1445
Technical Committee 203 /
Comité technique 203
Figure 5. The diagram of absolute value of displacements for grillage
slab in plane (visco-elastic model).
Forces in piles were reduced by a factor of ~3. The extreme
forces were located at the pile head (maximum compressive
reached 6.6
10
3
kN, tensile forces – 6.2
10
3
kN). But there are
piles where the maximum force was below the head. For the
majority of piles, the maximum forces were experienced during
the 15 – 20 second time period.
Preliminary calculations showed that for the given seismic
activity, oscillation magnitude for upper floors can reach around
1m, resulting in unacceptable operating conditions. In order to
reduce seismic wave impact, use of a seismic isolation system
has been advised. While such a system results in decreased
overall building rigidity, it significantly reduces the resonance
effect and helps to absorb oscillation energy by utilizing plastic
forces within the damper. In order to implement the seismic
isolation system, an additional damping layer was added into
the grillage slab. Thus, the resulting foundation design consisted
of a grillage slab, plate and damping layer. Grillage connected
piles to the bottom plate. The plate was lying on top of the
damping layer while being rigidly connected to the above-
ground constructions.
Based on the results of previous studies, it was determined
that increased local vertical stress values are primarily
associated with the presence of a weak soil layer (with increased
deformability). Conducted full-scale pile testing with shortened
piles showed sufficient carrying capacity to accept building
load. Taking into account that the building is located on the
slope which is fixed with two retaining walls from both
building’s sides (slope strengthening structures) the piles’
length was proposed to be decreased to 13 m not intersecting
weak soil layer. The undertaken researches of piles’ length
impact on construction’s stress condition corroborated this
solution.
Dynamic problems that take into account visco-elastic
plastic medium deformations while utilizing Newmark method
will need large amount of iterations to calculate stiffness matrix.
This prompts the use of direct explicit methods. In this work
Wilkins’ explicit method is used. One of the properties of the
direct methods is a very small integration time step, resulting in
a large number of iterations. However it allows the application
of a visco-elastic plastic model for all mediums without
significant algorithmic complexities or computational expenses.
Such an approach permits the stress-strain mode (state) to be
more exactly defined and research the interaction of the soil
base- foundation- above-ground construction system’s elements
while taking into account such effects as detachment of
structures from soil, consolidation of soil base etc. To describe
non-linear soil’s behaviour non-associative law of modified
Mises - Schleicher - Botkin’s criterion taking to account soil
structural strength is used (Boyko, Sakharov 2004, 2005) (Fig.
6). Model parameters can be defined with known methods.
Furthermore, soil properties measured on the actual site were
used to identify correct parameter values for the model.
Combined with dynamic viscosity with inherent parameters for
different mediums (Rayleigh’s model) this permitted to take
into account the real processes of the soil base’s deformation
when interacting with engineering structures during complicated
stressed mode and permits to project reliable and rational
structures as a result.
me
str
Figure 6. The modified criterion of Mises-Schleicher-Botkin with
structural strength (
me
,
str
– the limit of elasticity and structural
strength).
In this work, the traditional Mises - Schleicher - Botkin’s
model was supplemented with the condition of pre-boundary
plasticity and the effect of unloading for hydrodynamical
tensions.
Taking into account the unloading effect has significant
importance for solving problems with cyclical loading. The
realization presupposes than on the initial stage of loading up to
structural strength, the value of which has been taken to be 20%
more than natural hydrodynamic tensions (taking into account
the building), the soil is deformed with a modulus of elasticity,
the value of which is accepted from the results of labaratory
research to be E
el
=5
E
def
. At further load the soil is deformed
with module of deformation, which is why the shifts are
increasing. In the step of downloading the soil is deformed with
the module of downloading. Within this problem this module
was accepted as E
el
.
The shown model allows the consideration of the prelimit
plasticity, the visco-elastoplastic soil’s work and the damper’s
material and also to estimate the position of the building after
the end of the seismic load.
The specification of the work of soil and damper led to an
increase of shifts in the plane of the building. Maximal
amplitude increased from 8 to 10 sm. At the maximal amplitude
the shifts are oriented mainly along the action of the radial
component of the seismic load. At such conditions the
oscillations in the horizontal plane were close to the neutral
situation, which is why the building does not have tendencies
to horizontal shifts. The consideration of the plastic work of the
damping layer allows the calculation of the amplitude
decreasing of the oscillations of the top floors of the building.
These shifts differ significantly in character relative to the
previous calculations without the damping layer, and their
maximum value decreased 40% and became approximately 64
cm in the period of time from 15.1 to 24.5 seconds (Fig. 7).
Figure 7. The diagram of absolute value of displacements for upper
foundation slab (visco-elastoplastic model) and grillage slab (visco-
elastic model)
The process of irreversible building settlement was fixed
because of the consideration of the structure strength and the