Timbertanks manufacturer reviews standards

The impact of the Canterbury earthquake on tanks used in fire systems, town supply and industrial installations has been significant.

The earthquake was very localised and characterised by the underlying deep alluvial shale layer which created an unusual action of a strong pressure wave followed by a long period of low-frequency secondary shear waves that caused most of the damage in water tanks. This was well beyond any previous expectations specified by NZS 1170.5 at the time for the design and construction of such water storage tanks. It exceeded it by a large margin – three to four times more than would have been expected.

Base isolated tank at Papanui
which has ‘shuffled’ off its original position

Timbertanks’ managing director, Justin Jordan, and the company’s consultant engineer, Waldo Granwal, spent time with University of Canterbury and Holmes Consulting Group specialists, studying what occurred, and reached the conclusion that tanks in the Christchurch region will in future require special design.

Mr Jordan and Mr Granwal inspected a total of 25 tanks that had been installed over the last 30 years. Of these, seven tanks lost their water as a result of the quake. The remaining 18 performed as expected, shuffling approximately 20 mm, as base isolated tanks are designed to do.

Four out of the seven affected tanks had base isolation compromised by rigid pipe attachments or illegal ground obstructions, leaving three tanks that the company “would have liked to perform better”. Certain size diameter tanks only were affected; others around the epicentre that performed perfectly had a different diameter. Subsequent aftershocks exacerbated the situation at a couple of sites.

Controlling the slosh

“Everyone is trying to react quickly to this earthquake, but we also have to ensure we move correctly, because we may have to change our design as we learn more,” Mr Jordan says. “We were caught out by the extent of the sloshing – the wave velocity acceleration which caused the tank to move around. In all of this, we’re learning new things, and it takes a while to understand what happened and then translate into what we can do better – for us to set up our tanks so they can slide freely and control the wave slosh.”

All the damaged tanks were fire tanks, all of a similar size over 10–13 m across, which appears to equate to the sloshing frequency that occurred with the water over periods of 3–5 seconds. (Steel tanks of the same proportions had the same problems.)

An initial report by AON New Zealand – the sprinkler certification authority – on tanks inspected after the event, noted, however, that the current method of piping Timbertanks via a buried pipe rising into an anti-vortex plate in the centre of the tank “appeared sound”. The consequence of the pipework not being articulated only caused further damage.

Special attention

So is there an answer to this earthquake problem, given that the law requires that a fire tank be designed to retain its contents and be useable for purpose? “Anyone who claims that a tank designed to earthquake code 1170.5 would have survived this rogue event does not understand the whole picture,” Mr Jordan says. “Structures susceptible to these large secondary waves will need special attention beyond NZS 1170. In the past, base isolation has neutralised tank slosh – so in Christchurch, for example, a tank with a diameter at risk will need to have dampening baffles installed.

“We will also strongly recommend that companies have a maintenance contract which will provide a warrant of fitness that the tank meets all the requirements for performance in earthquake according to the existing standards.

“There is not a significant cost to upgrade a tank to meet new conditions, and we will be tackling it on two fronts,” he adds. “We are determined to hold on to our base isolation. The phenomenon that amplified the quake in Christchurch is not, to our current knowledge, present in any other location.”

Why did 18 tanks withstand the quake? Those that came through with minimal or no damage related to their locale e.g. they were on the hillside; they were base isolated and fit for purpose. Large Timbertanks water storage tanks in the region at West Moorland and Springhill, also one in Ashburton, suffered no damage or loss of function as a result of the earthquake.