Whenever the board of county commissioners of any county be of the opinion that it is necessary to construct or repair any county bridge or culvert it shall determine in what manner the same shall be built or repaired, and shall direct the county engineer to prepare complete plans for the same, and after the approved plans and specifications have been filed and before the notice of the letting is published the county engineer shall make and furnish to the board an accurate sworn estimate of the cost of the construction or repair of the same: Provided, That the board may employ an engineer to act in conjunction with the county engineer in preparing plans, specifications, estimates, and in superintending the construction of any bridge or culvert work: Provided, however, That no engineer shall be employed by the board to furnish or to prepare plans, specifications or estimates for any county or township bridge or culvert work, or to supervise the construction of any such work, who is in the employ of or represents any bridge company, bridge firm, or bridge contractor. Alternate plans may be put on file provided they are in accordance with the general specifications.