Weighty Matters

In gathering the material for this book, coordinating editor Lenore Coltheart was intrigued by her phone conversations with each of the four bridge engineers introduced here. All agreed to meet for a recorded discussion on the topic that is a weighty matter for many bridge engineers – how to ensure old as well as new bridges meet the continually increasing challenges of roads and their loads.

The participants were former chief bridge engineers Brian Pearson (in office from 1981 to 1987) and Ray Wedgwood (in office from 1987 to 2000, Director Bridges and Structures Wije Ariyaratne (2000 to present) and Amie Nicholas, bridge engineer with New South Wales (NSW) Roads and Maritime Services. The warm hospitality of Brian and Betty Pearson turned a damp and chilly Sydney winter Friday in 2017 into a memorable exchange of ideas, reflections and a wealth of experience.

The outcome is this chapter; the transcript of that lively conversation, amplified and edited by the participants.

Amie: It’s a rare opportunity to hear from three key people who have been responsible for meeting the challenges of economically delivering safe bridges for over half a century During this time, freight loads as well as environmental awareness (including heritage considerations) continue to increase. It is interesting too that in retirement, Brian and Ray are, like Wije, actively engaged in conserving the State’s heritage bridges. As it is seventy years since you first set out on your bridge engineering career Brian, I’m wondering what strikes you as the biggest change over that long period?

Brian: Timber bridges haven’t changed, but the type of bridge changed.You can see this in Sydney, where the Sydney Harbour Bridge, the Gladesville Bridge and the Anzac Bridge, constructed roughly thirty years apart, each represent state of the art bridge design for their eras. The 1964 Gladesville Bridge was the first major concrete arch bridge in the world to be built using precast sections. The Gladesville Bridge marked the transition from steel bridge technology as represented by Sydney Harbour Bridge, towards concrete design. Gladesville Bridge at the time had the world’s largest concrete arch span. It was also one of the first bridges designed with the aid of a computer. In 1995, the completion of the cable-stayed Anzac Bridge over Blackwattle Bay marked the emergence of yet another bridge technology in Australia.

Ray: We can contrast this period with road bridge building during the previous seventy years, the timber truss bridge era from the 1850s to the 1930s. Railways dominated for most of that time, with any iron or steel being reserved for railway bridge construction. The change to an emphasis on road infrastructure only started in the 1920s. For road bridges, timber took the load for a long time.

Brian: We should point out that when we talk about bridge loading, there are three types – dead load (the weight of the structure itself); live load (the traffic); and impact load (the effect of suspension, bumps and so on), above the live loading.

The biggest change for us was the change in live load from that 16-tonne farm tractor of 1900. The rapid increase started in the 1930s – look where it is now, and timber truss bridges are still in use. The design loading level stayed constant for many years. It was Spencer Dennis, Bridge Engineer from 1928 to 1951 (working initially for the Main Roads Board and then for the Department of Main Roads) who pointed out that with the rate of increase of traffic masses, design loading was inadequate.

Wije: The diminishing number of timber truss bridges brought increasing awareness and appreciation of their history, their ingenuity and their value for future generations. Publication of the first Timber Bridge Management Strategy in 2002 showed a commitment to keep at least a representative sample of timber truss bridges, and with this came an increased necessity for close collaboration between disciplines. That’s essential to keep the bridges of the highest heritage significance operative in an environment of increasing higher mass limits. To achieve this objective, in 1993 we tested two timber truss bridges, both over the Barwon River near Walgett. One was an Allan Truss bridge built at Eumenbah in 1920, and the other a Dare Truss, the 1930 Dangar Bridge (see Figure 7.2). The objective of testing these bridges was to develop mathematical models for a more accurate prediction of their behaviour under increasingly heavy loads and to develop standard strengthening methods to carry these heavy loads. I am confident that nowhere else in the world would anyone have conducted load testing of timber truss bridges with actual heavy loads in real time to determine their actual behaviour to these loads.

