39. Reinforced-Concrete Beams and Girders.—The depth of the beams and girders in reinforced-concrete construction varies, of course, with the span and loads to be supported. Their width enters little into the strength, and they may be made as narrow as possible in order to cover the reinforcing steel. It is the best practice to make beams and girders of the same width, for then the process of forming the molds is greatly simplified and the cost reduced.
In placing the reinforcement in the concrete, it should always be at least 2 inches from the outside surface, for a distance less than this is considered inadequate fireproofing. In order that the reinforcing metal [e, Fig. 16], may enter over the top of the reinforcing metal at f, it is usual to make the secondary girders, or beams, 3 inches less in depth than the main girders. To stiffen the building, brackets are customarily introduced between the column and girders, as illustrated at g. These brackets tend to greatly increase the rigidity of the connection and shorten the span of the girder somewhat.
Fig. 18
40. Construction at Window Heads.—Where it is necessary to have the window head near the top of the ceiling, reinforced-concrete construction lends itself readily to the requirements of this condition, for even where girders are supported over the window head, the construction may be followed out, as indicated at [h, Fig. 16]. Where it is desired to have the window head raised still higher, a construction similar to that shown in [Fig. 18] may be used. In this case, however, care must be taken to have the girders bear on the piers between the windows, and to have no intermediate beams.
(a)
(b)
Fig. 19