(1) In the preparation of specifications for the erection, construction and alteration of any public building, when the entire cost of the work shall exceed the base amount of $18,500, subject to adjustment under section 11903.1 (relating to adjustments to base amount based on Consumer Price Index for All Urban Consumers), the architect, engineer or other person preparing the specifications shall prepare only the following separate specifications:

(2) The project manager, construction manager or other person authorized by council to enter into contracts for the erection, construction or alteration of the public buildings shall receive separate bids upon each of the branches of work and council or the appropriate city officer shall award the contract to the lowest responsible bidder for each of the branches, including the balance of the work, in addition to the plumbing, heating, ventilating and electrical work and elevators and escalators. Where it is desired to install an air conditioning unit, the heating and ventilating may be regarded as one branch of work having only one set of specifications, and bids may be received and a contract awarded on that set as provided under this chapter.