for this condition.)
Strut ties placed below such
use of piles, caissons, or deep piers bearing on a firm
slabs will be cushioned or separated from the slab
stratum, may be the solution to this problem.
such that slab settlement will not damage the slab or
9-4.
Design of Elements.
strut ties. Alternatively, it may be more economical
to overexcavate the soil under the footings and
a.
General.
The mechanism used for the
recompact to control differential settlements under
transmission of horizontal forces may be friction
vertical loads, and to increase passive resistance of
between floor slab and ground; friction between
the sides of the footings under lateral loads so as to
bottom of footing and ground; and/or passive
eliminate the need for footing ties. Slabs on ground
resistance of earth against vertical surfaces of pile
when used as a foundation tie will have minimum
caps, footings, grade beams, or basement walls. The
reinforcing, according to ACI 318. As a minimum,
overturning effects, which require a careful analysis
a mat of #4 at 16 inches each way is recommended.
of permissible overloads for the combined effect of
vertical and lateral loads, must be considered in the
e.
Overturning. The overturning moment at
foundation design. Although rocking of buildings
the base of the building is resisted by the soil
about their foundations appears to have been
through the foundation. The total load on the soil is
beneficial in some instances, it is not permitted by
not changed, but there is a change in the distribution
this document because of the indeterminate nature of
of the soil pressure. For isolated spread footings, the
rocking as a means of energy dissipation.
Net
design requirement is simply to provide for vertical
upward forces must be resisted by anchorage into the
components
of
the
overturning
moment
in
foundation.
Stability against overturning must be
combination with the vertical forces due to dead and
provided for the short-time loading during an
live loads. For wall footings, there may be enlarged
earthquake (or wind) without creating disparities in
footings under the boundary members, and these will
the foundation configuration that would result in
have increased loads as indicated above for isolated
significantly different foundation settlements due to
footings, but there will also be loads on grade beams
gravity loads. These differential settlements could
or other connecting elements.
create more damage to the structure than the short-
time deformations that might occur under the highly
f.
Differential Settlement.
Earthquake
increased soil pressures due to earthquake effects.
vibrations may cause consolidation or liquefaction of
loose soils, and the resultant settlement of building
b.
Slabs on Ground. Slabs on ground are often
foundations usually will not be uniform. For rigid
thought of as nonstructural, but will in fact be
structures supported on individual spread footings
nonstructural only if detailed to be unconstrained by
bearing on such material, excessive differential
adjacent elements. In seismic design, the slab on
settlements
can
damage
the
superstructure.
ground should be used as a connecting, tying,
Stabilization of the soil prior to construction, or the
9 - 14
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