Grooves on water tank Why are there grooves on almost every water tank?
Just typing water tank in Google images would reveal what I am talking about. And here is a sample picture!
I think it would mostly be regarding water pressure.
Please share your knowledge.

 A: Of all the reasons I can think of, I came up with two advantages that the grooves may provide.  


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*The grooves increase the distribution of material away from the neutral axis. Therefore they increase the moment of area. An increased moment of area means that the material is more resistant to bending.
$\sigma_=\frac {My}I$
It's just like a sheet of paper. If you hold a sheet of paper from an end, it easily bends over. But if you roll it up into a cylinder, then hold it from an end; that should keep it straight like a cylinder.
But why don't they just bulge the whole think out? (That should provide even more resistance to bending). The answer is that though it does increase the resistance to bending, it also increases the shear stress between adjacent layers. The right balance is obtained by implementing an $I$-beam shape. They make multiple $I$ beam shapes in the form of grooves.
In short, the grooves increase resistance to bending and decreases shear stress between adjacent layers of the tank.

*The grooves provide holding support, allowing for better transportation of the tank.

