As Pam Andreasen watched the walls of the old Simi Valley Hospital begin to crumble and fall Monday, she thought of the 1994 Northridge earthquake.

The registered nurse was working that day in the building, which is being demolished after a new $75 million hospital tower opened last month.

It was 100 degrees. The emergency room was crowded with people who had been injured in their homes, many by falling objects. In the women and children's unit, where Andreasen worked, cesarean sections were complicated by aftershocks.

"I was frightened but I saw how people pulled together, and I knew it was OK," she said.

Many hospital employees couldn't wait for Monday, when oversize backhoes began to claw down the walls of a 43-year-old building that was obsolete and overcrowded, with as many as four patients sharing the same bathroom.

Andreasen, now nursing director of the women and children's unit, feels excited about the new beginnings too. But as she wore a blue hard hat and watched the building being gutted, she thought of her more than 19 years spent working there.

"I had my last three children here, and most of my nursing career was here," she said. "It's kind of mixed emotions, saying goodbye and starting something new."

Virtually all of the original hospital, which opened in 1965, is being torn down to make room for a 7,000-square-foot lobby with a vaulted ceiling, along with a chapel and gift shop.

On Monday, workers piloted huge tracked backhoes known as excavators, with booms that reach 30 feet high. As water was sprayed to control dust, the machines used their metal fingers to claw down exterior walls so they could get at rebar, copper pipes, wiring, wood and everything else inside the hospital in a part of the project labeled as soft demolition.

The big machines worked in concert with smaller vehicles to sort the materials and recycle about 80 percent of it.

The excavators plowed a path through the core of the building all the way to an old courtyard that will also be demolished. The plan was to pluck ficus trees from the soil and recycle them into mulch.

Hard demolition, or tearing down more walls, will continue in several days, and the project will reach into August. Demolition workers from J.R. Abbott Construction have already spent a month separating and capping water lines, piping and electrical systems that connected the demolished building to the rest of the hospital.

The noise and the sight of the building coming down attracted a handful of hospital employees. There was talk about the old building's lack of windows and leaks in the ceiling.

"I'm just glad it's going to be gone," said physical therapist Jeff Ehlenberger, "because our new building is the place where we're going to take the future of Simi Valley Hospital."