CA1160650A - System for protecting a body from motions transmitted through the ground - Google Patents
System for protecting a body from motions transmitted through the groundInfo
- Publication number
- CA1160650A CA1160650A CA000380290A CA380290A CA1160650A CA 1160650 A CA1160650 A CA 1160650A CA 000380290 A CA000380290 A CA 000380290A CA 380290 A CA380290 A CA 380290A CA 1160650 A CA1160650 A CA 1160650A
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- Canada
- Prior art keywords
- eigenfrequencies
- isolators
- construction
- fundamental
- body according
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
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- E—FIXED CONSTRUCTIONS
- E04—BUILDING
- E04B—GENERAL BUILDING CONSTRUCTIONS; WALLS, e.g. PARTITIONS; ROOFS; FLOORS; CEILINGS; INSULATION OR OTHER PROTECTION OF BUILDINGS
- E04B1/00—Constructions in general; Structures which are not restricted either to walls, e.g. partitions, or floors or ceilings or roofs
- E04B1/62—Insulation or other protection; Elements or use of specified material therefor
- E04B1/92—Protection against other undesired influences or dangers
- E04B1/98—Protection against other undesired influences or dangers against vibrations or shocks; against mechanical destruction, e.g. by air-raids
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- E—FIXED CONSTRUCTIONS
- E04—BUILDING
- E04H—BUILDINGS OR LIKE STRUCTURES FOR PARTICULAR PURPOSES; SWIMMING OR SPLASH BATHS OR POOLS; MASTS; FENCING; TENTS OR CANOPIES, IN GENERAL
- E04H9/00—Buildings, groups of buildings or shelters adapted to withstand or provide protection against abnormal external influences, e.g. war-like action, earthquake or extreme climate
- E04H9/02—Buildings, groups of buildings or shelters adapted to withstand or provide protection against abnormal external influences, e.g. war-like action, earthquake or extreme climate withstanding earthquake or sinking of ground
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- E—FIXED CONSTRUCTIONS
- E02—HYDRAULIC ENGINEERING; FOUNDATIONS; SOIL SHIFTING
- E02D—FOUNDATIONS; EXCAVATIONS; EMBANKMENTS; UNDERGROUND OR UNDERWATER STRUCTURES
- E02D27/00—Foundations as substructures
- E02D27/32—Foundations for special purposes
- E02D27/34—Foundations for sinking or earthquake territories
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- Engineering & Computer Science (AREA)
- Architecture (AREA)
- Environmental & Geological Engineering (AREA)
- Structural Engineering (AREA)
- Business, Economics & Management (AREA)
- Emergency Management (AREA)
- Civil Engineering (AREA)
- Electromagnetism (AREA)
- Physics & Mathematics (AREA)
- Life Sciences & Earth Sciences (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Mining & Mineral Resources (AREA)
- Paleontology (AREA)
- General Engineering & Computer Science (AREA)
- Buildings Adapted To Withstand Abnormal External Influences (AREA)
- Vibration Prevention Devices (AREA)
- Foundations (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A mechanically fully isolated body is connected by mechanical isolators, behaving elastically in all directions, with its basis. This basis is linked firmly to the surrounding formation. There exists a frequency range called hole of eigenfrequencies between the highest of the six lowest system eigenfrequencies - called funda-mental eigenfrequencies - and the lowest of all higher eigenfrequencies - called upper eigenfrequencies - of the oscillatory element consisting of the body and the isolators. In this range the oscillatory element presents no eigenfrequencies. The hole of eigenfrequen-cies shall cover the range of resonance of the relevant design response spectrum of the excitation. By these measures the body resists to the strongest earthquakes measured or to be expected at that site.
A mechanically fully isolated body is connected by mechanical isolators, behaving elastically in all directions, with its basis. This basis is linked firmly to the surrounding formation. There exists a frequency range called hole of eigenfrequencies between the highest of the six lowest system eigenfrequencies - called funda-mental eigenfrequencies - and the lowest of all higher eigenfrequencies - called upper eigenfrequencies - of the oscillatory element consisting of the body and the isolators. In this range the oscillatory element presents no eigenfrequencies. The hole of eigenfrequen-cies shall cover the range of resonance of the relevant design response spectrum of the excitation. By these measures the body resists to the strongest earthquakes measured or to be expected at that site.
Description
~ ;1 6~650 .
SYS~EM FOR PROTI:CTING A BODY FROM MOTIONS TRANSMITTED THROUGH
TH~: GROUND
BACKGROUND OF THE INYENTION
The invention refers to a mechanically in all directions isolated body, in particular a con-struction, machine or isolator station, which is connected to its base by mechanical isolators behaving elastically in all directions. The base is linked firmly to the subsoil.
By appropriate technical measures the body resists without damage the strongest earth-quakes ever measured or to be expected at a given site: Integral Earthquake Protection. By that is meant the ability of the protected part of the body to resist repeatedly extreme seismic excita-tions without elasto-plastic deformations of the structural frame.
It is a continuous concern of modern architecture to construct safely in seismic regions.
No satisfactory results could be produced up to now after years of intense research and develop-ment, and it has not been possible until now to protect structures completely against extreme earthquakes under all conditions. Of course, several new proposals of aseismic systems have been known in the last twenty years. One of these proposals deals with the destruction of energy by means of a vibration absorber installed in the top slab of the construction (Wirsching -P.H., Campbell G.W.: "Minimal structural response under random excitation using the vibra-tion absorber"; International Journal of Earthquake Engineering and Structural Dynamics~ Vol.
SYS~EM FOR PROTI:CTING A BODY FROM MOTIONS TRANSMITTED THROUGH
TH~: GROUND
BACKGROUND OF THE INYENTION
The invention refers to a mechanically in all directions isolated body, in particular a con-struction, machine or isolator station, which is connected to its base by mechanical isolators behaving elastically in all directions. The base is linked firmly to the subsoil.
By appropriate technical measures the body resists without damage the strongest earth-quakes ever measured or to be expected at a given site: Integral Earthquake Protection. By that is meant the ability of the protected part of the body to resist repeatedly extreme seismic excita-tions without elasto-plastic deformations of the structural frame.
It is a continuous concern of modern architecture to construct safely in seismic regions.
No satisfactory results could be produced up to now after years of intense research and develop-ment, and it has not been possible until now to protect structures completely against extreme earthquakes under all conditions. Of course, several new proposals of aseismic systems have been known in the last twenty years. One of these proposals deals with the destruction of energy by means of a vibration absorber installed in the top slab of the construction (Wirsching -P.H., Campbell G.W.: "Minimal structural response under random excitation using the vibra-tion absorber"; International Journal of Earthquake Engineering and Structural Dynamics~ Vol.
