Heavy Beams and Girders. For ordinary framed buildings there will be no difficulty in obtaining timbers large enough for every purpose, but in large structures, or in any building where heavy loads must be carried, it is often impossible to get a single piece which is strong enough to do the work. In this case it becomes necessary to use either a steel beam or a trussed girder of wood, or to build up a compound wood girder out of a number of single pieces, fastened together in such a way that they will act like a single piece.
Steel beams are very often employed for girders when a single timber will not suffice, and although they are expensive, the saving in labor helps to offset the extra cost of the material.
Wherever wood joists or girders come in contact with a steel beam they must bt cut to fit against it. The steel shape most commonly employed is the I-beam and the wood members must be cut at the ends so as to fit between its flanges. This is shown in Fig. 222. The joist B is sup ported on the lower flange of the I-beam C and the strap A prevents it from falling away from the steel member. The strap is bolted or spiked to the wood beam and is bent .over the top flange of the steel beam as shown. If two wood beams frame into the steel beam opposite each other, a straight strap may be used passing over the top of the steel beam and fastened to both the wood beams, thus holding them together. If a better support is desired for the end of the wood beam, an angle may be riveted to the web of the steel I-beam, as shown in Fig. 223, and the end of the wood joist may be sup ported on the angle. This is an expensive detail, however, and it is seldom necessary.
If a timber is not strong enough to carry its load, and if it is not desirable to replace it with a steel beam, it may be strengthened by trussing. There are two methods of trussing beams: by the addition of compression members above the beam, and by the addition of ten sion members below it. The first method should be employed when ever, for any reason, it is desired that there be no projection below the bottom of the beam itself. The second method is the one most commonly used, especially in warehouses, stables, and other build ings where the appearance is not an important consideration.
In Fig. 224 is shown a beam which is trussed by the first method with compression pieces A above the beam. All the parts are of
wood excepting the rods B, which may be of wrought iron or steel. The beam itself is best made in two parts E E placed side by side, as shown in the section at A. This section is taken on the line C D. The depth of the girder may be varied to suit the conditions of each case. In general the deeper it is made the stronger it becomes, pro vided that the joists are made sufficiently strong. Usually girders of this kind are made shallow enough so that the compression mem ber will be contained in the thickness of the floor and will not pro ject above it. A slight projection below the ceiling is not a serious disadvantage. The floor joists F may be supported on the pieces E, as shown at A.
In Figs. 225 and 226 are shown examples of girders which are trussed by the second method with tension rods D below beam. These rods are of wrought iron or steel, and the struts A are of cast iron. The struts may be made of wood if they are short, or if the loads to be carried are not heavy. Sometimes the girders are made very shallow and the struts A are then merely wood blocks placed between the beams C and the rod D to keep them apart. The girder shown in Fig. 225 is known as a king-post trussed beam, while the one shown in Fig. 226, with two struts instead of one, is known as a queen-post trussed beam. The beam itself, C, may be made in two or three pieces side by side with the rods mid the struts fitting in between them, or it may be a single piece, and the rods may be made in pairs, passing one on each side of the beam. The struts bear against the bottom of the beam, being fastened to it by bolts or spikes, as shown in the illustrations, so that they will not slip sidewise.
It sometimes happens that a heavy girder is required in a situa tion where trussing can not be resorted to, and where steel beams can not be readily obtained. In this case the only resource is to build up a compound beam from two or more single pieces. A girder of this kind can be constructed without much difficulty, and can be so put together as to be able to carry from eighty to ninety per cent of the load which a solid piece of the same dimensions will bear. There are many ways of combining the single timbers to form compound beams, some of the most common of which will be described.