The 12-span viaduct, which carries the A370 at Long Ashton, is in need of "essential maintenance" to the joints and bearings which have become worn.
Traffic flow on the bypass is being restricted for 23 weeks with two of the three lanes remaining closed.
North Somerset Council has encouraged motorists to avoid using the bypass and driving through Long Ashton village.
Traffic will be able to travel in one direction only along the bypass which is used by more than 19,000 vehicles each day.
Major work to repair the Yanley viaduct near Bristol has begun with long delays expected until the new year.