322. Although constructions of iron and steel do not properly come within the scope of this volume, there are so many places where metal work is used in connection with brick, stone and terra cotta that it has been thought desirable to briefly describe the most common forms of iron and steel construction used for supporting masonry walls, and the various minor details of metal work used in connection with the mason work.
Girders and Lintels. - All openings in masonry walls which it is not feasible to span with arches should have iron or steel lintels or girders to support the mason work above. The objections to wooden beams for supporting mason work are given in Section 255.
Since the price of rolled steel has been so greatly reduced, girders and lintels for supporting brick and stone walls are almost universally formed of steel I-beams, or girders built up of steel plates and angle bars. Except for very wide spans and exceptionally heavy loads, steel I-beams may be most economically used for such supports. As a rule, at least two beams should be used to support a 9-inch or 12-inch wall, and three beams for a 16-inch wall, the size of the beams, of course, depending upon the weight to be supported. The beams should be connected at their ends, and every 4 or 5 feet between with bolts and cast iron separators, cast so as to exactly fit between the beams. The girders should have a bearing at each end of at least 6 inches, and should also rest on cast iron bearing plates of ample size.
If the wall to be supported is of brick, the first course above the girder should be laid all headers. The width of the girder is generally made 2 inches less than that of the wall. In calculating the weight to be supported by a girder, much depends upon the structure of the wall above. If the wall is without openings, and does not support floor beams, only the portion of the wall included within the dotted lines, Fig. 205, need be considered as being supported by the girder. The beams in that case, however, should be made very stiff, so as to have little deflection. If there are several openings above the girder, and especially if there be a pier over the centre of the girder, as shown in Fig. 206, then the manner in which the weight bears on the girder should be carefully considered. In a case such as is shown in Fig. 206 the entire dead weight included between the dotted lines A A and B B should be considered as coming on the girder, and proper allowance made for the load being mostly concentrated at the centre.
Steel lintels for supporting stone or terra cotta caps and flat arches are described in Section 190.