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(8) Once the conditions of Step 7 are satisfied, evaluate the
results of the analysis in order to determine if any further modification of
the foundation dimensions is required. These added modifications are
required when the analysis indicates that the structure attains peak
rotations which are far less than what is permitted. In most cases,
analyses of the structure supported by both a soft and a compact soil are
required and generally, the foundation extension can be shortened until the
results of the analyses indicate that the structure rotates to 40 percent of
its overturning angle on the compact soil, provided the decreased length of
the foundation extension does not violate the design criteria in paragraph
3.b of this section.
In the event that more accurate soils data is available, the structure can
be allowed to rotate to the point of incipient overturning (see Figures 88
and 89).
(9) A detailed design of the foundation extension can commence
after satisfactory results have been obtained from the computer analysis.
Determine the location of the critical section for shear according to the
provisions of paragraph 2.c of this section.
(10) Determine from the printout of the soil bearing
pressuretime history, the peak shear (and corresponding bending moment for
thick sections, *ln/d < 5) at the critical section for shear and the peak
bending moment at the face of the support. These quantities (Vu, Mcr
and Mu) are computed by the program for a cantilever wall barrier. For
other structural configurations, these quantities must be computed manually.
Generally, the bearing pressure distributions at several time stations are
investigated to determine the peak shear and bending moments. When the
computation is performed by the computer program, the bearing pressure
distribution at every integration time station is investigated.
The peak shear usually occurs when the point on the foundation with zero
bearing pressure approaches the critical section for shear on the extension.
Figure 90 shows the configuration of the bearing pressure distribution
curve, and identifies the design parameters and critical sections for shear
and bending. The figure also illustrates the order in which the bearing
pressures are printed out by the computer program.
Figure 91 shows the free body diagrams for computing the peak shear and
bending moment on the foundation extension. The shear is determined by
computing the area within the bearing pressure distribution curve. The
bending moment is determined by computing the moment of the area within the
bearing pressure distribution curve about the desired location on the
extension.
(11) Determine the allowable shear stress that can be carried
by the concrete using the equations provided in paragraph 2.c. of this
section.
(12) Determine the thickness of concrete, d, required to carry
the peak applied shear load using Equation (160).
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