The erection, improvement and repair of all public buildings and works, shall be done by contract or may be done under the supervision of the city council by the city work force. In all said work to be done by contract and in furnishing or purchasing any supplies and/or material for the same or for any other use by the city, when the expenditure required for the same exceeds the sum of $15,000.00, subject to annual adjustment up or down in conformance with the Consumer Price Index, the contract therefor shall be awarded to the lowest responsible bidder pursuant to procedures established by ordinance adopted by the city council.