Here is the NHTSA advisory on Carpenter buses:
https://icsw.nhtsa.gov/people/injury/buses/carpenterbus/
Instead of forming the walls and ceiling from a single piece of channel, Carpenters had separate wall posts and roof beams, joined with longitudinal rails and all welded together. Buses built at the Mitchell, Indiana plant were prone to weld failures while those built at the Richmond plant (to almost the same design) were not. Were the Mitchell welders less experienced and competent? Unknown. Did they use different welding processes at the two factories? Unknown. Did the welds fail quickly or over time? Unknown.
It's interesting that when considering the strength of a roof raise, we usually seem to characterize it in terms of how well the structure would survive a serious accident that stressed the structure much more than ordinary usage (e.g. a rollover or a collision with another large vehicle). But the Carpenter buses out of Mitchell showed serious weld failures
just from ordinary usage (the structural failure that precipitated the investigation occurred because some of the welds had already failed before the accident).
