David Grieshaber drove across the idea last year. As he crossed the Bay Bridge with his wife, brainstorming unique ways to build an environmentally conscious house using recycled materials, he thought: What would become of the original eastern span once the new bridge opened? Neither he nor his wife had a clue, so Grieshaber decided to call Caltrans. After being rerouted to a half-dozen representatives, he was informed that the majority of the scraps likely…