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1985-ish Trek steel frame, seat tube failed near bottom bracket.
Service history unknown. The chainrings appear original.
The seat tube is essentially a lever that resists axial bottom bracket twisting under pedaling loads. Under long-term and vigorous use, seat tube failures at the bottom bracket are fairly common.
It is common to ovalize the seat tube side-to-side, and/or use lugs or gussets that go up the sides of the seat tube — rather than the front and back, as used on the frame above. (Slight reshaping does little to change frame stiffness; significant reshaping can increase bottom bracket stiffness.)
Below is a frame made for a rider who broke several frames, as above. The frame below has an ovalized seat tube that is also gussetted. It was photographed after about 200,000 km vigorous hilly riding. The paint is cracked from torch heat for repairs after the bottom bracket threads stripped, but the seat tube/bottom bracket junction is undamaged.
Below is a Bontrager "Race Lite" frame that uses gussets at the head tube (both top tube and down tube), seatpost clamp, and at the bottom bracket (shown here). Pictures from http://www.retrobike.co.uk/forum/viewtopic.php?t=92986 as of 2011/10.
See also FAIL-158.html.