Brian: It can require as much creativity and engineering skill to keep one of these bridges in service, as needed to design and build a new one.

Wije: Collaboration was also critical with the suppliers of the timber vital for the maintenance of bridges. With bridges to be retained in service for the long term, we needed to look at how timber might be supplied, especially the high strength, high durability timbers required for the most heavily loaded members of timber truss bridges. Roads and Martime worked closely with NSW Forestry and with timber mills throughout NSW, seeking new ways to reliably source this timber.

Ray: In terms of vulnerability to modern loads, some bridges are protected by being more out of the way; like the Victoria Bridge at Picton. It’s still used by local traffic and so has to be kept serviceable; but not to the heaviest loads. It has a five tonne limit and quite large barriers above the approaches (see Figure 7.3). Even for five tonnes it is a bit of work to keep the bridge in service, but this is one of the few timber truss bridges in the State that isn’t being asked to do more than it was designed for.

Wije: The new strategy certainly brought new engineering issues. Assessment of the timber truss bridges according to the only available Australian Standard (AS1720.1 Timber Structures) gave very poor results, and so in addition to load testing, collaboration between Roads and Maritime and the University of Technology, Sydney was necessary to develop realistic methods for the assessment of the strength of the different types of heritage timber truss bridges. Involving those at the workface of timber research enabled a better understanding of the strength of these bridges.

Ray: Like the knowledge gained for timber bridge engineering from the scientific testing of Australian hardwoods by Professor Warren at the University of Sydney in the 1880s.

Wije: Collaboration was also critical with the suppliers of the timber vital for the maintenance of bridges. With bridges to be retained in service for the long term, we needed to look at how timber might be supplied, especially the high strength, high durability timbers required for the most heavily loaded members of timber truss bridges. Roads and Martime worked closely with NSW Forestry and with timber mills throughout NSW, seeking new ways to reliably source this timber.

Ray: In terms of vulnerability to modern loads, some bridges are protected by being more out of the way; like the Victoria Bridge at Picton. It’s still used by local traffic and so has to be kept serviceable; but not to the heaviest loads. It has a five tonne limit and quite large barriers above the approaches (see Figure 7.3). Even for five tonnes it is a bit of work to keep the bridge in service, but this is one of the few timber truss bridges in the State that isn’t being asked to do more than it was designed for.

Amie: As long as the 20 tonne maintenance trucks don’t think their weight doesn’t count!

Brian: That was always the situation with the timber truss bridges, the drivers who think their overload somehow doesn’t count. Ray and I found this a decade ago, when we worked on the conservation of the 1895 Tharwa Bridge in Canberra; loaded 65-tonne sand trucks found the old bridge handy coming from loading sites on the southern side of the Murrumbidgee River to construction works in Canberra. This happened everywhere. For instance the harvesting of rice and wheat meant trucks in regional areas could be loaded to about 60 tonnes. At Deniliquin that was the usual tally at the weighbridge, where the trucks had travelled over the old truss bridges in that area; including the bridge over the Edwards River at Deniliquin, since replaced. They trundled over the truss bridges without any worry at all, despite carrying four times the live load design of the bridge. The same thing applied to wide loads too in harvesting time.

Ray: A lot of the harvester vehicles have arms that fold up vertically to get across bridges – they are still pretty wide though. The old bridges have six metres roadways and even four and a half metres between kerbs is common for the bridges still in service now. As well as their extra strength, another reason that these wooden truss bridges have lasted is that they were designed with a high factor of safety (of about seven, ultimate load to working load), whereas steel and concrete were designed for lower values. More recently, using limit state design principles, as opposed to working stress design principles, a smaller safety factor is used for the dead load for the live load. Because it’s a more known load, with older concrete bridges you can improve their load capacity just by applying limit state design principles, rather than the working stress design principles. A benefit from the lower factor of safety on the dead load is that it is generally about at least half or more than the total stress for which the bridge was designed.

