j.
Retaining Walls. Building walls retaining
k.
Mixed Systems. When subsurface conditions
soil should be evaluated for the effects of seismic
vary significantly across a site, it is sometimes
earth pressures. The seismic earth pressure acting
effective to use mixed systems, e.g., combinations of
on a building wall retaining nonsaturated, level soil
drilled piers and spread footings. Geotechnical
above the groundwater table may be approximated
consultation is especially important for mixed
as:
systems in order to control differential settlements.
The difference in lateral stiffnesses between the
∆p = 0.4khγ rw
spread footings and drilled piers must be considered
tH
(9-15)
in the foundation earthquake design. Nominal values
of the soil springs, determined in accordance with
where:
the requirements of this chapter, may be used in the
∆p =
additional earth pressure due to
analysis.
seismic shaking, which is assumed to be a uniform
pressure
kh = horizontal seismic coefficient in the
9-5.
Acceptance Criteria.
soil, which may be assumed equal to 0.5 SDS/2.5
a.
Performance Objective 1A. The response
γ= the total unit weight of soil
modification factors, R, for Performance Objective
t
1A, shall be in accordance with the structural system
Hrw = the height of the retaining wall.
identified in Table 7-1. The design of the foundation
shall be in accordance with Chapter 7 of NEHRP as
The seismic earth pressure given above should be
modified by this chapter.
added to the static earth pressure to obtain the total
earth pressure on the wall. The expression in
b.
Enhanced Performance Objectives.
Equation 9-13 is a conservative approximation of the
Mononabe-Okabe
formulation.
Seismic
earth
(1) Linear elastic analyses with m factors.
pressures much higher than summarized above may
Structural
foundation
components
should
be
develop on walls that are required to develop passive
considered to be force-controlled, and their lower-
pressures to resist lateral forces. In such cases, static
bound capacity, QCL, will be the nominal capacity, in
passive earth pressure formulations, neglecting
accordance with FEMA 302, multiplied by the
inertia forces in the soil, may be used to estimate the
appropriate capacity reduction factor, N. If soil
magnitude of total (static plus seismic) earth
springs are used in the analyses, the nominal
pressures on the wall. A triangular pressure
stiffness coefficients prescribed in this chapter are to
distribution may be assumed.
be multiplied by 0.5 for Life Safety, 1.0 for Safe
Egress,
and
2.0
for
Immediate
Occupancy
performance levels.
9 - 16
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