In large assembly halls it is often desirable to project the front of the gallery 10 feet or more beyond the line of posts. In such cases it will be necessary to support the front end of the gallery joists by a girder, which may itself be supported by a cantilever.
Fig. 462 shows approximately the framing of the lower gallery in the Mechanics' Hall in Boston. The large posts are placed about 25 feet apart and are carried up to support the roof trusses. On each side of the posts are placed heavy 12-inch iron channels (in line with the 2x12 joists) which are bolted to the post, and also supported by the bracket or brace shown in the drawing. The channels thus act as cantilevers, and support at their inner ends horizontal trusses, which in turn support the ends of the gallery joists. The other end of the joists rests on a partition under the gallery. The middle of the joists is supported by an ornamental open truss extending between the posts. Truss A is concealed in the gallery rail. This method of framing is applicable to many places where it is desired to use large posts placed from 20 to 25 feet apart, with a considerable projection to the gallery. Careful calculations of the various strains must be made, however, to insure safety.
In some cases galleries may be advantageously supported by means of trusses, similar to truss A, hung by rods from the roof trusses. When the depth does not exceed 16 feet the gallery may be supported in this way without posts.