Ground Motions and Structures - Earthquake Engineering for Structural Design - page 299

Ground Motions and Structures 299

interaction and structural dynamics. The bad behavior of many foundations during the
above-mentioned earthquakes confirms the complexity of the phenomena produced
during the ground motions. The Soil Dynamics is a new branch of Soil Mechanics,
which deals with the behavior of soil and foundations under dynamic loads. Earthquake
ground motions constitute a class of these dynamic loads which usually challenges the
engineers in their design of different kinds of foundations.
One main problem of the seismic response of the foundation is related to the change
of soil properties for seismic loads. Even for static conditions, the correct evaluation of
the soil strength is a challenge for the geotechnical engineers. Additional factors must
be considered for seismic conditions, depending on the soil type, which can be
characterized as cohesive or cohesionless (Pecker, 1996):
- the rate of loading may significantly affect the value of the statically determined
strength. The modification is not very high for cohesionless soils, but can by
very important also for cohesive soils, increasing the static strength of 30% to
60%;
- the repetition of alternate cycles of loading may cause a degradation of the soil
and a subsequent decrease in its strength. Once again the cohesionless
soils are insensitive to this effect and the cohesive soils experience degradation
when they are strained beyond a given strain threshold which is soil type
dependent;
- the saturated cohesionless soils experience an increase in their pore water
pressure due to the cyclic loading, which may lead to a liquefaction condition,
unless the drainage conditions allow for a rapid dissipation of this pressure.
The seismic behavior of foundations depends on their conformation. The following
main foundation types are used in seismic areas:
- spread shallow individual or continuous foundations , used for the structures
insensible to differential settlements;
- mat foundations (plate foundations), applicable for structures sensible to the
differential settlements;
- pile foundations , used in the case of soft soils and structures sensible to
differential settlements.
In general case, the seismic forces have six components acting on the foundations:
- vertical forces, which can be very important if the seismic vertical components

are high (such in case of near-source earthquakes). Only in the far-field areas
can these components be neglected, since their magnitude is small with respect
to the static one;

- two shear forces, introducing an inclination of the resultant force. These forces

can be very important in the zones where seismic horizontal components are
dominant;

- two overturning moments, introducing an eccentricity of the resultant force and,

in some cases, the uplifting of the structure;

- torsional moment, very important in case of asymmetric structures.