All bids shall be made on the proposal blanks furnished by the county, signed by the bidder, sealed and presented by the bidder, the bidder’s agent or attorney or sent by mail, to the county clerk. The board shall conduct the letting of all contracts for bridge and culvert work in such a manner as to give free, open competition, and all qualified bidders shall be given an equal opportunity to bid upon the plans and specifications on file, and they shall be required when bidding a lump sum on two or more bridges or culverts to also submit a separate proposal on each of the bridges or culverts included in their proposals.