2, 1974). Others propose to put the superstructure of the construction on horizontally moving roller bearings with elastic elements building up elastic return forces (Matsushita K., Izumi M.:
Studies on mechanismes to decrease earthquake forces applied to buildings"; Proceedings of the 3rd World Conference on Earthquake Engineering, London, 1965). Finally, there are pro-posals to suspend the superstructure in order to separate it from the direct influence of ground excitations (Oto Lanios C.J. et al.: "Study of the behavior of a hanging building under the effect of an earthquake"; Proceedings of the 4th World Conference of Earthquake Engineering, Santi-ago de Chile, 1969).
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Need for an improved earthquake protection All these and similar ideas did not influence very much the coGventional or so-called 'earthquake-adapted' method of construction, used for earthquakes up to medium intensity.
With smaller earthquakes, this method grants an efficient protection for the human lives as well as a minimum safety for the construction, though structural damages up to a wrecking state have to be put up with. But this protection is insufflcient for high earthquake intensities.
Dangerous situations to be dealt with arise especially for seismic high risk constructions in highly seismic regions. Such constructions are considerably endangered by the sudden and unexpected occurrence of earthquakes or other quake-like excitations. Especially concerned are facilities for public supply with vital functions (hospitals, administrative headquaters, command centers); for transportation (important bridges, railway stations, tunnels); for energy supply (coffer dams, power stations, fuel storage); of the industrial sector (chemistry plants, explosive manufactures); for military needs; as well as structures with high concentrations of people (sky-scrapers, convention buildings, movie theaters, schools, shel~ers). The existence of some of these constructions situated in highly seismic regions depends basically on the technical capa-city to realize the integral earthquake protection. The need for improved earthquake protective systems is thereby clearly established.
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1 ~ 6~650 Problem to be solved There is the underlying understanding for the conventional, 'earthquake-adapted' method of construction that the fundamental structural eigenfrequencies lie most inevitably in the range of resonance of typical earthquake response spectra. The kinetic energy transrnitted to these constructions by the soil excitations is transformed into structural deformations. As long as these structural deformations remain in the elastic range, the structure will not.be damaged. In case of excitations resulting in structural deformations exceeding the yield limit of the material however, there appear ruptures which may end in the collapse of the construction.
Full base isolation of constructions An important progress in the seismic safety of constructions could be achieved, when the structural deformations, occurring mainly in the superstructure of conventionally designed con-structions, were successfully removed from this rupture endangered zone. To achieve this goal, the Swiss Patent # 450.675 proposed to use highly elastic isolators as flexible zones, placed in between the superstructure of the construction and its foundation. This produces a so-called 'fully base isolated' or 'elastically noating' construction.
This procedure is well known from the mounting of machine foundations on elastic bear-ings, and its application to constructions has already been described in specialized periodicals (Hubacher C., Staudacher E., Siegenthaler R.: "Erdbebensicherung im Bauen"; Neue Zurcher Zeitung, Technikbeilage, Febr. 9, 19~0). The concern is full base isolation (3-dimensionally floating), to be distinguished from the horizontal base isolation. While the isolators of the latter are highly elastic, merely in the horizontal plane (Delfosse G.C.: "The GAPEC System -A new highly effective aseismic system"; Proceedings of the 6th World Conference on Earth-quake Engineering, New Dehli, 1977), the former are flexible in all directions, i.e. also verti-cally.
Integral earthquake protection by full base isolation Although the advantages of this aseismic system were generally known, there was no awareness of the fact that the concept of the full base isolation of constructions could be improved, by additional, appropriate technical measures, up to their integral protection against the strongest earthquakes known yet.
Judging the conYentional methods to verify the safety against earthquakes For the exact numerical verification of a construction, in principle, the mechanical proper-ties of all elements of the system, i.e. construction, isolators, foundation and soil, have to be known. The mass, damping and stiffness distributions of the superstructure, the possibly present isolators, the foundation and the soil are introduced into the mathematical model of the system, to determine the dynamic response corresponding to a given seismic load reliably.
But the today most commonly applied numerical methods using very much simplifiedfinite element models, which are proposed in the legal construction standards, do not allow to meet the requirements of the integral earthquake protection. Thus, the current spectral analysis method using mean value response spectra is resulting in a possible error margin of several hundred percent, while the cost of a modal or linear incremental analysis become comparable with the cost of the raw construction. It is important to keep in mind that, in order to achieve the integral earthquake protection, those exposed elements of the structural frame have to be Iocated whose failure could first initiate the collapse of the structural frame. - How then, the design according to the requirements of integral earthquake protection can it be actually verified in practice ?
Also, the technique for defining the loading case "earthquakes" has an important ~, .
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._ deficiency: There is no direct mathematical relation between the seismological quantities to indi-cate the strength of a given earthquake deduced from estimated intensities or measured magni-tudes and quantities directly used by engineers, based on the acceleration functions of a signal.
- How then, a relation between seismic risk maps and legal loading case prescriptions can it be established ?
The criticism is based on the fact - that in the earthquake protection of structures one operates with an insufflciently known loading case and with numerical methods simplifying the actual situation in an inadmissi-ble way;
- that the commonly used finite element models are not detailed enough fos the exact verification of the structural frame, respectively that they are too expensive in actual applications wh~n detailing it correctly (i.e. uneconomic models versus insufficiently exact results);
- that the laws describing the construction material behavior (e.g. reinforced concrete) under seismic excitation are not sufficiently known.
Thus, the seismic forces in a structural frame, computed by conventions according to the standards, must be looked at as an unreliable estimate of the actual forces which, in reality, can exceed them several times.
The philosophy of limited earthquake protection as found in modern earthquake standards corresponds to these observations. It requires - that constructions resist small earthquakes, occurring periodically at the construction site, with as little damage as possible and - that, when intensive respectively extreme earthquake excitations occur, the protection of human life has the priority over the protection of the construction.
The reason, why the philosophy of protection has been limited in this way, was not merely because it would be uneconomic to request more consequent structural protection meas-ures for all constructions in highly seismic regions. In practice, the tools did actually not exist to realize technically, respectively to verify numerically, if the integral earthquake protection was achieved. Finally, there existed a considerable legal insecurity in the determination of the loading case "earthquakes" for which science did not yet provide approved bases for its definition.
Goal The goal of the present invention is to eliminate the mentioned disadvantages and to render practicable the integral earthquake protection of constructions or similar bodies. By the proposed technical measures, they shall resist repeatedly and without damage to the strongest earthquakes ever measured at the construction site, respectively to be expected there from pre-viously defined or known cases. The technical measures shall be such that the integral protec-tion aimed at be proven by computation or experimentation in a simple, reliable and economic way.
