Michael Arad, the architect for World Trade Center Site Memorial was a Georgia Tech Alumni and he gave a talk regarding the designing of the memorial. One of the major theme of his design was that he wanted to commemorate the absence of the towers. The design instantly makes sense as it relates loss with absence, which is an universal expression of loss.
Above is the completed memorial with museum still under construction.
This paper describes the process of architectural design. The paper says that design constraints evolve with each iteration of the design. It further says that initial design constraints are few in number, so architect selects an initial design based on individual choice and initial constraints, but with the first design itself the constraints evolve. So the paper describes an evolving process of architectural design wherein the constraints help in creation of a design while a design evolves the constraint through design appreciation.
Micheal Arad describes a slightly different process of architectural design. He had the concept of building a memorial using the theme of absence, two years before the competition was announced. He had made some initial sketches of the design as he was personally interested in building the design. After the competition was announced, he just elaborated his initial design. Although the minor changes were made in the design once the construction began, but there was no evolution in the basic design. Another striking feature of his design is the simplicity with which it conveyed the message.