Learning from Earthquakes - Earthquake Engineering for Structural Design - page 82

3.3LESSONSFOR CONSTRUCTION VULNERABILITY

During the last important earthquakes, the difference between good and poor
building performances can deliver many lessons regarded from different points of
view (McClure, 1989, Mazzolani, 2002, Gioncu and Mazzolani, 2002, 2003).
The analysis aspects refer to the lessons for improving the analysis
methodologies.
- For a proper seismic design the estimation of seismic actions must
consider all characteristics of earthquakes: interplate, intraplate, intraslab,
near-field or far-field position and site influence, in function of site
position related to the source.
- Incorporating in the analysis the checking of rigidity, strength and ductility
is a measure, which mostly affects the code attitude to reduce the damage
and to prevent collapse.
- Special distinction must be made between required values (resulting from
earthquake characteristics) and available capacity (function of the structure
features).
- Designing structures using code provisions does not always safeguard
against damage in case of exceptional earthquakes, because they refer to
normal earthquakes only. Therefore, an improved robustness (overall load
bearing capacity which a structure is able to provide) must be provided to
the structure when such exceptional earthquakes are foreseeable in the
building site.
- Exceptional earthquakes can be defined as the ones not considered in the
codes, or considered in a wrong way, but also when the actual intensity of
the earthquake is higher than the design one. After this definition, the
majority of damaging earthquakes can be framed into this category
(Mazzolani, 2002).
- In case of exceptional earthquakes, the design code does not safeguard the
structure against excessive plastic deformations. Potential failure modes
must be identified and ductility should be provided at all locations where
plastic deformations occur. Therefore, detailing for ductility and
redundancy provide safety against collapse.
- Stiffness and strength of some secondary structural elements (e.g.
staircases) and non-structural elements (e.g. stiff infilled walls), which are
not considered as a part of the lateral force-resisting system, may strongly
affect the seismic response of buildings, especially in case of uncontrolled
distribution of such elements.
- Buildings that experience successive earthquakes may suffer progressive
weakening or eventual collapse. From the 33 high buildings collapsed in
Bucharest during the 1977 Vrancea earthquake, 31 were partial damaged
during the 1940 earthquake.

- Traditional code provisions consider methodologies based on seismic

forces, but in the last period the interest seems to move towards
methodologies based on seismic displacements.