Abstract: An air-cooled heat exchanger has a frame assembly that has a plurality of keyed structural members, where each of the plurality of keyed structural members is interconnected to a corresponding one of the plurality of keyed structural members with a unique mortise and tenon connection joint. Also disclosed is a method for manufacturing and assembling the frame assembly that begins with the step of providing a plurality of keyed structural members that each have a first connection joint component that is uniquely matched to a second connection joint component. The method continues with the step of assembling the frame assembly by interconnecting each of the plurality of keyed structural members using the first and second connection joint components. The method ends with the step of permanently connecting the frame assembly by permanently fastening each of the plurality of keyed structural members.