Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 458

using the wind force provisions. It was only in 1927 that the Structural Engineers
of California (SEAOC) developed the Recommended Lateral Force Requirements
(known as the Blue Book) in the frame of the UBC. Here, the concept of lateral
forces proportional to the mass was firmly introduced into practice. In addition to
promulgating lateral force levels, seismic design regulations asserted that the
design lateral forces must be applied in two orthogonal directions and distributed to
all floor levels in proportional way (Diebold et al, 2008). Strong motion
accelerogram data were recorded during the 1940 El Centro earthquake. The El
Centro ground motion data were used as basis for seismic force levels introduced
in the further codes for many years. In 1943 Los Angeles recognized the influence
of the flexibility of a structure on the amount of the earthquake design forces. San
Francisco engineers developed a relationship, stating that seismic forces are
inversely proportional to the structure period. In 1971 SEAOC created the Applied
Technology Council (ATC) as an independent organization for improving design
practice and codes. The first results of this new organization were the 1978 UBC,
the 1988 UBC and the 1988 Blue Book. Later on, the ATC was incorporated into
the national model building codes as the National Earthquake Hazard Reduction
Program (NEHRP), which is now the prompter of development of seismic
regulations for new buildings. This code is elaborated under the patronage of an
agency created in 1979, the Federal Emergency Management Agency (FEMA).
The primary purpose of this agency is to coordinate the response to all disasters
occurring in the USA. The American Institute of Steel Code was published in 1990
for steel structures and similar codes were elaborated for other structural materials.
All the codes were published by the Building Officials and Code Administration
(BOCA).
After 1990, especially after the 1989 Loma Prieta and the 1994 Northridge
earthquakes, the improvement of the code provisions continued. In 1994, three
model codes were issued by the International Code Council (ICC). In 2000, the
ICC issued the first set of comprehensive and coordinated codes for the built
environment, known as the International Building Codes (IBC) series. In addition
to these code series, there are other professional societies or institutions, which
elaborated their general or specialized codes. The situation of the existing codes is
the following:
- Uniform Building Code (UBC), elaborated by SEAOC, which, historically
speaking, was the earliest seismic design provision in USA. During the last
period, the editions of 1991, 1994 and 1997 must be mentioned. Traditionally,
the UBC code is used in the Western States. In California, this code is used as
Californian Building Code (CBC).
- National Earthquake Hazard Reduction Program (NEHRP) provisions,
elaborated by FEMA, with the editions of 1994, 1997, 2000, 2003 and 2009.
- InternationalBuildingCode (IBC), elaborated by ICC on the basis of the 2000
NEHRP provisions, with the editions of 2000, 2003 and 2006.
The evolving process in the USA codification is now oriented to have only one
seismic standard implemented everywhere in all the states, without any exception
(Bonneville and Bachman, 2002, Gioncu, 2006).