Designers and contractors wishing to keep projects conventional obviously must avoid the unconventional. That's one of the main objectives of the pre-proposal feasibility research. It’s no fun explaining to clients the need for expensive structural analysis after they agreed to a proposal based only on conventional design (of course, proposals that don’t allow for unexpected discoveries later on should also be avoided).
Neither is it any fun for a contractor to make what seems like a minor client-requested design change in the field, only to be told by the Building Inspector that the project must now be engineered. Something about that minor change caused the project to cross an invisible line into the unconventional zone.
It can be a very fine line indeed between conventional and unconventional, so a thorough understanding of the determining factors is important. Previous posts have discussed a few of the general factors. Also, some situations are more perilous than others, so we'll try to cover as many of those as possible. 'Dividing line' situations fall into three categories: materials, methods and conditions.