This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
238. Architects sometimes specify a ledge to support the floor joists, by using a continuous brick corbel of three or more courses. This mode of construction is shown in Fig. 99. At a is the 4-inch brick corbel, of three courses of brick; b, b shows the wooden floor joists; c is a strip of plank placed between the floor joists to nail the rough flooring to; d is the rough floor, and e the tongued-and-grooved flooring; f is the 1" x 2" furring on the wall, and g the lath and plaster over the furring strips. Corbeling out for the floor joists has several advantages, one being that, in case of fire, the corbels act as a fire-stop, largely preventing the spread of the flames from story to story; and, in case the floor joists fall, they are inserted such a short distance in the wall that they will not have so much tendency to pull the wall over as if anchored. The wall is also much stronger when corbeled out, for whenever timbers extend into a wall, they lessen the section, or bearing area, by just the amount of space taken up by the ends of the floor joists, and in partition and party walls this is very considerable.
When walls are corbeled out in this way, a plaster or wooden cornice is required, to give a proper finish to the junction of wall and ceiling', and to form the angles of the rooms. This cornice is shown on the dotted line at h, Fig. 99.
All corbeling for floor joists should be executed in good hard brick; no salmon or soft brick should be allowed in the construction, for, as much of the weight of the floor joists and floors bears on the corbel, any structural weakness might cause serious results.
239. In Chicago, the building law provides that all walls of warehouses, 16 inches or less in thickness, and all walls of dwellings, 12 inches or less in thickness, shall have ledges 4 inches wide, corbeled out to support the floor joists, and in all cases when the ledges are built, they are to be carried to the top of the joists, as shown in Fig. 99.