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 their
      networks. 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
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 in
      Figure 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                         < 25m spans are generally adequate except
      (1976-NAASRA        Bridge   Design for some road trains
      Specification)                      > 25m spans are generally adequate except
                                          for road trains and multiple B doubles
      MS18 bridges                        < 20m simply supported spans are generally
      (1953 NAASRA          Bridge Design adequate except for U-slab bridges without
      Specification)                      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 low
      classification 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 major
      superstructure 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
        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 design
        process 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 and
        determine 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.
          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 solutions
      During 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 of
      access 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;
        •     Widening of pier crosshead.

        For superstructures strengthening methods include:
        • Carbon fibre strengthening;
        • Strengthening of halving joints;
        • Reinforced concrete U-Beam Overlay
        • Externa l 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 tensioning 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
            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 pier
        crosshead, 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
        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    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 MLR loads, the crossheads were deficient in flexure and
        shear.
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 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 widening
      to 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.
3.2      Strengthening of Substructure Elements


3.2.1    Carbon fibre 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, 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.
     Photograph 5 – Carbon Fibre Strengthening – Bass Highway Off-ramp


Arden Street Bridge forms a critical part of Melbourne’s 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 the
section unless the beams can be fully wrapped. On Arden Street Bridge, as the
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 to
reinstate the connection between the beam and the deck.
                   Photograph 6 – Carbon Fibre Installation – Arden Street Bridge


3.2.2       Strengthening of Halving Joints

        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 steel
        composite 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 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 5 – Steel Girder Halving Joints - Mersey River Bridge

3.2.3    Reinforced 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 the
        beams. 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.
                         Figure 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
3.2.5    Wrought Iron 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 Melbourne’s 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.




                 Figure 7 –Elastic Model                 Figure 8 –Comparison of Model and
                                                                  Field Test Results.
      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 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.

       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

				
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