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

spread footings and drilled piers must be considered

(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

additional earth pressure due to

analysis.

seismic shaking, which is assumed to be a uniform

pressure

soil, which may be assumed equal to 0.5 *S*DS/2.5

modification factors, *R*, for Performance Objective

1A, shall be in accordance with the structural system

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

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, *Q*CL, 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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