Brian: Apart from the fact of inherent strength, there are potential factors of weaknesses in timber, like rot and white ants, with deterioration evident at the ends of a member and the rest being quite sound. But we still have to replace the whole length, say six metres or so, because the end 100 mm has deteriorated.

Ray: Brian’s other point about traffic width of lanes means a lot of the older timber truss bridges are now too narrow for safe use, particularly with trucks on them. Some trucks now have very small clearances on the carriageway, which makes it a problem with timber trusses.

Amie: Training to meet these challenges today is vital. I wanted to ask if this was less an issue when removal of the timber truss bridges was a ready option. Maybe start with the present and work our way back?

Wije: The change in emphasis from reactive bridge maintenance to proactive bridge upgrading meant not only the need to train the carpenters of the bridge crews in both new and old timber construction techniques, but also to provide training and guidance to engineers in the science of timber design.

At the inaugural RMS Annual Bridge Conference in 2004, one of the papers presented was on the strengthening of the heritage timber truss bridge over the Abercrombie River south of Bathurst. At every following conference, there have been a number of papers presented on timber bridges; sharing learnings on either design, construction, specifications, heritage aspects, inspection or maintenance.

Brian: In my time, training was in-house as we did all our designs internally, with 160 staff in the bridge section. There were no consultants and all the work was done by staff. All complete designs were undertaken in-house, with rare exceptions. Towards the end of my era, we had two years of producing 160 new bridge designs a year.

Ray: That would now be reversed, with only an occasional in-house design.

Brian: We had regular, maybe quarterly meetings of the bridge foremen, and in fact it was pressure from people in the field, the foremen and engineers behind our 1987 revision of Manual No.6, the 1962 version. They thought it should recognise the Dare Truss which until then wasn’t included as a design type, though they were all familiar with Harvey Dare’s bridges. They frequently referred to the one at Dungog as the ‘Dare Bridge’.

I thought about it a long time. What finally convinced me was a conversation a morning tea that I arranged when Sir Ralph Freeman visited Sydney for the 50th anniversary of the Sydney Harbour Bridge in 1982. He reminisced with Gordon Stuckey who had been the principal design engineer for the Bridge. They talked about Dr Bradfield, constantly busy with calculations in his corner office at the Public Works head office in Bridge Street, and it struck me then that when we talk about engineering design we should recognise the ‘stress man’, the one who makes it all work but who is forgotten in legal as well as lay definitions of a bridge designer. That’s quite hard on engineers!

And so we now have the Dare Truss with its steel bottom chord, finally identified in the February 1987 revision of the manual.

Wije: The Roads and Maritime Timber Bridge Manual remains a valuable resource. The current manual is a very comprehensive document, first published as a draft in 2000, then issued in final form in 2008. In eight chapters it covers in great detail, many aspects of bridge design, construction and maintenance, and has been widely used both by Roads and Maritime and by local councils in their timber bridge maintenance projects.

Our most recent manual is a new draft Roads and Maritime guide for design and assessment of heritage timber bridges, part of which has also been incorporated into the new Australian Standards for Bridge Strengthening and Rehabilitation (AS 5100.8).

Ray: In my time Don Carter ran training for the bridge crews – foremen and leading hands – we used to get all the bridge guys down from the country. These were residential, I recall one at Kurrajong for instance. They were a chance for the crews to share tricks of trade between them all, as well as the workshops on types, timbers and techniques.

Amie: Don Carter is still involved in our Timber Bridge schools and we also have an annual get together for updates. Wije: Yes, in the last ten years we’ve held six Timber Bridge schools, since our first week-long residential Roads and Maritime Timber Bridge School in 2005, my second year as Principal Engineer. As part of the Bridge Workforce Skills Project, the need for training in timber bridge maintenance for engineers and supervisors was a high priority. These Timber Bridge schools cover all aspects of timber bridge maintenance including timber technology, decay and termites, durability and preservatives, inspection of timber, structural behaviour, repair and maintenance methods and strengthening techniques. An important and popular aspect of this training is the opportunity for hands on experience in test boring of timber to detect decay and also site visits including inspections of timber truss bridge strengthening projects.

