There is a widespread perception that timber bridges have a shorter service life than those
constructed of other material. However, with a thorough understanding of the properties of timber and the proper techniques in the design, inspection and maintenance; timber bridges are capable of hundreds years of service at an effective cost.
This is proven by studying the fact that the average age of a timber bridge in Australia is 82 years. How many 82 year old concrete bridges has anyone seen? The of use advanced non-destructive testing techniques to find the degradation in the timber elements coupled with advanced restoration methods provides an excellent solution for local governments as they seek to reduce life cycle and maintenance costs for their timber bridges while extending life. To increase the life of a timber bridge even 15 years leads to a significant saving for local governments which are hard pressed to maintain level of service for rate payers. Particularly in low population areas which have large numbers of old timber bridges.
Modifying the following top ten maintenance practices will prevent premature deterioration and add years of useful service life to these structures.
1. Change vertical through bolting to horizontal connections and verticals that do not pass
through the top surface to prevent moisture from entering the heartwood of the timber.
2. Stop the improper use of malthoid barriers that trap moisture on the top of the timber
elements where it is unable to evaporate.
3. Insure positive drainage from the deck that does not fall directly onto the structural
4. Accommodate dimensional change in timber elements due to changes in moisture
content. Provide slots in brackets to allow for expansion and contraction of the timber.
5. Stop the use of banding. Steel banding is no longer a recommended practice for
stabilization of timber piles that are degraded by decay, splits and/or cracking.
6. Stop the use of near end drift pinning in an attempt to reduce end splitting.
7. Provide adequate clearance for timber elements to breathe and dry out and regular clear
the structure of debris build ups around the timber elements.
8. Stop the use of heavy percentage solid coatings (over 29% solids). Moisture penetrates
the coatings through cracks in the surface and then becomes trapped in the timber behind
the coating thereby promoting decay.
9. Stop the use of heavy notching and ensure a slope cut of a minimum 1:6 gradient is used
to prevent stress concentrations and re-entrant cracking.
10. Use properly sized timber in pile bents and place loads within D (depth of cross
head/cap) of the pile to prevent horizontal shear cracking in undersized cross heads.
Advanced Rehabilitation and Maintenance Methods:
Restoration methods include the use of high-strength fibre retrofits to restore the capacity of structural elements in-situ under live loading. This may be combined with targeted replacement of highly decayed elements or complete replacement of the superstructure and/or deck. See example of this type of restoration below.
Figure 1: Substructure and superstructure restoration of Meacham’s Bridge for Cassowary Coast Regional Council, FNQ. The repairs performed by Timber Restoration Systems and design by Tingley included reinforcement of log girders with high-strength fibre, high strength reinforcement and posting of the piles, high strength fibre reinforcement of crossheads and deck replacements with vertically laminated incised 9 kg/m3 Pentachlorophenol pressure treated decks. The existing timber deck remained in place throughout the restoration and was replaced many years after the first restoration. This bridge was restored from 12T to T44T HML loading and instead of being replaced many years ago it is still in service today. This saved the community close to 2 million dollars in borrowed funds. The savings to date through saved interest is already enough to build a new concrete bridge. The community has now restored many timber bridges. In 2018 they have replaced 5 steel bridges that were less than 20 years old with new timber bridges that will last 5 times longer than steel in the coastal environments. A total of 32 old timber, concrete and steel bridges have been restored at an estimated savings of 20 million dollars over the last 8 years. Tingley designed these restoration and replacement solutions.
Dan Tingley will be presenting at the upcoming Bridge Asset Management & Renewal Conference, running on the 19th & 20th September 2018 in Sydney. Download the brochure for the full program.