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 1130 bridges 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
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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