If you are willing to sink the time into it it it not impossible for a smart person to do legal work that appears to require a professional lawyer.
can a dedicated lawyer pick up some engineering books and build a bridge that can survive SF's 'Big One'?
As for surviving "the big one"... it depends on how big? I'm not sure humanity in general knows how to build a bridge that could survive a dozen meters of shift between its piers. But that is a possibility in the largest conceivable earthquakes.
I can tell you from witnessed experience that a "car engineer" can custom design a house, do most of the code-related work himself, and pass all relevant inspections on the first try (with flying colors and compliments from the inspector), which seems much closer in scope.