Quantcast Section 7. Foundations

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1.  INTRODUCTION.  The structure's foundation is designed for both
conventional loads (dead and live) and blast load conditions.
a.  Conventional Loads.  Procedures of designing foundations for
conventional loads are adequately described in several textbooks, two of
which are Design of Concrete Structures, by Winter and Nilson, and
Foundation Engineering Handbook, edited by Winterkorn and Fang.
b.  Blast Loads.  The design for foundations for protective structures
is based on providing a means of transmitting the blast loads from the
structures to the underlying soil, without a soil shear failure (i.e., a
plastic flow or a lateral displacement of soil from beneath the
foundation), or causing excessive total settlements of the soil or
differential settlements of various parts of the structure under the
impulse loads.  To limit settlements, the load on the soil should be
transmitted to a soil stratum of sufficient stiffness and the load should
also be spread over a sufficiently large area.
(1) In designing a structure's foundation for blast loads, an
analysis has to be performed to determine if the foundation will slide or
overturn, by calculating its peak response and the time history of the
bearing pressures acting on it.  Consider, for example, a retaining wall
under the action of blast-induced forces (Figure 82).  The foundation will
tend to slide in the direction of the blast loads and rotate also.  It can
be seen that the structure's foundation is subjected to horizontal
frictional forces as the building tends to slide and to active bearing
pressures as the structure tries to rotate, thus concentrating the vertical
load toward the rear end of the foundation.  The changing load distribution
is illustrated in Figure 82.  If the overturning moment about the edge of
the foundation exceeds the stabilizing moment, then the structure will
rotate until the stabilizing moment becomes equal to, or greater than, the
overturning moment.  Unless the resistance to overturning is developed
fairly rapidly, the structure will overturn.  The structure will also slide
if the horizontal frictional and passive earth forces are exceeded.
(2) Sliding and overturning of the structure reduce the resistance
needed by the structure to resist the blast loads.  The extent to which the
structure can resist sliding and overturning will depend on the capacity
of the structure for utilizing vertical dead, live and blast loads to
prevent such motion.
a.  Introduction.  Design procedures and criteria vary according to the
type of foundation, the use of the structure and the design loads.  A
foundation for a radar tower, for example, must provide stability against
excessive motions and rocking, whereas the foundation for a rotating or
reciprocating machine has to be designed against progressive settlement of
the underlying soil.


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