The collapse of the tank may be due to vacuum. Can't tell from the picture if there is a roof on the tank. It is also odd that there is no coating on the tank. The tank looks like a bolted tank, not riveted.
More pictures, more information .....and of course you get better answers !!!
..... As is typical in this forum, the OP begins a long wasteful guessing game
I believe that this is a riveted tank, based on its double and triple row of fasteners. The fact that there is no exterior coating or shiny fasteners also leads me to believe this.
This tank may have a vacuum failure, but it does not look like a typical vacuum failure. Most vacuum failures show a collapse that begins near the top of the shell and forms vertical "valleys"
Where is the tank located ? How old is the tank ??? Is there a large operating tank vent ???
Riveting of tanks ended sometime in the 1930's. That and I have never seen a riveted tank of this size makes me doubt that this is a riveted tank. Of course, this is more like a game of twenty questions.
Here is a picture of a bolted tank:
It is unusual that the 2nd picture shows evidence of numerous leaks while the 1st picture shows none.
If this is a water tank, the corrosion will have been most severe at the water line, so it is possible that may have been caused by vacuum failure. The question is; does the tank have a roof.
Don't think this is tree damage because the extent of damage is too great to have been caused by a single tree.
It looks to me like primarily a corrosion failure. I've seen where tanks that were held at nearly a constant level would could have a band of corrosion at the interface.
Usually, vacuum or wind blow-in would be a large area on one side of the tank, not a band around it like that. Usually, seismic buckling would be at the bottom.
Looks like a riveted tank to me. I remember one guy saying the last tank he had seen riveted was in the 1950's (in the US) and it was done that way to match an existing tank. However, that transition could have varied depending on local labor and practices.
To me it this does not at all look like a vacuum failure. The tank wall has buckled from vertical force not radial. Prehaps corrosion lowered the buckling strength but it looks like wind loading could have caused this.
I can buy the corrosion and buckling from the weight of the tank above the thinning, but it definitely is not a tree-fall since there is no impact mark on the top of the tank.
To me this looks like a pretty small riveted ( or perhaps bolted ) tank. I've worked on many that were over 100' diameter. At first I thought it might be seismic failure but I'm liking vacuum failure more, but not your usual one where it is discovered quickly - the roof was sucked in and continued vacuum caused shell failure until the roof to shell seam finally split and released the vacuum. The OP will need to post more information.
Looking at the bottom upper 2 courses, the bolts indicate to me that several sheets were replaced and instead of riveting as originally done, bolts were used.
The history of this tank would be interesting. I'm wondering if the top three course were replaced or even added at some point as they show no rust streaks compared to the bottom set.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
With the number of patches on the third course from the top this is leaning (pun intended) towards a corrosion failure IMO. Might have just happened or a bit of extra wind load but without any further info on operation and history it's difficult to go much further.
BTW is it me our is this tank slightly conical?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
Question: is this tank located where a large snow load from a recent storm may have suddenly accumulated on top ( assuming there is a fixed cone roof on top )?
