Strengthening of Tasmanian Highway Bridges

Graeme Nichols

Department of Infrastructure, Energy and Resources

David Coe and Andrew Goelst

Pitt and Sherry

SYNOPISIS

The Tasmanian Department of Infrastructure, Energy and Resources (DIER) is responsible for

the management of 1130 bridges located on National and State highways. Arising from the

Mass Limits Review process, DIER has instigated a programme of upgrading the bridge stock

with approximately 170 bridges targeted in the current programme.

Pitt & Sherry were engaged to undertake the detailed structural assessment of the

understrength structures, develop alternative concept strengthening solutions and provide the

detailed design and documentation of the preferred solution.

A number of unique methods have been developed for strengthening bridge components.

Methods for strengthening substructures include:

• External post-tensioning of pier crossheads;

• Widening of blade piers;

• Widening of pier crosshead;

• Infill walls between pier columns;

• Bonding of steel plates to crossheads.

For superstructures strengthening methods include:

• Strengthening of steel girder halving joints;

• Providing shear connection between steel girders and deck slab;

• Use of carbon fibre to strengthen the deck slab.

1 INTRODUCTION

The Tasmanian Department of Infrastructure, Energy and Resources (DIER) has

responsibility for the management of the Tasmanian classified road network, including 1130bridges located on National and State highways.

In 1996 the National Road Transport Commission Mass Limit Review proposed the

implementation of increased allowable axle mass limits to achieve regional productivity

benefits. The Review, which included the estimated costs of strengthening or replacing

deficient structures, concluded that increasing vehicle loads would significantly improve

transport efficiency. Following implementation of the new Mass Limit Review (MLR) design

loads, DIER were required to identify deficient structures and implement a program of

strengthening measures.

DIER undertook an initial desktop review of categories of bridge types and spans to identify

understrength bridges to the MLR design loads. Subsequently a number of consultants were

engaged to undertake a desktop analysis of the understrength bridges and identify components

requiring strengthening works.

Following the desktop analysis, approximately 170 bridges have been identified as requiringpotential strengthening works with an estimated cost of $35 million. DIER has instigated a

program of upgrading the deficient bridges. With significant constraints on available funds,the process of assessing the priority for investigation and strengthening was based on the

following criteria:

• Existing load capacity of structure;

• Strategic heavy load route designation;

• Traffic intensity;

• Specific heavy load access requirements;

• Funding sources.

Generally, strengthening activities to date have concentrated on bridges located on strategicheavy load routes. For bridges located on the National Highway, the CommonwealthGovernment has funded the strengthening work. For bridges located on state highways, work

has currently been funded by the State Government to date, although there is a possibilitysome of the work may be jointly funded by Commonwealth and State Governments.

Where the Mass Limits Review process has identified understrength bridge substructures and

isolated superstructure components it has generally proved to be cost effective to proceed

with strengthening. Where analysis shows major superstructure elements, such as bridge

girders, to be understrength the cost of practical strengthening measures is greatly increased.

In these cases, DIER has decided the costs of undertaking bridge performance load testing is

warranted in order to obtain more refined load capacity information.

2 STRENGTHENING DESIGN METHODOLOGY

Pitt & Sherry were engaged by DIER to undertake the detailed investigation and design of

strengthening measures on critical deficient structures. The work involved the following

stages:

i) The detailed structural assessment of the structures;

ii) Development of alternative concept strengthening solutions;

iii) Detailed design and documentation of the preferred solution;

iv) Preparation of tender specification.

It was recognised there is pressure for further mass increases to be introduced in future. As a

result the bridges were assessed for the current standard traffic design loading, current heavy

platform design loading and foreseeable future load increases as proposed in the Draft

Revised Section 2 AUSTROADS Bridge Design Code.

During the detailed assessment the structures were analysed for seven different vehicle load

cases. These were split into the two following general load groups for the purposes of

developing different strengthening options:

Load Group A – Austroads Bridge Design Code (ABDC) T44 and W, and MLR Load

Option F

Load Group B - ABDC HLP320 and 400, and Draft Austroads Loads M1600 and

S1600.

The strengthening options were developed based on the principle that structures should be

strengthened to Load Group A as a minimum but where practical and justifiable within the

funding available to Load Group B.

The design development process has been quite extensive requiring close liaison between

DIER and Pitt & Sherry in order to deliver practical, cost effective solutions. During design

development, it has been usual for the proposed strengthening measures to be reviewed by an

experienced bridge construction engineer to assess potential buildability issues.