In an extended context of the invention, the protection of bodies against excitations of general nature shall be achieved. Besides natural earthquakes, artificial earthquakes resulting from nuclear or conventional bomb explosions as well as blastings, airplane crashes, shell impacts and other shock-like eft^ects can be included in the invention goal. Apart from bodies like constructions, shelters, powerplants, and military facilities, big machines and isolator sta-tions can also be protected. Not only the actual foundation soil of detached free-standing con-structions, but also a cavern rock, machine chassis or a structural part supporting endangered instruments, can assume the role of the excited medium. The preceding enumeration does not claim to be exhaustive, of course. As an endangering event is to be considered also e.g. an air-plane crash on a reactor building which contains a big machine.
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-,r _ ........................................................... . _ _ 1 1 6~50 SUMMARY OF THE INVENTION
., For a mechanically in all directions isolated body of the nature mentioned, the goal set is achieved according to the invention in such a way that there is a frequency range called "hole of eigenfrequencies" between the highest of the six lowest system eigenfrequencies (called funda-mental eigenfrequencies) and the lowest of all higher system eigenfreuqencies (called upper eigenfrequencies) of the oscillatory element consisting of the superstructure D and the isolators C. In this frequency range the oscillatory element presents no eigenfrequencies. Also, the hole of eigenfrequencies should lie in the "resonance range of the endangering design response spec-trum" of the excitation.
In the case of the body being a construction, the mechanical isolation in all directions is reached by means of horizontally and vertically highly elastic damping elements (mechanical isolators) which are inserted between the superstructure and its foundation after having separated both parts of the structure. By appropriate mass distribution of the superstructure and stiffness distribution of the isolators, the six fundamental eigenfrequencies of the oscilla-tory element can be placed in a range which lies beneath the range of resonance of the design response spectrum of the excitation and all upper eigenfrequencies can be placed in a range which lies higher than the range of resonance.
The superstructure is to be of rigid conception. For that, it is designed box- or honeycomb-like, with continuous and supporting exterior walls. By that measure, it will have the ability to cope with the arbitrarily directed dynamic forces within the structural frame.
Thus, an earthquake-adapted conception of the structure is reached which, as such, is also required by seismic standards for conventional constructions. This measure results in the upper eigenfrequencies of the structure rising above the upper limiting value of the spectral range of resonance, if it stands on in all directions highly elastic isolators at the same time.
The foundation is linked rigidly to the surrounding formation, e.g. the surrounding soil forms with it a rigid unity wherefore the foundation can be designed as a continuous slab or a hod or a specially designed interrnediate story. The vertical stiffness of the soil should be at least six ~o nine times greater than the vertical stiffness of the isolators, so that the influence of the soil flexiblility can be neglected for numerical verification needs.
As a consequence of such a conception of the construction and the isolators, a hole in the structural eigenfrequencies in the range of resonance of typical seismic strong-motion response spectra is formed.
Through appropriate design of all components of the isolated construction, in accordance with the invention, the construction resists without damage to the greatest strong-motion earth-quakes ever measured or to be expected at the construction site. It is therefore integrally pro-tected against earthquakes. With that is meant that the protected part of the construction be able to resist repeatedly extreme seismic excitations without elasto-plastic deformations (des-tructions) in its structural frame~
, Brief description of the drawings Fig. 1 schematically shows a structure standing on a foundation soil sensitive to seismic exci-tations; -- -Fig. 2 a Fourier amplitude spectrum of an earthquake;
Fig. 3 schematically illustrates a mechanical isolator constructed in accordance with theinvention;
Fig. 4 shows in cross-section a shelter in a cavern which is isolated from ground motions in accordance with the invention;
-.. . . . . ... ,~ . , ... .,.. ,., , , . _ . ... .. . . ,,,, , _ -t ~6~50 -Fig. 5 is an exploded perspective view of a model of a standard structure for the numerical or experimental verifiaction of the integral earthquake protection.
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DE~;CRIPTION OF THE PREFERRED E~IBODIMENT
Excitation in the medi~m A
The endangered structure D represented in figure 1 is connected over an isolator zone C
to the foundation B and the surrounding formation A, e.g. to the foundation soil. P(1) and P(2) define the reference points of a measurable excitation wave E, e.g. an earthquake, expressed in the time domaine T as a 3-dimensional acceleration function with the translations 4,' and rotations [DE(T)].
~i4 The notion 'excitation' stands for any (3~dimensional) dynamic motion in the surrounding medium A, which transmits itself onto the isolator zone C of the body D. These motions can turn out to be shock-like, periodic, or pseudo-periodic oscillations. oscillations.
As 'intensive and extreme excitations' are to be understood such kinds of dynamic motions of the isolators C which can endanger the mechanical integrity of the body D or res-train its normal use in an inadmissible way or render it impossible, as long as no protection -measures are taken.
The excitations can be defined in a deterministic way as time or frequency functions, or probabilisticly as design spectrum forms. The specific character of the excitation is described as a combined Fourier amplitude and a phase spectrum or a so-called response spectrum. The decisive quantities are then the frequency content of the signal, i.e. the spectral distribution of the amplitudes as a function of frequency, and the maximum acceleration of the signal, for cali-bration purposes.
A seismic response spectrum similar to figure 2 is considered as a typical seismic design spectrum, wherein the possible fundamental eigenfrequencies and lowest upper eigenfrequen-cies of the proposed oscillatory element, consisting of the body D and the isolators C, are shown. This kind of a design response spectrum is computed from one or several standardized time functions. By means of a well known numerical procedure, the maximal response of a simple (un)damped oscillator to a given excitation is established and shown graphically as a function of the oscillator eigenfrequency. For typical seismic excitations on soil level, there ._q results a central frequency band with high amplitudes. This band is called the range of reso-nance of the response spectrum.
In figure 2, F stands for the frequency and Sa for the acceleration response spectrum of . ~ the excitation. Sa(max) is the peak or maximum value of the spectral acceleration in the range of resonance, and Sa(R) is its reference or limiting value, whereby F(l) and F(2) are designed as corresponding limiting frequencies of the range of resonance.
The upper and lower limitation of the complete spectral frequency range F results from the specific character of the excitations against which the proposed technical measures are taken. The range of resonance covers the resonance endangered structural eigenfrequencies. It is determined as that central range 11 of the amplitude spectrum whose spectral accelerations exceed the limiting or reference value Sa(R). This limiting value Sa(R) can be expressed as a function of the maximum value Sa(max), e.g. Sa(R) = 0.8 x Sa(max). Beneath range of reso-nance 11 is the range of the lower, fundamental eigenfrequencies and above is range 111 of the - upper eigenfrequencies.
The fundamental eigenfrequencies of most constructions fall generally in range II of the schematic Fourier amplitude spectrum. The values delimiting the range of resonance for firm soils lies between 1.6 Hz and 6.0 Hz (values by experience varying with the soil quality). This results in the following widths of the ranges:
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Range 1: < 1.6 Hz Range 11: between 1.6 Hz and 6.0 Hz Range 111: > ~.0 Hz It is to be considered that all statements concerning structural eigenfrequencies refer to the oscillatory elemen~ (body D with isolators C, Fig. 1) as a whole, and not to parts of the body D alone.
