The present invention relates to a load dissipating and limiting element, which has been specifically designed for antiseismic devices and apparatus for bridges, viaducts, buildings and the like.
In prior Patent Applications in the name of the same Applicant (Italian Patent Applications No. 19538 A/86 and No. 19539 A/86 filed on February 26, 1986), there are illustrated antiseismic devices comprising specifically designed pivot pins, shown in figures 1 and 2, which are substantially provided, at one end thereof, with a cylindric portion to which there is coupled a variable cross-section portion tapering toward the opposite end where a ball shaped portion ends.
The pivot pins, restrained in the cylindric portion, are deformed so as to be bent according to a given curvature, under the action of seismic forces applied to the ball shaped portion.
These embodiments provide multiple advantages, for example a comparatively reduced cost, a simple construction, reduced size, as well as multidirectional characteristics.
On the other hand, the mentioned pivot pins allow only rather limited sliding movement to occur, which drawback can be at least partially removed by using the arrangements shown in figures 3, 4 and 5, by diagrams which are statically and dynamically equivalent.
As it should be apparent, these constructions, while being operative, are very complex and expensive.
The document IT-A-19538/86 (or 19539/86) discloses a load dissipating and limiting element for antiseismic devices having the features of the pre-characterizing portion of claim 1.
Accordingly, the aim of the present invention is to solve the above mentioned problem by providing a dissipating element which, owing to its specifically designed construction, affords the possibility of overcoming all of the difficulties associated with the mentioned limited sliding movements.
In fact, as is known a seismic event generates abrupt displacements of the soil and, accordingly, abrupt accelerations which, as they are transmitted through foundation assemblies, will generate inertia forces because of the involved large masses forming the bridge, viaduct, building and the like construction.
Also known is that, by using suitable de-coupling devices, as properly arranged, it is possible to reduce the inertia forces.
For example, in the case of bridges and viaducts, resilient restraining means have been adopted, which define a swinging element system, practically operating has a spring for the construction mass which has a proper resonance frequency which can be displaced away from the characteristic frequencies of the seismic events so as to greatly reduce the involved forces.
However, in the characteristic curve of the above mentioned restraining elements, after a first portion of proportionality between the stresses and strains, follows an abruptely raising portion thereby it is not possible to "control" the maximum force involved.
For solving this problem there have been designed further antiseismic restraining elements in which the above mentioned proportional portion is followed by a curve portion in which the force is independent from the strain, that is these elements have a substantially "plastic" performance.
In particular, these elastic-plastic performance devices allow the possibility of reducing as well as controlling the maximum inertia forces transmitted by the restrained masses (such as a viaduct mass) to the supporting constructions (for example the pile and shoulder construction), with self-evident advantages in the case of a high intensity seismic event.
From a dynamic standpoint, the use of horizontal restraining elements, with substantially elastic-plastic characteristics, corresponds to the introduction, near a series type of "spring", of a power dissipating element provided with a very high power dissipating capability and which, owing to its plastic deformations, provides a very advantageous ductile performance.
Accordingly, by the above mentioned dissipation, it is possible to obtain a great reduction, under a high intensity seismic event, of the spectrum response, with respect to the accelerations of the overall construction system, with a consequent great reduction of the stresses on the supporting structures.
In this connection it should be pointed out that the antiseismic devices have been mainly used in the road and railway bridges since the present technique is to made the latter with a continuous truss type of construction.
To the foregoing it is to be added that it is not possible to define the main direction of the seismic movements, thereby it will be necessary to designe multidirectional antiseismic devices.
Within the scope of the above mentioned aim, a main object of the present invention is to provide a load dissipating and liming element which can be directly associated with the supporting structures and which allows to easily make antiseismic devices, differentiated from the supporting structures or apparatus, and adapted to provide an efficient restraining system exclusively on a horizontal plane.
Another object of the present invention is to provide such a load dissipating and limiting element which is very simple construction-wise and can be easily made and assembled with a comparatively reduced cost.
