Challenges for the Next Generation of Seismic Codes - Earthquake Engineering for Structural Design - page 452

- If it ruins good, the builder shall make compensation for all that has been
ruined and, because he did not construct properly this house which collapsed,
heshallrebuiltthehousefromhisownmeans.
- If the builder built a house and does not make its constructions meeting the
requirementandawallfallin,thatbuildershallstrengthenthewallathisown
expense.
Period of observations. This period introduced for the first time the
observations of damage that occurred during an earthquake. The effects of some
disastrous events were examined in order to learn lessons about the damage and to
elaborate some rules to prevent undesirable behaviors. The tragic 1755 Lisbon
earthquake constrained the authorities to elaborate the first building code
provisions for seismic resistance, focused on prohibiting certain types of
construction, which were observed to poorly behave in past earthquakes, and
requiring the use of given constructional details and techniques, which were
observed to provide a better performance (Hamburger, 2003). After the 1908
Messina earthquake, a special commission of practicing engineers and engineering
professors was commissioned to recommend improved constructional
requirements. The resulting report included, for the first time, recommendations for
the lateral distribution of seismic loads. Similar activities were developed in Japan,
after the 1923 Kanto (Tokyo) earthquake, when a 33m building height limit was
imposed. In the USA, it was observed that certain types of constructions
(especially unreinforced masonry buildings) consistently performed in a poor way,
so rules were developed to regulate the features of these construction types in order
to improve their performance. This period is characterized by the fact that the
regulations were strictly based on experiences learned during the earthquakes.
Early prescriptive codes. The Californian earthquakes that occurred in the early

th

20 Century (especially the 1906 San Francisco earthquake) were the reason for

the start of the early seismic code developments. The basic requirement was
introduced that structures intended to resist earthquakes must be provided with
sufficient strength to resist specific lateral forces. Therefore, the use of lateral
seismic forces in design became widely used and the code development refers to
the evaluation of these lateral forces. During the first period, these forces depended
on the building weight only, but this approach was improved in the further
development of provisions. For the first time in the 1943 provisions for the Los
Angeles City, the lateral forces were determined in function of the first vibration
mode and the response spectra of ground motion. These preliminary concepts were
based on grossly simplified physical models, engineering judgments and a number
of empirical coefficients. Influenced by the conventional design concepts, the
earthquake actions were considered as static loads and the structures as elastic
systems. This simple concept was the standard design methodology for several
decades. There are good reasons to understand the success of this design approach.
This methodology has been well understood by structural engineers, because it is
relatively easy to be implemented (Gioncu and Mazzolani, 2002).
Modern period. The next step in code development was the study of the
dynamic response of structures, leading to the shear base, being distributed through
the height of the building according to the shape of the fundamental mode. This