Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 459

Main code specifications. Regarding the design earthquakes, it was recognized
that the design rules are constantly changing via new ideas, methods and research.
The SEAOC Seismology Committee continues to review the UBC code with the
intent to improving the recommendations. Each new important earthquake
produces new provisions in UBC editions (Diebold et al, 2008):

Earthquake UBC Edition Code improvements

1971 San Fernando 1973-1976 Seismic zone 4 with increased base shear
requirements

1985 Mexico City 1988 Separation of building to avoid pounding

Design of steel columns for maximum axial
forces
Restriction for irregular structures
Ductile detailing of perimeter frames

1987 Whittier 1991 Revision of site coefficients

Narrows Revision to spectral shape

Increased wall anchorage forces for flexible
diaphragm buildings

1989 Loma Prieta 1991 Increased restrictions on chevron-braced

frames
Limitation on b/t ratios for braced frames
Ductile detailing of piles

1994 Northridge 1997 Restrictions on use of battered piles

Requirements to consider liquefaction
Near-fault zones and corresponding base shear
requirements
Revised base shear equations using 1/ T
Spectral shape
Redundancy requirements
Design for overstrength
More realistic evaluation of design drift
Steel moment connection verification by test.

One can see that the Northridge earthquake, due to the severe damage produced
generated the most important changes in the UBC provisions. Among them, the
effects of near-source earthquakes, especially for strike earthquake types, as
emphasized by the Northridge earthquake, which generated a crucial changing in
seismic design philosophy. The 1997 UBC code is the only code which directly
considers these effects, by increasing the seismic forces in function of the distance
from the fault.
NEHRP was created in 1990, with the mission:
“… to develop and promote knowledge, tools and practice for earthquake risk
reduction, through coordinated multidisciplinary, interagency partnerships among