Upgrading of Railway Bridge at Woll I Creek Sydney
Upgrading of Railway Bridge at Woll I Creek Sydney
Upgrading of Railway Bridge at Woll I Creek Sydney
1. INTRODUCTION
Sydney Trains engaged Arcadis (formerly Hyder Consulting) to undertake an options study and
subsequent detail design for the superstructure replacement of the railway bridge located in Southern
Sydney at Guess Avenue, Wolli Creek 7.684km, Illawarra line.
The original single-span bridge comprised two (2) side-by-side separate transom-top riveted steel half-
through girder superstructures, supported on brick abutments. Each half-through girder superstructure
supported two (2) tracks and had a span (centre to centre of bearings) of 13.1 metres.
The full four-track wide brick abutments (in anticipation of future quadruplication) were constructed around
1915, however, only one (1) superstructure was installed then, being for the two (2) tracks that were
existing at that time. This bridge was constructed as part of a new road connection through the railway
embankment.
When quadruplication of the Sydenham to Rockdale section of the Illawarra line occurred in 1923, the
second superstructure was installed on the existing extended abutments.
The vertical clearance prior to superstructure replacement was approximately 4.13 metres (signposted as
4.0 metres). As such, the original bridge had a substandard vertical clearance, below the preferred
standard height of 4.6 metres, as specified in AS 5100.1 Bridge design for clearances of bridges above
local roads.
Figure 1 shows Guess Avenue underbridge shortly before superstructure replacement work.
Rail Level
1285mm (approx)
Based on the above, it was obvious that a ballast-top replacement superstructure comprising standard
Sydney Trains pretensioned girders was not suitable for this site, given one of the criteria was to increase
the vertical clearance to the road below. Even if transverse post-tensioning was used to create the
structural benefits of plate-type deck behaviour, for the anticipated span length of 13m, the overall
proposed construction depth would be 1820mm (that is, 600mm (track & ballast) + 20mm (waterproof
membrane & rubber ballast mat) + 1200mm (girder depth)). This far exceeds the approximate existing
dimension from rail level to underside of through girder shown in Figure 2.
This lead to consideration of the filler beam form of construction for the replacement superstructures,
together with direct rail fixation, thereby eliminating ballast.
Filler beam bridge decks are classified as a shallow deck system, having relatively high slenderness (span
to depth), however, they possess high stiffness. As such, they are particularly suited to projects where
construction depth (that is, rail level to underside of superstructure) is to be minimised.
4.1 Features
To the author’s knowledge, this project was the first time a segmental (in the transverse direction) match
cast filler beam deck unit, with direct rail fixation, has been used in New South Wales, if not Australia.
Deck units either comprise two (2) or four (4) concrete encased steel sections, depending on their width.
Transverse tie bars (VSL CT Stressbar, Macalloy Bar, or equivalent) ensure effective shear key behaviour
is achieved and prevents differential vertical displacement between adjacent deck units (refer Figures 4
and 5).
A filler beam deck unit superstructure was found to have an overall construction depth of 865mm (that is,
235mm (60kg rail + Alt.1 fastening system + 15mm thick epoxy mortar pad) + 630mm deck unit thickness).
The proposed vertical clearance from the underside of the filler beam superstructure to Guess Avenue
was found to be around 4550mm, and with a track lift of 50mm, the preferred 4.6 metre vertical clearance
was achieved.
Full-penetration butt welds are required by Sydney Trains at the flange/web connections of the built-up
encased steel I sections. As such, commercially available Welded Column sections are not appropriate,
as these have fillet welds on each side of the web.
The match-cast deck unit side faces were then ‘cemented’ together with a thin (approximately 0.5mm to
1mm) layer of epoxy bonder. Apart from compensating for minor imperfections in the combined surfaces,
the epoxy provides a waterproof seal in the joints.
AS 5100 Bridge design does not cover filler beam design theory. As such, for the design of the filler
beam superstructure, the following codes were used:
BS EN 1994-2:2005 (British Standard version of Eurocode 4 – Design of composite steel and concrete
structures – Part 2: General Rules and Rules for Bridges).
UK National Annex to Eurocode 4 – Design of composite steel and concrete structures – Part 2:
General Rules and Rules for Bridges (December 2007).
