Keeping costs down with clever ‘reverse’ engineering
Queensland’s Kilkivan to Kingaroy Rail Trail opened in 2017
Posted on 19.03.2018
Queensland’s 89km Kilkivan to Kingaroy Rail Trail opened last year, constructed by the Gympie and South Burnett Regional Councils. Rail Trails Australia notes the good standard achieved for a relatively low budget of $2 million for the trail surface and bridges. Below is a summary of the South Burnett Regional Council engineer’s technical explanation about how he achieved low cost outcomes for the South Burnett section, which may be of interest to others planning rail trail construction.
I designed the bridges based on the old railway style of log/timber deck, but with handrails added. I scaled down the timber dimensions (see details below) and then had engineering consultants (GHD) prepare the drawings and determine load capacities at the end. By approaching the construction in a new way we not only saved costs, but expect to reduce maintenance costs. The structure We started with just the concrete abutments and centre piers, from there I looked at the measurements of the old bridges, which was a 425mm corbel and a 425mm girder and 100mm sleepers – a total of 950mm. I simply reverse-engineered them by working out the size of the girder needed for the longest span (7.4m) of all the bridges, which came out at 300mm diameter to achieve the load requirements. This reduced the dimensions needed to achieve the overall 975mm of the old railway structures.
It also helped with efficiency – as the sawmill had only one size to produce. For those with an engineering or mathematical bent, you might think that my maths is out because we have 25mm too much. However on the ground there was typically a slight variance, depending if the bridge was on a slight curve (top side always higher) or due to how the original builders dealt with the onsite construction. The result was that timbers were fitted with or without a check-in at the bottom. The sawmill put the raw logs through a single cut to effect the flat side on which they could process then used a paring saw to make them all even. This immediately gave us three flat sides to work with, from just the basic preparation process.
The decking is all 75mm thick however the width varies from around 100 to 250mm. This significantly reduced waste when the sawmill was cutting the timber, as they simply cut as much from each log as it could yield, with boards of varying width. This reduced the overall cost of decking timber. My research suggested that the greatest threat to timber bridges is the ingress of water through the vertical holes and decking fastened from the top. The resulting dry rot and decomposition is evident when you strip the old sleepers from the top of a timber bridge – the top 125mm is degraded, yet the 300mm below is still solid and structurally sound. We confirmed this with ultrasounds on the Tingoora Bridge. Based on these findings, we developed new construction drawings for typical plank fixing detail. While it takes longer, the benefits are reduced water ingress into the main bearers. The fixing involved:
- Angle brackets fastened with a horizontal threaded rod. The 300mm bearers had a horizontal hole every 1 metre.
- On top of the angle bracket we bolted a 125mmx50mm bearer.
- The decking was affixed to the bearer from underneath – to the 125×75 fixing plank, so no holes on top (this slower process added an extra two days, but is expected to extend the life of the bridge).
- The middle bearer was set 5-10mm higher than the others, which also assisted in drainage and therefore fewer wet surface, mould, moss or slip issues.
In most cases decking needs to replaced at least once or twice over the life of the bridge, as the it is most exposed to environmental effects and usage. Typically the replacement of the decking will not be due at the same time as the bearers underneath. Our construction method has an additional advantage – sections of decking can be lifted off to allow for the replacement of bearers, corbels or any part of the super structure without disassembling the whole deck.
This is achieved by disassembling the through bolts on the angle plates to the bearers. The decking remains affixed to the 125×75 fixing plank and sections can be lifted as a complete component from the bearers.
The hand-selected timber is local Eucalyptus crebra (Narrow-leafed Ironbark). Queensland Forestry lists this species as being naturally termite resistant with an above ground life of 40+ years and untreated stress grades of F11, F14, F17, F22, making it perfect for timber bridges
The handrails were also one of my own designs, comprising a steel panel and post bolted together. This is similar to pool balustrade to provide added safety for young children. We felt that an inside bike rail, which is not mandatory in Queensland, may increase the ability for curious children (and even some adults) to stand on to obtain a better view of the waterway below. It would also have increased the complexity and therefore cost. The height is approximately 1350mm above the deck, a little higher than the standard 1200mm; even if a young child stood on the bottom rail it was still 1150mm to the top.
The panels were standard across all bridges, making it cheaper to manufacture and much simpler if we needed to replace any in future. However, this was a little lost in actual production as the manufacturer thought that we may need a quantity of smaller sizes. The bolt-together panel attachment point is flat steel, allowing for the bridge to be either straight or curved without distortion of the panels. Longer and bigger curves only add strength to the handrails and the overall base width is significantly increased (Tingoora Bridge is a good example – refer photo).
We ended up with a certified load limit of 3.5 tonnes, enough to carry council inspection/maintenance vehicles, an ambulance, or a police/State Emergency Service 4×4. The load limit is very conservative – the bridges are capable of 17 tonne loads without movement, as demonstrated when the contractor put his heavy roller (17t) over each one a number of times. The limit was kept low in part to discourage use by Council slasher drivers and local farmers using it as a shortcut with ploughs, etc.
A slasher or agricultural equipment is at more risk of coming into contact with the handrails, so the load limit was reduced to cater for smaller vehicles.
We engaged a local fencing contractor who regularly works with raw timber and had the work checked and signed off by (GHD) construction engineers.
This reduced the costs significantly and avoided us having to find scarce timber log bridge carpenters.
In total we built eight log bridges from scratch, refurbished the old curved timber log bridge at Tingoora and constructed a new low level timber bridge for Council slashers across Barambah Creek.
Total cost of the ten bridges, including drawings, certification, survey, timber, fixing and construction was $612,916 ex-GST.
In all, a heap of reverse engineering to make it ‘fit’ the budget – plus I don’t know how many hundreds of hours at night, at work and during weekends with several notebooks full of reverse engineering to bring the costs back to a level which could be funded with our $1.4m share of the grant.
Rail Trail Connections
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