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12-70 Project. Specifically, ground motions associated with the basis also needed to be considered by the NCHRP 12-70 Proj-
1,000-year return period could be used to identify the following: ect or at least be coordinated with future work being done to
implement the NCHRP 20-07 Project recommendations:
· Geographic areas that will not require special seismic design
studies. For these areas there will be enough margin in the · The shape of the spectrum to be used for design. Significant
static design of retaining walls, slopes and embankments, differences in spectral shapes occur between CEUS and
and buried structures to accommodate seismic loading, WUS. These differences in spectral shape affect soil response
unless special conditions (such as liquefaction) occur. in terms of either peak spectral acceleration or time histories
· The type of analyses that will be required in more seismically from which design computations or response analyses are
active areas. For example, the decrease from the 2,500-year conducted. The previous AASHTO LRFD Bridge Design
return period proposed in the NCHRP 12-49 Project to the Specifications made no distinction between spectral shapes
1,000-year return period resulted in smaller increases in within the CEUS and WUS. The updated maps use the
ground motions. This meant that nonlinear behavior of soil USGS Seismic Hazard Maps for a 1,000-year return period,
was not as significant in any proposed design methodology thereby accounting for differences in spectral shape of
as it would have been for the original NCHRP 12-49 Project characteristic earthquakes in CEUS versus WUS.
recommendations. · The method of introducing site effects on the rock motions
developed for the 1,000-year earthquake return periods. The
Another important recommendation made as part of the former site categories in the AASHTO LRFD Bridge Design
NCHRP 20-07 Project was to follow an NCHRP 12-49 recom- Specifications were too qualitative in description to allow
mendation to use the spectral acceleration from a response consistent use. The new site factors followed recommenda-
spectrum at 1 second (S1), rather than the PGA, as the param- tions given in the Federal Emergency Management Agency's
eter for defining the seismic performance category. The spec- (FEMA) National Earthquake Hazards Reduction Program
tral acceleration at 1 second was used for determining both the (NEHRP) reports and the International Building Code
level of and the requirement for design analyses. Part of the (IBC) documents, similar to what was recommended by
motivation for this change was the observation that damage the NCHRP 12-49 Project and consistent with South Car-
olina Department of Transportation (SCDOT) guidelines
during earthquakes was better correlated to S1 than to PGA.
prepared by Imbsen & Associates.
By adopting S1 as the parameter for determining the level of
· Performance expectation for the retaining walls, embank-
and the requirements for design, the region where the thresh-
ments and slopes, and buried structures under the 1,000-year
old of seismic demand would be sufficiently low to avoid the
event. For this event the amount of acceptable deformation
need for specialized seismic demand analyses increased. There
depended on factors such as the potential consequences of
have been significant developments in the seismological com-
the deformation (that is, to the retaining wall, roadway
munity in the past 10 years which concluded that the seismo-
embankment or cut slope, or culvert), the potential need for
logical environment in CEUS differs from WUS in regards to
and cost of repair, and the additional design requirements
the long-period content of earthquake ground shaking. For
associated with the performance evaluation. A single set of
the same PGA, ground motion records from CEUS have much design guidelines that captured all of these factors was not
lower shaking intensity at longer periods of ground motion. easily developed.
The choice of using spectral acceleration at 1 second held the
potential for minimizing the need for dynamic response
analyses for many transportation structures. 2.2 Literature Search
In order to simplify integration of the results of the NCHRP Literature reviews were conducted for the three primary
12-70 Project with future editions of the AASHTO LRFD technical areas of the Project: retaining walls, slopes and
Bridge Design Specifications, developments resulting from the embankments, and buried structures. The goal of the literature
NCHRP 20-07 Project served as the basis when formulating review was to do the following:
analysis requirements for retaining walls, slopes and embank-
ments, and buried structures. The relevant analysis require- · Identify the state-of-the practice in each of the areas of
ments included typical levels of ground shaking and spectral consideration,
shapes for WUS and CEUS, which then defined the demand · Understand the basis for the methods being applied,
requirements for completing the design of retaining walls, including their assumptions and limitations,
slopes and embankments, and buried structures. · Investigate alternative approaches that might be adopted
While the preliminary decision on return period addressed during the development of analytical methodologies,
one critical design need for the NCHRP 12-70 Project, the · Establish some of the desirable features of analytical meth-
following additional changes regarding the earthquake design ods that should be considered for development, and
