Mayor Bloomberg and Fire Commissioner Nicholas Scoppetta on Wednesday announced widespread changes in the upper ranks of the Fire Department.

Top commanders will now work more regular hours, and new commanders will be assigned to oversee each of the five boroughs. Bloomberg and Scoppetta said they hope the new structure, though decentralizing some responsibilities, will help the commissioner work in closer concert with his management team as a whole.

“We will be able to take advantage of the expertise, the experience of our most senior chiefs in the department and bring them back into the daily management of every aspect of the department,” Scoppetta said.

Previously, fire chiefs worked one 24-hour shift followed by three days off, meaning they were only at work 87 days a year. Now they will work a regular five-day week, covering a 24-hour shift once every two weeks.

To accommodate the new positions and workload, eight more regional commanders will be appointed for a new total of 18.

The changes come amid reports that several high-ranking officers are retiring and just before the release of a consultant’s review of the FDNY’s rescue effort at the World Trade Center, which is said to find fault with coordination.

Scoppetta, however, said the overhaul should not be seen as an attempt to correct any supposed shortcomings on September 11.

“We keep coming back to what could have avoided all those events on 9/11,” Scoppetta said. “What would have avoided it was if two planes didn’t crash into the World Trade Center, loaded with jet fuel — that’s what would have avoided it. There are a lot of lessons from 9/11 — I’m not being casual about that — and we all are learning some lessons from it. It’s going to be an even stronger, greater department than it is now.”

More management changes are expected after the performance review is completed.

As it shuffles the hierarchy, the Fire Department is faced with replacing nearly a quarter of its top officers, between early retirements and heavy losses suffered in the terrorist attack. Emotional burnout and the prospect of higher pensions — inflated for retirees this year because of the extra overtime associated with the disaster — are said to be driving a trend of nearly double the usual rate of retirement.

The highest ranking uniformed officer, Chief of Department Daniel Nigro, plans to retire soon. Nigro, 53, was sworn in shortly after the previous chief, Peter Ganci, died on September 11, and he is credited with helping the department cope with the loss of a total of 343 firefighters in the terrorist attack.

The 33-year veteran reportedly decided to step down after Scoppetta denied him tenure, which would have allowed him to retire with a pension based on the chief of department’s salary, instead of the lower rank of chief of operations, his previous position.

Despite the pain of the losses, the commissioner said, the department is in good shape.

Wednesday’s shake-up is just the latest chapter in Bloomberg’s efforts to revamp city government to fit his corporate management style. The mayor has tremendous faith that smart, creative managers can solve any problem. He said the FDNY overhaul “reflects my commitment to always being open innovation, to finding new ways to take the things we did yesterday and do them better today.”