Amie: We now have a good knowledge of how timber truss bridges work, and why they failed, but I’m wondering whether this was the case earlier.

Ray: It was pretty obvious where failure was to do with rot, or white ants, and the bridge foremen were experienced in locating white ant nests. For a while we experimented with using epoxy to replace the damaged areas, but that didn’t work very well.

Amie: We still come across bits of timber with epoxy in that has failed and we think what is this!

Ray: We had eleven or twelve divisions across the state, all experienced in timber truss bridge maintenance – I don’t think there would still be that level and spread of experience now, would there?

Amie: A lot of work has gone into trying to get a consistency of approach across the regions, but that’s not easy, there are pockets of expertise but the aim is a consistent approach.

Brian: Our structure was a strictly controlled pyramid where the decision makers drew on depths of knowledge and experience and were appointed from within the ranks. It worked very smoothly, with the final decision worked out the best possible one. This successful and longstanding structure was completely demolished by the practice of bringing in senior staff and chief executives from outside and the flattening of the pyramid of experience and expertise.

In my day our budget income was split between the road engineer and myself and we met every three months with the accountant. Before we made out the annual program we worked out the jobs in order of priority, the two of us would sit down and work out the roads and bridges program for the whole organisation. With ongoing smaller components such as the legal section that had to have funds; the planning worked well. Both the road engineer and I had the same approach, we wanted more than we could have, so we had to negotiate and agree. Then the program would go to the Commissioner, he’d make his adjustments, then to the Minister who would make some adjustments. Then the jobs started for the year and every three months we looked at how we were going. It worked pretty well, we knew what was happening across the state and across the organisation.

Wije: The strategic directions paper for Roads and Maritime’s Technical Capability almost ten years ago is an example of the longer-range planning we need now. In that paper, rehabilitation design for timber bridges was recognised as an area to be strengthened. Although much had been learned through successful rehabilitation and strengthening projects such as the multi-award winning work on the Allan trusses on Hinton Bridge, there were also less successful attempts at strengthening, such as the work on the trusses at Clarence Town. Although some of the best and most experienced timber engineers were involved with the design and construction of the bridge strengthening, available copies of original drawings were of poor quality. That meant the engineers misunderstood the original design intent, which resulted in the bridge not having the capacity to take the design loads. Bailey bridging, the portable system discussed in Chapter 4 ‘Timber truss technology’, quickly had to be installed (see Figures 7.6a&b).

Ray: The old Manual still had the method for replacing truss members, but in my day because of wartime experiences, we were using Bailey bridges.

Brian: Bailey bridges were very important in my time too.

Ray: We’d put the Bailey bridge under or beside (with reduced clearance for traffic) and support the load from the cross girders to relieve the cross girder load on a truss, either one side or both sides, then repair the truss, transfer the cross girder load back to the truss and remove the Bailey.

Brian: The triple single Bailey bridge was the most important Bailey for major repairs. Ray: That’s three Baileys side by side. After the 1939-45 war there was plenty of surplus Bailey around and it was certainly put to use for civilian bridges. Some canny bridge foreman possibly thought how useful it would be to support bridges under repair.

Amie: At Glennies Creek before the current work on the de Burgh Truss bridge began, it was held up by Bailey bridging (see Figure 7.7). The critical thing is without that, the 1904 bridge would probably have fallen into the river before the current upgrade work could be done. The old way had its problems –

Ray: Percy Allan’s scheme of using pairs of elements in main truss members was to allow a half member to be used to support the bridge under low loads while an element was replaced, then do the other half.

