Concrete has been poured for the Yerba Buena Island Transition Structure. It was a massive undertaking taking several months and requiring three phases, and hundreds of truck loads of specially engineered concrete. That's not the end of the story. Once the concrete cures, or hardens, then nearly a hundred tendons of steel are snaked through the bridge. (That's in addition to the many tons of rebar already inside.) The tendons are stretched the length of the bridge, and then pulled tight on both ends — a process that strengthens the bridge even more, and actually gives it lift.