3 STRENGTHENING SOLUTIONS

Arising out of the program, a broad range of bridge strengthening methods have been

developed. These methods include:

i.) External post tensioning of pier crossheads;

ii.) Widening of blade piers;

iii.) Bonding of steel plates to pier crossheads;

iv.) Infill walls between columns;

v.) Widening of pier crossheads;

vi.) Strengthening of steel girder halving joints;

vii.) Carbon fibre to strengthen bridge deck;

viii.) Provision of new shear connectors.

The strengthening solutions have been developed to address deficiencies identified from the

detailed assessment to suit each structure and site constraints. Most of the adopted solutions

have proved successful and can be transferred to bridges with similar deficiencies. The

following section provides further details on the strengthening methods listed above.

3.1 External post tensioning of pier crossheads

At Hellyer River Bridge the hammerhead pier crosshead to the 2 span steel girder

superstructure was identified to be understrength:

• For flexure, with a maximum action/capacity = 1.25 for Load Group A vehicles with 50%

of the top reinforcement not anchored sufficiently;

• For shear and torsion for Load Group B vehicles.

In order to restore flexural strength to the crosshead, DIER accepted an external post

tensioning option consisting of high strength Macalloy bars stressed against prefabricated

steel stressing heads located at either end of the crosshead, as shown in the Figure 1. Although

located in a benign environment all steelwork, including the Macalloy bars, were coated with

two coats of epoxy primer. Due to a lack of depth in the crosshead, the moment capacity

could only be increased to accommodate Load Group A design vehicles.

Figure 1 – Post Tensioned Pier Crosshead

Photograph 1 – Post Tensioned Crosshead - Hellyer River Bridge.

The approximate cost of the work was $57,000. The work proceeded smoothly with minimal

disruption to traffic using the bridge. During post tensioning, traffic was limited to a single

central lane with a 10km/hr speed restriction enforced. The as constructed strengthening on

Hellyer River Bridge is shown in Photograph 1.

3.2 Widening of blade pier

Stitt River Bridge is a 2 span steel girder structure, with a hammerhead pier. The pier

crosshead, which is supported on a blade type column, was found to be understrength for

Load Group A vehicles with:

• Flexural maximum action/capacity = 1.64 with the top reinforcement not anchored

sufficiently;

• Shear maximum action/capacity = 1.04;

• Failure for combined shear/torsion.

Photograph 2– Blade Pier Widening– Stitt River Bridge

Photograph 2 shows the adopted strengthening solution of widening the blade pier to improve

the bending and shear properties of the crosshead and also remove the problem of torsion.

Dowels were grouted into the existing crosshead and pier at 300mm spacing, alternately

located to both faces of the wall. The design considered concrete shrinkage effects against the

existing pier, with the specification detailing requirements for casting sequences and

programming. A gap was left between the top of the widening and the underside of the

crosshead. After a reasonable period to allow for further shrinkage effects, the gap was filled

under pressure with a non-shrink grout.

The approximate cost for undertaking this work was $86,000. During construction the

majority of the work was able to proceed without traffic restrictions on the bridge. Prior to

grouting the traffic lane on the side of the bridge to which grouting was to occur was closed.

It remained closed until the strength of the grout was 20MPa. A speed restriction of 10km/hr

was applied to the open lane during this period.

3.3 Bonding of steel plates to pier crossheads

The piers to Little Forester River Bridge consist of three hexagonal concrete columns

supporting an 800mm deep crosshead. The crosshead, which supports a precast concrete

inverted U-beam superstructure, was identified as having inadequate shear capacity.

In addition to a standard deck overlay to strengthen the superstructure, steel plates were

bonded to the crosshead to increase the shear capacity for Load Group A vehicles, as shown

in Photograph 3. Steel angles were fixed to the top and bottom corners of the crosshead and

the vertical steel plates were fixed to the sides at regular spacing. The steelwork, which was

galvanised, was fixed to the crosshead with an epoxy bonding agent.

The approximate construction cost was $35,000. The bridge was closed to traffic while

undertaking the remedial work as there was insufficient width to install the deck overlay by

keeping one lane open to traffic. As a result a bypass was constructed and remained in place

while work to the piers was carried out.

