The recent sudden failure of a timber truss bridge in Gudbrandsdalen Valley, Norway highlights the need to consider whether using more sustainable materials in buildings and structures could increase the risk of catastrophic failure.
A car plunged into the water as the bridge fell apart, while a lorry became stuck on a collapsed section. Both drivers escaped unharmed. Tretten Bridge, which only opened in 2012, runs between Norway's main north-south motorway and county road 254 in Øyer municipality, just north of Lillehammer.
Constructed of timber and steel, it is almost 150m long and 10m wide and has a two-lane road and a pedestrian walkway and had an intended design life of one hundred years.
Tretten Bridge was inspected along with other wooden truss bridges in 2016 following the collapse of Perkolo Bridge, another timber crossing in Gudbrandsdalen.
As reported by New Civil Engineer (NCE) , Arup's global timber specialist Andrew Lawrence, who is also a visiting professor in timber engineering at Cambridge University, said while everything at this stage was speculation, the bridge's timber parts looked to have been exposed to the rain meaning they were at risk of decay, and its main load path relied on timber tension connections.
While Norway has a long tradition of building timber bridges across rivers or streams the country embarked on a new era of larger road bridges in the 1990s.
According to a 2017 report on the durability of timber bridges, the failure of Perkolo Bridge was blamed on a design engineer "misreading" the transfer of forces in the joint, a mistake that was not picked up by either the internal, nor external design controls.
It is concerning that despite Norway having a long history of constructing bridges in timber, the previous failure of the Perkolo Bridge and subsequent inspection of all other wooden truss bridges failed to identify any serious deficiencies with the Tretten Bridge.