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 (1) The design of a foundation subjected to dynamic loads,
however, is a trial-and-error procedure. The initial size of the
foundation is estimated, considering such factors as the forces acting on
the structure and the static bearing stresses in the soil. The trial
design is then analyzed to determine its response to the design dynamic
loads.
(2) The types of foundations usually encountered in
blast-resistant structures are shown in Figures 83, 84 and 85. As stated
in the report by W. Stea titled Overturning and Sliding Analysis of
Reinforced Concrete Protective Structures, the design of cantilever walls
and single cell barriers (Figures 83 and 84) must consider the motion of
the structures on the supporting soil. These structures rely completely on
the soil to provide the required resistance to overturning and sliding
motions. In the multi-cell barrier of Figure 80, the blast loads are
resisted and confined by the blast-resistant walls, and the overall
structure is restrained from overturning by the massive walls and
foundation slab. Here, the motion of structure is not critical and is,
therefore, not considered in the design.
(3) The main emphasis of this section is upon the design of
foundations for protective structures susceptible to overturning, but the
method of analysis and criteria is generally applicable to other structures
encountered in blast resistant design.
b. Preliminary Design. In the design of the structures shown in
Figures 83 and 84, the sizes of the blast-resistant walls are determined
by the procedures and criteria outlined in NAVFAC P-397. In the design of
the foundations, however, the following guidelines, as presented in Stea's
report, should help in estimating the required size of the foundation:
(1) Cantilever wall barrier (Figure 86) usually requires a long
foundation extension. The foundation thickness should be approximately
1.25 times the wall thickness and the length should be 45 percent of the
height of the wall.
(a) The slope of the bottom face of the foundation should not
exceed 5 degrees, since an increase in the slope reduces the moment arm of
the resultant of the soil pressures about the center of gravity of the
structure.
(b) Cantilever barrier foundations should be symmetric about
the centerline of the blast-resistant wall.
(2) Single cell barriers (Figure 87) usually do not require long
thick foundations to prevent overturning. The dimensions of the
foundation are established by providing the foundation with sufficient
bending strength to completely develop the ultimate strength of each blast
wall. To achieve this, the distance between the compression and tension
reinforcement, dc, should be equal to the maximum dc of the blast
walls. The amount of concrete cover should conform to the ACI Code. The
length of the foundation extension is established by providing the
anchorage required for the reinforcing steel in the concrete.
With these estimated dimensions of the foundations, a dynamic analysis can
now be performed. A computer program, OVER, written by Stea, et al., and
described in Section 8, is suitable for such an analysis.
2.08-299
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