11359_SA_Design of an Innovative Support Scheme to Strengthen Tarban Creek Brdg at Hunters Hill in Syd

9th Austroads Bridge Conference, Sydney, New South Wales 2014

© ARRB Group Ltd and Authors 2014 1

DESIGN OF AN INNOVATIVE SUPPORT SCHEME TO STRENGTHEN TARBAN CREEK BRIDGE AT HUNTERS HILL IN SYDNEY Salah Assi, Roads and Maritime Services (RMS), Australia Lindsay Brown, Roads and Maritime Services (RMS), Australia Mark Bennett, Roads and Maritime Services (RMS), Australia

ABSTRACT

The bridge on Burns Bay Road over Tarban Creek in the inner western Sydney suburb of Hunters Hill is an iconic structure in Sydney Harbour. The main span of this bridge is 90m long and comprises five prestressed concrete ribs. Each rib consists of two inclined portal legs supporting a cantilever beam. The beam and the portal legs form an elegant arch shaped structure. Cantilever beam extensions support the adjacent approach spans at half joints. Severe corrosion was identified in the tendons of each portal leg and a retrofit was carried out in 1970. This retrofit was investigated in 2004 because of maintenance concerns and increasing traffic loading on the bridge.

Due to the high risks it was decided that further strengthening of the portal leg structure was justified. Bridge inspections have also revealed that the bearings and cross girders at the half joints located at the end of the cantilever beams are severely damaged and require replacement. RMS Bridge and Structural Engineering initiated and developed an innovative support scheme design. In this design, the cantilevered beams of the main span will be supported and jacked from new permanent Pier frames to precompress the portal legs. This will balance unacceptable tensile stresses under serviceability loadings and thereby strengthen the arched shaped structure. This design is currently under construction and has substantially lower cost and provides easier constructability than other proposals considered in the past. The new Pier frames provide a permanent working platform for rehabilitating the half joints and for their inspection in the future.

This paper outlines the lessons learnt from the various retrofit design options investigated and describes the adopted innovative support scheme design to strengthen the structure.

Figure 2: Cantilevered main span beams and half joint with existing cross girder

Figure 1: Existing Tarban Creek Bridge



EXISTING BRIDGE DESCRIPTION

The bridge was completed and opened to traffic in 1965. The overall bridge length of 230m includes a main central span of 90m and eight approach spans of approximately 17m each. The deck width of 28m provides for six traffic lanes, a median and one footway.

Figure 3: Existing bridge general arrangement

The prestressed concrete structure forming the main 90m span comprises five parallel ribs, 3m wide and spaced at 5.8m centres. Each rib consists of two inclined portal legs supporting a cantilever beam. The beam and portal legs form an elegant arch shaped structure. At the end of each cantilever, there is a corbel that supports the dapped end of the adjacent spans. The finger plate deck joint above each half joint has no drainage trough beneath it. Dirt build-up and water leaking through the joint has clogged up the half joint and lead to significant deterioration of the concrete cross girders.

The beam and portal legs were constructed from tapered precast concrete box segments. A system of post tensioned tendons joins all the segments of each rib together. The ribs are connected transversely by reinforced concrete diaphragms between the beams above the portal legs, and at the cantilevered ends.

The four approach spans either side of the main span are 17m in length and consist of precast prestressed concrete girders with a reinforced concrete deck slab. The girders of the approach span nearest the main span have dapped ends that rest on elastomeric bearings located at the half joint. The dapped ends of the girders were connected with reinforced concrete cross girders and were not designed for jacking to replace the bearings even under dead loads only.

The piers that provide the remaining supports for the approach spans consist of slender reinforced concrete rectangular columns, and the bridge abutments are also constructed from reinforced concrete.

The main bridge span was constructed as follows:

• The precast portal leg units were erected on temporary supports with flat jacks installed in two rows at the tops of the portal legs.

• The main beam precast segments were erected on falsework and post tensioned.

• The main beams were hoisted and traversed into position over the portal legs.



• The flat jacks were extended to lift the main beam, induce axial thrust, and stress the portal leg tendons.

• The flat jacks and post tensioning ducts were grouted.

• The adjacent approach span beams were erected, supported on the half joint, and precast deck slab panels were installed.

• The post tensioning of the main beam was completed and the cross girders and in-situ deck slab were cast.

