The No. 2 and No. 3 lanes just west of Seventh Street are closed as the CHP and Caltrans work to remove the big-rig and its load, CHP Officer Peter Van Eckhardt said.
A second big-rig is headed to the scene to remove the portable building, but it is not expected arrive before 8:30 a.m., he said.
With only one lane of traffic open, the CHP is advising motorists to find an alternate route. Motorists headed to the East Bay are advised to use the San Mateo Bridge instead.
Van Eckhardt said the big-rig's driver apparently lost control around 6:20 a.m. It is not clear why, but Van Eckhardt said the truck might have had a flat tire. A Sig-alert was issued at about 6:30 a.m.