When such bridge or viaduct is built by a municipality and does not cross any place over which the county is authorized to construct a bridge but crosses merely railroad or railroads and private property, the county commissioners may contract to pay an amount of money, not exceeding thirty per centum of the entire cost of the proposed bridge or viaduct. Such bridge or viaduct shall thereafter be maintained as a municipal structure, and the county shall not be liable for any part of the cost of maintenance or repair thereof.
16 P.S. § 5853