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Adding to More Floor Loading for large tanks..

Just adding to what Scott.H said about floor loading with timber joists
and columns etc - it is well worth noting that the long term strength of
timber is reduced and failure stresses are 57% of the short term
strength. Thus for long-term loading situations (i.e. a big mother
tank), the beams and supporting columns' dimensions stated in the
standards need to be well exceeded. The general 'standards' available
for residential housing construction are generally determined using
average short term random or uniform loading patterns. With a long term
stress concentration at one point, structural integrity and
serviceability will be an issue if the standards are used.

Stresses due to shear can most likely be neglected unless the tank is
situated closely to a column, where the tank is not over the column. The
beams that the tank sit on will experience a large shear stress if the
tank weight is not supported by having columns attached to the beam
directly beneath the tank.

Stresses due to bending (bending shear) are the concern with larger
spans where the tank is not very close to a column (i.e. within say,
200mm). This is the most likely cause of problems. Scott said that
bending may be a problem not necessarily structurally, but only with
leveling the tank. Wood is a natural fibre and like I said above, long
term failure stresses are 57% of the short term strength. This is due to
'unravelling' of the fibres (even in the densest hardwoods). So
structural significance may be a larger problem than initially thought -
the wood may appear fine, but over a few years the tank will sink more
and more as the beam bends more and more, possibly until failure.

There is a lot to add to the topic, and there are equations that can be
put into use to calculate the size beams, columns etc (My/I, Euler
buckling load, etc) but for simplicity I will only say this; if you have
a large tank and are at all unsure about the flooring system and the
capacity of the floor structure to bear the loads - I strongly recommend
that you employ the use of a qualified structural engineer. 

Adam Shaw