The newly-installed seismic baffle in the tank at Northlands Mall

The newly-installed seismic baffle in the tank at Northlands Mall

"Timber structures behaved remarkably well"

Tuesday, 21 August 2012

The December 2011 NZ Society of Earthquake Engineering (NZSEE) Bulletin covering technical papers written about the effects of the February 2011 Christchurch earthquake, includes a report on the performance of engineered timber structures, including timber water tanks.*

The authors, led by Professor of Wood Design at Canterbury University Andrew Buchanan, and including our managing director Justin Jordan, examined the impact of the two earthquakes on timber stave tanks in the region.

Seismic Base Isolation protection for Timbertanks was developed after the 1987 Edgecumbe earthquake, with the assistance of Trevor Kelly of the Holmes Consulting Group.

Following the Christchurch earthquakes, as previously reported, we developed a slosh wave spoiling seismic baffle and are currently developing a computational fluid dynamics (CFD) programme which will model earthquake shaking on any style of tank design. The report concludes: "Engineered timber structures behaved remarkably well in the Canterbury earthquakes. The seismic design of future timber structures will be influenced by the excellent observed performance and lessons learned in 2010 and 2011."

This builds upon the recommendations of the study group that produced the NZSEE report on 'Seismic Designs of Storage Tanks' in 2009, which stated: "The seismic performance of storage tanks is a matter of special importance, extending beyond the economic value of the tanks and contents. Without an assured water supply, uncontrolled fires subsequent to a major earthquake may cause substantially more damage than the earthquake itself, as occurred in the great San Francisco earthquake. Safe supplies of drinking water are also essential immediately following destructive earthquakes to avoid outbreaks of disease. Consequently, water supply reservoirs must remain functional after earthquakes."

At the recent NZSEE conference, Justin and TTEL consulting engineer Waldo Granwal were delighted to hear an affirmation that base isolation - that Timbertanks has engineered into its installations since Edgecumbe - reduces movement in a structure by three to seven fold.

In a paper presented in a technical session, Ron L Mays, a United States based Kiwi engineer confirmed that: "seismic isolation has the ability to significantly improve the seismic performance of existing buildings through a seismic retrofit..."
"Installation of baffles in timber water tanks has proved a useful tool in retrofitting all or any tanks," says Justin Jordan. "Existing tanks not built to the current seismic code can be fitted with our baffles. It's about thinking smart. The damage to concrete tanks from earthquake or other movement can also be ameliorated by fitting these baffles."
The recent Wellington earthquake caused water damage to a high-rise building as the water tanks in the top of the building sloshed out water, flooding the top two floors.

Fire-fighting capacity enhanced

Meanwhile in Hawera, Fonterra have added to their plant's fire fighting capacity with a new 1100m3 Timbertank, chosen because of its ability to withstand seismic disturbance. The tank will feed fire-fighting sprinkler capability for their new coolstore, which has a height of 18.04 metres at its apex and a floor area of 6,700m2.

The recent 7.0 earthquake off the coast of Opunake, although deep underground and causing little damage, reinforced the view that the correct decision was made in choosing to install a timbertank for this purpose.

The tank has the new seismic baffles installed that assist in minimising 'sloshing', which if unchecked can lead to catastrophic tank failure. Fonterra already has five timber water storage tanks at its Southland plant at Edendale, the largest of which is 2150m3.

Earlier this year, we also rebuilt a clarifier tank for the lactose operation at the Kapuni gas plant. The original tank was too large for it's purpose, so we raised the roof and rebuilt the tank to smaller specifications underneath. This has proved to be an excellent economical solution. The tank also has a connection to be used by the Fire Service should the need arise. Additionally, a Timbertank has a fire resistance rating (FFR) of 1.5 hours, because with staves 65mm thick, 90 minutes of charring will occur before structural integrity is threatened.

NZ Fire Service Fire Risk Management Officer based in Hawera, Matt Crabtree, said the new Petrocorp tank and the Kapuni tank are essential tools for such major infrastructure, particularly when remembering the tragic catastrophic fire in the Tamahere coolstore near Hamilton.

* Vol 44, no.4, P399 - 400