Collapsed skyscrapers absolutely would be a good source of steel. Depending on the apocalypse, there would also be a good argument for intentionally collapsing

Collapsed skyscrapers absolutely would be a good source of steel. Depending on the apocalypse, there would also be a good argument for intentionally collapsing any skyscrapers that were still standing, as depressing as that is. An unmaintained skyscraper will be shedding glass panels fairly frequently, and once the window glass is gone it will start accumulating rainwater and corroding the structural steel. Uncontrolled collapses also have a risk of knocking down neighboring buildings. Walking through a downtown area 50 years after it was depopulated would be fairly dangerous. You would want crews stripping empty buildings and knocking them down asap. Not to mention large cities are located at ports where you're going to want to operate anyway, once civilization is rebuilt enough to engage in ocean freight again.