Читать книгу Strength Of Beams, Floor And Roofs - Including Directions For Designing And Detailing Roof Trusses, With Criticism Of Various Forms Of Timber Construction - Frank E. Kidder - Страница 6

In the rules hereinafter given the breadth and depth of the beam are always supposed to be measured in inches and the span in feet. The meaning of the terms referred to is clearly shown in Fig. 1. Beams are also sometimes supported at three or more points, in which case they are called continuous beams. These will be considered in their proper place. There is also the cantilever beam, or a beam fixed at one end. The cantilever portion of the beam is that which projects beyond the support. The other end may be fixed in a wall, as at A, Fig. 2, or it may be held down by its own weight and the load on it, as at B. A beam supported at the center only, as at C, is a double cantilever, each side being considered as a cantilever. All three cases are met with in building construction, although that shown at B is the most common.

There are also different ways of loading a beam, although loads are usually classed either as distributed or concentrated. A distributed load is one that is applied over the entire length of the span, and when the load is uniform, as in the case of a plain brick wall of uniform height, the load is called uniformly distributed. Floor loads, although as a matter of fact not absolutely uniform, are generally considered as such. Floor joists resting on a girder may be considered as a uniformly distributed load, when the joists are not spaced more than 2 feet on centers. When they are spaced 4 feet or more on centers they should be considered as a series of concentrated loads.

A concentrated load is one that is applied at a single point of a beam, although in practice the “point” may be perhaps 3 feet long. An iron safe resting on the center of a beam 10 feet or more in length would be considered as a concentrated load. The end of a header framed to a trimmer is also a concentrated load, as is also a partition extending across a series of beams or joists.

The effect of a concentrated load applied at the center of a beam is just twice as great as if the load were uniformly distributed. When the load is applied between the center and the end the effect may be greater or less than that of a distributed load, according as the point of application is nearer to the center or to the support.