arguably cost an additional E100 000 and E1 Sm, respectively. The incorporation of ‘integral’ bridges (Hambly’s method) may arguably cost an additional E l 50 000 and E2m, respectively.

During the heady days of road construction in the 197Os, some 800 bridges were built annually in the country. Even during the recent times of much reduced activity in this sector, it is thought that some 150 bridges connected with new roads and replacement of old bridges are probably being constructed each year. It is further estimated that about a fifth of these (i.e. some 30 bridges p.a.) could directly benefit from the application of this method of continuity towards eliminating joints in such bridges. Remaining bridges could be of other forms of construction or may utilise other existing methods of continuity, as appropriate.

Considered nationally and assuming Elm to be the average cost of a typical multispan bridge, the savings in construction costs shown in Table 2 could accrue by the adoption of the deckslab continuity method as compared with alternative forms of construction:

In addition, the asterisked (*) methods involve disadvantages and/or maintenance costs, traffic disruption costs and difficulties to the public which are unquantifiable. These apart, from the application of this method of continuity, there could be initial cost savings of the order of E4.5m to &9m p.a. as

TABLE 2 Initial cost saving as

alternative methds

E1 Smlyear

Alternative methods of construction compared to the

Jointed simply supported spans (conventional) method* Jointed tied-deck (Maunsell’s) method* Monolithic diaphragms (Mattock’s) method

E3mlyear

&9m/year Integral bridges (Hambly’s) method* E6mlyear Overhang diaphragms (Pritchard’s) method E4.5dyear

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.

Global utilisation of this approach to continuity should result in enormous 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 remaining life, something which does not appear to be readily feasible with other methods. The Appendix to the paper delineates a comparative 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 problem in new composite bridge construction, whilst making significant economies in construction and maintenance costs, particularly as compared with the other ‘equivalent’ methods. Possibilities exist for its application in some existing multispan bridges and during the strengthening of a few others.

References l .

2.

3.

4.

5.

6.

Kumar, A.: ‘Composite concrete bridge superstuctures’, Wexham Springs, British Cement Association, Publication No. 46.505, 1988 Kumar, A.: ‘Detailed design of composite concrete bridge superstruc- tures’, Wexham Springs, British Cement Association, Publication No. 46.506, 1988 Kumar, A: ‘Locally separated deckslab continuity in composite bridges’, Proc. IABSE Henderson Colloquium: ‘Towards joint-free bridge decks’, July 1993, Cambridge, E & F N Spon Kumar, A.: ‘Connecting slabs for multispan composite bridges’, London, The Patent Office, Patent No. 2 183 700, 1988 Mattock, A. H., Kaar, P. H., Hanson, N. W., Hognested, E., and Kriz, L. B.: ‘Precast concrete bridges’ (a series of seven papers published in the journal of research and development laboratories of the Portland Cement Association, USA, 1960-61) Pritchard, B. P.: ‘The use of continuous precast beam decks for the M1 I Woodford Interchange viaducts’, The Structural Engineer, 54, No. 10, October 1976


Paper: Kumar

Fig 36. Deck reinforcement for Grifin 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 Stand- ard BD 57/95, London, HMSO, August 1995

10. Department of Transport: ‘Design for durability’, Departmental Advice Note BA 57/95, London, HMSO, August 1995

1 I . BS 5400 Steel, concrete and composite bridges: Part 4 ‘Code of prac- tice for design of concrete bridges’, London, British Standards 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

APPENDIX

Comparative view of continuity methods In view of the diverse range of applications, techniques, parameters and requirements, such a comparison has to be broadly based, largely subjec- tive in nature, and mainly indicative of trends. Cost indications can be noto- riously 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 a typical medium-sized bridge are indicated in the brackets below. In this context, it has 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 cost of the bridge. The materials content is unlikely to be significantly different for the various forms. In addition, there are also many qualitative and maintenance-related factors pertaining to each method of construction. These largely unquantifiable and generally detrimental aspects are noted 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 at all supports are time 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 is complicated by nosings, and piers also require drainage arrangements as waterproofing and drainage often fail.

Fig 37. Finished view of Gnfin Lane Bridge

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

Jointed tied-deck (Maunsell’s) method (bridge cost: approx. El . 1 m) ( 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 are time-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 to cover 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) Potential for damage to suflacing due to the 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 at the beam ends. (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) because of the large sagging moments which could arise at the beam ends. (4) Precast beams may require temporary bearings under each beam end at piers during construction which could be replaced by 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) 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, shear and torsion effects are complex 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. (1 0) Bearings are designed to accommodate cumulative longitudinal movements. ( l 1) Expansion joints of adequate capacity are provided at the abutments. (1 2) Drainage arrangements for the leaking water at the piers are avoided. (1 3) Waterproofing of the deck is simplified. (14) Excellent riding quality of the deck. (15) No risk of corrosion of reinforcement from the leaking joints at the piers. (16) No long term-maintenance commitment or disruption to traffic 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 ends of the bottom flanges (which are difficult to accommodate in the beam manufacturing process) because of the large sagging moments which could arise at the beam ends. (4) Precast beams would require temporary trestle support under each beam end during construction which could be replaced by 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, about 5%) in the depth of beams required. (9) Considerable increase in number of operations, complexity and construction period. (10) Bearings are designed to accommodate cumulative longitudinal movements. (1 1) Expansion joints of adequate capacity are provided at the abutments. ( 13) Drainage arrangements for the leaking water at the piers are avoided. ( 1 3) Waterproofing of the deck is simplified. (1 4) Excellent riding quality of the deck. (1 5 ) No risk of corrosion of reinforcement from the leaking joints at the piers. (1 6) No long-term maintenance commitment or disruption to traffic 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) because of the large sagging moments which could arise at the beam ends, particularly at 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 required in the top slab to cope with the hogging moments above the piers. (7) Shorter abutments and/or vertical piles may 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 space is difficult and time-consuming. (9) Diaphragm reinforcing details and construction for combinations of 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 number of operations, complexity and construction period. (1 2) The backfill behind abutments and surfacing undergo cyclic disturbances due to expansion-contraction of the deck; therefore, to avoid excessive damage, a run-on slab and some form of compressible joint is recommended if concrete pavements are present. (1 3) Bearings are provided at piers for rotations only. (14) All expansion joints are eliminated. (1 5 ) Drainage arrangements for the leaking water at 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 quality of the deck but approaches could be adversely affected. (1 9) Lack of knowledge about the state offorce distribution within the various parts of the bridge as they are dependent upon the unknown structure-soil interaction. (20) Long-term maintenance commitment to 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 deckslab strips. ( 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 provided at the abutments. Potential exists for the elimination of 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 quality of the deck. ( 12) No risk of corrosion of reinforcement from the leaking joints at the piers. ( 13) No long-term maintenance commitment or disruption to traffic during repairs.