Although the structure appears to be an arch it is actually a portal frame with inclined legs and cantilever extensions.

For a construction sequence drawing, refer to Figure 14 attached to the end of this paper.

CORROSION OF TENDONS AND INITIAL REPAIR IN 1970 When the ducts in the portal legs were grouted with cementitious material, the tendons at the joints between precast portal leg units were incorrectly packed with a high sulphate cement grout compound (Plaster of Paris), which was highly corrosive to the tendons. Inspection four years after construction revealed damaged tendons with section loss of 80% to 100% in all portal legs of the arch span.

While a solution was developed, temporary props were installed under the half joints. A retrofit system was designed in 1970 and installed at two joints in each portal leg. This system consisted of external prestressing strands across each joint. The anchorage plates were bonded to the inside of the existing concrete box section with epoxy adhesive. The wall thickness of the precast box segments is only 6 Inches (152 mm) and did not allow the mechanical attachment of the anchorage plates to the box segments. These plates and strands were encased in concrete, and the temporary props were removed

NEED FOR REHABILITATION

With time there was increasing concern as to the long term performance of the epoxy bonding of the pretressing anchorage plates. Even though the risk of failure was considered to be low, the potential consequences are extreme. Further inspection in 2008 identified severe deterioration of the cross girders and bearings at the half joints.

The decision was made to proceed with a major rehabilitation of the bridge.

Figure 5: View of main span ribs Figure 4: 1970 retrofit repair inside portal leg



Portal Leg Retrofit System

In 2004, due to maintenance concerns and the increasing traffic load on the bridge, RMS undertook an investigation and assessment of the 1970 retrofit system. This investigation identified the following:

• The durability and long term behaviour of the epoxy adhesive could be an issue as there is no long term information available on this material.

• The possibility of debonding of the epoxied anchorage plates, although unlikely, would result in the potential instability of the structure.

• The above findings warranted further rehabilitation of the structure to eliminate uncertainties in the performance of the bridge, and the possibility of the development of a high risk situation due to potential structural instability.

A structural assessment was carried out and several options for strengthening the portal leg of the arch were investigated in detail by experts from both Consultants and RMS Bridge and Structural Engineering, in consultation with RMS Sydney Asset Section and RMS Sydney Project Services.

Cross Girders and Bearings at Half Joint An inspection carried out in 2008 identified that some of the bearings and most cross girders at the half joints located at the end of the cantilever beams of the arch shaped structure are severely damaged and require replacement. There is also an ongoing need to flush out the gap behind these cross girders below the deck joint.

Figure 6: Damaged cross girder at half joint Figure 7: Elastomeric bearing at half joint

INVESTIGATED OPTIONS

RMS engaged a consultant to assess the capacity of the existing structure, and to investigate options for the rehabilitation of the portal legs. Their preferred option was the installation of new external prestressing. RMS Bridge and Structural Engineering reviewed this design, and it was not considered suitable due to the infeasibility of attaching anchorages to the thin walls of the ribs. Moreover, the identified adverse effect of additional prestress forces on the existing structure could not be solved with a feasible engineering solution. Alternative options were also investigated by RMS, some of which were:

• Strengthening with Fibre Reinforced Polymer. This was discussed with specialists from the University of NSW and it was agreed that it was unsuitable.

• Splicing of the original tendons. This was discussed with VSL Australia. This was also considered unsuitable.



• Construction of a reinforced concrete infill at each joint. The weight of the infill will impose unacceptable stresses on the existing structure and it was considered unsuitable.

RMS INITIAL DESIGN SOLUTION

RMS’s initial design solution to strengthen the portal legs involved installing a grouted steel plate jacket across the segment joints where the tendons were corroded. A 30 mm thickness of cement mortar grout was to be pumped between the steel jacket and the existing structure. The steel jackets would be made composite with the existing section in the locations away from the joints where the original tendons have retained their full strength. This would be achieved using high strength steel bolts to develop clamping forces between the steel box, cement mortar, and existing portal legs.

While feasible, the construction of this option, with initially estimated construction cost of $10M, will require additional work such as, upgrade the access inside the ribs, control of traffic and the final estimated construction cost was very high ($40M).