Delimitating the danger resulting from excitations The kind of danger resulting from excitations for conventionally designed constructions which are to be protected by the proposed technical measures, is delimited as following:
a) Danger of collapse, for constructions of conventional design having resonance-endangered eigenfrequencies in range 1. The excitation of the foundation B can cause unadmissibly big deformations in the structural frame of the construction D. This results in destruc-tions on the most exposed structural elements: Threat to the mechanical integrity (col-lapse of the construction or being of no use anymore) of parts of or the whole construc-tion.
b) Danger of resonance, for constructions of a conventional design having resonance-endangered eigenfrequencies in range 11 or for instruments inside the construction D.
The resonance effects produced in the construction result in an over-loading of the activated structural elements: Damages in parts of, or on the whole construction or on objects inside the construction.
c) Danger of brittle fracure, for constructions of conventional design having resonance-endangered eigenfrequencies in range 111. The excitations of the soil have a frequency content which results in shock-like strain releases: brittle fracture destructions. Brittle fracture destructions suppose a brittle response behavior of the used construction materi-als within the frequency range of resonance.
d) Danger of over-loading due to differential movements of the bearings. The passing of an excitation wave from P(1) to P(2) (Fig. 1) can result in big differential bearing displace-ments: Damages resulting from the tendancy of the construction to pull itself locally from the bearings.
Included are damages from loading combinations of the above-mentioned loading cases a) to d); many of the constructions of conventional design typically have resonance-endangered eigenfrequencies in ranges 11 and 111.
.; . , P~actical application For a better understanding, an example of a construction exposed to extreme earthquakes illustrates the proposed technical measures which are introduced to achieve the goal of the integral earthquake protection, as well as the possibility of a simple way to verify it numerically.
These measures apply directly to other applications.
The superstructure D, which is to be protected, is propped over the isolators C on the foundation B, the same linked firmly with the soil A (Fig. 1). The schematized soil movement is called E.
The protected superstructure D comprises ordinarily all parts of the construction which are not firmly bound in the soil. The superstructure D can prop itself directly on the founda-tion or lie on the basements, which are bound to the surrounding soil, acting in this case as an intermediate foundation.
Mechanical isolators C have a dual function: on the one hand, they control the oscillatory behavior of an elastic system and on the other hand, they exert a damping effect. Accordingly, the schematically illustrated isolator of figure 3 presents a spring element C.1 and a damping ~ , .
w ''1 ., .
I ~ 6~650 element C.2. In one embodiment the isolator has a top and a bottom plate bonded together with rubber sheets so that the isolators firmly connect the superstructure with the foundation.
The rubber sheets among themselves are glued together so that they can absorb the deforma-tions in tension/compression, and in shear. Isolators constructed in this manner are highly elastic in all directions.
Natural rubber bearings are preferred to steel springs due to their considerably superior damping and because they correspond ordinarily - with the required durability - to the special elasticity requirements. Vertically supporting steel springs, according to the present state of the art, are not considered able to cope with the big horizontal displacement capabilities required for systems safe against extreme earthquakes.
In the position at rest of the oscillatory system, isolators have to transmit the static forces of the superstructure to the foundation. Their geometric positions and individual stiffness are basically determ;ned by the mass distribution in the superstructure and cannot be influenced without considerable technical measures; i.e. that vertical loads on foundation level have ordi-narily to be absorbed where they appear, whereby the following design types are possible: A
"carpet" of isolators consisting of merely identical elements; a "carpet" of isolators consisting of individually adapted elements: a terraced disposition; free positioning.
The following foundation design variants are possible: A classic foundation with additional elements to integrate the aseismic system (foundation directly bound to the soil); an intermedi-ate storey separating the basements from the upper stories and containing the additional ele-ments of the aseismic system.
Independently of the chosen variant, the foundation design has to meet the following requirements: As zone designed to support loads and to be very stiff in all directions; uncon-trolled relative displacements between the supporting areas must be prevented; isolators are to be protected against damaging environmental infJuences; to check, maintain and replace the iso-lators, the access to them mus~ be easy; it must give a full-proof acceptance of the classic load-ing cases (dead weight, live loads, wind and snow).
As another execution form of the invention, figure 4 shows a shelter D propped over iso-lators C surrounded by the walls of a cavern.
It has been proven by computation that, in order to protect a fully base isolated construc-tion in an integral way, the following three technical measures have to be applied:
- The six fundamental eigenfrequencies (1 - 6) of the system have to be transferred in the frequency range I (Fig. 2). This happens by means of mechanical isolators which are highly elastic in all directions (i.e. also vertically). Either of the lowest fundamental eigenfrequencies (1 and 2) shall not be higher than about 40 % of the lower limiting fre-quency F(1) of the range of resonance, to fully isolate the system. The fundamental eigenfrequency (6) may rise just beneath the level of the limiting frequency F(1), to transmit correctly the static forces to the foundation.
- The superstructure has to be designed in such a way that all of the upper eigenfrequencies (7, 8 ...) of the base isolated system lie in the frequency range III. For this purpose, a box- respectively honeycomb-like conception of the superstructure was chosen. The exte-rior walls of the construction have to be supporting, continuous, solid and contain not more openings than necessary. Their joint action, together with the slabs, inner walls and columns has to be guaranteed by we11 known construction measures. Appropriate design will exclude relative movements of element joints, element boundaries and construction sections. A honeycomb-like conception results, when the slabs and the supporting inner walls are included in the bracing of the box-like outlay of the construction.
- The foundation and the foundation soil together, have to form a rigid unity. For that pur-pose, a firm foundation soil has to be chosen, e.g. rock, bedrock or appropriate consoli-dated sediments of good quality, whose vertical stiffness is at least six to nine times supe-rior to the total isolator stiffness. The foundation itself has to be designed generally as a continuous slab or as a hod. By this, the relative movements between the supporting ., :.
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areas can be reduced to a negligible quantity and the influence of the soil flexibility can be neglected in the numerical model.
These technical measures have the following effects:
- In the range of resonance of the design spectrum (range II, Fig. 2), a zone without struc-tural eigenfrequencies is formed: hole of eigenfrequencies in the range of resonance of the seismic response spectrum.
- Owing to the fact that all of the fundamental structural eigenfrequencies are very low, only considerably reduced parts of the higher frequency content of the soil excitation E
are transmitted to the superstructure. Thus, the danger of brittle fracture in the super-structure is averted.
- Thanks to the rigid conception and to the high mechanical isolation, no danger of collapse . exists anymore for the superstructure.
- Possible differential displacements of the supporting areas during the passing of a seismic wave are seized at their source (relative movements between the highly elastic isolators) and thus reducing considerably the danger of a local over-loading in the superstructure.
