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