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View of the interior of one of the corridors of the upper, second floor of the two-storey Market of Trajan. There are two such upper corridors—one on each upper side of the "courtyard." The door(s) on the left side of this image open into shops that line this corridor.
The arches that are visible above the corridor are very interesting. These arches transfer the "load" from the heavy roof that is over the central courtyard to the walls that separate the shops. Without this support, the structure would collapse. "These arches represent an astonishing structural innovation for which Apollodorus of Damascus is seldom given credit: the flying buttress" (Ressler, p. 70). Yes, the same structural concept that was used in the grand cathedrals of the Middle Ages!
Steven Ressler, Understanding Greek and Roman Technology: From Catapult to the Pantheon—Course Guide Book. The Great Courses — The Teaching Company, Chantilly, Virginia, 2014.
The Market of Trajan was designed by Apollodorus of Damascus, Trajan's architect, and built in A.D. 100–110. It is built into the Quirinal Hill, which had to be excavated away. It is preserved in places to six–storeys and contained 170 rooms!
The "market" had a variety of functions: shops for merchants, administrative offices for the government, and apartments. Today it houses a museum—Museu dei Fori Imperiali—in which artifacts from the various Imperial Fora are on display.