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 15

A continuous beam is one which extends over three or more supports.

The formulas for determining the strength and stiffness of such beams are too elaborate to be reduced into simple rules, but it is worth while to know how the strength and stiffness of such beams compare with the strength and stiffness of a single span.

The strength of a continuous girder of two spans, Fig. 12, is the same as if the girder were cut over the support, when the load is distributed over the full length of the beam, but when the load is applied at the center of each span the strength of the beam is increased one-third by making it continuous over the center support, provided that the spans are equal. If the spans are unequal the increase in the strength will be less.

Fig. 12.—Continuous Girder of Two Spans.

Fig. 13.—Continuous Beam of Three Equal Spans.

The stiffness of a continuous beam of two equal spans is more than doubled by having the beam continuous over the center support, whether the load is distributed or concentrated.

In the case of a continuous beam of three equal spans, Fig. 13, the strength is increased one-fourth when the load is distributed, and two-thirds when equal loads are applied at the center of each span, and the stiffness is increased about 90 per cent. in both cases. It is therefore desirable to make beams continuous whenever practicable.

When beams are made continuous over three or more supports the points of greatest strain are those which come over the center supports or support, hence the beam should not be cut into at those points.