The Sarah Mildred Long Bridge over the Piscataqua River will be closed nights for construction for three weeks beginning on or about July 9. The work involves installing prefabricated steel barriers about 3 feet high along the length of the bridge. The work was required by a structural study that indicated that the guardrails are so rotten, they are a safety risk. The bridge will be closed from 10 p.m. to 5 a.m. Monday through Thursday; from 10 p.m. Friday to 8 a.m. Saturday; from 8 p.m. Saturday to 9 a.m. Sunday; and from 8 p.m. Sunday to 5 a.m. Monday. Those who use the bridge — which connects Portsmouth to Kittery, Maine — during those hours will have to switch to the Interstate 95 bridge.