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Understanding the role of the geotechnical and structural withstand about 0.12g pseudo-static loading, based on a
engineers is rather important, and this Project needed to clar- very conservative capacity associated with first yield, with
ify these roles in the process of preparing the LRFD specifica- the most conservative assumption on wave scattering
tions. These roles also need to be understood in the definition (that is, 1.0 as discussed in Chapter 6), and the most con-
of load and resistance factors to use during design. Since in- servative nonyielding structural performance criteria.
dependent groups often are responsible for the design elements, 3. Under a less conservative interpretation, more suitable for
each group needs to have a basic understanding of what is correlating to historical structural damage from past earth-
being conveyed by the load or resistance factor that is being quakes, the inherent capacity is likely to be much higher, to
used for seismic design. a PGA at the ground surface as high as 0.68g. This case cor-
responds to a scattering factor (see Chapter 6) equal to 0.5,
1.2.1.3 Example of LRFD Reserve Capacity Concept and nominal yielding is allowed.
4. Even for a nonyielding limit state, a scattering factor equal
In formulating the LRFD guidelines, consideration needs
to 0.5 can be justified for most design situations, espe-
to be given to a prevalent consensus among practitioners, es-
cially for much of the central and eastern United States
pecially in state highway departments, that retaining walls,
(CEUS), where the characteristic ground shaking has
slopes and embankments, and buried structures generally have
lower, long-period ground motion content. In this situ-
performed very well during seismic events--even though many
ation the retaining wall can withstand a site-adjusted
constructed structures have not been designed for the earth-
PGA of 0.24g.
quake load case. The main reason for this relates to the fact
that the capacity of most retaining walls, slopes and embank- For the 1,000-year return period ground motion criterion
ments, and buried structures provides sufficient reserve to re- that was adopted by AASHTO in July of 2007, most regions
sist some level of earthquake loading when they are designed in the CEUS, other than the New Madrid and the Charleston
for static loading. This observation needed to be kept in mind regions, will be required to design for a PGA at the ground
when formulating the LRFD specifications in order that the surface of about 0.1g or lower. For much of the Western
proposed approach was determined to be reasonable to engi- United States (WUS), outside of California, Alaska, and the
neers using the methodology. Pacific Northwest, design would be for a PGA at the ground
As an illustration of this point, Dr. Lee Marsh, who served surface of about 0.2g. Based on the above cited reserve struc-
on the Technical Advisory Panel for the NCHRP 12-70 Project, tural capacity study, along with results from dynamic analy-
quantified the level of reserve structural capacity for a hypo- ses of retaining walls, many of the regions in the CEUS and
thetical wall, to put the design process in perspective. In the WUS can use simplifying screening criteria to eliminate the
course of a design, retaining walls are designed for global and need for overly complicated seismic analyses.
external stability (that is, the process of checking for sufficient
soil capacity for the global system), as well as for internal stress 1.2.2 Overview of Conclusions
in the structural components. Dr. Marsh conducted a set of from Initial Phase of Work
analyses to determine the reserve structural capacity for a
standard wall that had been designed for a static load condi- The initial phase of work involved Tasks 1 through 5 of the
tion. For simplicity, Dr. Marsh conducted the analyses for a Working Plan. A number of conclusions were reached in this
nongravity cantilever sheet pile wall to focus on structural in- early work, and these conclusions formed the framework for
tegrity issues, rather than involving additional complexity the work plan that was implemented in Task 6 and reported
associated with other nonstructural failure modes such as in the 1st Interim Report. Highlights from Tasks 1 through 4
sliding failure through the soil at the base of a semi-gravity are summarized here:
wall. Such mechanisms introduce an additional load fuse which
might further reduce the earthquake design load to a lower · Task 1: Data Collection and Review. The conclusions from
value than the case associated with sheet pile walls. Results of this task were that the methodologies available to design
these analyses are included in Appendix B. professionals within departments of transportation (DOTs)
The sensitivity study conducted by Dr. Marsh indicates the and consultants for the DOTs are primarily limited either
following: to pseudo-static methods, such as the Mononobe-Okabe
(M-O) method for the design of retaining structures and
1. Most existing retaining walls, even when they only are de- the limit equilibrium method of slope stability analysis, or
signed for static loading, have sufficient reserve structural ca- to simplified deformation methods (for example, New-
pacity to withstand an appreciable level of earthquake load. mark charts or analyses). Although these methods have
2. If a retaining wall has been designed to satisfy typical re- limitations, as discussed in later chapters of this Draft Final
quirements for static loading, the inherent capacity will Report, improvements in these methodologies still offer
