bridge strengthening paper

7/28/2019 Bridge Strengthening Paper

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Strengthening of Bridges

David Coe - Pitt and Sherry

1.0 INTRODUCTION

Understanding the load capacity of bridges should be the fundamental requirement of

all road authorities. This knowledge is essential for proper management of traffic on

any transport network. It is surprising how many authorities have a poor

understanding of bridge load capacity and hence, by inference, little appreciation of a

major risk on their road network.

There is pressure for road authorities to increase legal loads of vehicles across theirnetworks. The transportation industry has invested heavily in vehicles with increased

axle mass with road friendly suspensions. While access for a number of years has

been limited to designated routes, road authorities are being pressured to provide

increased access so that the great economic benefits highlighted in the 1996 National

Road Transport Commission report on Mass Limits Review (MLR) can be realised.

Figure 1 illustrates the new vehicle loads following the MLR process.

Figure 1 Mass Limit Review Loads

As a result of these needs to improve bridge management processes and to provide

access to heavy axle mass vehicles, road authorities are under increasing pressure to:

Determine, and further refine, the load rating of bridges

Develop cost effective strengthening solutions


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2.0 LOAD RATING

Following acceptance of the recommendations of the Mass Limits Review, Austroads

developed Guidelines for Bridge Load Capacity Assessment, through a Bridge

Assessment Group, comprising representatives from the state road authorities. These

guidelines focused on assessing bridges for the live load configurations shown inFigure 1, which represented the increased axle mass vehicles.

The Bridge Assessment Group collected, summarised and distributed bridge rating

information and produced guidelines such as that shown in Table 1 below.

Design load Comments

T44 bridges

(1976-NAASRA Bridge Design

Specification)

< 25m spans are generally adequate except

for some road trains

> 25m spans are generally adequate exceptfor road trains and multiple B doubles

MS18 bridges

(1953 NAASRA Bridge Design

Specification)

< 20m simply supported spans are generally

adequate except for U-slab bridges without

concrete overlays

Pre MS18 bridges Review all bridges

Table 1 Bridge Rating Guidelines

This table demonstrates that, in general, many bridges constructed after 1953 should

be adequate for the higher mass vehicles. However, many bridges located on lowclassification routes were only designed for 75% of the full design load.

In 2004 the Australian Bridge Design Code was superseded by AS5100 Bridge

Design, including Part 7: Rating of Existing Bridges. The methodology used to assess

the load capacity of a bridge in the code is based on ensuring the same level of risk in

a specific case as required for the general case.

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 majorsuperstructure elements, such as bridge girders, to be understrength the cost of

practical strengthening measures is greatly increased. In these cases, the costs of

undertaking further investigation and analysis, including bridge load testing is often

warranted in order to obtain more refined load capacity information. It is frequently

proved through load testing that a bridge has more capacity than originally calculated

in a simple desk top analysis.

With significant constraints on available funds, the process of assessing the priority

for further investigation and strengthening needs to be aligned with the communities

demands for improved level of service with regard to load capacity of designated


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routes, or road hierarchies. The criteria for determining the priority of selecting

structures may include:

Existing load capacity of structure;

Strategic heavy load route designation;

Traffic intensity;

Specific heavy load access requirements;

Funding sources.

2 STRENGTHENING DESIGN METHODOLOGY

When a structure has been identified through a desktop assessment to be understrength

and is required to be capable of carrying the higher loads in accordance with the

priorities described above, it is important to undertake an extensive engineering designprocess to achieve an optimised solution. This process should involve 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 and preparation of

tender specification.

2.1 Detailed Structural Assessment

A desktop analysis that initially identifies a structure as being understrength is usually

based on the existing drawings, making the same assumptions for the analysis as an

engineer would make for a new design. This tends to be a conservative approach

where:

The elastic model is usually relatively simplistic,

The material properties are based on lower bound characteristic values code based

values, and

Factors are adopted from the design code tend to be conservative.

