Pre-nineteenth-century wide-span buildings
In many ways a more efficient alternative to the simple barrel vault for covering a large rectangular hall was the groined vault, in which the main barrel vault running along the length of the hall was intersected by a series of shorter ones crossing it at right angles [12.3]. This was the favourite type of vault for the largest halls of Imperial baths, and well-preserved examples remain in the Baths of Diocletian (where the frigidarium was converted in the Renaissance into the Church of S. Maria degli Angeli). It was also used for the main hall of Trajan's market adjoining his Forum not far from the Basilica Ulpia, and, finally, for the last of the large basilicas - the Basilica Nova started by Maxentius and completed by Constantine. It had the advantage of concentrating the need for support at only a few widely separated points along the longer sides of the rectangle. This allowed both for clerestory lighting below the transverse barrels and for the opening out of the main room at floor level into smaller side-rooms below the windows. Of the Basilica Nova, only the supports of the main vault on the north side and the side rooms contrived between them now stand to their full height [12.4]. But some impression of its original scale can be gained from these.