The damage to the cross girder and bearing was identified, after the preparation of the initial design to strengthen the portal legs, and subsequently a design to replace the damaged cross girders was also prepared, and required the building of a safe temporary access, estimated to cost $3.5M. There is also an ongoing cost associated with future maintenance, jacking and replacement of bearings located at the half joint.

Figure 8: Photomontage of initial solution

RMS FINAL DESIGN SOLUTION

In response to these challenges, RMS Bridge and Structural Engineering initiated and developed an innovative and cost effective design solution that involves the construction of new permanent Pier frames under the half joint at the ends of the main span cantilever beams.

Jacking up and supporting the ends of the cantilever from the new Pier frame beams precompresses the portal legs and eliminates the tensile stresses in the portal legs at the serviceability limit state for all load combinations.

This solution alters the flow of forces in the structure, eliminating the need to rehabilitate the portal legs at the joints. It also eliminates the need to build a temporary access to replace the cross girder and bearings. It provides substantial savings in construction cost and improved constructability compared with other designs and provides a safe work platform for future maintenance work at the half joint.



This design has the following advantages over other options:

• The cost saving compared to the previously prepared strengthening option is dramatic, as the estimated construction cost is only $11M, compared to the estimated cost of strengthening the arch of $40M.

• The construction of the cross girder can be carried out concurrently with the construction of the support scheme, with an estimated cost saving of $3M.

• Substantial improvement is provided in the structural behaviour of the portal legs that eliminates the potential high risk situation that could arise in the event of failure of the epoxy adhesive used in the 1970 joint repair.

• Safe access is provided to repair damaged cross girders and bearings, and permanent support is provided for ongoing maintenance including flushing and cleaning the gap behind the cross girder under the deck joint to eliminate further damage of these girders.

• The solution is easy to construct, eliminates traffic delays and ensures safety of road users.

• The necessity to work in a confined space is eliminated, which is safer for workers.

For sketch drawings of the new support scheme including general arrangement and elevation of portal frames, refer to Figures 15 – 17 at the end of this paper.

Figure 9: Pile cap and piles during construction

Figure 10: Construction under traffic from a barge

COLLABORATION WITH URBAN DESIGNERS

The highly scenic nature of this part of Sydney Harbour, the character and aesthetic qualities of the bridge, and the many residential properties around it, necessitated investigation of options to improve the aesthetics of the support scheme.

The configuration of this support scheme was developed by RMS Urban Design Section, and their consultant Hassell Studio in collaboration with RMS Sydney Asset Section, RMS Sydney Project Services and RMS Bridge and Structural Engineering.

A set of options were presented to the public whose feedback was sought. The vast majority agreed with the preferred option presented to them. It comprises a simple portal frame pier that straddles the existing portal legs, with columns that are tapered to help match the form of the other bridge components. This option best respects the significant architectural qualities of the bridge and minimises the visual impact on the elegant form of the arch. Additionally, situating the new piers outside and away from the existing arch also minimised visual complexity and interference with the existing bridge.



Figure 11: Photomontage of early option Figure 12: Photomontage of adopted scheme after collaboration with urban designers

CHALLENGES IN STRUCTURAL DETAILING The preferred shape after urban design input presented some challenges that needed to be overcome:

• The portal frame with columns outside the existing portal legs required a longer clear span for the pier headstock, so more post tensioning and a deeper section were required to control deflection.

• The tapered pier columns meant that the width available for post tensioning anchorage was limited.

These challenges were overcome by using two vertical columns of six rows of draped post tensioning tendons. The anchorage type incorporated anchor plates on a recessed surface, which were later cast in. This required less width than embedded anchorages, without compromising durability. The tendons were draped such that the end segments of each one was level and parallel, enabling the anchorage plates to be placed in the same plane, making the most efficient use of the limited room. In addition, a self-compacting concrete mix was specified for this design for part of the headstocks above the columns to overcome the limited space in the anchorage zone and to eliminate the need for manual compaction in this area.

Figure 13: Half height end of beam constructed for testing of self-compacted concrete mix



CROSS GIRDER REPLACEMENT DESIGN

The design of the replacement cross girder has to overcome the following constraints:

• The functions for which the existing cross girder was designed needed to be performed, including stiffening of the edge of the deck slab, and distribution of lateral forces to the bearings.