It is important to review where the desktop analysis is showing deficiencies anddetermine if further investigation will improve the understanding of the actual load

capacity of a structure. It is usually warranted to undertake further detailed

investigation and assessment including:

Inspection of the structure to identify elements of the structure that may affect the

structural performance of the bridge. For example, the barriers on a structure will

often attract load and improve the capacity of a structure. Similarly, there is

usually some form of fixity at a support which will frequently enhance the

structural performance of a structure.


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In many cases it will be difficult to identify the extent such items may contribute

to the structural performance of a bridge. Depending on the areas where the

structure is understrength, a load test may be warranted.

It is usually worthwhile undertaking testing to better understand the properties of

the material actually used in the structure, particularly for older structures

By undertaking a detailed investigation, the dimensions for a structure will be

better understood and it should be possible to reduce factors in the load assessment

process.

2.2 Alternative concept strengthening solutionsDuring the design development process there needs to be a close liaison between the

client and designer in order to deliver practical, cost effective solutions. As it usually

impossible to close any structure for any significant period, the constraints to install

any proposed strengthening work will usually drive the strengthening design solution.

It is likely there will be pressure for further mass increases to be introduced in future.

As a result it is advisable to assess structures for the current standard traffic design

loading and the SM1600 loads specified in AS5100.2. Strengthening options should

be developed based on the principle that structures should be strengthened to current

standard traffic design loading as a minimum but where practical and justifiable within

the funding available to Load Group B.

During design development, it is advisable for the proposed strengthening measures to

be reviewed by an experienced bridge construction engineer to assess potential

buildability issues and also provide guidance on cost estimates, where the cost ofaccess and labour usually far outweighs the cost of materials.

2.3 Detailed design

During the detailed design process, a thorough risk assessment should be undertaken

of the proposed works. It will often be more economic to accept that some issues will

need to be finally resolved during the construction work, rather than fully appraising

and eliminating all risks during the design process. However, it is important for

authorities to include reasonable contingencies when undertaking strengthening, or

rehabilitation work.

3.0 STRENGTHENING SOLUTIONS

In Tasmania and Victoria there has been a campaign by road authorities to strengthen

a significant number of bridges. 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;

Bonding of steel plates to crossheads. Infill walls between pier columns;


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Widening of pier crosshead.

For superstructures strengthening methods include:

Carbon fibre strengthening;

Strengthening of halving joints;

Reinforced concrete U-Beam Overlay

External Post Tensioning

Strengthening of wrought iron structures

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 Strengthening of Substructure Elements

3.1.1 External post tensioni ng of pier crossheads

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

superstructure was identified to be understrength in flexure for MLR vehicles and in

shear and torsion for MS1600 vehicles.

The strengthening works involved external post tensioning consisting of high strength

Macalloy bars stressed against prefabricated steel stressing heads located at either end

of the crosshead, as shown in the Figure 2. 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 MLR design vehicles.

Figure 2 Post Tensioned Pier Crosshead


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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.

3.1.2 Widening of blade pier

Stitt River Bridge is a 2 span steel girder structure, with a hammerhead pier. The piercrosshead, which is supported on a blade type column, was found to be understrength

for MLR vehicles in flexure and shear, and failure for combined shear/torsion.

Photograph 2 Blade Pier Widening Stitt River Bridge


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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.

3.1.3 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.

3.1.4 I nf il l 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 MLR loads, the crossheads were deficient in flexure and

shear.


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Photograph 3 Shear Capacity Strengthening - Little Forester River Bridge

Photograph 4 Infill Walls Bass Highway Off-ramp


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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 MS

1600 loads.

3.5 Widening of pier crossheads

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 MLR vehicles in flexure, shear and torsion.

Figure 3 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 for MLR vehicles by wideningto both sides in order to minimise the site disturbance, as shown in Figure 3. 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 3. 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.


