Page 453 - Advanced Design Examples of Seismic Retrofit of Structures

Examples of Nonengineered Buildings Chapter 6 399

experiences in seismic retrofitting of adobe buildings which centers on several
years of research by GSAP are presented [5].
The objectives of seismic retrofit measures that satisfy conservation criteria
are ranked in order of importance are:

(a) provisions for life safety during the most severe earthquakes;
(b) limitation of damage to repairable levels during the most severe
earthquakes; and
(c) minimizing damage during moderate earthquakes.
It is noteworthy that different retrofit measures may be used to satisfy each of
these objectives. The life-safety objective must be ranked first, but the second
and third objectives are interchangeable depending on the goals of the decision
makers [5].
Although this part can be considered as examples of conservation of historic
buildings, the methods and approach implemented here can be easily extended
to other residential buildings of this kind.

6.4.1.2 Law Enforcement and Retrofit Measures
In California, construction of new unreinforced masonry buildings was prohib-
ited in 1933, and state law (enacted in 1986) required seismic retrofitting of
existing structures. Nowadays, most URM buildings have undergone retrofit-
ting [24]. There is particular cause for concern in regions which can generate
strong earthquakes, but only rarely. Such regions may not have regulations lim-
iting the construction of URMs, or have only implemented them recently.
For example, the Wasatch Fault in the US state of Utah closely parallels the
state’s most populous metropolitan area, the Wasatch Front. This has a popu-
lation of 2 million, and contains 200,000 URMs compared with the entire state
of California’s 25,000 [25]. Utah has recently retrofitted many public URMs to
withstand earthquakes better, but most URMs in the state are private homes.

6.4.1.3 Survey of Damage to Adobe Buildings
The 1994 Northridge earthquake offered a rare opportunity to observe the huge
loss that could occur to the remaining historic adobe buildings in California.
The field data collected following this earthquake has led to a better general
understanding of the seismic performance of adobe buildings. A total of 19 his-
toric adobe buildings were surveyed after the earthquake and the observed
structural damage were classified into several groups according to Table 6.1.
Examples of damage to the surveyed buildings are presented in Fig. 6.7. The
GSAP team also classified the severity of the different damage types in terms
of their effect on both life safety and the historic fabric of the building. More
information about this damage may be found elsewhere [26].
The extent of damage to an adobe structure subjected to an earthquake is
a function of four major factors: (a) the severity of the ground motion;