QUINCY — With a rumbling boom and a burst of smoke, a large section of the Long Island Bridge was demolished Monday as contractors imploded a 750-foot-long piece that connects to the island, officials said.
The controlled demolition happened at 11 a.m., sending a dark billow of smoke into a whipping breeze.
The Coast Guard had set up a 1,000-yard safety zone between Moon Island and Long Island, and marine units from State, Boston, and Quincy police were on scene to steer boaters away.
The demolition targeted three sections of the bridge — sections 12, 13, and 14 — each 250 feet long, Mayor Martin J. Walsh’s office said.
Part of the ailing bridge over Boston Harbor was already removed in February.
The bridge, which opened in 1951, was closed abruptly Oct. 8 by Walsh because of concerns about its condition.
Before the closing, hundreds of homeless people, those struggling with addictions, and troubled teens traveled across the bridge to receive services on the 225-acre island.
The closure set off a scramble to find alternative spaces for them.
The demolition, by Walsh Construction Co. of Chicago, is expected to cost $20.6 million and to be finished by the end of April.
The city has issued a request for proposals for design of a new bridge.
The design is expected to cost $9 million.
Building a new bridge is expected to cost at least $80 million.