NEW DEVELOPMENTS IN BRIDGESUPERSTRUCTURES

F.A. RAPATIONI, BE(CML), MIE(AUST), CP ENo.SENIOR BRIDGE DEVELOPMENT ENGINEER

VlcROADS

E.J.B. WELLS, BE(CML)DESIGN ENuINEER - VlcROADS

P. GRAN, M.ScBRIDGE STANDARDS ENGINEER - VlcROADS

Frank Rapattoni is the Senior BridgeDevelopment Engineer in the Principal BridgeEngineer's Department of VicRoads, Victoria. Hejoined the organisation in 1971 and has extensiveexperience in the design of bridges and other roadsnuctures. Other experience within VicRoadsincludes planning, road design, geotechnicalinvestigations, road and bridge consnuction. Henow manages and directs the activities of theDevelopment Section involved in research,development of standards and standardcom~nents for road structures and providesspecIalist support to the Design Department andother areas of VicRoads.

Jim Wells is an En~ineer in the DevelopmentSection of the Pnncipal Bridge Engmeer'sDepartment of VicRoads. He joined theorganisation in 1970 and has extensive experiencein the design of bridges and other road snuetures.Prior to joining VicRoads he worked with theBrisbane City Council in the areas of road andbridge design and construction. He is nowworking on development and design of standardbridge components.

Peter Gran is the Bridge Standards Engineer inthe Development Section of the Principal BridgeEngineer's Department of VicRoads. He has aMasters' qualification in Civil Engineering fromthe Bmo Technical University, Bmo,Czechoslovakia. His early experience in bothdesign and construction has been with designdepartments in Czechoslovakia before gaining acontract position on projects in the Republic ofIraq where he was engaged in construction andcontract administration of bridges. Following hisarrival in Australia, Peter briefly managed theCivil Centre project for the City of Essendonbefore joining the Roads Corporation. He iscurrently involved in research and developmentactivities and provides specialist support to theDesign Department and other areas of VicRoads.

PAPER NO. 39

ABSTRACf: This paper describes two new VicRoads developments in bridgesuperstructures which have led to significant cost savings and increased safety in bridgeconstruction.

The first one is a new range of prestressed concrete beams which have been developed forbridge spans between 19 to 35 metres. The beams, named "SUPER T-BEAMS" provide anattractive economical alternative to I-beams, Trough beams, Bulb T-beams and steel beamsin this span range.

The second development concerns the adaptation of -rRANSFLOOR" decking, aproprietary precast reinforced concrete permanent formwork system. The decking consistsof mass produced 3-dimensional trusses partly embedded in a thin concrete base slab whichforms the permanent formwork. The decking becomes an integral pan of the deck slab andacts compositely with the slab.

ACKNOWLEDGMENTS: The authors with to thank the Chief Executive of VicRoadsfor his permission to publish this paper. The views expressed in this paper are those of theauthors and do not necessarily reflect the views of VicRoads.

PAPER NO. 39

NEW DEVEWPMENTS IN BRIDGESUPERSTRUcruRES

1. INTRODUCTION

This paper describes two new VicRoads developments in bridge superstructures whichhave resulted in significant cost savings and increased safety in bridge construction.

The first development is a new range of prestressed concrete (P.S.c.) beams for bridgeswith spans between 19 and 35 m. The bearns, named Super T-bearns, are a "big brother"to the currently used T-Slabs.

The second development concerns the adaptation of Transfloor decking, a proprietaryprecast reinforced concrete (R.C.) as permanent formwork system for composite beamand deck slab bridge superstructures.

The close co-operation between VicRoads, taking the lead role in setting standards andco-ordinating developments, and industry facilitating implementation, ensured aneffective and smooth introduction of the new developments.

