SAN FRANCISCO (BCN) — Caltrans is removing the fourth of five 504-foot sections of the old eastern span of the Bay Bridge Wednesday and Thursday, Caltrans officials said.

Work lowering the 504-foot truss started at about 6:30 a.m. and will take two days with crews working 12 to 14 hours per day. The section will be lowered onto barges and hauled away.

Constructed in 1935, the bridge was decommissioned in 2013 when a new self-anchored suspension span was finished to replace it. Crews have slowly been deconstructing the obsolete span since then and demolition is scheduled to continue until 2018.

One reason for the long process is because of extensive measures taken to mitigate impact on the surrounding environment, according to Caltrans. As the truss is lowered to barges in the bay, work crews are being advised by a bird deterrence contractor to make sure there is little impact on wildlife.

“Although we have already successfully completed this truss lowering operation on three recent occasions, we are diligent in our continued commitment to safety and the environment,” chief bridge engineer Brian Maroney said in a statement.

If weather conditions are favorable with no high winds, work on the truss could be finished by the end of Wednesday.