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3.2 Strengthening of Substructure Elements

3.2.1 Carbon fi bre strengthening

Carbon fibre is being used increasingly to improve the load capacity of reinforced

concrete bridge superstructures. It is predominantly used to improve the flexural

capacity of beams and decks. For example, 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%.

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, severerestrictions 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.


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Photograph 5 Carbon Fibre Strengthening Bass Highway Off-ramp

Arden Street Bridge forms a critical part of Melbournes road network connecting the

Central Business District to the inner suburban and industrial areas of Kensington. The

bridge crosses the Moonee Ponds Creek. The 47m long, 7 span structure was

constructed in 1923. It consists of an in-situ reinforced concrete deck with 5

downstand beams. Each beam is cast integrally into a reinforced concrete pier.

The bridge was found to have inadequate capacity in flexure and both vertical and

longitudinal shear in the regions close to and over the piers. Plastic Analysis allowing

moment re-distribution at supports did not provide any significant benefits. The low

rating in the region of the supports was exacerbated by reinforcement detailing which

is no longer considered acceptable.

A critical constraint during the development of bridge strengthening options was that

there should be minimal disruption to the traffic using the bridge. In effect, this

required all strengthening proposals to be installed under the bridge.

It was proposed to install a folded steel plate to the underside of the deck and the side

of the downstand beam. To ensure structural continuity the folded plate was epoxy

bonded to the concrete substrate along with chemical anchors. The combination of the

plate and the anchors provided increased capacity for both flexure and longitudinal

shear over the supports.

Increasing the shear capacity of the downstand beams in the vicinity of the piers was

more of a problem. The use of carbon fibre strengthening for shear strengthening has

been very limited, because it is very difficult to mobilise the full shear planes in thesection unless the beams can be fully wrapped. On Arden Street Bridge, as the


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downstand beams were cast integrally into the deck it was not possible to wrap the

carbon fibre around the beam to provide the necessary anchorage lengths.

Nevertheless, the folded steel plates that were proposed for strengthening the beams

for flexure, provided the opportunity to fully anchor carbon fibre shear strengthening

at the deck/beam interface. As a result, the carbon fibre strengthening detail shown in

Figure 4 was proposed. A high modulus carbon fibre was chosen in this case, so that

the minimum movement in shear would mobilize the most resistance force within the

fibre, maximising the benefit to the bridge beams.

Figure 4 Arden Street Bridge Strengthening Detail

Once the Contractor had thoroughly cleaned the bridge and provided access for closer

inspection there was a significant crack identified at the interface between the

underside of the deck and the beam, forming a structural discontinuity between the

deck and the beam. For the strengthening work to be fully effective, it was essential

that the continuity between the downstand beam and the reinforced concrete deck was

reinstated. Extensive crack injection was undertaken along the length of the bridge toreinstate the connection between the beam and the deck.


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Photograph 6 Carbon Fibre Installation Arden Street Bridge

3.2.2 Strengthening of Halving Join ts

With increased vehicle loads, the increase in shear force at supports often causes

capacity problems. For example, Mersey River bridge is a 186m long, 5 span steelcomposite plate girder bridge. At the piers, the girders to both spans have a halving

joint, as shown in Figure 5. The analysis showed the halving joints were overstressed

in the following areas for MLR vehicles:

Halving joint web panel;

First full depth web panel;

Lower halving joint load bearing stiffener.

It was decided to strengthen the halving joints 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 5.

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 kerblocated on the side of the bridge away from the girder undergoing strengthening. A


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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.

F igure 5 Steel Gi rder H alving Joints - Mersey River Br idge

3.2.3 Rein forced Concrete U-Beam Overlay

There have been a significant number of bridges constructed from precast reinforced

concrete U-beams. The beams are usually bolted together, with a grouted shear key at

deck level. The poor connection details between the beams means that there is very

little distribution of load between the beams. As a result most U-Beam bridges do not

have sufficient capacity for MLR vehicles.

