Y2012-02 A collapse of a water tower in Jyväskylä on 3 November 2012
A water tower at Kangasvuori, Jyväskylä, collapsed at 6:07 am on 3 November 2012, upon which the control room of the water utility noticed a communications break and a pressure drop. A duty officer sent to the location observed that the water tower had collapsed, scattering concrete ele-ments over a wide area of the surrounding terrain. The water tower was almost full and some 2,000 m3 of water gushed into the area.
The Rescue Services were alerted slightly over two hours after the collapse. They and the police were mainly tasked with cordoning off the area and ensuring that no one had been buried under the rubble. Although outdoor exercise trails cross the area, no one appears to have been present when the collapse occurred.
The water tower was completed in 1976. Around that time, a new, prefabricated water tower was introduced to the market, which offered municipalities a cost-efficient, easy overall purchase. The towers were based on the fairly new pre-tensioning method also used in bridge construction. This method involved the use of strong post-tensioned steel tendons.
The water tower collapsed because a large, ring-shaped girder and the attached steel tendons snapped. These steel tendons were rusty on the surface and, more importantly, some had been markedly weakened by cracks. An investigation revealed that the collapse was caused by what is known as delayed hydrogen cracking, which sometimes occurs in strong, permanently tensioned steels. This phenomenon is relatively unknown in the construction sector. In this case, the hydrogen cracking was probably due to rainwater causing the steel tendons to rust. The hydrogen cracks were due to hydrogen released during the rusting process.
The investigation also examined the water tower’s equipment and revealed that the system was not single fault tolerant. A collapse in the tank could also have occurred due to overfilling caused by malfunctioning automation. Crucially, the roof could have been displaced if water had reached the ceiling of the tank. Such a scenario did not occur in this case, but account should be taken of this possibility in water tower arrangements elsewhere.
Together with municipalities, the Accident Investigation Board recommends that the Ministry of the Environment and the Finnish Transport Agency chart all structures which are susceptible to hydrogen cracking and ensure that the parties in charge of the structures’ safety perform the necessary inspections and repairs. Further, the basic data on all water towers in Finland should be compiled and used to help water supply organisations to ensure that towers are appropriately maintained and safe, by informing these authorities about good practices, technical problems and solutions. It is also worth noting that attention should be paid to the management of ageing water tower structures and systems, as well as their fault tolerance. In every case, account must be taken of the original planning principles in the planning of essential changes, expansions and maintenance. Structures and equipment should be regularly compared to planning principles.