1.3G
B = footing width
=
k sv
(9-9)
B(1 - v )
L = footing length in direction of bending.
(b) Capacity parameters.
2.
The lateral capacity of a footing
1. In the absence of moment loading, the
should be assumed to be attained when the
vertical load capacity of a rectangular footing of
displacement, considering both base traction and
width B and length L is
passive pressure stiffnesses, reaches 2.0 percent of the
thickness of the footing. Upper and lower bounds of
Qc = q c BL
(9-10)
twice and one-half of this value, respectively, also
apply.
For rigid footings subject to moment and vertical
load, contact stresses become concentrated at footing
(3) Pile Foundations.
edges, particularly as uplift occurs.
The ultimate
moment capacity, Mc, is dependent upon the ratio of
(a)
General.
Pile foundations, in the
the vertical load stress, q, to the vertical stress
context of this paragraph, refer to those foundation
capacity, qc.
Assuming that contact stresses are
systems that are composed of a pile cap and
proportional to vertical displacement and remain
associated driven or cast-in-place piles, which
elastic up to the vertical stress capacity, qc, it can be
together form a pile group. A single pile group may
shown that uplift will occur prior to plastic yielding
support a load-bearing column, or a linear sequence
of the soil when q/qc is less than 0.5.
If q/qc is
of pile groups may support a shear wall. Generally,
greater than 0.5, then the soil at the toe will yield
individual piles in a group could be expected to be
prior to uplift. This is illustrated in Figure 9-6. In
less than 2 feet (0.6m) in diameter. The stiffness
general, the moment capacity of a rectangular
characteristics of single large-diameter piles or
footing may be expressed as:
drilled shafts are described in Paragraph 9-2c(4).
LP
q
(b) Stiffness parameters. For the purpose
1 -
Mc =
(9-11)
2 qc
of simplified analyses, the uncoupled spring model
as shown in Figure 9-1b may be used where the
footing in the figure represents the pile cap. In the
where:
case of the vertical and rocking springs, it can be
assumed that the contribution of the pile cap is
P = vertical load
relatively small compared to the contribution of the
piles. In general, mobilization of passive pressures
P
q=
by either the pile
BL
9 - 10
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