With the number of inmates at Cook County Jail falling below 9,000, Sheriff Tom Dart said he has shuttered one building and will soon close another, saving up to an estimated $15 million a year and easing staffing woes.

Surrounded by rows of empty metal bunk beds in closed Division 2, Dart told reporters he believed this marked the first time ever that the sprawling West Side jail had been downsized. The closings lower the jail's capacity to about 9,100, down some 900 beds.

A construction binge took place in the early to mid-1990s after the jail population soared, forcing hundreds of inmates to sleep on mattresses on the floor and leading to a public-safety nightmare as many inmates won quick release on signature bonds to ease overcrowding.

Until recently, the complex near the Criminal Courts Building at 26th Street and California Avenue had been the nation's largest single-site jail for years. The number of inmates peaked at more than 11,000 in 2002 and has steadily declined since 2004 to about 8,600, Dart said. A jail in Texas is now bigger, he said.

The sheriff said a drop in arrests, particularly in Chicago, was the principal reason for the jail's population decline. Its annual intake plummeted 18.6 percent from 2004 through 2009, he said. Arrests by Chicago police account for about four-fifths of the jail's detainees.

Overcrowding has been an endemic problem at the jail for more than three decades, forcing a series of federal judges to keep a watchful eye over operations. Dart is under pressure to boost staffing after U.S. District Judge Virginia Kendall ordered the county to hire an additional 210 corrections officers and fill nearly 300 vacant positions.

With the closings of the two buildings, Dart said about 175 correctional officers will be reassigned elsewhere in the jail, a move he hopes will satisfy part of the judge's order. Earlier this week, the County Board approved other hirings as well.

Dart said Division 2, a 55-year-old building that once served as a kitchen, will be converted into a cafeteria. Division 3, which was built in 1973, will be mothballed but kept ready to handle any uptick in population.

"We have no delusions that this is going to be the case forever, but we feel comfortable with what we're building into this, that we are going to be able to maintain this and to keep some of these divisions closed, some hopefully forever," he said.

mwalberg@tribune.com