Amie: Yes, Allan did design his truss for easier maintenance by duplicating members so each could be removed from the truss, but it’s a risky business. Instead, the Bailey bridge takes all the load, allowing replacement of any part. Without the Bailey bridge, many more timber truss bridges would probably have been demolished as it wouldn’t be safe to make a big repair.

Ray: The Bailey picks up the load from the cross girders that would have gone on the truss members, they jack it up a bit and it relieves that load, taking the load itself.

Amie: A new bridge to hold up the existing bridge – Ray: And it can be left there for a month or six months –

Amie: Or even years, like Glennies Creek Bridge which was once on a quiet back road but is now the route used by the area’s expanding mining industry. Brian: There was always Bailey available, we would order from our Central Workshop where Bailey bridges were stocked and maintained, use it and return it to the Central Workshop for cleaning and storage.

Amie: The technique is just starting to change, with some projects now using an under-truss system, so instead of putting the Bailey on top of the bridge and pulling the bridge up to meet the Bailey, they put a more modern steel truss under the bridge and support the bridge from that. Obviously you can’t do that if there will be floodwaters up to that level, or if the temporary support is going to stay on for five or ten years. But for actual repair works where you can’t afford to narrow the bridge, with safety becoming a bigger issue every day, this technique has been used on two bridges so far, Colemans over Leycester Creek in the northern region, and at Morpeth in the Hunter region.

Ray: Morpeth gets floods though doesn’t it?’

Amie: Yes, it does – in this case it was an urgent repair and it was only there for about a month, and I think the work was done in a low-flood risk period. You certainly wouldn’t be able to leave it there for a year.

Wije: As a result of the perceived skills shortage, close collaboration with the University of Technology, Sydney (UTS) led to an extensive research program which shed significant new light on exactly how the different heritage timber truss bridges functioned and what made each type susceptible to failure. As well as laboratory testing at UTS, there was also full scale load testing of timber bridges in service, such as Vacy Bridge in the Hunter Region which was strain gauged and load tested to provide a better understanding of exactly how the stresses flow through the different timber elements in the bridge as a heavily laden truck travels from one side of the bridge to the other. A general inspection of all remaining timber truss bridges, owned by both Roads and Maritime and Councils was undertaken to verify that the desktop analysis matched actual behaviours of real bridges. This work resulted in the new draft Roads and Maritime Guide for design and assessment of heritage timber bridges which covers the functioning and failure modes of the different truss types in great detail. This knowledge has been essential in enabling the production of a suite of strengthening solutions that can be adopted with confidence on heritage timber truss bridges to strengthen them for the future.

Brian: In my time the actual road approach alignment to a bridge could be a problem that might mean removal of a timber truss bridge. The standard procedure for the designers was a right angle approach, as the pier structure had to be square to the flow of the river.

Ray: With the road usually alongside the river, with the truss bridges you had to make an angle at the approach as it was very difficult to skew the end.

Brian: It wasn’t such an issue for a horse-drawn coach, which could get around the approach angle as it was just crawling along, but at motor car speed you brake heavily or go off road. This is one of the reasons that a bridge in good condition would be replaced, it didn’t suit modern road speeds or design and had to go. This was the case before my time too. Although it is easily assumed that in my time and Ray’s, we were the ones getting rid of the timber truss bridges at a fast pace, the figures don’t show that and I’d question the assumed rate of loss of timber truss bridges in our time. It’s an easy calculation – with a tally of 407 timber truss bridges built from the 1850s and 65 remaining in 2007, that’s an average of six lost each year over the 57 years. Our rate of loss was less than that, as we never reached six removals a year during our chief engineerships!