In addition, important simplifications result for the mathematical model:
- For mathematical purposes, the superstructure may be treated as a rigid body even in the main stage of the numerical analysis. It has merely the six degrees of freedom of a rigid body in space. In practice, and compared to 'exact solutions', the quality of the results is barely influenced.
- The influence Or the soil flexibility can be neglected in the mathematical model, if the unloaded, so-called 'free-field eigenfrequencies' of the soil amount at least to two and a half to three times the highest fundamental eigenfrequencies of the base isolated con-struction: Thus, the interaction construction/soil can be neglected in this case.
- The influence of the upper eigenfrequencies (from the 7th system frequency on) on the excitation load of the protected part of the construction may be neglected in the mathematical model.
- Problems of resonance in the protected part of the construction can be trea~ed with locally due to its rigid conception. There is no interaction possible between locally resonance-endangered parts and the protected part of the construction due to its rigid conception.
As a consequence, a tool has been developed to verify numerically if a structure meets the requirement of the integral earthquake protection against extreme earthquakes. The follow-i`~ ing procedure is considered appropriate to furnish this proof for loads corresponding to extreme earthquakes. It is based on the fact that, by an appropriate definition of the loading case, extreme earthquakes can be distinguished by their intensity and character from standard earth-quakes occuring periodically in a highly seismic region. (This way of defining the loading case can be transposed appropriately to the other dynamic loading cases mentioned.) Thus, the numerical verification of earthquake-safety is realized in the following steps:
Step 1 Definition of the loading cases "extreme earthquakes'i and "standard earthquakes" valid for the construction site Step 2 Standard earthquake safety verification of the construction-with an inactive earthquake protection system, according to the prescriptions of the earthquake standards of the concerned region and assuming a restriction to elastic deformations Step 3 Location of the most unfavorable directions of incidence by rotating 3-dimensional design earthquakes around the foundation of the construction with an active earth-quake protection system - Approximative spectral or exact modal/incremental analysis in the most unfavorable directions of incidence ~ .~, , .
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-1 1 6~50 , g -Step 4 Analysis of the influence of technically possible variations of the mass, damping, and stiffness distribution of the construction with an active earthquake protection system -Parameter analysis by repetitive spectral analysis in the most unfavorable directions of incidence Step 5 Determination of the extreme response values (movements and forces) of the global construction and of parts of it with an active earthquake protection system - Modal or incremental analysis with extreme design earthquakes in the most unfavorable direc-tions of incidence Step 6 Verification of the design of the structural frame with the most unfavorable force com-binations of the static and dynamic loading cases - Actual safety verification Step 7 Analysis of local resonance problems in distinct parts of the construction - Analysis ~, with simple finite element models using the resulting kinematical responses of step 5 ;~ as input functions.
For steps 3 to 6, the structure has to be defined as a 3-dimensional model. The structural analysis has to be so detailed that the analysis allows to locate actually those exposed structural elements which undergo first elasto-plastic deformations.
To confirm the numerical verification of a fully base isolated construction by experimenta-tion, a model of a standard construction has been chosen and exposed to an excitation corresponding to the rnost intensive earthquakes known yet.
This standard construction, presented in exploded view in figure 5, shows a clear static and dynamic conception. The ground-plane has a punctual symmetry and the design is monol-ithic, compact, and box- and honeycomb-like. It guarantees a strong rigid construction. Again, the superstructure is called D, the isolators C and the foundation B. The storey slabs D.1, the }oof D.2, the core D.3 with the staircase, the inner and exterior walls D.4 and the-columns D.5 contribute all to the inner bracing of the construction.
This standard building is representative for a tower-like building.
In applying the proposed technical and analytical means to this building, the feasibility has been established of a structure meeting the requirements of the Integral Earthquake Protection.
Thus, it has been proven by an extended numerical and experimental verification that it is pos-sible in practice to create a hole of eigenfrequencies covering the range of resonance of the seismic design spectrum.
: .. `1 _ _ _ _ , . _ _
Studies on mechanismes to decrease earthquake forces applied to buildings"; Proceedings of the 3rd World Conference on Earthquake Engineering, London, 1965). Finally, there are pro-posals to suspend the superstructure in order to separate it from the direct influence of ground excitations (Oto Lanios C.J. et al.: "Study of the behavior of a hanging building under the effect of an earthquake"; Proceedings of the 4th World Conference of Earthquake Engineering, Santi-ago de Chile, 1969).
.
Need for an improved earthquake protection All these and similar ideas did not influence very much the coGventional or so-called 'earthquake-adapted' method of construction, used for earthquakes up to medium intensity.
With smaller earthquakes, this method grants an efficient protection for the human lives as well as a minimum safety for the construction, though structural damages up to a wrecking state have to be put up with. But this protection is insufflcient for high earthquake intensities.
Dangerous situations to be dealt with arise especially for seismic high risk constructions in highly seismic regions. Such constructions are considerably endangered by the sudden and unexpected occurrence of earthquakes or other quake-like excitations. Especially concerned are facilities for public supply with vital functions (hospitals, administrative headquaters, command centers); for transportation (important bridges, railway stations, tunnels); for energy supply (coffer dams, power stations, fuel storage); of the industrial sector (chemistry plants, explosive manufactures); for military needs; as well as structures with high concentrations of people (sky-scrapers, convention buildings, movie theaters, schools, shel~ers). The existence of some of these constructions situated in highly seismic regions depends basically on the technical capa-city to realize the integral earthquake protection. The need for improved earthquake protective systems is thereby clearly established.
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1 ~ 6~650 Problem to be solved There is the underlying understanding for the conventional, 'earthquake-adapted' method of construction that the fundamental structural eigenfrequencies lie most inevitably in the range of resonance of typical earthquake response spectra. The kinetic energy transrnitted to these constructions by the soil excitations is transformed into structural deformations. As long as these structural deformations remain in the elastic range, the structure will not.be damaged. In case of excitations resulting in structural deformations exceeding the yield limit of the material however, there appear ruptures which may end in the collapse of the construction.
Full base isolation of constructions An important progress in the seismic safety of constructions could be achieved, when the structural deformations, occurring mainly in the superstructure of conventionally designed con-structions, were successfully removed from this rupture endangered zone. To achieve this goal, the Swiss Patent # 450.675 proposed to use highly elastic isolators as flexible zones, placed in between the superstructure of the construction and its foundation. This produces a so-called 'fully base isolated' or 'elastically noating' construction.
This procedure is well known from the mounting of machine foundations on elastic bear-ings, and its application to constructions has already been described in specialized periodicals (Hubacher C., Staudacher E., Siegenthaler R.: "Erdbebensicherung im Bauen"; Neue Zurcher Zeitung, Technikbeilage, Febr. 9, 19~0). The concern is full base isolation (3-dimensionally floating), to be distinguished from the horizontal base isolation. While the isolators of the latter are highly elastic, merely in the horizontal plane (Delfosse G.C.: "The GAPEC System -A new highly effective aseismic system"; Proceedings of the 6th World Conference on Earth-quake Engineering, New Dehli, 1977), the former are flexible in all directions, i.e. also verti-cally.
