In the highrise projects of 1921 and 1922 Mies was mostly concerned with the relation between structure and enclosure and the properties of the glass as a skin. In the 1923 "Concrete Office Building" project for Berlin, he shifts its attention to the loadbearing structure as the main element, actually the only element, of his architectural vocabulary.
Most important, here the material is not generic but rather specific. Mies gives a great deal of thought to the column grid, the spans, and other dimensional considerations of the structure. He is also concerned with the program in a specific way--file cabinets are supposed to be placed against the upturned beams that make the enclosure of the building, etc.--and other more detailed, if minimal, aspects of the project, like the entrance or the top and bottom of the structure.
Once more, the rendering of the project (an imposing charcoal and graphite perspective almost 3 meters long!) is carefully framed within the fabric of the city. Unlike Le Corbusier, who tried to reinvent (Ville Radieuse,) overpower (Plan Voisin,) or wish the city away (Marseille,) Mies seemed quite happy to work within the existing city, yes, in a contrasting manner, but almost contextual. In the 1923 project the building is barely taller than its neighbors, as if to complete--more than to compete with--the surrounding fabric.
In the three projects of 1921, 22 and 23, Mies articulates a game of solids and voids, realism and abstraction, that set his position towards the city and informed most, if not all, of his subsequent urban projects, from the competition entry for Alexanderplatz in Berlin (1928) to the Seagram Building in New York (1958.)