According to the present invention, the above mentioned aim and objects, as well as yet other objects, which will become more apparent hereinafter, are achieved by a load dissipating and limiting element, for antiseismic devices and apparatus for bridges, viaducts, buildings and the like, having the features of claim 1. Further andvantageous features are claimed in the dependent claims.
Further characteristics and advantages of the present invention will become more apparent hereafter from the following detailed description of a preferred, though not exclusive, embodiment of a load dissipating and limiting element, according to the invention, which is illustrated, by way of an indicative but not limitative example, in the accompanying drawings, where:
With reference to the figures of the accompanying drawings, the load dissipating and limiting element, which has been particularly designed for antiseismic devices and apparatus for bridges, viaducts, buildings and the like according to the invention, comprises an elongated body, overally indicated at the reference number 1, which, advantageously, is made of a steel material having a high ultimate elongation characteristic (elongation at rupture).
At the end portions thereof, the body 1 is provided with restraining elements which, preferably though not necessarily, comprise cylindric portions 2.
At the central portion thereof, each load dissipating and limiting element, which will be more simply called "spindle" is provided with a double tapering region 3 converging from the ends toward the central portion thereof.
Each spindle will be designed so as to provide the desired displacements and reactions, and its shape is so designed as to use, under plastic deformations, the main portions of the material forming the stem thereof, that is the tapering region thereof, so as to provide the desired ductile performance.
The "spindles" are restrained by means of the end restraining elements 2 to the elements or plates 10 of the antiseismic devices or supporting constructions subjected to a relative displacements during the seismic event.
One of the two restraining elements can be withdrawn in order to allow the above mentioned relative displacements of the plates, without causing the spindle stem to be elongated.
The cross-sectional shape of the stem can be circular, in which case an isotropic performance will be obtained with respect to the displacements and reactions, or any desired shapes (for example an elliptical shape) in which case it will be obtained an anisotropic type of performance, while preserving the important features of a multi-directional characteristic.
Figures 9 and 10 show an antiseismic device made by using the above disclosed spindles, and comprising two end plates 10 restraining said spindles.
More specifically, the bottom plate is anchored to the understructure by means of restraining elements 11, whereas the top plate is coupled to the overstructure through a cup element 12 in turn anchored by restraining elements 13.
The mutual movement under the structure and above it will deform the spindles 1 and the provision of the cup element 12 allows to absorb the displacements toward one another of the end plates, which displacements is produced by second order effects deriving from the spindle deformations, without subjecting the latter to any tension stresses.
As stated, the same effect can also be obtained by using sleeve removable restraining elements.
In the embodiments shown in figures 9 and 10 there is provided a circular geometry; of course any arrangements of the spindles will be possible, for example a square geometry, rectangular geometry and the like.
The use of the above disclosed spindle provides a solution equivalent to that of the prior art shown in figures 3 to 5 with much simpler constructional means.
In fact, in the prior art diagram, the top and bottom hinges will rotate, by symmetry, for the same amount, under the effect of the displacements of the two end plates; likewise, the load dissipating and limiting element according to the invention provides the same effect since it is adapted to increase the mutual displacements, the length being the same, with respect to the pins and with the same stiffness of the system.
To the foregoing it is to be added that the device according to the present invention can be also provided with other means, such as set-breakage small keys, in order to remove from the characteristic curve the resilient portion, as well as with plungers or other hydraulic means for absorbing the slow movement (for example due to thermal effects) without stressing the spindles.
The spindle elements according to the present invention can be applied to any supporting apparatus (for example made of neoprene, steel, polytetrafluoroethylene and so on) in a like way to that shown in the above mentioned patent applications IT-A-19538/86 and 19539/86.
The invention as disclosed is susceptible to several modifications and variations all of which will come within the scope of the invention as defined in the appended claims.
In practicing the invention, the used material, as well as the specific size and shapes can be any according to requirements within the scope of the claims.