Wooden bridges stand the test of time

14 May 2018

Wooden bridges have been an important part of road and rail networks in New Zealand.

Tripti Singh and Dave Page from Scion’s Wood and Fibre team have looked at the history and use of timber bridges in New Zealand.

Most of the older bridges were constructed using Australian hardwood and native durable New Zealand timber. In the 1950s–1960s, bridges were built with glue-laminated and preservative-treated timber.

From the 1950s, the Forest Research Institute, now Scion, monitored a range of bridges as part of their in-service testing programme. The authors have used test data to look at the durability and maintenance requirements of wooden bridges in New Zealand.

They found that copper-chrome-arsenic (CCA) treated decking seldom decayed. However, deterioration through mechanical damage was common. Of seven public highway bridges included in the testing programme, the decks required repair and replacement due to mechanical damage in less than 25 years.

While wooden structures have largely been replaced by concrete and steel on major arteries, they still have a place where lightweight, easily assembled structures are needed. These timber bridges are also an alternative to other materials in roads which carry relatively low traffic loads.

In the last 10 years, there has been a resurgence in the installation of engineer-designed wooden structures. Contrary to the traditional method of construction, these prefabricated components are manufactured in a factory and brought to the job site where they can be assembled quickly with a minimum of cutting and drilling.

Singh, T., & Page, D. (2017). Case studies on the history and use of timber bridges in New Zealand. Wood Material Science & Engineering, 1-8.