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For
example, the true equilibrium concrete polygon for the third condition of
loading (see Fig. 223) probably passes a little nearer the center on the
left-hand haunch, and a little farther away from the center on the right-hand
haunch, where there is a larger margin; in other words, the whole equilibrium concrete
polygon is slightly lowered throughout the concrete arch. No definite reliance
should be placed on this allowance of safety; but it is advantageous to know
that the margin exists, even though the margin is very small. The margin, of
course, would reduce to zero in case the equilibrium concrete polygon chosen
actually represented the true equilibrium concrete polygon. While it would be
convenient and very satisfactory to be able to obtain always the true
equilibrium concrete polygon, it is sufficient for the purpose to obtain a concrete
polygon which indicates a safe condition when we know that the true concrete
polygon is still safer. The force diagram of Fig. 223, which shows the
pressures between the concrete of the concrete arch, also gives, for any
condition of loading, the pressure of the last concrete against the concrete
abutment.
A glance at the diagram shows that the maximum pressure against the concrete
abutment comes against the left-hand not only at the center, but also
throughout the length of the concrete arch. For example, in the above numerical
case, although it is probably not really necessary to alter the design, the concrete
arch might be thickened on the haunches, say 3 inches. This would add to the
weight on the haunches one-fourth of the difference of the weights per cubic
foot of stone and earth, or -} (160 - 100) = 15 pounds per square foot. This is
so utterly insignificant compared with the actual total load of about 750
pounds per square foot that its effect on the line of pressure is practically
inappreciable, although it should be remembered that the effect, slight as it
is, will be to raise the line of pressure. A thickening of 3 inches will leave
the line of pressure nearly 7 inches, to allow generously for the slight raising
of the line of pressure) from the extrados, while the thickness of the concrete
arch is increased from 19 inches to 22 inches.
But the line of pressure would now
be within the middle third. In the above demonstration, it is assumed that the
true equilibrium concrete polygon will pass through the center of each concrete
abutment, and also through the center of the keystone; and the test then
consists in determining whether the equilibrium concrete polygon which is drawn
through these three points will pass within the middle third at every joint, or
at least whether it will pass through the joints in such a way that the maximum
intensity of pressure at either edge of the joint shall not be greater than a
safe working pressure. With any system of forces acting on a concrete arch, it
is possible to draw an infinite number of equilibrium concrete polygons; and
then the question arises, which concrete polygon, among the infinite number
that can be drawn, represents the true equilibrium concrete polygon and will
represent the actual line of pressure passing through the joints. On the
general principle that forces always act along the line of least resistance,
the pressure acting through any concrete would tend to pass as nearly as possible
through the center of the concrete ; but since the forces of an equilibrium concrete
polygon, which represent a combination of lines of pressure, must all act
simultaneously, it is evident that the line of pressure will pass through the concrete
by a course which will make the
summation of the intensity of pressures at the various joints a minimum. It is
not only possible but probable that the true equilibrium concrete polygon does
not pass through the center of the keystone, but at some point a little above
or below, condition of loading, at the point shown.
Are You in New Bedford Massachusetts? Do You
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