3.4 Infill walls between columns

The steel girder bridges forming the on and off ramps to the Bass Highway on the western

side of the Mersey River in Devonport are relatively complex with varying span lengths,

widths and skews along the length of both bridges. The piers consist of 675mm square

reinforced concrete columns supporting 1050mm deep reinforced concrete crossheads. For

Group A loads, the crossheads had the following deficiencies:

• For flexure, maximum action/capacity = 1.4 with inadequate reinforcement detailing;

• For shear, maximum action/capacity = 1.3.

Photograph 3 – Shear Capacity Strengthening - Little Forester River Bridge

Photograph 4 – Infill Walls – Bass Highway Off-ramp

It was decided to strengthen the piers by constructing a new 300mm thick concrete wall

between the columns. The new wall is dowelled into the existing column and pile cap to

develop monolithic behaviour. The gap between the top of the infill wall and the underside of

the crosshead is grouted under pressure injection after a suitable curing period. The

strengthening increases the capacity of the piers to include Load Group B vehicles.

At the time of writing this work is currently being undertaken as shown in Photograph 4.

3.5 Widening of pier crossheads

Similar to the bridge in Section 3.3 above, Treehawke Creek Bridge has a precast concrete

inverted U-Beam superstructure with a hammerhead pier. The pier crosshead, which is

supported on a blade type column, was found to be understrength for Load Group A vehicles

in the following areas:

• For flexure, maximum action /capacity = 1.9 with inadequate reinforcement detailing;

• For shear, maximum action /capacity = 1.15

• Inadequate capacity for combined shear and torsion.

Figure 2 – Crosshead widening – Treehawke Creek Bridge

The bridge is located in an environmentally sensitive area with the pier being partially

submerged. It was decided to strengthen the crosshead to Load Group A vehicles by

widening to both sides in order to minimise the site disturbance, as shown in Figure 2. The

widening process involved drilling and grouting dowels into the existing crosshead, preparing

the existing concrete surface and casting new reinforced concrete bolsters to the side of the

crosshead. The concrete mix included a super plasticiser to facilitate concrete placement and

reduce shrinkage.

The approximate cost of the crosshead widening works was $64,000. During construction,

the Contractor proposed to anchor the dowels in epoxy mortar instead of the detail shown in

Figure 2. Difficulty was experienced with fixing the reinforcement in the confined space and

applying the specified bonding agent to the surface of the existing concrete crosshead with the

reinforcement for the widening in position.

3.6 Strengthening of steel girder halving joints

Mersey River bridge is a 186m long, 5 span steel composite plate girder bridge. At the piers,

the girders to both spans have a halving joint, as shown in Figure 3. The girders to one span

are supported on the pier, and the girders to the other span are supported on the same

alignment through the halving joint. The analysis showed the halving joints were overstressed

in the following areas for Load Group A vehicles:

• Halving joint web panel;

• First full depth web panel;

• Lower halving joint load bearing stiffener.

Although the girders have inadequate flexural capacity for the heavier design loads, it was

decided to strengthen the halving joints to Load Group B vehicles by providing:

• Additional web panel plating;

• Additional vertical intermediate web stiffeners to reduce effective panel sizes;

• Increased bearing stiffener thickness in the lower halving joint.

Details of the strengthening measures are shown in Figure 3.

The approximate cost of the works was $170,000. The bridge forms part of the National

Highway and it was required that one lane should remain open at all times. Traffic was

restricted to a single 3m wide lane immediately adjacent to the kerb located on the side of the

bridge away from the girder undergoing strengthening. A speed restriction of 20km/hr was

also applied immediately prior to welding commencing until 15 minutes after completion of

the weld. Extensive weld inspections demonstrated the required quality of the welds was

achieved even though the Contractor had difficulty slowing the traffic to 20km/hr.

Figure 3 – Steel Girder Halving Joints - Mersey River Bridge

3.7 Carbon fibre to strengthen bridge deck

The reinforced concrete deck to the Bass Highway “on-ramp” on the western side of the

Mersey River in Devonport was found to be deficient in sagging moment by up to 47%, with

only marginal difference between Load Group A and B vehicles.

Carbon fibre laminates were specified to be adhered to the underside of the deck to improve

the flexural capacity of the slab by supplementing the existing steel reinforcement. The 2.0m

long laminate strips span between the steel girders. The 80mm wide, 1.2mm thick strips are

installed at a spacing of 650mm along the deck.

Prior to installation, the substrate must be carefully prepared by patch repairing any unsound

areas and removing concrete laitance. The preparation of the substrate must be verified by

undertaking pull-off tests as the substrate integrity is critical to the success of the process. The

structure must be closed to traffic during placement of the carbon fibre laminates and during

curing of the adhesive. The curing time can be reduced by applying heat to the adhesive.

