nstrations, to
justify their use.

It is also of the utmost importance to consider the elastic behavior of
structures, whether of steel or concrete. To illustrate this, the writer
will cite a case which recently came to his attention. A roof was
supported by a horizontal 18-in. I-beam, 33 ft. long, the flanges of
which were coped at both ends, and two 6 by 4-in. angles, 15 ft. long,
supporting the same, were securely riveted to the web, thereby forming a
frame to resist lateral wind pressure. Although the 18-in. I-beam was
not loaded to its full capacity, its deflection caused an outward
flexure of 3/4 in. and consequent dangerous stresses in the 6 by 4-in.
angle struts. The frame should have been designed as a structure fixed
at the base of the struts. The importance of the elastic behavior of a
structure is forcibly illustrated by comparing the contract drawings for
a great cantilever bridge which spans the East River with the expert
reports on the same. Due to the neglect of the elastic behavior of the
structure in the contract drawings, and another cause, the average error
in the stresses of 290 members was 18-1/2%, with a maximum of 94 per
cent.

Mr. Godfrey calls attention to the fact that stringers in railroad
bridges are considered as simple beams; this is theoretically proper
because the angle knees at their ends can transfer practically no flange
stress. It is also to be noted that when stringers are in the plane of a
tension chord, they are milled to exact lengths, and when in the plane
of a compression chord, they are given a slight clearance in order to
prevent arch action.

[Illustration: FIG. 3.]

The action of shearing stresses in concrete beams may be illustrated by
reference to the diagrams in Fig. 3, where the beams are loaded with a
weight, _W_. The portion of _W_ traveling to the left support, moves in
diagonal lines, varying from many sets of almost vertical lines to a
single diagonal. The maximum intensity of stress probably would be in
planes inclined about 45 deg., since, considered independently, they produce
the least deflection. While the load, _W_, remains relatively small,
producing but moderate stresses in the steel in the bottom flange, the
concrete will carry a considerable portion of the bottom flange tension;
when the load _W_ is largely increased, the coefficient of elasticity of
the concrete in tension becomes small, or zero, if small fissures
appear, and the concrete is unable