The three-day closing of the Mississippi River bridge to Hastings has been postponed again.

The closing was to start late this week, but now is not expected until Sept. 18 at the earliest. It will allow the twin-arch main span of a new bridge to be floated and hoisted into place atop piers, just upstream of the old bridge.

The delay is necessary to give crews more time to ensure that welds and railings are adequately tested on the barge platform built to float the new bridge structure toward waiting abutments extending from both river banks, the Minnesota Department of Transportation (MnDOT) said Monday.

The first step of the move was completed last week. The 330-ton span and skeletal steel bridge deck was rolled on four flatbeds across a just-finished causeway onto a flotilla of eight barges. The two biggest, 72 by 260 feet, were brought up from the Gulf of Mexico. Six smaller barges were locked into place between the two big ones at the ends of the 545-foot-long span.

The massive flotilla will be moored out of the river navigation channel until the historic move, delayed three times, happens. When all the work is completed in early 2014, the bridge will have the longest free-standing tied-arch span in North America, MnDOT said.

While the 98-foot-tall span is floated in and hoisted, the old Hwy. 61 bridge will be closed for up to three days, said MnDOT spokeswoman Kirsten Klein.

She said the closing is a precaution to avoid possible accidents on the old bridge caused by gawker slowdowns to watch the main span installation. Resulting traffic jams would hinder emergency vehicles from responding to fires or other incidents on either side of the river.

Jim Adams • 952-746-3283