The provisions of sections 234.310 and 234.320 shall not abrogate or impair any right or power which such cities may now or hereafter have to build and construct bridges and viaducts and their approaches thereto, or to acquire, in whole or in part, by purchase, lease, gift, condemnation proceedings or otherwise, bridges, viaducts and the approaches thereto already constructed, together with the necessary lands, rights-of-way, easements, leaseholds, other property and rights, upon which any such bridge, viaduct or its approaches are now or may hereafter be constructed, operated and maintained by private parties, and to own, operate and maintain any such structure for public use as and for a public highway for all kinds of vehicles, pedestrians, railroads and streetcars.