2. SUPER T-BEAMS

2.1 Background

The catalyst for the development of the Super T-beams was provided by thesuccess of the T-Slabs used for spans up to 19 metres. The details of these beamsare described in a paper by Rapattom and Wells (1). :

The market demand for a new type of beam, with similar features to the T-Slabs,to span in excess of 19 metres was confirmed by a number of manufacturers andbridge builders. It was considered that such a new beam would lead to lower coststhan the then current standard P.S.C. beams used for these larger spans, as shownin Figs. I, 2 and 3. One manufacturer, who described the T-Slabs as a "dream forprecasters", was concerned about the high establishment cost required to supplythis segment of the market. In fact, a number of moulds were required to caterfor spans up to 45 metres and the cost was often beyond the reach of the smallermanufacturers, especially in view of the small market size for precast beams inthis span range. Fig. 4 shows that up to three different moulds were needed foreach type of beam to cover spans between 19 and 35 metres. A total of ninedifferent standard moulds were currently in use to cover all the beams in this spanrange. This compares with one fixed mould and removable plinths required forthe Super T-beams.

2.2 Development Strategy

2.2.1 General

The strategy used for the development of the beams was designed toensure the earliest possible implementation, as a considerable number ofbridges in this span range were being planned at that time in Victoria.

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It was considered important to firstly finalise the external dimensions ofthe beam. This ensured that designers wishing to adopt the beams couldbegin design immediately and manufacturers could fabricate the mouldearly with confidence, secure in the knowledge that this would be a longterm industry standard. The internal void dimensions which do not affectthe fIXed mould dimensions could then be chosen to suit particular designs.

2.2.2 Objectives and Criteria

The brief for the development of the Super T-beams included thefollowing objectives and cri teria:

•

•

•••

•

2.2.3

Beams to have similar characteristics to the T-Slabs, enabling easymanufacture and handling, maximum safety and minimumformwork for the bridge deck construction.

Varying top flange width to suit bridges of any geometry includingcurved alignment.

Straight tendons to be used and debonded at the ends as required

Beams to cater for spans between 19 and 35 metres.

Only one fIXed outer mould to be used for the complete range.Different depths to be achieved by using removable plinths insertedin the moulds as shown in Fig.5.

Design to minimise the overall cost of the bridge rather thanoptimise any particular part

Consultation

Close consultation with all interested parties was an important part of thedevelopment strategy. This was achieved by inviting comments at the initialstage from all manufacturers, some bridge contractors and the relevantVicRoads engineers. Consultation continued throughout the developmentprocess and implementation to ensure that the needs of all concerned weresatisfied and that ensuing problems were identified and corrected as soonas possible.

2.2.4 Detailed Design

The detailed design for the first Super T-beams was carried out to suitindividual bridge sites, using the adopted external beam dimensions. Someof the features adopted in the early designs, by both VicRoads engineersand consultants who had also adopted the beam cross-section were asfollows:

• Minimum web thickness of 90 mm to ensure adequate concretecasting and compaction.

• Bearings placed to follow the bridge deck crossfall and to behorizontal in the longitudinal direction. This required lateralrestraints to counter the transverse component of the gravity forces.

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• No intermediate or end diaphragms. This does not permit the fullutilisation of the beams torsional characteristics, but it leads toreduced overall cost.

2.2.5 Implementation

Both VicRoads engineers and consultants adopted the concept in earnestand the first beam was cast within six months of the start of development.The first bridge was completed within twelve months.

2.2.6 Beam Manufacture

The manufacture of the beams posed few problems. The forming of thevoid was an important consideration. A number of options wereinvestigated including collapsible formwork, sacrificial formwork and voidfillers such as polystyrene. Manufacturers finally opted for the lattersolution, as shown in Fig.6, which was successfully implemented.

One of the early problems included hairline cracking in the end blocksfollowing the release of prestress after steam curing. An investigationcarried out by VicRoads revealed that this was due to excessive tensilestresses caused by a combination of spalling stresses, due to prestressing,and the thermal gradient across the end block at the time of release. Thethermal gradient was due to the much faster rate of cooling of the beamexterior with respect to the interior following steam curing, leading totensile stresses (2). The cracking has now been reduced by introducingadditional reinforcement in the face of the end block. Further, it isproposed to limit the thermal gradient by insulating the end blocks oncompletion of the steam curing. The reduced rate of cooling should lowerthe tensile stresses to an acceptable level.

