(4) Drilled shafts. In general, drilled shaft
transmitted into the ground.
Simple three-
foundations or piers may be treated similarly to pile
dimensional
free-body
diagrams
of
whole
foundations.
When the diameter of the shaft
substructures may be of great help in defining the
becomes large (>24 inches), the bending and the
design conditions.
lateral stiffness and strength of the shaft itself may
b.
Column Base. If a column is assumed to be
contribute to the overall capacity. This is obviously
necessary for the case of individual shafts supporting
fixed in the analysis of the superstructure, the
isolated columns. In these instances, the interaction
foundation system must have the strength and
of the soil and shaft may be represented using
stiffness required by this assumption.
Winkler-type models.
c.
Development of Forces into the
Foundations.
Foundations must be detailed to
9-3.
General Requirements.
develop the horizontal and vertical components of
seismic forces imparted by columns, shear walls, and
a.
Base. The base of the building is the level
braces. In instances where footing are subjected to
at which the earthquake motions are considered to be
lateral thrusts due to applied vertical loads, such
imparted to the structure. From the point of view of
horizontal thrust will be added to the lateral seismic
design, the base is the level at which the base shear
force indicated above. An example of this case could
is resisted. In a building without a basement, this is
be the outward thrusts on footings of a rigid gable
simply at grade, where footings develop lateral
bent due to applied vertical loads.
resistance. In a building with a basement, the base is
at grade if grade-level framing or the upper portion
d.
Interconnection of Foundation Elements.
of the basement wall is capable of developing the
Foundation ties shall be provided as required by
required lateral resistance, or at the basement level,
Chapter 7 of FEMA 302. The ties can be formed by
if the lateral resistance cannot be developed at grade
an interconnecting grid network of reinforced
level. On sloping sites, the level at grade may be
concrete struts or structural steel shapes encased in
unrestrained at the downhill side, but restrained, like
concrete. As an alternative, a reinforced concrete
a basement, at the uphill side.
The base of a
floor slab, doweled to walls and footings to provide
building is determined by judgment, considering the
restraint in all horizontal directions, may be used in
mechanism for developing lateral resistance. The
lieu of the grid network of ties. Slabs on grade will
base should be taken at the highest level where the
not be used as ties when a potential for liquefaction
building can transmit lateral forces into the ground
has been identified, or when significant differential
on all sides.
Partial basements and sites with
settlement is expected between footings and slab. In
varying subsurface conditions are also potentially
such cases, slabs on grade will be cut loose from
troublesome. The engineer should consider how the
footings and made free-floating (note that the
forces enter the substructure, and how they are
effective unsupported height of the wall is increased
9 - 13
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