Strengths and weaknesses
Across Europe and North America, thousands of these road bridges are in need of significant structural maintenance every year, having been built perhaps four, five or even six decades ago. Over half of the highway bridges in the Netherlands are 40+ years old, for example, and this is similar for assets in the U.S., according to the Infrastructure Report Card 2017. These bridges are coming under strain as they take on much heavier and intense traffic loads than originally designed for. In parallel, continuing urbanisation and increasing prosperity is leading to the construction of brand new transport infrastructure and the issue of embedded carbon in these structures is an ever-growing problem. It’s clear that alternative materials are required and using timber in new infrastructure construction will reduce embedded carbon now, which is preferable to reusing material in the future.
The advantages of timber as a building material are well-known: plentiful in supply, cost-effective and able to absorb CO2 from the atmosphere. When correctly designed it can last for 100 years and be swapped out at the end of its useful life in a bridge to be reused in another context.
Due to its lightweight nature, a bridge’s superstructure can be replaced by timber while retaining the substructures. A major question is of course timber’s relationship with water – any material in a road bridge needs to retain its strength and remain resistant to the long-term effects of the weather. Protecting the timber from direct contact with water is key to avoid fungal attack and resulting degradation. Ventilation is also important for timber, it must be able to dry after humid periods. It must be inspectable too, to keep an eye on any degradation. Factoring these into our design process, it was clear timber remained viable when used as a structural element, not exposed to the elements.
Fire safety is an important consideration when designing with timber. In many National codes there are no requirements for fire loading, but still the structure needs to be safe in such situation and prevent casualties. Unlike steel, the strength is not influenced by temperature. The incineration speed of glulam is approximately 0.7mm/min. This means that for a large block glulam bridge girder (1x1.5m) the cross section reduces by only 4% after half an hour of fire and 10% after an hour. This reduction can be considered in the design for extra safety.
Helpfully, timber doesn’t degrade from road de-icing salts like concrete and steel. And our design anticipates disassembly so that bridges could be easily widened or reused in future. This is circular economy thinking, designing for a structure’s evolution, rather than making a non-adaptable one-time bet on materials, that commits you to ‘use-demolish-waste-repeat’.
Proof of concept
Our BoLT design offers a theoretical lifespan of at least 100 years, since it is fully protected from rain and aligned with future Eurocode standards for timber bridges. While timber-concrete composites are being increasingly used on bridge superstructures, our version goes a step further by replacing the traditional concrete superstructure with a full mass timber one. This results in 75% of the superstructure’s (including surfacing) total weight being a renewable material, resulting in a climate positive superstructure.