2.2.7 Site Construction

The transport and erection of the Super T-beams for the first bridges werecarried out successfully. The construction of the bridge superstructureproceeded as planned, however the sloping bearings led to some problems.The lateral shearing of the bearings, due to the transverse component ofthe gravity forces, were significant. In order to control the deflection, itwas necessary to install lateral restraints before the erection of the beams,and progressively transfer the transverse forces from each beam as it iserected to the lateral restraints but this has proved difficult to carry out.It has now been decided to specify bearings horizontal in both directions.Two recommended alternative details are shown on Figs. 7 and 8. Theestimated cost of either option is similar and the choice of the detail is atthe discretion of designers.

2.3 Design Features

The design features for Super T-beams, shown on Fig.9, can be summarised asfollows:

• Top flange width can vary from 1120 to 2000 mm to suit any bridge widthand curvature. Width may be up to 3000 mm for pedestrian bridges.

• Top flange thickness 75 mm minimum

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• Minimum web thickness of 90 mm

• Straight tendons debonded at ends as required

• Beams placed to follow bridge deck crossfall

• Bearings horizontal in both directions. Alternative bearing details as shownon Fig. 7 and 8.

• Overlay deck thickness 140 mm minimum with two layers of reinforcement

• No intermediate diaphragms. Shallow end diaphragms may be required ifrecess for expansion joints reduces deck thickness below 140 mm

• The bridge deck can be made continuous at the piers to eliminateexpansion joints. Beam continuity can be achieved over piers in a similarmanner to alternative beams.

2.4 Future Developments

It is proposed to further refine the design of the Super T-beams. The loaddistribution characteristics are being closely studied using finite element modelsand the use of diaphragms will be further investigated as an optional arrangementto extend the span limi t.

The design and preparation of drawings for the full range of beams will becompleted in the near future in order to facilitate the rapid design of bridges ofany width and span within the range considered.

2.5 Benefits

2.5.1 Uniform Standards and Efficiency

The development of the Super T·beams in close co-operation with industryhas provided an opportunity to establish a new standard beam which willimprove the efficiency and competition in the bridging industry. Thestandard cross-sections for the beams are shown in Fig. 9.

2.5.2 Cost Savings

The cost of bridges for the span range covered by the Super T-beams hasbeen significantly lower since the introduction of the Super T·beams(although the number of bridges is too small to arrive at definiteconclusions at this stage). The current depressed state of the economy isa major contributing factor in lowering costs but in our opinion, the newbeams have made a significant impact as follows:

•

•

•

Competition among manufacturers has increased as a result of thisdevelopment. Some manufacturers (at least two) have entered thismarket segment for the first time creating greater competition.

The fixed mould reduces manufacturing costs. The establishmentcost can be amortised across a larger number of beam sizes.

Manufacturing time is reduced as beams can be simply lifted fromthe moulds after manufacture without demoulding.

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• Site construction costs are reduced. Forming of the deck is largelyeliminated and speed of construction increased significantly. Inaddition, relatively unskilled labour can be used for deckconstruction.

There is clear evidence that the Super T-beams are now thefavoured beam type in Victoria.

2.5.3 Safety

The safety for site workers is increased when compared with other beamtypes as the Super T-beams provide a working platform immediately aftererection. In addition, edge deck formwork and safety rails can be attachedto the beams prior to erection, further enhancing safety on site.

2.5.4 Aesthetics

The aesthetics of the Super T-beams is considered to be superior to 1­beams and Bulb T-beams. The appearance is similar to that of thefavoured box girder bridges and has found immediate acceptance byclients.

2.6 Conclusions

The Super T-beams have a number of important advantages over the currentrange of standard beams which lead to reduced cost of bridging. One fixed mouldwith removable plinths can be used for all beams within the 19 to 35 m spanrange. This leads to lower manufacturing costs by reducing investment in mouldsand decreasing manufacturing time. Savings in cost and time are also accrued insite construction works by largely eliminating deck formwork and diaphragms andusing simple details. The width of the top flange can vary to suit any bridge widthand geometry, affording much flexibility. Safety for site workers is increased as thebeams provide a safe working platform from which to complete the constructionof the bridge. The fine aesthetics of the beam should make it a favoured solutionfor bridges in this span range.

