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RE: Water Tanks

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Thanks David,

Actually the only reason we got involved with the tanks, is because a wind
storm picked up a newly installed empty tank, and threw it over the top of a
hill. Typically here in SA, domestic rainwater tanks, don't need development
approval, which generally also means no engineering evaluation. The tanks
have otherwise been in use for years, and barring being picked up by the
wind: are generally sufficient for purpose. The builders have no problems or
issues regarding being on the roof whilst constructing.

For some reason the tanks are now going through development approval:
possibly because of increased footprint. The tanks are available in a range
of sizes. Since the tanks already exist all over the place the objective has
to be to demonstrate the tanks have acceptable performance. However we have
no mandatory code defining acceptability. I'm not doing the design, just
seeking information to pass on: but checking the wind loading was the first
issue checked, and the corrugated skirt was also checked as a tank wall:
which as I say the wall is largely there to hide what is otherwise a big
plastic bag. I believe an American water association code was used for part
of the assessment of the skirt as a tank. Some modifications have been
imposed on the design to address the wind loading problem.

The roof over the tank was described as a cover. The tank is round, and the
cover comprises a series of trusses, parallel to one another rather than
radial, then corrugated steel cladding is sprung over to form a dome. Since
roof live loading wasn't an issue, the reactions from were not transferred
to the steel skirt. The question has thus arisen whether the steel skirt can
support the loads. History shows the walls can support real world loads.

Testing maybe the best approach, but with a range of different tank sizes,
such may not be practical in the short term. Adding extra roof supports may
be acceptable for the current project seeking approval, but really need to
demonstrate that the current design is adequate. Or find the limitations of
the steel skirt to support the roof load, and thus when additional roof
supports do need to be provided. (Fortunately we don't have snow.)

Since any portion of the wall is restrained by adjacent wall, it seems like
a complicated problem. Treating a portion of the wall as a simple column is
too unrealistic, it will simply generate a need for additional support:
conservative for design but poor assessment.

We also want to avoid using FEM software, which I don't believe can check
buckling in any case. What we are looking for is a code of practice or
specification with simple empirical rules.

Failing that I guess we will have to recommend some testing. Which I guess
is not too difficult, applying a known point load to the top edge of the
skirt. For the most part I believe localised deformation of the edge of the
sheet is more the issue than collapse of the wall. It's just a problem of
proving it. Easier said than done.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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