The title of this post comes from a discussion – well actually a series of discussions – that I have been having with a bridge engineer.
We were standing under the timber truss road bridge outside of Lithgow, NSW when a car drove over it. It is a McDonald type truss road bridge built in 1893 (for a bit more detail about different timber truss types have a look here or here .
The discussion was focused on works that are needed to repair the bridge and ensure that it could cope with predicted traffic loads.
Let me give you two versions of the same event.
Version 1 – The Heritage Professional (me – excited and rambling)
Wow. When you are under here you really hear that rumbling and clacking. I remember this from drives when I was younger. I’ve been speaking to people and that sound is one of their most recognisable and consistent memories of these bridges. In fact, I would go so far as to say that the sound of a car going over these bridges and how people respond to it is almost as significant as the construction technology and social value.
Version 2 – The Engineer (not me – exasperated and measured)
Wow. That sound makes me want to not stand under here. All I hear is loose timbers and bolts that need tightening. If you needed any further justification for why the deck needs to be replaced that should be it. Do you know that originally when the decking was laid it ran diagonally and was tight? It would have been as quiet as crossing over a modern concrete bridge. The decking that is here now is probably the third or fourth deck laid. It has been considerably modified and doesn’t reflect the original design intent or aesthetic of the McDonald trusses.
As time goes on and places are used and experienced, people’s perception of them change. What is considered important or significant invariably changes.
When it comes to timber truss bridges we have a situation where people have developed a connection with something that, from an engineering and design perspective, is considered a flaw.
Timber truss bridges are a class of artefact in which different values inform how they are viewed and what is considered appropriate treatment.
For the local who needs to cross the river to access the town or move produce or stock the importance of the bridge is in its function. Crossing the river is the most important thing. If the bridge needs repairs or modification to continue this, any solution, including demolition and building a new bridge, is valid.
For the bridge engineer who appreciates the history of timber truss design and recognises the rarity of this element, keeping the trusses on a working bridge is the primary goal. This means that all other bridge elements may be able to be modified, including the decking, piers and buttresses.
For the tourist or casual visitor, usually in a car, their remembered experience of the bridge may be that few seconds of driving over it. The clumping of tyres on timber and metallic rattle of loose bolts and stays. If you change this, you change the very nature of the bridge for them.
These types of bridges show us that what is important or significant about a place can change over time. Many places have significance today for reasons that were never thought of at the time of construction.
What is significant about heritage places is a moving feast and being able to recognise and manage this change in types and levels of significance is always a challenge; particularly if what is considered to be significant, is ultimately a design flaw.