I know the general formula for water pressure is related to the height of the water column. But imagine the next situation: A vertical glass tube, 1.0 m high and 100 mm wide, full of water, can support the water inside even with a 0.5 or 1.0 mm thickness wall, but if I have a pool, 1.0 m high and 5 m diameter, I doubt the same wall will be enough to contain the water inside. Why?.The pressure will be the same, as for 1.0 m water column, so how is mass affecting that? I think the basic formula accounts for unlimited back-span. How can I calculate the required wall for a narrow container? What is the back-span from where the formula is OK?