On Monday afternoon, engineers Peter Kim and Tony Jen, on their way back to their office after a long day inspecting the Schuylkill Expressway, pulled off I-95 in Port Richmond for some late lunch at a takeout. After eating sausage sandwiches in their car, they drove north on Richmond Street.

Something caught Kim's eye.

"Oh, my gosh," Kim said.

There it was: a huge crack in a column supporting the six lanes of I-95 overhead.

Kim, a structural engineer who does contract work for PennDot, knew the viaduct well. He had inspected the column twice in the last three years and seen reports on the column since 2003. Its condition had always been stable.

But the deterioration he saw Monday was a "dramatic" change from when he'd last seen the column, in December.

"We were kind of taken aback by it, how much it had changed," said Kim, who works for Specialty Engineering, a Bristol consulting firm. "You don't usually see that, that much change."

Kim took pictures on his cell-phone camera and called PennDot immediately. It was about 4:30 p.m. In two hours, the cracked column was swarming with state officials and contract engineers, who shared Kim's alarm. They concluded that the city's busiest highway had to be shut down immediately.

"I just felt like I was doing my job," said Kim, 40, of Horsham. "Definitely anybody who saw it would have reported it."

Chuck Davies, PennDot's assistant district design executive, said officials were grateful for Kim's alertness. "I think the fact that Peter called it in - usually when an inspector does that, it's an urgent situation."

As a bridge inspector, Kim is constantly looking at structures, even casually, on his time off. "It's your line of work," he said. "That's what you do. You're always thinking about it."

Kim has been an inspector for 14 years - mostly bridges. Monday, he and Jen had spent the entire day on a crane examining the Schuylkill Expressway bridge that crosses the river near University Avenue. With all the equipment in place, they did not stop for lunch. So they were famished when they started back to their office in Bucks County.

Motorists inconvenienced by yesterday's road closure may curse Kim's urge for a hot sausage - the structure was not due for another official inspection until October 2009. But Kim said the center column appeared to have cracked recently - "maybe a matter of days" - and had it failed, two sections of I-95 in both directions might have collapsed onto the street below.

Kim last officially inspected the cracked column in October. But he remembers stopping by in December and looking it over informally because it had shown wear.

"Typically, I wouldn't have been back in that area in quite some time," he said yesterday. "The seriousness of that column - you just never know. I guess it was good luck I was in that area."