UIC Code 773, 4th Edition, 1st January 1997 Recommendations for the design of joist-in-concrete
railway bridges, International Union of Railways.
There are a number of geometric criteria in relation to the filler beam form of construction, as outlined in
Appendix 1 of this paper.
As previously noted, the superstructure was transversely post-tensioned using high-tensile tie bars
(Macalloy Bar, 40mm diameter), to assist the structural interaction between adjacent deck units. There
are fifteen (15) transverse tie bars, spaced at 850mm.
ACES bridge analysis software was used to determine the theoretical design actions along each line of
longitudinal ‘girders’, whereby each ‘girder’ comprised an individual steel section and associated width of
concrete encasement. Appendix 2 of this paper shows the determination of the theoretical bending
moment capacity of an individual filler beam section, whereby the plastic moment capacity is used.
Initially, continuous hinges were modelled between adjacent deck units (shown as rectangles in Figure 7)
to simulate the transverse load distribution mechanism to the longitudinal girders, whereby transverse load
distribution between adjacent units is effected by transmission of load through the continuous shear keys
by shear action only. As such, for this initial modelling, the superstructure behaved as an articulated plate,
which by definition has zero transverse bending stiffness (E I) across the longitudinal shear key.
However, with the introduction of transverse post-tensioning across the superstructure, the initial
articulated plate model was transformed into orthotropic plate structural behaviour, effected by the removal
of the member end releases for transverse bending. This brought about a superior load distribution
system, whereby the entire deck width is more effectively engaged to contribute to the load-carrying
capacity.
The primary purpose of the post-tensioned transverse tie bars was to ensure the shear key connected
deck units remain in compression, thereby preventing separation. As such, the post-tensioning
arrangement (that is, bar spacing and bar force) was determined to be sufficient to generate a contact
pressure between deck units to overcome the applied load effects (transverse bending moments).
As can be seen in Figure 8, the new superstructure has a reduced construction depth of 420mm compared
with the original steel half-through girder superstructure.
420mm
6. SUPERSTRUCTURE INSTALLATION
During two (2) consecutive weekend track possessions of approximately 48 hours each, one at the end of
June 2015 and the other at the beginning of July 2015, the separate superstructures were installed.
Prior to the first track possession, a 750 tonne crawler crane was assembled adjacent to the Sydney
abutment, beside the railway tracks.
Horizontal saw cutting of the abutment ledges to accommodate the proposed precast concrete headstocks
was carried out on the Friday afternoon of each track possession. Temporary steel stitching plates were
installed across the cut lines to prevent any differential movement/separation due to the action of rail traffic
in the period leading up to the actual possession.
Following track occupation, slewing of the overhead wires and cutting of the rails, the existing steel
superstructure was removed by crane and cut up into pieces near the site. The upper sections of the
previously saw cut abutment ledges were removed and the precast reinforced concrete headstocks
installed on a levelling bed of mortar (Figure 9). Vertical dowel bars were then inserted to provide shear
connection between the new headstocks and trimmed down existing brick abutments.
Figures 10, 11, 12, 13 and 14 show various stages of the construction work.
Following installation of elastomeric bearing strips along the ledges of the new headstocks, the filler beam
deck units were installed (Figure 12).
Following installation of all filler beam deck units, the entire deck was transversely stressed and the rails
installed.
7. CONCLUSIONS
The upgrading of Guess Avenue underbridge with filler beam deck units superstructures represented an
appropriate solution for this particular site.
It satisfied Sydney Trains’ project objectives, primarily being to increase the vertical clearance to the road
below, re-use the existing substructures and to construct a bridge upgrading form of construction that
represents reduced maintenance.
8. ACKNOWLEDGEMENT
The author would like to thank Sydney Trains for providing access to the site during the superstructure
installation work and also supplying the time-lapse video of the construction work.
Extract from BS EN 1994-2:2005 (British Standard version of Eurocode 4 – Design of composite steel and
concrete structures – Part 2: General Rules and Rules for Bridges).
Extract from UIC Code 773, 4th Edition, 1st January 1997 Recommendations for the design of joist-in-
concrete railway bridges, International Union of Railways.