EXOSKELETONS ARE USEFUL. The fire at Chicago's Dexter Building appears to have been started by wrecking crews removing scrap from the basement.

Fire Department spokesman Larry Langford said the building's owner told him the crew was using acetylene-oxygen cutting torches before the fire that gutted the Wirt Dexter Building at 630 S. Wabash Ave.

"If they're cutting pipes and stuff, there's generally enough heat to get insulation or structural components burning," Langford said.

Although service on the Chicago Transit Authority is still disrupted, an unusual design feature of the building might have prevented an even more disruptive collapse onto the new L tracks behind it.

It was built as a commercial loft structure and later housed the George Diamond Steakhouse. The building’s most distinctive feature was its perforated cast-iron beams, which run up and down the rear façade. They were structural support devices that were decades ahead of their time, but Tuesday night, they were believed to be the only thing keeping the rear wall from collapsing onto the ‘L’ tracks.

Stylistically, these rear beams give the aspect of a New York tenement to the alley side. But photographs of the fire and cleanup show the rear wall still standing even as the other walls are being removed.

There was once a George Diamond steakhouse in Milwaukee, where my parents would sometimes go. It closed some years before the original closed in Chicago. They took us there once after a visit to the Museum of Science and Industry. Good grub, but no longer the place to see and be seen that the Chicago Tribune mourned thismorning.

Tuesday's blaze also took a toll on downtown's vanishing stock of social landmarks. The Dexter Building once housed George Diamond's Steakhouse, a classic celebrity-watching site in the great age of Chicago nightlife in the 1950s and 1960s. Outliving that era, its trademark red carpeting was threadbare when the restaurant closed a few years ago. Fire officials said the building was not occupied at the time.

Take note: the end of the 75-minute Hiawatha and the North Shore Line were not the only symptoms of the encroaching tawdriness of the 1970s and beyond.

RUNNING EXTRA: More about the steak house (yes, there was a George Diamond), the Wirt-Dexter building and the fire, and the aspirations of the building's owner, an heir to George Diamond. Under a George Diamond nostalgia photo, a comment gives the history of the steakhouses (Palm Springs and Milwaukee before Chicago, forsooth. I have eaten at the Wirt-Dexter restaurant. The North Shore Line passed a previous location a block north.)