Integral earthquake protection by full base isolation Although the advantages of this aseismic system were generally known, there was no awareness of the fact that the concept of the full base isolation of constructions could be improved, by additional, appropriate technical measures, up to their integral protection against the strongest earthquakes known yet.
Judging the conYentional methods to verify the safety against earthquakes For the exact numerical verification of a construction, in principle, the mechanical proper-ties of all elements of the system, i.e. construction, isolators, foundation and soil, have to be known. The mass, damping and stiffness distributions of the superstructure, the possibly present isolators, the foundation and the soil are introduced into the mathematical model of the system, to determine the dynamic response corresponding to a given seismic load reliably.
But the today most commonly applied numerical methods using very much simplifiedfinite element models, which are proposed in the legal construction standards, do not allow to meet the requirements of the integral earthquake protection. Thus, the current spectral analysis method using mean value response spectra is resulting in a possible error margin of several hundred percent, while the cost of a modal or linear incremental analysis become comparable with the cost of the raw construction. It is important to keep in mind that, in order to achieve the integral earthquake protection, those exposed elements of the structural frame have to be Iocated whose failure could first initiate the collapse of the structural frame. - How then, the design according to the requirements of integral earthquake protection can it be actually verified in practice ?
Also, the technique for defining the loading case "earthquakes" has an important ~, .
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-!
._ deficiency: There is no direct mathematical relation between the seismological quantities to indi-cate the strength of a given earthquake deduced from estimated intensities or measured magni-tudes and quantities directly used by engineers, based on the acceleration functions of a signal.
- How then, a relation between seismic risk maps and legal loading case prescriptions can it be established ?
The criticism is based on the fact - that in the earthquake protection of structures one operates with an insufflciently known loading case and with numerical methods simplifying the actual situation in an inadmissi-ble way;
- that the commonly used finite element models are not detailed enough fos the exact verification of the structural frame, respectively that they are too expensive in actual applications wh~n detailing it correctly (i.e. uneconomic models versus insufficiently exact results);
- that the laws describing the construction material behavior (e.g. reinforced concrete) under seismic excitation are not sufficiently known.
Thus, the seismic forces in a structural frame, computed by conventions according to the standards, must be looked at as an unreliable estimate of the actual forces which, in reality, can exceed them several times.
The philosophy of limited earthquake protection as found in modern earthquake standards corresponds to these observations. It requires - that constructions resist small earthquakes, occurring periodically at the construction site, with as little damage as possible and - that, when intensive respectively extreme earthquake excitations occur, the protection of human life has the priority over the protection of the construction.
The reason, why the philosophy of protection has been limited in this way, was not merely because it would be uneconomic to request more consequent structural protection meas-ures for all constructions in highly seismic regions. In practice, the tools did actually not exist to realize technically, respectively to verify numerically, if the integral earthquake protection was achieved. Finally, there existed a considerable legal insecurity in the determination of the loading case "earthquakes" for which science did not yet provide approved bases for its definition.
Goal The goal of the present invention is to eliminate the mentioned disadvantages and to render practicable the integral earthquake protection of constructions or similar bodies. By the proposed technical measures, they shall resist repeatedly and without damage to the strongest earthquakes ever measured at the construction site, respectively to be expected there from pre-viously defined or known cases. The technical measures shall be such that the integral protec-tion aimed at be proven by computation or experimentation in a simple, reliable and economic way.
In an extended context of the invention, the protection of bodies against excitations of general nature shall be achieved. Besides natural earthquakes, artificial earthquakes resulting from nuclear or conventional bomb explosions as well as blastings, airplane crashes, shell impacts and other shock-like eft^ects can be included in the invention goal. Apart from bodies like constructions, shelters, powerplants, and military facilities, big machines and isolator sta-tions can also be protected. Not only the actual foundation soil of detached free-standing con-structions, but also a cavern rock, machine chassis or a structural part supporting endangered instruments, can assume the role of the excited medium. The preceding enumeration does not claim to be exhaustive, of course. As an endangering event is to be considered also e.g. an air-plane crash on a reactor building which contains a big machine.
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-,r _ ........................................................... . _ _ 1 1 6~50 SUMMARY OF THE INVENTION
., For a mechanically in all directions isolated body of the nature mentioned, the goal set is achieved according to the invention in such a way that there is a frequency range called "hole of eigenfrequencies" between the highest of the six lowest system eigenfrequencies (called funda-mental eigenfrequencies) and the lowest of all higher system eigenfreuqencies (called upper eigenfrequencies) of the oscillatory element consisting of the superstructure D and the isolators C. In this frequency range the oscillatory element presents no eigenfrequencies. Also, the hole of eigenfrequencies should lie in the "resonance range of the endangering design response spec-trum" of the excitation.
In the case of the body being a construction, the mechanical isolation in all directions is reached by means of horizontally and vertically highly elastic damping elements (mechanical isolators) which are inserted between the superstructure and its foundation after having separated both parts of the structure. By appropriate mass distribution of the superstructure and stiffness distribution of the isolators, the six fundamental eigenfrequencies of the oscilla-tory element can be placed in a range which lies beneath the range of resonance of the design response spectrum of the excitation and all upper eigenfrequencies can be placed in a range which lies higher than the range of resonance.
The superstructure is to be of rigid conception. For that, it is designed box- or honeycomb-like, with continuous and supporting exterior walls. By that measure, it will have the ability to cope with the arbitrarily directed dynamic forces within the structural frame.
Thus, an earthquake-adapted conception of the structure is reached which, as such, is also required by seismic standards for conventional constructions. This measure results in the upper eigenfrequencies of the structure rising above the upper limiting value of the spectral range of resonance, if it stands on in all directions highly elastic isolators at the same time.
The foundation is linked rigidly to the surrounding formation, e.g. the surrounding soil forms with it a rigid unity wherefore the foundation can be designed as a continuous slab or a hod or a specially designed interrnediate story. The vertical stiffness of the soil should be at least six ~o nine times greater than the vertical stiffness of the isolators, so that the influence of the soil flexiblility can be neglected for numerical verification needs.
As a consequence of such a conception of the construction and the isolators, a hole in the structural eigenfrequencies in the range of resonance of typical seismic strong-motion response spectra is formed.