Ray: One of the big changes from the 1960s was bypasses, which didn’t require removal of the existing bridge in a town. A new site and a new bridge design were included in the design of the bypass, in fact there are now standard designs to suit a whole lot of different sites. It makes me shudder when I drive up to the north coast with every major bridge on the highway looking the same. The adoption of concrete removed the requirement for a straight bridge and one of the first curved concrete bridges, at Mount White Interchange on the M1, was one of my early designs. The planners wanted a curved bridge and we said we can do that – they were very delighted, it was not what they were used to. You can see the whole history in a place like Kempsey, which had an Allan Truss in 1900 (see Figure 7.8), then a steel truss in 1959, and then the Macleay River concrete bypass bridge that carries the highway around the town.

Brian: I was associated with that job, at Kempsey, replacing the timber truss bridge with the new steel truss. When the new bridge was opened I had a lot of complaints, the long-term residents of the adjacent pub were irate and said they couldn’t sleep at night, as the new concrete deck didn’t rattle! Kempsey’s timber truss bridge was the largest in Australia when it opened in 1900 and fifty years later it rattled all the way across – the longest deck and the longest rattle (see Figure 7.9).

Amie: I wondered whether you had any particular reaction from people about the replacement of their timber truss bridge.

Brian: In my experience the public attitude was mixed. Farmers generally didn’t like them, with nearly all through trusses that were difficult for farm equipment, and as well the load limits for bridges – and for approaches too, if there was no good turning point for unsuitable vehicles – were regarded as a nuisance.

Many had load limits, the road authority could impose load limits under their Act. For the farmers, they’d had their day, especially with heavy loads going through the decking system which was the weakest part of the bridge

Ray: I think the timber truss bridges might have been designed like that, with the decking a safety valve to protect the trusses.

Brian: That’s a very good point, I’d never thought of that.

Ray: Particularly with timber cross girders, and what we did more recently was to replace the hardwood cross girders with steel box sections in the strengthening of the bridges – so whether that safety valve effect will continue to be available is a question.

Amie: And that’s because as loads increase, margins for safety are decreasing, so we need that extra capacity. The truss has that capacity with cross girders strengthened, so now the girders and deck have the strength too.

Ray: Better to break the deck than break a truss.

Brian: While farmers let us know the trouble they had, I’d say in contrast townspeople liked their timber truss bridges.

Ray: It was the same in my time – there was a lot of affection for the local timber truss bridge. This was the reason you set up the heritage bridge committee, Brian?

Brian: The history was seen as important to local identity – this was the impulse behind the heritage recognition and why I set up the heritage bridge committee – though of course I was forced into it by Judy Birmingham and the National Trust! We were writing letters to each other unsuccessfully, as truss bridges were being removed and no one was satisfied. There was a lot of correspondence about various sites – but the responses were never very satisfactory to the Trust. Eventually Judy Birmingham said it would help to know the reasons for our decisions and invited me to give a talk and deliver our side of things. That worked, and then she suggested a joint committee, so we did that and began many productive years of discussion. Every couple of months after work, always over white wine and sandwiches as I recall – the perfect recipe for seeing a different view point and finding common ground. The committee is still going, but in a slightly different format.

Ray: In some places the early passion for retaining bridges was very strong – for instance the Morpeth, Hinton and Dunmore bridges in the Hunter region always had passionate local champions (see Figures 7.10 – 7.13).

Brian: They are classic examples, part of the histories of the towns. This happens when a locality or a town appreciates its history and is aware of the part that transport played. And, of course, if their bridge is functioning properly.

Amie: Yes – it has to be functioning, that’s the big thing. That’s the problem with another Hunter region bridge, at Clarence Town, the one Wije mentioned before. It hasn’t been a properly functioning bridge across all our eras – maybe not since the 1920s. It just can’t perform its function properly and that’s not anybody’s fault, just what’s happened. So there, people want to appreciate the history of the bridge as part of the history of the town, they want to like it, but the frustration of putting up with a non-functioning bridge has boiled over into a feeling of just replacing it with a concrete bridge that will work. It gets to the point where the only way to get rid of the frustration seems to be getting rid of bridge. This is why it is so critical to the long-term conservation of these bridges that we have them serve the purpose people need them for.

