All contracts for the making of any public improvements in the city, to be paid for in whole or in part by special assessment or special taxation, and any work or other public improvement, when the expense thereof shall exceed the sum of twenty thousand dollars ($20,000.00), shall be let to the lowest responsible bidder therefor, in the manner hereinafter prescribed, and be approved by the mayor. (Ord. 1726, 5-7-2002; amd. Ord. 1854, 1-3-2006)