Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 457

Because it is not possible to present all details of the analyzed code provisions,
only the development, the basic philosophy and the main characteristics will be
reported.
10.2.2 United States of America
The presentation of the progress in seismic design codes starts with the USA,
because the evolution of the world's codes was strongly influenced by the
development of the USA code. The Uniform Building Code (1997) provisions have
been recognized throughout the world as the leading reference for the design of
earthquake resistant structures.
Seismological aspects. The territory of the USA is characterized by two very
distinct seismic zones. So, the USA code must cover two different seismic
demands for structural design. Intraplate (inland) crustal earthquakes shake the
Eastern part, where the largest earthquake ever to hit the USA was the 1811-1812
New Madrid (Tennessee) one, with the magnitude M 8.6, due to the presence of an
important rift system. The very high magnitude of these earthquakes, unusual for
the intraplate faults, is due to very bad soil and liquefaction. There are also some
intraplate crustal earthquakes produced in South Carolina, but the most active
seismic regions are located along the Western shore of the country, where the
Pacific and North American tectonic plates meet. Due to the almost complete
oblique subduction of the Farallon plate under the North American plate (only two
corners remain from this old tectonic plate, the Juan de Fuca plate at North and the
Cocos plate at South), a system of strike fault lines has been developed. Some of
these faults are known to be very active (such as the famous San Andreas fault),
while others are presumed to be inactive, but they can give unexpected surprises
(s.g. the Northridge earthquake, produced in an unknown fault belonging to the
Los Angeles fault system).
Along the Alaska coast, the earthquake types are subduction-thrust, produced
by crustal and intraslab faults. The majority of the sources are situated in shallow
crust.
Considering all these aspects, the dominant earthquake types are situated in
shallow crust, so the near-source effects are dominant for structural design.
Code evolution. It was strongly influenced by the last Californian earthquakes.
The USA code system is a very special one, because each of the 41 states
constitutionally has its jurisdiction for the regulation of construction. The evolution
of codes is unique in the sense that the different organizations, agencies or
institutions, on volunteer bases, are autonomously producing their codes, so that
there are so many code models. Until 2000, each government usually adopted a
model building code by ordinance, choosing one of the existing model codes,
elaborated and published by private organizations (Bonneville and Bachman, 2002,
Mahoney, 2002, Ghosh, 2002).
The development of codes for seismic-resistant constructions has been evolving
over a number of years and it is applicable to all materials (Popov, 1991).
Californian engineers and legislators were the initiators in the USA of the ever-
improving seismic codes. San Francisco was rebuilt after the 1906 earthquake