The approximate construction cost for the strengthening was $150,000. As the bridge forms

an integral part of the link between East and West Devonport, severe restrictions were

imposed in the contract regarding when the bridge could be shut to traffic.

Difficulties were experienced during construction with irregularities in the deck soffit because

the as-constructed detail varied from that shown on the drawings. As a result the pull-off tests

failed and it was necessary to apply an epoxy grout to the underside of the deck in order to

achieve an adequate surface for adhering the carbon fibre laminates.

3.8 Provision of new shear connectors

Hoblers Bridge is an 8 span steel bridge with spans 1, 5, 6, 7 and 8 simply supported. The

girders in spans 2 and 4 extend over the piers into span 3 to support a simple “drop-in” span.

The original drawings show there were no shear connectors attached to the steel girders in

spans 2 and 4. Therefore the girder cannot act compositely at ultimate limit state. For Load

Group A vehicles maximum flexural design action/capacity values of approximately 1.2 were

calculated.

It was proposed to make the deck composite with the steel girders by installing new shear

connectors, in the form of M24 bolts, through the concrete deck and fixing into the top flange

of the steel girders as shown in Figure 4. The shear connectors were to be installed by:

• Initially locating the existing reinforcement within the deck;

• Cutting concrete cores in the deck, avoiding the reinforcement, to the top of the steel

girder.

• Fixing the bolt to the top flange of the girder;

• Grouting the core with the bolt installed.

Figure 4 – Installation of Shear Connectors - Hoblers Bridge Road

To date this work has not been carried out because of the perceived high risk of clashing with

reinforcement when coring the deck. Additionally there are extensive traffic management

problems associated with coring in the deck.

In light of these problems it has been decided to load test the structure prior to implementing

any strengthening measures.

4 TESTING PROGRAMME

In the 1999/2000 financial year testing is being carried out on the following three bridges:

• Tasman Bridge, Tasman Highway, Hobart

• Ringarooma River Bridge, Tasman Highway, Branxholm

• Hoblers Bridge, Tasman Highway, Launceston

Both Tasman and Ringarooma River Bridges are post-tensioned precast concrete segmental

bridges. Preliminary analysis indicated that, although both bridges are capable of carrying the

nominated Option F and T44 vehicles, the requirement of a 1 MPa minimum residual

compression at joints between segments is not satisfied in some parts of the structures.

As outlined above, for Hoblers Bridge, construction drawings showed that the girders in spans

2 and 4 were not fitted with shear connectors and therefore are deficient at ultimate limit state.

In each case it was considered appropriate to install instrumentation and load test the

structures to determine actual strains and deflections under live load and relate the findings to

those predicted by analysis.

Additionally, all 3 bridges will have a bridge monitoring system applied to monitor the

structural responses to every day loading events. By collecting load event data over a

sufficient period, it is anticipated the information will allow the in-service performance of the

bridges to be further understood and provide a valuable input for risk assessment and

management by the asset manager.

5 CONCLUSIONS

Following the introduction of MLR vehicles, a significant number of bridges have been

identified as understrength. With limited funds available, DIER has initiated a program of

strengthening or further investigation by focussing on structures located on the heavy load

strategic road network.

In general it has proved more cost effective to strengthen bridge substructures and isolated

superstructure components. Strengthening options have been developed based on the principle

that structures should be strengthened to current design loads, including MLR vehicles, as a

minimum. In recognition of the pressure for further design load increases, where

economically justifiable the strengthening measures were increased to accommodate the

actions from proposed higher design loads.

DIER and Pitt and Sherry have developed a number of effective strengthening solutions to

suit a wide range of structural deficiencies and site constraints. The majority of the solutions

have proved to be successful and will be transferred to other structures with similar

deficiencies.

The construction issues need to be carefully assessed for all proposed strengthening works

and in particular for relatively new techniques, such as carbon fibre strengthening. In addition

to the construction methodology, management of traffic on the bridges while the work is

being carried out is a critical issue.

6 ACKNOWLEDGEMENTS

This paper is presented with the permission of the General Manager of Roads and Public

Transport, Department of Infrastructure, Energy and Resources.

7. REFERENCES

1. NRTC, National Road Transport Commission (1996)

Mass Limits Review, Report and Recommendations, Melbourne, Victoria.

2. AUSTROADS, Standards Australia and Australasian Railways Associations (1996)

Australian Bridge Design Code, Sydney, New South Wales