Figure 1

Figure 1

Earthquake Research for Timber Water Tanks

Thursday, 18 August 2011

The Sustainable Option for Water Storage The 7.1 Canterbury earthquake in September last year exceeded New Zealand's water tank engineering expectations, with the consequence that all styles of tank construction were affected.

As a result, Auckland based Timbertanks Enterprises Ltd (TTEL) managing director Justin Jordan and consulting engineer Waldemar Granwal set about finding an answer to the extreme sloshing of water in tanks, so they could perform better if affected by another 'Christchurch style' earthquake.

The first challenge was to develop a mechanical baffle that would ameliorate the sloshing effect of waves in which sidewall reflections are significant, and then develop a software programme to commercially useful state-of-the-art predictive modelling.

As they could not find any other Computational Fluid Dynamics (CFD) work regarding earthquakes striking water tanks, TTEL decided to fund a CFD research programme. University of Sydney graduate engineer Hamed Riazi, was contracted to work on the development of design software for tanks under seismic disturbance, or the development of sloshing prediction for water tanks.

"What we want with this research is to establish mathematical proof of what happens inside our tanks," says Justin. "We have a wave testing tank that gives a very clear visual demonstration of what happens, but no numbers or facts that can be crunched by engineers.

"We can see our baffle works. Modelling and proving it gives us the ability to model an existing reservoir that may be a different shape - square or rectangular in the case of some concrete tanks. They were all damaged to a greater or lesser degree in the 'quake and their engineers are looking at putting baffles in those tanks to help stop the sloshing. It is a significant study."

The interaction between contained fluid (water) and solid (tank wall) is generically known as Fluid Structure Interaction (FSI) and is a basic process for determining the effect of earthquake on tanks in general.

TTEL is funding the eight months study following the installation of baffles in four tanks - two at Lincoln (rainwater harvesting tanks), Office Max and Northlands Mall (fire fighting tanks) - that with a baffle in place came unscathed through the more severe February earthquake.

"Eye witnesses at the Northlands Mall tank site during the second 'quake reported the positive outcome of having a baffle installed. They saw the tank 'wiggle and shake' as designed on its base isolation, but it did not move at all at the wall to base interface, we assume because the baffle dissipated all the energy. I had confirmation by text two hours after the February 'quake that it was all OK."

In the CFD research, a prototype tank 10m diameter and 6m water height has been used in the baffle simulation runs. A seven cycle run found sloshing to a maximum of 600mm with peak water surface times of 1.25sec, 11.5 sec and 24 sec. (See figure 1)

Tests in April, modelling with two and three meter diameter cylindrical baffles, have shown encouraging results of sloshing amplitude reduced by approximately 20% and 50% respectively. (Illustration: Figure 1)

The study will continue to the end of the year, with a next important consideration porosity of the tank baffles.

Justin says if clients are keen to ensure that their fire fighting tanks in particular will withstand a quake of some severity or whatever nature throws at them, baffles can be retrofitted. They are fixed to the floor of the tank with floats attached to move independently during a shake.

"Installation of a baffle in a water tank really relates to the whole country," says Justin, "but known earthquake regions are probably keener to have them fitted. So it's a matter for those who rely on tanks to decide - is it critical that this tank/water supply will remain working in a disaster?"

Figure 1 shows extended run time of undamped tank to 50 seconds elapsed. The graph shows convincingly that with 2m and 3m diameter centre baffles, sloshing amplitude is reduced by approximately 20% and 50% respectively.

"The fire fighting industry has asked us to prepare a checklist because the law as it stands does not say you have to retrofit improvements to tanks. It will fundamentally be a Timbertank list, but when completed, it could be applied to any style of tank. AON provides quality checks for such tanks and offer the best advice to sprinkler owners and the industry to ensure they meet regulations."