Barrier islands are, by their very nature, separated from the neighborhoods, business districts and communities of the mainland. To get to the paradises of Longboat Key, Lido Key, Anna Maria Island, Siesta Key and the like, you’ll need to get on a boat or cross a bridge.More than 100 years after John Ringling led a circus parade across the then newly built, timber-supported Ringling Causeway, a concrete fixed-span bridge that graces most Sarasot…