Every time an earthquake affects a contemporary city, numerous lessons arise regarding the performance of modern buildings. However, lessons included in international postearthquake reconnaissance reports regarding the influence of architectural features on building seismic performance barely reach either architectural and city planning practice, or decisions taken by city officials and politicians that continue including in the design of urban zoning regulations (UZR) some modern building configurations categorized in seismic codes as “irregular.” Irregularities, in terms of building configuration, mean that the structural design and analysis require the application of special considerations, as well as rigorous official controls on the construction site for the appropriate application of seismic detailing requirements.
With the endorsement of Modernist individual building and the laisser faire doctrine (let each one to do what she/he wants), UZRs relaxed. Now each modern building is placed in the existing traditional block as a structurally independent unit as in Fig. 3. However, several problems arise when neighboring buildings are not taken into consideration.
If neighboring buildings’ load resisting system consists of load bearing shared walls, special care has to be taken not to weaken the lateral force strength of those buildings when they are split by a property line, and consideration must be made of the effects that those existing structures can produce in the new one, and the new one on the existing ones (Fig. 4).
Most UZRs encourage the use of soft story irregularity because the area enclosed by a soft first story is rewarded to the owner since it is neither computable as part of the maximum allowable built area, nor for tax control, but they are computable for selling purposes. These areas at the first story are mostly used for social events, parking garages, and other activities that require layouts free of walls.
Arnold and Reitherman  comment that until 1973 the Uniform Building Code (UBC) didn’t include any seismic regulation or recommendation for irregular configurations. In the 1970’s, due to the destructive effects produced by recent earthquakes in modern buildings (Anchorage, 1964; Caracas, 1967; San Fernando, 1971; Managua, 1972; Guatemala City, 1976; Imperial Valley, 1979), a group of architects in California participated in significant studies with earthquake engineers, to promote the inclusion in seismic codes of some special recommendations for the design and construction of buildings with modern architectural configurations. Also, several articles and books were published with recommendations for architectural design in seismic zonas.
However, until 1974 building codes didn’t include special provisions for buildings with irregular configurations. In 1975 SEAOC included in Recommended Lateral Force Requirements and Commentary of the Blue Book 1974 twenty examples of irregularities that had to use dynamic analysis methods instead of the traditional equivalent static force method and recommended them to be included in the next version of UBC. These recommendations were not considered for the 1976 UBC. In 1978 SEAOC published the ATC306: Tentative Provisions for the Development of Seismic Regulations for Buildings that included a section on Building Configuration with a series of drawings that were supposed to be included in the 1979 UBC but in the end, were not. At last, after the Mexico earthquake of 1985, the 1988 UBC edition included two tables defining some parameters for the identification of “irregular” configurations, in plan and elevation.
Since then many recent seismic codes around the world have included special provisions for the following configurations: Plan: torsional irregularity; extreme torsional irregularity; reentrant corners; diaphragm discontinuity; outofplane offsets; nonparallel systems. Vertical: stiffness irregularity – soft story; stiffness irregularity – extreme soft story; weight (mass) irregularity; vertical geometric irregularity; inplane discontinuity in vertical lateralforce resisting elements; discontinuity in capacity – weak story; extreme weak story.
 Allan Lavell, “Local Level Risk Management. Concepts and Expe-
rience in Central America,” ponencia presentada en Disaster Pre-
paredness and Mitigation Summit, 21-23 de noviembre de 2002,
Nueva Delhi, India, pp. 2-3.
 Kenneth Frampton, “Modern Architecture: A Critical History,”
Thames & Hudson; 4th. Edition, London 2007.
 R. Reitherman, “The Effects of the 1906 Earthquake in California
on Research and Education,” in Earthquake Spectra, April 2006,
22:S2, S207-S236, Oakland, CA, 2006.
 G. V. Berg, “Seismic Design Codes and Procedures,” EERI, Berke-
ley, CA, pp. 23-24, 1983.
 Ch. Arnold, and R. Reitherman, “Building Configuration and Seis-
mic Design,” John Wiley-& Sons, Inc., New York, 1982.