Each highway bridge existing on July 1, 1921, maintained in whole or in part by the state, shall be so maintained as to permit the safe passage of a two-axle vehicle having a gross weight of not less than eight tons, and each bridge constructed thereafter shall be so built and maintained as to permit the safe passage of a two-axle vehicle having a gross weight of not less than sixteen tons or a three-axle vehicle having a gross weight of not less than twenty tons, provided each bridge constructed by the commissioner on a state highway after July 1, 1955, shall be so built and maintained as to permit the safe passage of any vehicle or combination of vehicle and trailer or semitrailer or any other object not exceeding the weights specified in section 14-267a.