The Manhattan Bridge is held up by four major cables and hundreds of suspenders connecting the cables and the bridge deck; the cables need repairing and the suspenders will be replaced. Work on the two cables toward the center of the bridge -- "B" and "C" on the above diagram -- will disrupt some vehicular lanes but won't affect the bike or pedestrian paths.
Additionally, repairs along the southernmost cable, over the walkway, shouldn't be too disruptive. At a presentation to Brooklyn Community Board 2 [PDF], DOT reported a plan to cover the path with protective sheds under any active construction. The walkway would be a bit cramped, as the sheds are only three feet wide, but people could walk or bike across the bridge as normal.
However, when the northern cable is under construction -- a job that's expected to take almost a full year to complete -- both pedestrians and cyclists could see their trips disrupted. The sheds aren't wide enough for bikes to pass each other, so DOT is considering rerouting cyclists to what is normally the pedestrian side and putting pedestrians under sheds on the bike side. That could pose a number of problems for the city's second-most popular bike route over the East River.