Construction on the new Tappan Zee Bridge will affect some Metro-North train service this weekend as new girders are installed.

A total of 31 miles of giant steel beams will eventually support the new $3.9 billion bridge's concrete road deck.

On the Westchester side, the girders will connect the future westbound span's on-land connection to the first offshore support pier in the Hudson River.

Construction crews will actually push the beams over the Metro-North Railroad tracks in Tarrytown toward the first pier in the river. As the operation progresses, additional girders will be connected to the assembly and pushed farther westward.

This part of the process is expected to take about three months, with work over the Metro-North tracks occurring during three Friday overnights this fall. The first day of work is scheduled for this Friday, Oct. 23, with the two remaining dates still yet to be set in November and December.

The plan is for Metro-North to reduce overnight train service starting at 9 p.m. this Friday and that will run until 10 a.m. Saturday. During that time, the bridge crews will stop construction activities when trains pass through the area and then start up again when they get the all clear from Metro-North.