New Challenges in Seismic Design - Earthquake Engineering for Structural Design - page 15

- Existence of relief and basin irregularities, knowing that they can produce
important amplification of ground motions.
- Possibility of occurrence of landslides or ground liquefaction.
- Determination of influence of traveled path and attenuation curves.
- Effects of site condition types.
- Description of spatial movements, directivity, incoherence, etc.
- Modification of ground motions due to topographic irregularities and
alluvial valleys.
- Effects of soil liquefaction.
- Establishing the main ground motion types.
- Digitalization of recorded accelerations, velocities and displacements.
- Establishing the dominant natural period of ground motions.
- Determination of ground motion duration.
- Determination of number of significant pulses.
- Characteristics of near-field and far-field earthquakes
Finally, it is very important to obtain the c haracteristics of design earthquake
for the analyzed site, by means of:
- Presentation of possible accelerogram sets.
- Maximum possible earthquake magnitude.
- Magnitude recurrence.
- Dominate natural period.
- Earthquake duration.
- Attenuation curves in function of the distance from epicenter.
One must have in mind that all information given by Engineering Seismology
has a great character of incertitude, due to the difficulties to know in detail the
crust and mantle characteristics and to transform these characteristics in
quantitative values for design purposes. The limited number of records, obtained in
the same place during different earthquakes, originating more or less from the
same source, impedes the use of statistical procedures to determine the values of
these characteristics.

1.5 EARTHQUAKEENGINEERING VERSUS STRUCTURAL
ENGINEERING

1.5.1 EarthquakeLoadsversus Static Loads

In order to understand the difficulties in defining the earthquake loads, it can be
useful to compare them with conventional loads, for instance the wind loads, both
being horizontal but physically different (Murty, 2003):
- According to the common design philosophy the basis are: the pressure on

the exposed surface areas in case of wind and the inertia forces, resulting
from the random motions of the ground at the base of structure, in case of
earthquakes (Fig. 1.5a).

- Wind forces on the structure have a non-zero component superposed with

a relatively small oscillating component. Thus under wind forces, the
structure may experience small fluctuations in the stress field, but reversal