I'm dredging up the old stories but my memory won't cooperate. College student is moving into an apartment and meets two of his new neighbors, two girls. Various things happen and they end up a triad. Guy goes girl, which means as a would-be engineer she has to prove herself to her coworkers. Working for an engineering firm, she saves the day when new stress detectors of her own design detect that a city bridge is going to give way soon. The old design detectors weren't picking up the problem, but with the new detectors they have enough time to prevent any injuries and damage.

Seems like this was written by one of the greats in TG fiction.