53 Pa. Stat. § 2509
In case the county commissioners do not unite in any such contract as is hereinbefore provided for, or in case said viaduct does not cross any river, creek, stream, or place, over which the county is authorized to build bridges, it shall be lawful for the said city to contract for the construction of the viaduct, as aforesaid, and to pay for the entire work; or to contract with all, or any other, of the aforesaid parties, in manner aforesaid, in which case the plans and specifications for the said work shall be prepared by the city, and advertisement shall be made for bids, and the contract let, in the manner hereinbefore provided. The contract for the work shall be based upon the city's appropriation, and upon the amounts agreed to be paid by the other parties, as provided in the preceding section.
53 P.S. § 2509