Back to Boston's Big Dig...
It appears the concrete ceiling panel fell because:
- it was held up by bolts that were held in place by epoxy.
- the bolt holes had to be dust-free in order for the epoxy to hold.
- it's pretty difficult to remove dust from a construction site.
- bolts with built-in epoxy "exploders" were probably too expensive.
- alternatives to the concrete panel were explored but (1) they cost the same or more; and (2) there wasn't time to switch construction techniques.
- there was great pressure on keeping down costs and completing the project that was first estimated to cost $4 billion but exploded to $14 billion (I think) as its scope expanded over the years.
So CAD was not involved.
You can read an interesting (and informative) discussion on slash.dot: Big Dig - One of Engineering's Greatest Mistakes?