Following on from last month’s piece, where we discussed road infrastructure, this month I thought I’d try to provide a little insight into the challenges associated with bridge maintenance in our shire. Palerang is by no means the only shire impacted by the need to replace aging bridge infrastructure. It is a common problem in rural areas, where bridges may have been built to fill a need at a particular time in history, but with heavier vehicles and increased usage are no longer up to the task.
I mentioned in the post on road infrastructure that we still had 25 timber bridges nearing the end of their useful life. In each case, we ultimately have two options—simply declare the bridge unserviceable, or replace it with a more durable structure. The former is rarely an acceptable option from a social perspective. The latter is invariably a serious strain on council’s budget.
There are generally two replacement options: a box culvert or a full bridge structure.
The recent replacement of St Omer’s Bridge on the Nerriga Road is a good example of the former option. As is typical of timber structures, the deck and main bearers (all timber) were deteriorating badly under heavy traffic loads (in the photo, those are temporary steel props holding up the middle of the old bridge).
The replacement concrete box culvert structure cost a little over $500,000, but can now handle the increasing traffic loads on the road.
The Molonglo River bridge at Captains Flat is a more challenging proposition.
Being so high above the river, there is no alternative to a replacement bridge structure. With the quality of the concrete in the existing piers unknown, the entire structure will need to be replaced. While it’s a little difficult to see in the photograph, the piers are currently strapped with additional bracing, and massive steel girders had to be fixed to the deck of the right hand span to compensate for the failure of the timber beams below the deck. This structure has seen better days.
The replacement of the Molonglo River bridge is due for completion towards the end of October, and the progress photo (taken in late September) shows the new piers and headstocks already in place. Next comes the new deck.
In this case, the replacement project is likely to run to around $1.2m. This is well beyond the capacity of Palerang’s annual infrastructure budget and we, like many other rural councils are dependent on special grants from the State and Federal governments for projects such as these. In this case, the NSW government will contribute half the cost of the replacement, but council must borrow the remaining $600,000 and make the repayments from its own reserves (i.e. rates revenue).
Update: The new Molonglo Bridge at Captains Flat was completed as planned in October 2014, at a cost of just under $1.2m.
Now, that leaves just 24 more timber bridges to deal with…