A common method of strengthening these bridges is to provide a reinforced concrete

deck overlay, as shown in Figure 6 below. The deck overlay not only increase the

structural depth of the superstructure, but provides good load distribution between thebeams. It can also be seen in Figure 6, that provision of new kerbs provides an

excellent opportunity to upgrade the bridge barriers as the existing barriers will rarely

meet current code requirements.


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F igure 6 Typical Deck Overlay Detail

3.2.4 External Post Tensioning

Following the accident on the Tasman Bridge, in addition to the replacement of the

damaged spans and piers, the number traffic lanes on the bridge were increased. This

resulted in additional traffic loading on the outer beams, for which it had not been

designed. As a result external post tensioning was provided to the outside beams to

increase the structural capacity, as shown in Photograph 7 below.

Photograph 7 External Post Tensioning Tasman Bridge


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3.2.5 Wrought I ron Structures

Strengthening of wrought iron bridges is particularly difficult and significant problems

are frequently encountered including:

High cost access is usually expensive and the strengthening work inherently

slow and labour intensive. Material compatibility wrought iron has a laminar structure that provides high

strength in the longitudinal direction but is weak in the transverse direction.

Strengthening of components by means of welding is potentially dangerous.

Disruption to the community any extensive strengthening proposals require

prolonged lane closures and possibly closure of the bridge for considerable

periods.

Heritage issues developing a strengthening solution sympathetic with the

heritage values of the bridge would be difficult.

Princes Bridge is Melbournes grandest bridge linking the southern commercial and

art centres to the commercial heart of the City. It is one of the busiest bridges in

Australia servicing vehicular, trams and extensive pedestrian traffic. Built in 1888 it

has significant heritage value.

A desktop analysis of the bridge load capacity showed the bridge required extensive

strengthening to meet current legal loads. Pitt & Sherry and Van Ek Contracting

offered an alternative proposal to carry out a performance load test on the bridge with

the objective undertaking a more rigorous analysis by developing a calibrated

structural model to optimise strengthening requirements to meet current legal loads.

The Performance Load Test involved attaching strain gauges to critical structural

members to measure the response of the structure under a test vehicle, developing an

elastic model in a structural analysis program, as shown in Figure 7, and modifying

the parameters in the model so that it has a similar response to that measured in the

actual structure in the field, refer Figure 8.

F igu re 7 Elastic Model F igure 8 Comparison of Model andF ield Test Resul ts.


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The analysis using the calibrated model showed that the bridge acted in a significantly

different way to the original desktop analysis and the vast majority was deemed to

have adequate strength for the new design loads. As a result the strengthening work

comprised predominantly of replacing wrought iron rivets with high strength bolts.

It is quite common for rehabilitation work on such structures that additional work is

required once access is provided and the extent of damage is understood. It was

recognised there was a high risk of repair work being required once access was

provided and the pigeon guano removed to allow detailed inspection of the structure

and there was a reasonable contingency for the repair of these different deteriorated

members.

The calibrated structural model was used to determine the extent of degradation that

was permissible before intervention was required. In this way the extent of repair work

was optimised.

Photograph 8 Structural Repair Princes Bridge 4.0 CONCLUSIONS

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

been identified as understrength. With limited funds available, road authorities have

initiated programs of strengthening or further investigation by focussing on structures

located on the strategic road network.

In general it has proved more cost effective to strengthen bridge substructures andisolated superstructure components. Strengthening options have been developed based


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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.

Pitt and Sherry has 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.

5.0 REFERENCES

1. NRTC, National Road Transport Commission (1996)

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

2. STANDARDS AUSTRALIA, AS5100.7 Bridge Design Rating of Existing

Structures, Standards Australia, New South Wales, 2004

3. AUSTROADS BRIDGE ASSESSMENT GROUP, Guidelines for Bridge Load

Capacity Assessment, AUSTROADS, Sydney New South Wales, 1997

bridge strengthening paper

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