Wood Research and Development has designed many modern timber bridges during the past 5 years in both North America and Australia. The author has been fortunate to participate in the construction of three of those bridges in Canada in that time and offers these as case studies in support of the premise that the humble timber bridge would be part of any community's infrastructure asset portfolio in a carbon neutral future. This paper will also refer to case studies from the USA and Australia.
Modern timber bridges have recently been compared to the "mass timber" revolution occurring in the vertical structures building market on a global scale. The reality is that before "mass timber", timber bridges were manufactured from large dimension timber including engineered timbers such as glulam (glue laminated timber). Glulam has been in the marketplace for over 60 years in North America and there were many Laminators in Australia around 40 years ago, some of who still operate today but they generally remain focussed on the indoor building market.
Timber is the most sustainable building material. What other raw material can grow again from the stump of a harvested tree or be replaced by a seedling of another tree which will grow in the same space as the initially harvested tree. Through photosynthesis, trees extract Carbon Dioxide from the air around us and sequester this carbon in the timber structure of the tree. Growing structural timber trees locks up more carbon from our atmosphere and this contributes substantially, in a carbon accounting sense, to generating the highly sustainable outcomes when constructing timber structures. Modern timber bridges carry the same load case vehicles as other material bridges ranging from 5kPa to HL625 in Canada, right through to SM1600 here in Australia. This paper will discuss the carbon accounting conducted on one of the case study projects which demonstrates that modern timber bridge projects are not only carbon neutral but are in fact carbon negative. This leads to the conclusion that several timber structures added to an organization's asset portfolio will offset some of the structures on the register which were carbon positive, thus creating carbon neutrality through the accounting process. This is the course of action currently being undertaken by Waka Kotahi (NZ Transport Agency) as a means for the government to reduce their carbon emissions as they move towards net zero in 2050.
In a public tender scenario where the specification is performance based and does not impose material restrictions on suppliers, the modern timber bridges built recently in Canada have proven to be the lowest cost for the client as well bringing environmental benefits for the worksite and the immediately surrounding area. The costs from the case studies will be discussed to demonstrate that modern timber bridges are often the lowest project cost supported by improved construction environmental and community satisfaction outcomes. These arguments clearly support the addition of the modern timber bridge to any asset owner's bridge asset register.