Brian: Could Clarence Town have a solution like the success with conserving Tharwa Bridge and keeping it in service?

Amie: There are things we could do at Clarence Town but because we got so tied up getting heritage approvals and various processes, a solution has been delayed ten years and people currently are so angry they have been fundraising themselves, to build a new concrete bridge beside the 1880 bridge. Which will of course take away its final functionality, or any future functionality we can provide for the timber truss bridge.

Brian: A particular problem, isn’t it, with Clarence Town Bridge one of only two remaining Bennett Truss bridges, the original old PWD truss.

Amie: And it’s just so needless, if we had been able to go ahead properly ten years ago. You mentioned widening it Brian, one of the things at Clarence Town is it has the standard 16-foot (4.8 metres) roadway of that truss type. It is even narrower as it has been under Bailey bridging for so long and you feel squished in driving along it. It’s one of the only timber truss bridges that I won’t walk over, it is not safe for pedestrians and it’s certainly not unreasonable for people to want to walk over their lovely old local bridge. If they can’t drive over either, what is it for? In response to a big outcry for a wider bridge I came up with a concept, and so did Ray, along with the others developed, but it was too little too late. The Clarence Town community was fed up.

A story that is not looking good, but it tells an important truth about bridge conservation, that the bridge needs to work. Its function is critical to its heritage value.

Brian: It’s particularly unfortunate, with Clarence Town our oldest NSW timber truss bridge, maybe the oldest in Australia.

Amie: There is a worse case though, with Swan Hill Bridge. Swan Hill is a very, very busy town and the bridge is narrow and causes all sorts of queueing. Because it is so busy it is really hard to do maintenance as you have to close the bridge and people won’t put up with that, so the bridge gets more run down until work can’t be put off any longer. So Swan Hill people have another legitimate claim against the bridge, that it is not being looked after properly – and for the poor maintenance crew their reception is a bit like parking officers, they have had insults and even eggs hurled at them, so intense is local frustration over the functioning of Swan Hill Bridge. ‘When will we get a concrete bridge?’ Their bridge is not working for them anymore.

Ray: You can put load limits on, but you can’t have a bridge that isn’t a working bridge.

Amie: And cases where a new bridge is built and the historic bridge turned into a pedestrian bridge or cycleway don’t provide the long-term solution a heritage bridge needs. The Bendemeer Bridge in the New England region is the other example, a Dare Truss with the steel bottom chord, being looked after by the local Council. They are doing their best, but the cost of conservation will always be a factor, now and for the future.

Brian: Five Day Creek Bridge on the road up from Kempsey to Armidale is another example.

Amie: It was.

Brian: There was a strong move to keep it, but this was never logical because of the degree of deterioration of the structure. The site was in a fairly isolated place and maintenance of the bridge was difficult because of its location.

Amie: Five Day Creek was a McDonald Truss, now replaced by a concrete bridge. Another example is also on that road, the Styx River Bridge, an Allan Truss built in 1900. It is beautiful in its setting, but perhaps faces the same fate. We were talking about vehicle accidents because of sharp approach angles before and that’s what happened here. The road comes down to a very steep 90° turn and someone didn’t take the turn well and smashed into the truss. At a site like that the quickest way to get the road back open for traffic is not to reconstruct a heritage timber truss, it’s to build a concrete bridge.

Amie: Before we finish, I’m wondering if there are memorable incidents in the long experience of bridges gathered for our conversation?

Ray: When a timber truss bridge on the Pacific Highway north of Grafton got hit in an accident in the 1990s, Igor Vaulin, a wellknown bridge engineer for the Grafton district was in charge. When he did the sums, he found the accident had left the bridge minus one member, but it was still standing. It’s all the inherent strength that you don’t allow for in your calculations.