Through appropriate design of all components of the isolated construction, in accordance with the invention, the construction resists without damage to the greatest strong-motion earth-quakes ever measured or to be expected at the construction site. It is therefore integrally pro-tected against earthquakes. With that is meant that the protected part of the construction be able to resist repeatedly extreme seismic excitations without elasto-plastic deformations (des-tructions) in its structural frame~
, Brief description of the drawings Fig. 1 schematically shows a structure standing on a foundation soil sensitive to seismic exci-tations; -- -Fig. 2 a Fourier amplitude spectrum of an earthquake;
Fig. 3 schematically illustrates a mechanical isolator constructed in accordance with theinvention;
Fig. 4 shows in cross-section a shelter in a cavern which is isolated from ground motions in accordance with the invention;
-.. . . . . ... ,~ . , ... .,.. ,., , , . _ . ... .. . . ,,,, , _ -t ~6~50 -Fig. 5 is an exploded perspective view of a model of a standard structure for the numerical or experimental verifiaction of the integral earthquake protection.
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DE~;CRIPTION OF THE PREFERRED E~IBODIMENT
Excitation in the medi~m A
The endangered structure D represented in figure 1 is connected over an isolator zone C
to the foundation B and the surrounding formation A, e.g. to the foundation soil. P(1) and P(2) define the reference points of a measurable excitation wave E, e.g. an earthquake, expressed in the time domaine T as a 3-dimensional acceleration function with the translations 4,' and rotations [DE(T)].
~i4 The notion 'excitation' stands for any (3~dimensional) dynamic motion in the surrounding medium A, which transmits itself onto the isolator zone C of the body D. These motions can turn out to be shock-like, periodic, or pseudo-periodic oscillations. oscillations.
As 'intensive and extreme excitations' are to be understood such kinds of dynamic motions of the isolators C which can endanger the mechanical integrity of the body D or res-train its normal use in an inadmissible way or render it impossible, as long as no protection -measures are taken.
The excitations can be defined in a deterministic way as time or frequency functions, or probabilisticly as design spectrum forms. The specific character of the excitation is described as a combined Fourier amplitude and a phase spectrum or a so-called response spectrum. The decisive quantities are then the frequency content of the signal, i.e. the spectral distribution of the amplitudes as a function of frequency, and the maximum acceleration of the signal, for cali-bration purposes.
A seismic response spectrum similar to figure 2 is considered as a typical seismic design spectrum, wherein the possible fundamental eigenfrequencies and lowest upper eigenfrequen-cies of the proposed oscillatory element, consisting of the body D and the isolators C, are shown. This kind of a design response spectrum is computed from one or several standardized time functions. By means of a well known numerical procedure, the maximal response of a simple (un)damped oscillator to a given excitation is established and shown graphically as a function of the oscillator eigenfrequency. For typical seismic excitations on soil level, there ._q results a central frequency band with high amplitudes. This band is called the range of reso-nance of the response spectrum.
In figure 2, F stands for the frequency and Sa for the acceleration response spectrum of . ~ the excitation. Sa(max) is the peak or maximum value of the spectral acceleration in the range of resonance, and Sa(R) is its reference or limiting value, whereby F(l) and F(2) are designed as corresponding limiting frequencies of the range of resonance.
The upper and lower limitation of the complete spectral frequency range F results from the specific character of the excitations against which the proposed technical measures are taken. The range of resonance covers the resonance endangered structural eigenfrequencies. It is determined as that central range 11 of the amplitude spectrum whose spectral accelerations exceed the limiting or reference value Sa(R). This limiting value Sa(R) can be expressed as a function of the maximum value Sa(max), e.g. Sa(R) = 0.8 x Sa(max). Beneath range of reso-nance 11 is the range of the lower, fundamental eigenfrequencies and above is range 111 of the - upper eigenfrequencies.
The fundamental eigenfrequencies of most constructions fall generally in range II of the schematic Fourier amplitude spectrum. The values delimiting the range of resonance for firm soils lies between 1.6 Hz and 6.0 Hz (values by experience varying with the soil quality). This results in the following widths of the ranges:
:`j . i ~,~
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5 ~
Range 1: < 1.6 Hz Range 11: between 1.6 Hz and 6.0 Hz Range 111: > ~.0 Hz It is to be considered that all statements concerning structural eigenfrequencies refer to the oscillatory elemen~ (body D with isolators C, Fig. 1) as a whole, and not to parts of the body D alone.
Delimitating the danger resulting from excitations The kind of danger resulting from excitations for conventionally designed constructions which are to be protected by the proposed technical measures, is delimited as following:
a) Danger of collapse, for constructions of conventional design having resonance-endangered eigenfrequencies in range 1. The excitation of the foundation B can cause unadmissibly big deformations in the structural frame of the construction D. This results in destruc-tions on the most exposed structural elements: Threat to the mechanical integrity (col-lapse of the construction or being of no use anymore) of parts of or the whole construc-tion.
b) Danger of resonance, for constructions of a conventional design having resonance-endangered eigenfrequencies in range 11 or for instruments inside the construction D.
The resonance effects produced in the construction result in an over-loading of the activated structural elements: Damages in parts of, or on the whole construction or on objects inside the construction.
c) Danger of brittle fracure, for constructions of conventional design having resonance-endangered eigenfrequencies in range 111. The excitations of the soil have a frequency content which results in shock-like strain releases: brittle fracture destructions. Brittle fracture destructions suppose a brittle response behavior of the used construction materi-als within the frequency range of resonance.
d) Danger of over-loading due to differential movements of the bearings. The passing of an excitation wave from P(1) to P(2) (Fig. 1) can result in big differential bearing displace-ments: Damages resulting from the tendancy of the construction to pull itself locally from the bearings.
Included are damages from loading combinations of the above-mentioned loading cases a) to d); many of the constructions of conventional design typically have resonance-endangered eigenfrequencies in ranges 11 and 111.
.; . , P~actical application For a better understanding, an example of a construction exposed to extreme earthquakes illustrates the proposed technical measures which are introduced to achieve the goal of the integral earthquake protection, as well as the possibility of a simple way to verify it numerically.
These measures apply directly to other applications.
The superstructure D, which is to be protected, is propped over the isolators C on the foundation B, the same linked firmly with the soil A (Fig. 1). The schematized soil movement is called E.
The protected superstructure D comprises ordinarily all parts of the construction which are not firmly bound in the soil. The superstructure D can prop itself directly on the founda-tion or lie on the basements, which are bound to the surrounding soil, acting in this case as an intermediate foundation.
Mechanical isolators C have a dual function: on the one hand, they control the oscillatory behavior of an elastic system and on the other hand, they exert a damping effect. Accordingly, the schematically illustrated isolator of figure 3 presents a spring element C.1 and a damping ~ , .
w ''1 ., .
I ~ 6~650 element C.2. In one embodiment the isolator has a top and a bottom plate bonded together with rubber sheets so that the isolators firmly connect the superstructure with the foundation.
The rubber sheets among themselves are glued together so that they can absorb the deforma-tions in tension/compression, and in shear. Isolators constructed in this manner are highly elastic in all directions.
