Where any county is responsible for the construction or maintenance of any highway, such highway shall, if and when the county commissioners consent thereto by resolution, a copy of which shall be filed in the office of the clerk of the courts of the proper county and with the Department of Highways, be adopted by the Commonwealth as a State highway and shall thereafter be constructed, repaired and maintained at the expense of the Commonwealth as State highways are now constructed, repaired and maintained under the provisions of existing laws: Provided, That no highway shall be taken over by the Commonwealth under the provisions of this act, unless such highway was a county highway on the twenty--eighth day of May, one thousand nine hundred forty-three, nor if a contract for the improvement thereof has been entered into between a contractor and the Commonwealth or local authorities, or both, until such contract shall have been completed: Provided further, That the Commonwealth shall not become responsible under the terms of this act for any bridge, viaduct or other structure, except drainage structures, having a total spanned length of ten feet or less, measured along the center line of the highway.