The introduction of the Super-T Beam as an alternative to current standardbeams has found ready acceptance among designers, manufacturers and bridgecontractors. Some of the early problems have been rectified. The good co­operation between VicRoads and industry has ensured an effective and smoothimplementation.

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3. TRANSFLOOR DECKING

3.1 Background

Bridge superstructures comprISing either steel or concrete beams actingcompositely with a reinforced concrete deck slab are very efficient structurally.However, forming of the deck, particularly the cantilever, is difficult and costlyand can be unsafe. Many different types of formwork have been used over theyears ranging from plywood, steel (Bondek), prestressed concrete planks andreinforced concrete planks. The forming of the deck slab cantilever requires aframe connected to the exterior beam, and therefore it is the most difficult andexpensive part of the formwork. Specialised labour is also required and the safetyrecord over the years for this type of construction has not been good.

VicRoads, in venture with Transfloor Australia, carried out a feasibility study andtesting of the Transfloor decking system to determine its suitability for use onbridges. The results of the investigation are reported below.

3.2 The TransOoor System

Transfloor decking consists of mass produced three-dimensional wire trussespartly embedded in a thin concrete slab, as shown in Fig. 10. The base slabcontains all the required bottom reinforcement. The balance of the required topreinforcement is added after placing the Transfloor decking and before casting theremainder of the deck slab. A number of different truss sizes are available anda variety of modifications (eg. polystyrene block outs) are possible depending onjob specific requirements.

The Transfloor decking system has been used successfully in the Australian andEuropean building industry for some years.

FIG. 10 • PHOTO OF TRANSFLOOR DECKING

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3.3 Investigation Objectives

The investigation involved a theoretical analysis of the system, testing to verify theexpected behaviour and cost estimates to determine the competitiveness withalternative systems as follows:

•

•

•

•

••

3.4

Structural design to ensure adequacy of the decking for a range of deckslab spans from 1200 mm to 1800 mm and cantilevers to 600 mm.

Testing to verify the strength and failure mode of the cantilevered sectionof the decking. This was considered to be the only test required to confirmthe expected behaviour.

Analysis of the composite action behaviour and detail design to ensureeffective shear transfer.

Consideration of the site construction method including transport, handlingand installation of the decking and casting of the deck slab.

Cost estimates to ascertain the competitiveness of the system.

Construction of a prototype bridge comprising the system and finalevaluation.

Design Details

The design was carried out assuming full composite action between the bridgedeck slab 'cast-in-place and the supporting beam. This is ensured by the provisionof studs welded to the top flange for steel beams (and cast-in shear connectors forconcrete beams) after the installation of the decking units.

The decking becomes an integral part of the finished deck slab and actscompositely with the cast-in-place slab, thus utilising all the materials efficiently.The precast slab discontinuity at the transverse joints is not considered to be ofsignificance as the longitudinal compressive forces are easily resisted by the cast­in-place deck slab. Before the cast-in-place deck slab is constructed the cantileverstrength is provided by the trusses with the top wires resisting the resulting tensileforces and the concrete taking the compressive forces.

The design of the slab reinforcement was carried out in accordance with the 1992AUSTROADS Bridge Design Code. At the joints, consideration was given todesigning the slab as a one way slab transversely in view of the discontinuity.However research carried out by Bryan (3) for similar slabs shows that this is aconservative approach and the joints can be disregarded, eliminating the need foradditional reinforcement. It is considered that the latter approach is satisfactoryas the deck slab membrane action provides ample reserve of strength.

TrPical design details for an interior span and a cantilever span are shown inFIgs 11, 12 and 13.

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3.5 Manufacture

The Transfloor decking is manufactured with gaps in the precast concrete slabwhich will end up directly above the beams. The trusses are continuous over thewidth of the bridge including over the gaps in the concrete slab. The cast-in-deckslab concrete ensures an effective connection between the beams and the deckslab. The decking is manufactured to a desirable maximum width of 2500 mm andmaximum length of 12500 mm (desirable limits for transport) and is made to suitactual bridge widths. Skew units can also be made as required.

