Static structures (e.g. – buildings) – Means compensating earth-transmitted force – Relative motion means between a structure and its foundation
Reexamination Certificate
2001-06-28
2003-10-14
Friedman, Carl D. (Department: 3637)
Static structures (e.g., buildings)
Means compensating earth-transmitted force
Relative motion means between a structure and its foundation
C052S167100, C248S562000, C248S638000
Reexamination Certificate
active
06631593
ABSTRACT:
FIELD OF THE INVENTION
1. Field of the Invention
The present invention relates to directional sliding pendulum seismic isolation systems and articulated sliding assembly therefor, and more particularly, to directional sliding pendulum seismic isolation systems and articulated sliding assemblies therefore, that can reduce seismic load applied to structures, such as bridges or general buildings, through directional pendulum motion and frictional sliding.
2. Description of the Related Art
Recently, multi-span continuous bridges are widely used. In general, such a multi-span continuous bridge is designed to have a single fixed point in the longitudinal direction of the bridge.
FIG. 1
a
shows an example of the conventional multi-span continuous bridge. In the conventional 4-span continuous bridge, a fixed support
102
is installed on a fixed support pier
103
, which is located in the middle of the 4-span continuous bridge, to restrict the longitudinal movement of the superstructure
101
of the bridge. Movable supports
107
are installed on movable support piers
104
,
105
and
106
to permit free longitudinal movement of the superstructure
101
of the bridge.
FIG. 1
b
is a schematic view illustrating the deformation of the 4-span continuous bridge of
FIG. 1
a
when a seismic load is imparted thereto. Referring to
FIG. 1
b,
the seismic load is applied to the superstructure
101
of the bridge in the arrow direction “b” by an earthquake ground motion expressed in the arrow direction “Ug”. The superstructure
101
of the bridge moves in the longitudinal direction of the bridge due to the seismic load. If the frictional force is negligible at the movable supports, the seismic load imparted to the superstructure
101
of the bridge would be transmitted solely to the fixed support pier
102
through the fixed support
103
. The fixed support pier
102
provided with the fixed support
103
would withstand the whole seismic load transmitted from the superstructure
101
of the bridge, and finally be forced to deform as shown
FIG. 1
b.
If an excessive seismic load is applied to the fixed support pier
102
, the bridge itself as well as the fixed support
103
of the fixed support pier
102
will be seriously damaged, consequently resulting in possible failure of the fixed support pier
102
.
In traditional earthquake resistant design of bridges and general structures, the structural members, components and systems are required to have adequate amount strength and ductility in the event of strong earthquakes. However, the structures designed according to this strength design principle tend to experience severe damage or excessive deformation in the event of very strong earthquake even though they may not collapse. Therefore alternative methods have been developed that can protect structures from earthquakes within predetermined deformation limit. One of the most widely used protection methods is seismic isolation system. Because it has been proved to be very effective in the reduction of seismic load in recent earthquakes, the use of seismic isolation systems is on an increasing trend.
The basic principle of the seismic isolation system will be explained in connection with the earthquake actions. However, the seismic isolation systems according to the present invention are not restricted to the earthquake motion, and can be applied also to various kinds of dynamic loads applied to the structures.
If a structure
201
is fixed to the ground
202
as shown in
FIG. 2
a,
it can be modeled as a single degree of freedom system as shown in
FIG. 2
b.
The response of the structure to the earthquake action, such as base shear force and relative displacement can be estimated using response spectra.
FIGS. 2
c
and
2
d
show graphs of acceleration response spectra and graphs of displacement response spectra respectively as examples. The drawings show response spectra for two values of damping ratio. In the graph of
FIG. 2
c
, the vertical axis indicates the spectral acceleration and the horizontal axis indicates the period. In the graph of
FIG. 2
d
, the vertical axis indicates the spectral displacement and the horizontal axis indicates the period. The base shear force acting between the structure and the ground by the horizontal ground motion can be estimated from the acceleration response spectrum shown in
FIG. 2
c
. That is, if the natural period and the damping ratio (&xgr;
1
or &xgr;
2
) of the single degree of freedom are given, the spectral acceleration is read from the curves shown in
FIG. 2
c
. If the obtained spectral acceleration value is multiplied by the mass of the structure, the base shear force is approximately found.
The relative displacement between the superstructure and the ground can be estimated from the displacement response spectrum shown in
FIG. 2
d
. If the natural period of the single degree of freedom and the damping ratio are given, the spectral displacement is read from the curves shown in
FIG. 2
d
. The obtained spectral displacement shows the relative displacement of the ground of the single degree of freedom.
As can be seen from the graph shown in
FIG. 2
c
, generally, if the period becomes longer, the spectral acceleration is reduced. Moreover, in the same period, if the damping ratio becomes larger, the value of the spectral acceleration is reduced.
In the case of the spectral displacement, as can be seen from the graph shown in
FIG. 2
d
, if the period becomes longer, the relative displacement is increased. Furthermore, in the same period, if the damping ratio becomes larger, the value of the spectral displacement is reduced.
In conclusion, if the period is longer and the damping ratio is higher, the spectral acceleration is reduced, and thereby the seismic force, i.e., floor shear force, becomes small. The seismic isolation systems adopt the above mechanical principle. For example, the seismic isolation system such as a high damping lead rubber bearing has mechanical properties that the horizontal stiffness is very small but the damping capacity is high.
As shown in
FIG. 3
a
, if a seismic isolation system
203
is installed between the base frame and a ground
202
, the natural period of the whole structural system becomes even longer, and also the damping ratio increases. Like this, if the natural period T becomes longer period T
e
or the damping ratio &xgr; is increased to a ration &xgr;
e
, then the seismic force can be reduced significantly, as can be seen from the graph shown in
FIG. 3
b.
However, as shown in
FIG. 3
c
, if the natural period becomes longer, the relative displacement increases. To restrict the increase of the relative displacement, dampers can be installed in addition to the conventional seismic isolation system having low damping capacity. One of the seismic isolation systems having high damping capacity and the long natural period, which do not require the additional dampers, is a sliding pendulum seismic isolation system. However, the sliding pendulum seismic isolation system used presently has a structure that a slider moves on a dish having a concave surface, and therefore if the seismic isolating period becomes longer, the diameter of the dish becomes even larger. In the case of bridges, generally, an area to install a seismic isolator on a pier or an abutment is extremely restricted. Therefore, a long span bridge requiring the seismic isolating period of a long-term has a difficulty in using the conventional sliding pendulum seismic isolation system of the dish type.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a sliding pendulum seismic isolation system having a new configuration, which can be easily installed without limitations in an installation area.
It is another object of the present invention to provide a sliding pendulum seismic isolation system, which does not use dampers additionally employed in a conventional seismic isolation system that has low damping capacity.
It is a further object of the present invention to provide a sliding pendulum seismic
Friedman Carl D.
Glessner Brian E.
Head Johnson & Kachigian
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