OAKLAND (AP) — The new, $6.4 billion eastern span of the San Francisco-Oakland Bay Bridge is done, and demolition of the 1930s-era bridge has begun.

The tricky, years-long demolition of the old span presents myriad engineering challenges of its own, Brian Maroney, the California Department of Transportation's bridge engineer, told the Contra Costa Times.

"The old bridge is the world's largest armed bow and arrow," Maroney said. "We have to de-string it very carefully or it will go boom!"

Crews conducting the $281-million demolition will first dismantle the old bridge's steel frames, called trusses, roadway deck and piers that are closest to the new span, allowing the new bridge's bike and pedestrian path to be completed.

In all, about 7 ½ million tons of asphalt, concrete and steel will be removed.

Contractors are concerned with how the Depression-era span's old steel will behave as heavy equipment is rolled onto it.

Crews will use gauges that measure strain in each piece before it is cut, and will use 500-ton jacks to help balance loads.

Work was delayed on the demolition planning after a series of seismic safety bolts on the new span failed, requiring extensive engineering work and repair before its opening.

The agency was also delayed in getting a permit for handling toxic lead paint debris on the old span.