3.6 Installation

The handling of the decking is performed with slings. Lifting points are designedto suit particular precast slab sizes. When in place the deckmg is supported byhigh density polystyrene strips of appropriate compressibility which provideuniform temporary support and also seal the gap for concreting. The supportingstrip plays an important role in the behaviour of the slab under ultimate loadingas reported by Bryan (3). Unyielding support materials was found by Bryan tolead to premature cracking and failure of the deck slab (3).

3.7 Prototype Testing

The prototype testing was limited to the cantilever section of the Transfloor asthere was some doubt about its behaviour under ultimate load. Of particularconcern was the strength of the truss over the beam, at the gap in the precastslab, during the casting of the deck slab. The testing confirmed the failure modeas the buckling of the transverse reinforcement under compression. The test loadswere increased up to 4.5 times the calculated maximum construction load whenthe impending failure was clearly identified. Testing was stopped before actualfailure.

The testing confirmed that no temporary support of the cantilever is necessaryduring construction of the cast-in·place deck slab. The maximum deflection of thecantilever (600 mm) during construction will be less than 1 mm.

3.8 Construction of Prototype Bridge

The first bridge constructed using the Transfloor decking is situated on a lowtraffic volume road in the Shire of Dundas (Victoria). The bridge cross-sectioncomprises three steel I-Beams as shown in Fig. 14. The construction of the bridgeproceeded as planned with the decking units delivered to the site and installed;the shear studs were then welded using the flash welding technique. Theconstruction of the deck slab was completed soon after.

The bridge was inspected some months after construction and no problems wereobserved. The Shire Engineer was satisfied with the end result which wasachieved at a very competitive price with a short construction time and with fewconstruction problems. Following the success with the first bridge, the Shireconstructed another bridge using the same system. A number of other bridgesusing the system are being planned for other Municipalities.

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3.9 Cost Savings

The cost savings that can be achieved by using the Transfloor decking fora particular bridge will depend on many factors, including type of beamused, remoteness of location, required construction time, availability ofmaterials and resources and other local factors. It is therefore difficult toproduce an accurate estimate. However it is estimated :that cost savings(when compared to a deck formed with alternative systems) would be upto $15 per square metre of bridge deck without accounting for the shorterconstruction time.

3.10 Safety

The Transfloor decking provides a ready made safe working platform forsite workers constructing the bridge. The side forms (precast panels orformwork) for the deck slab and temporary railing can be added beforeinstallation of the decking, if desired, to further enhance safety.

3.11 Future Use

The Transfloor system can be adapted for use on both steel and concretebeams provided that it is engineered and detailed for the particularapplication. The rehabilitation of old bridge timber decks is an area ofpotential application which will be investigated in the near future, in viewof the reduced time of closure for the bridge.

3.12 Conclusions

The Transfloor decking system has been investigated for use on compositesuperstructures for road bridges. It is concluded that the decking can beadapted for the construction and rehabilitation of bridge deck slab. Thedecking becomes an integral part of the bridge deck and acts compositelywith the cast-in-place deck slab to provide a cost effective structuralsystem.

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The ability of the decking to cantilever beyond the external beam is amajor feature which leads to significant savings in cost and constructiontime for bridges. The system provides a safe working platform for siteworkers constructing the bridge. The simplicity of the system enables theuse of relatively unskilled labour for the construction of the deck slab withreliable quality.

REFERENCES

1. Rapattoni F.A and Wells EJ.B. - New bridge superstructure using P.S.c.T-Slabs. Proceedings of the AUSTROADS Bridge Conference, Brisbane(Nov. 13-15, 1991) pp. 743-758.

2. Wells EJ.B. - Super-T Beam end block cracking investigation. Vicroadsinternal report (1993).

3. Ross Bryan Associates Inc. Recommended Practice for Precast PrestressedConcrete Composite Bridge Deck Panels. PCI Journal (March-April 1988)pp. 67-109

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