You may want to try reading your local building code. This may not get into engineering principles at the level you’re seeking, but it may effectively provide t

You may want to try reading your local building code. This may not get into engineering principles at the level you’re seeking, but it may effectively provide the functional understanding of how (if not why) a certain material/size has been specified in its application- esp. if you’re working from (or modifying) similar plans.