Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 460

the NEHRP agencies and their stakeholders, that improve the Nation's earthquake
resilience in public safety, economic strength and national security” (NEHRP,
2008).
The NEHRP agencies have established three long-term strategic goals:
(i) Improve the understanding of earthquake processes and impacts.
(ii) Develop cost-effective measures to reduce earthquake impacts on
individuals, the built environment and society-at-large.
(iii) Improve the earthquake resilience of communities nationwide.
Therefore NEHRP, after its creation, became to be the most active agency in
mitigation of earthquake effects in the USA. The NEHRP 2003 recommended
provisions are based mainly on the UBC provisions, but with some important
improvements. So, due to the difficulties to determine the distance of site from the
fault, the NEHRP 2003 code eliminates the UBC provisions to increase the near-
source accelerations. In exchange, a seismic hazard map of the USA was
elaborated. For structural design, national maps of earthquake shaking hazard
provide essential information for determining the maximum accelerations, even for
near-source events. The main problem of the USA codes is the definition of the
maximum accelerations for Eastern and Western seismic areas with the two very
different source types: interplate (strike-slip) and intraplate ones. For the second
source, there is an important set of data, which is useful for the definition of the
design acceleration without problems. Contrary, for the first source the data are
scarce, due to infrequent occurrence of these earthquakes, so the definition of the
design acceleration is very difficult and other criteria must be used. The intraplate
faults give rise to low or moderate earthquakes, but in some exceptional cases
(possible to be detected only for long return periods), the magnitude can be higher.
These important differences give many problems in defining the design
accelerations for the two seismic zones of the USA. In 1997 the NEHRP
provisions stated that the design earthquake corresponds to one with a 2%
probability of exceeding in 50 years (a return period of about 2500 years). But for
Eastern USA, the design earthquake is defined as 2/3 of the maximum considered
earthquake. The use of an earthquake with a return period of 2500 years as the
basis for the design has created several criticisms, because the seismic hazard in
various USA cities is different. It is well known that great uncertainty is involved
in the estimation of ground motion with a long period (e.g. 2500 years). Thus, it is
not appropriate to use such an earthquake as a basis for the design of buildings.
Since seismologists have more confidence in the estimation of ground motion from
earthquakes with shorter return periods, it should be more reasonable to use such
an earthquake as the basis for defining the design earthquake. Therefore, it is
proposed that the buildings situated in Eastern USA should be designed for
serviceability against a design earthquake with a short return period, which can be
estimated with less uncertainty. Then, the building should be provided with
sufficient ductility to resist large infrequent earthquakes (Hwang, 2000). Due to
these criticisms, one of the tasks for elaboration of the 2009 NEHRP provisions
was the reassessment of hazard curves especially in low and moderate seismic
areas (e.g. is a return period of 2500 years too long?) (Kircher et al, 2008).