Brian: And sometimes damage was no accident – there were bridges deliberately set on fire, from farmers’ frustration at load limits, or the functional shortcomings of the bridge itself, perhaps. As well as looking at bridges removed for development of the road system, we should also count the loss of our timber truss bridges to floods – this was the most common cause. These were mostly on coastal rivers but there were some inland too. The bridges on high timber trestles were particularly vulnerable, especially if there was lots of timber upstream that would be uprooted and come down with the flood at great force.

You’d expect coastal bridges to be more vulnerable than western bridges. When the timber truss bridges were built, there were no systematic waterway calculations to set theoretical 1-in-100-year flood levels. Instead flood limits were set by asking locals to recall where the highest flood had reached. Quite often that wasn’t the highest flood, that one came after the bridge was finished and washed it away.

Ray: The timber trestle piers at the Tharwa Bridge were very vulnerable when the Murrumbidgee flooded and they were replaced with concrete piers in the 1930s. You can see the marks of the formwork on the concrete piers. They were constrained to build the concrete pier inside the timber trestle pier, using the trestle as a framework to support the formwork.

Brian: That reminds me that we can lose bridges underwater without a flood – I’m thinking of another timber truss bridge in the ACT, the 1925 Commonwealth Avenue Bridge which spanned the Molonglo River and was drowned by the waters of Lake Burley Griffin (see Figure 6.28).

Amie: There are quite a few timber truss bridges under water like that; usually the trusses are removed, probably so they are not a hazard, but the approach spans and the rest are left there, submerged under water in various places.

Brian: An accident I won’t forget is when a motorcyclist (with the aid of bottle of whiskey as it happens) managed to ride off the running planks we used to install over damaged decking. We designed a system of 18-inch running planks to provide for single lane traffic while protecting the planking underneath, which was laid transversely.

When our man ran his motorbike off the running planks towards the end, he came off the bike and was injured. There was a court case where eventually the judge ruled in favour of the motorcyclist, deciding that despite the bottle of whiskey, the planks were more dangerous. It was during the 1950s and as a result I sent out a memo for all the State regions, that all running planks were to be extended to the full width of deck.

Ray: There was an odd accident at Condobolin in the 1970s when a semi-trailer with a bulldozer on board tried to cross the Allan Truss bridge built over the Lachlan River in 1900. The 3.6m blade of the bulldozer struck the first truss on the downstream side, the blade lodging in the end diagonal member and partially severing the truss. The driver got the blade out and proceeded across the bridge – but when he reached the middle, the weight caused the truss to fail, causing the semi-trailer, bulldozer, and driver to fall into the river. Although others were on the bridge at the time, no one was hurt and eventually the truck and bulldozer were towed up the slope of the fallen span and onto the collapsed approach, where a crane finally hauled them clear. If nothing else it is a lesson in how a timber truss can look so graceful and yet have such strength.

Amie: Conserving bridge heritage by working out changes compatible with safe operation and historic values teaches us a lot. These are creative challenges. For instance the decking of a timber truss bridge might look and sound rustic and rattling and please people, but the original decking might have been quite different. Photographs of the timber truss bridge over the Darling River at Wentworth show a decking surface that would not have had that ‘typical’ rattle. The SLT decking we are using in timber truss bridges today is remarkably similar in appearance to that original decking. The work is in this sense restoring the original functionality and aesthetic lost along the way in these bridges’ many decades of service.

Wije: I’d like to emphasise that collaboration between design engineers and heritage practitioners is a key aspect of producing solutions for strengthening these bridges, while also conserving and displaying their heritage significance. This means many different solutions are developed and discussed to attain the right balance between structural safety considerations and sympathetic heritage conservation.

Equally important now is collaboration with our customers – the community – generally an increasing emphasis within Roads and Maritime, with management of timber truss bridges no exception. New construction and maintenance techniques have been developed in consultation with the local communities in order to minimise inconveniences and maximise safety, for both Roads and Maritime workers and road users.