Jimmy Gully Pedestrian Bridge crosses Jimmy Gully along the Brisbane Valley Rail Trail (BVRT) near Harlin, Queensland. Jimmy Gully Bridge was a 6 span timber railway bridge that was decommissioned in the early 1990’s. Each span consisted of three girder lines with the two middle spans comprised of double stacked hardwood log girders. Each pile bent has three hardwood timber piles with the middle two bents consisting of five piles with the outer (batter) pile on each side providing lateral stability to the structure.
An extensive inspection of Jimmy Gully Bridge was completed by Wood Research and Development (WRD) technicians on 24 May 2016. The aim of the inspection was to complete the visual and EPHOD® (Electronic Pulse Highlight and Outline Diagnostic) compression wave technology analysis of the in-situ condition of the major structural elements of the bridge. The design of any restoration refurbishment, upgrade and repair strategies was then taken into account based on the inspection data collected.
WRD engineers determined that Jimmy Gully Bridge required a new superstructure and deck system along with several of the substructure elements requiring repair works. It was designed with a new pentachlorophenol-treated glulam girder and deck system with connecters that do not penetrate through the top surface which greatly increased the longevity of the structure as well as reduced maintenance costs. In addition to this Decaystop® (Borate Salt) rods were installed in the remaining hardwood elements in order to resist further deterioration of the timber caused by fungi growth.
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TRS began removing the existing deck and girders using a lightweight crane truck. As expected as per the inspection report many of the girders have undergone significant decay due to the present of vertical bolts that allow moisture to travel into the heartwood of the timber log girders.
Once the girders were removed the pile tops were cut-off and removed along with the existing split headstocks and corbels due to decay in each of the timber elements. As the pile tops were lowered the bridge was reduce from 6 spans to 4 spans. The existing timber abutment configuration was removed and backfilled in.
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Once all of the existing deck, girders, corbels and headstocks were removed the pile tops were trimmed down and levelled so the new finished deck height would be approximately 1 metre lower than the existing deck height.
After the hardwood piles were trimmed down to the correct level they were treated with a wood preservative (Copper Naphthenate) and coated with a wax barrier treatment Anchorseal®.
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The transverse glulam deck panels were then laid out on top of the girders and arranged tightly to allow for any natural shrinkage in the timber during the different seasons of the year.
The handrails were installed and attached to the underside of the deck and the side of the girders.
Modulus galvanized steel balustrade panels and cycle rail were installed as per the Australian Standards for pedestrian and cycle tracks.
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Decaystop® (Borate Salt) rods were installed in the remaining hardwood elements in order to resist further deterioration of the timber caused by fungi growth. Note the red bungs are the decay ‘hotspots’ where the timber is prone to have a higher moisture content. These holes are required to be checked for rod depletion as per the issued maintenance manual (typically every 2-5 years depending on environmental conditions).
Completion of Construction – May 2018
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