• A wider gap at the half joint is required to permit inspection, flushing and maintenance beneath the expansion joint.

• The girder spacing is approximately equal to lane width, which necessitates the replacement of one cross girder at a time to eliminate closure of more than one traffic lane.

• Prolonged closure of traffic is not permissible.

• The construction is to be carried out concurrently with the construction of the support scheme.

To overcome the restriction on traffic closures, it was determined that the cross girder should be fabricated from steel, allowing fast installation using bolted connections. The need to connect to the existing precast girders presented the following challenges:

• The drawings of the original bridge do not give sufficient detail of the post tensioning anchorages in the dapped ends of the existing precast girders.

• Non destructive testing of the dapped ends that are accessible without demolishing the existing cross girders did not result in locating these anchorages with sufficient confidence to permit drilling into these dapped ends.

• Lack of adequate room around this anchorage to drill holes for replacement steel embedment for new cross girder.

PREPARED CROSS GIRDER REHABILITATION DESIGN

RMS developed a design to replace the damaged cross girder during the construction of the support scheme.

This design consists of a replacement steel edge stiffening beam under the edge of the deck slab, which will be supported at each end by steel girders placed alongside the prestressed girders, which in turn will be supported by brackets clamped to the side of the prestressed girder away from the end anchorage.

This design overcomes the constraints while minimising interruptions to traffic. It also requires no drilling into the dapped ends of the girders. For sketches and perspective drawing of the new cross girder arrangement, refer to Figures 18 and 19 at the end of this paper.

CONCLUSIONS

An innovative support scheme design to strengthen the Tarban Creek Bridge at Hunters Hill has been developed to provide a cost effective and constructible solution. This design is currently under construction and resulted in a potential saving of approximately $30 million, compared with other suitable strengthening options. The design is easy to construct, eliminates traffic delays, eliminates work in confined spaces, and ensures the safety of the road users.

This support scheme could be used to strengthen other bridges with half joints located away from the supports.

The new permanent Pier frames will serve as a work platform for rehabilitating the half joints and bearings and provides a safe access for future inspection and any required maintenance of the half joint.



APPENDIX

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Figure 18: Concept sketch showing new cross girder configuration

Figure 19: 3D perspective of new cross girder arrangement between existing beams



ACKNOWLEDGMENTS

We acknowledge the contribution to the project team members and Wije Ariyartane, Martin Gormley, Neil Forrest and Gareth Collins from RMS.

AUTHOR BIOGRAPHIES Salah Assi, Bridge Engineer (New Design), Bridge Engineering, RMS. Salah is a Civil/Structural Engineer with more than 27 years experience in bridge and structure design and construction supervision, of which more than 24 years were in Bridge Engineering of the RMS. He has extensive experience in the design of complex bridge projects, including prestressed concrete and steel bridges and bridge widening and rehabilitation. His wide experience also covers review of consultants’ complex designs and provision of training and technical advice during design and construction of numerous bridge projects. He has been working as a design team leader in Bridge Engineering for the last sixteen years.

Lindsay Brown, Project Engineer, Bridge New Design. Lindsay Brown graduated from the University of Sydney in Civil Project Engineering and Management in 2002. Joining the RTA as a graduate engineer, he was involved in the construction of the Pacific Hwy Karuah Bypass, and worked for a short time as a site engineer on small road construction projects. He then worked for four years as a project engineer for Bridge Assessment & Evaluation Section, specialising in bridge assessment of heavy loads and higher productivity vehicles, and also conducting structural inspections, detailed analyses, and bridge performance testing. Since 2008, he has worked in New Bridge Design Section doing concept and detailed design of steel and concrete bridges, along with some complex rehabilitation design.

Mark Bennett is the Senior Bridge Engineer (New Design), Bridge and Structural Engineering, Roads & Maritime Services, NSW. He has been chairman of the sub-committee responsible for the bearing and deck joint Part of the Australian bridge codes since 1990. He has over 35 years’ experience in the design, construction and maintenance of bridges. He was the Resident Engineer on the construction of the Bridge over the Murray River at Mildura and has worked in the design teams of many major bridges in NSW including the Anzac Bridge and Alfords Point Bridge.

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11359_SA_Design of an Innovative Support Scheme to Strengthen Tarban Creek Brdg at Hunters Hill in Syd