bridge strengthening paper
TRANSCRIPT
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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