You CLEARLY did not understand the failures in both Shuttle accidents.
Challenger was caused by a cultural need to keep flying and stick to the schedule. They ignored the warning signs from the SRB O-rings. They conitinued to blow vent holes in the field joint insulating putty despite evidence that the O-rings were getting singed.
Columbia was a similar failure of management to ignore debris damage issues until too late.
Pre-calculated safety factors were irrelevant.
And Bob (as usual) is correct about the nylon bolts. And way back when we were always told to use the nylon bolts cause they would shear off easily and save the wing, like rubber bands.