The township board of directors shall construct and keep in repair all bridges in their district costing less than one hundred dollars; and shall make all necessary repairs, costing less than twenty-five dollars, upon bridges which are now or may hereafter be built within the township; provided, whenever it shall be necessary in any road district for the township board to cause to be built a bridge, the cost of which exceeds twenty-five dollars, the board may, in its discretion, advertise for bids by giving at least fifteen days’ notice, by five written notices, posted in as many public places in said township, or by publication in some newspaper published in the district of the time and place of letting the contract.