This extension of the Liberty St. pedestrian bridge started taking shape this week and will open in mid-March and is likely to stay for at least a few years. Rather than entering the bridge at Liberty and Greenwich Sts., pedestrians will have to walk down to Albany St. and over to the West Side Highway, where stairs to the bridge will rise in front of 90 West St. The change will allow the Port Authority to demolish part of the current bridge to build the vehicle security center for the World Trade Center site. The construction of the bridge extension will require traffic changes on West St. between Rector and Liberty Sts. this weekend: From 10 p.m. Friday to 10 a.m. Saturday, two of three northbound lanes on West St. will be closed, and then from 10 a.m. Saturday to 5 a.m. Monday, one of three northbound lanes will be closed.