§ 1621-g. Construction and purchase contracts. The authority shall let contracts for construction in the same manner, so far as practicable, as is provided by law for contracts for the city, except that where the estimated expense of a contract does not exceed five thousand dollars such contract may be entered into without public letting. Nothing in this section shall be construed to limit the power of the authority to do any construction directly by the officers, agents and employees of the authority. Contracts for the purchase of supplies, materials and equipment shall be let in the same manner as is provided by law for contracts of the city.