Natural rubber bearings are preferred to steel springs due to their considerably superior damping and because they correspond ordinarily - with the required durability - to the special elasticity requirements. Vertically supporting steel springs, according to the present state of the art, are not considered able to cope with the big horizontal displacement capabilities required for systems safe against extreme earthquakes.
In the position at rest of the oscillatory system, isolators have to transmit the static forces of the superstructure to the foundation. Their geometric positions and individual stiffness are basically determ;ned by the mass distribution in the superstructure and cannot be influenced without considerable technical measures; i.e. that vertical loads on foundation level have ordi-narily to be absorbed where they appear, whereby the following design types are possible: A
"carpet" of isolators consisting of merely identical elements; a "carpet" of isolators consisting of individually adapted elements: a terraced disposition; free positioning.
The following foundation design variants are possible: A classic foundation with additional elements to integrate the aseismic system (foundation directly bound to the soil); an intermedi-ate storey separating the basements from the upper stories and containing the additional ele-ments of the aseismic system.
Independently of the chosen variant, the foundation design has to meet the following requirements: As zone designed to support loads and to be very stiff in all directions; uncon-trolled relative displacements between the supporting areas must be prevented; isolators are to be protected against damaging environmental infJuences; to check, maintain and replace the iso-lators, the access to them mus~ be easy; it must give a full-proof acceptance of the classic load-ing cases (dead weight, live loads, wind and snow).
As another execution form of the invention, figure 4 shows a shelter D propped over iso-lators C surrounded by the walls of a cavern.
It has been proven by computation that, in order to protect a fully base isolated construc-tion in an integral way, the following three technical measures have to be applied:
- The six fundamental eigenfrequencies (1 - 6) of the system have to be transferred in the frequency range I (Fig. 2). This happens by means of mechanical isolators which are highly elastic in all directions (i.e. also vertically). Either of the lowest fundamental eigenfrequencies (1 and 2) shall not be higher than about 40 % of the lower limiting fre-quency F(1) of the range of resonance, to fully isolate the system. The fundamental eigenfrequency (6) may rise just beneath the level of the limiting frequency F(1), to transmit correctly the static forces to the foundation.
- The superstructure has to be designed in such a way that all of the upper eigenfrequencies (7, 8 ...) of the base isolated system lie in the frequency range III. For this purpose, a box- respectively honeycomb-like conception of the superstructure was chosen. The exte-rior walls of the construction have to be supporting, continuous, solid and contain not more openings than necessary. Their joint action, together with the slabs, inner walls and columns has to be guaranteed by we11 known construction measures. Appropriate design will exclude relative movements of element joints, element boundaries and construction sections. A honeycomb-like conception results, when the slabs and the supporting inner walls are included in the bracing of the box-like outlay of the construction.
- The foundation and the foundation soil together, have to form a rigid unity. For that pur-pose, a firm foundation soil has to be chosen, e.g. rock, bedrock or appropriate consoli-dated sediments of good quality, whose vertical stiffness is at least six to nine times supe-rior to the total isolator stiffness. The foundation itself has to be designed generally as a continuous slab or as a hod. By this, the relative movements between the supporting ., :.
.j ... . .
~ 31 6~650 .
areas can be reduced to a negligible quantity and the influence of the soil flexibility can be neglected in the numerical model.
These technical measures have the following effects:
- In the range of resonance of the design spectrum (range II, Fig. 2), a zone without struc-tural eigenfrequencies is formed: hole of eigenfrequencies in the range of resonance of the seismic response spectrum.
- Owing to the fact that all of the fundamental structural eigenfrequencies are very low, only considerably reduced parts of the higher frequency content of the soil excitation E
are transmitted to the superstructure. Thus, the danger of brittle fracture in the super-structure is averted.
- Thanks to the rigid conception and to the high mechanical isolation, no danger of collapse . exists anymore for the superstructure.
- Possible differential displacements of the supporting areas during the passing of a seismic wave are seized at their source (relative movements between the highly elastic isolators) and thus reducing considerably the danger of a local over-loading in the superstructure.
In addition, important simplifications result for the mathematical model:
- For mathematical purposes, the superstructure may be treated as a rigid body even in the main stage of the numerical analysis. It has merely the six degrees of freedom of a rigid body in space. In practice, and compared to 'exact solutions', the quality of the results is barely influenced.
- The influence Or the soil flexibility can be neglected in the mathematical model, if the unloaded, so-called 'free-field eigenfrequencies' of the soil amount at least to two and a half to three times the highest fundamental eigenfrequencies of the base isolated con-struction: Thus, the interaction construction/soil can be neglected in this case.
- The influence of the upper eigenfrequencies (from the 7th system frequency on) on the excitation load of the protected part of the construction may be neglected in the mathematical model.
- Problems of resonance in the protected part of the construction can be trea~ed with locally due to its rigid conception. There is no interaction possible between locally resonance-endangered parts and the protected part of the construction due to its rigid conception.
As a consequence, a tool has been developed to verify numerically if a structure meets the requirement of the integral earthquake protection against extreme earthquakes. The follow-i`~ ing procedure is considered appropriate to furnish this proof for loads corresponding to extreme earthquakes. It is based on the fact that, by an appropriate definition of the loading case, extreme earthquakes can be distinguished by their intensity and character from standard earth-quakes occuring periodically in a highly seismic region. (This way of defining the loading case can be transposed appropriately to the other dynamic loading cases mentioned.) Thus, the numerical verification of earthquake-safety is realized in the following steps:
Step 1 Definition of the loading cases "extreme earthquakes'i and "standard earthquakes" valid for the construction site Step 2 Standard earthquake safety verification of the construction-with an inactive earthquake protection system, according to the prescriptions of the earthquake standards of the concerned region and assuming a restriction to elastic deformations Step 3 Location of the most unfavorable directions of incidence by rotating 3-dimensional design earthquakes around the foundation of the construction with an active earth-quake protection system - Approximative spectral or exact modal/incremental analysis in the most unfavorable directions of incidence ~ .~, , .
. ~
-1 1 6~50 , g -Step 4 Analysis of the influence of technically possible variations of the mass, damping, and stiffness distribution of the construction with an active earthquake protection system -Parameter analysis by repetitive spectral analysis in the most unfavorable directions of incidence Step 5 Determination of the extreme response values (movements and forces) of the global construction and of parts of it with an active earthquake protection system - Modal or incremental analysis with extreme design earthquakes in the most unfavorable direc-tions of incidence Step 6 Verification of the design of the structural frame with the most unfavorable force com-binations of the static and dynamic loading cases - Actual safety verification Step 7 Analysis of local resonance problems in distinct parts of the construction - Analysis ~, with simple finite element models using the resulting kinematical responses of step 5 ;~ as input functions.
For steps 3 to 6, the structure has to be defined as a 3-dimensional model. The structural analysis has to be so detailed that the analysis allows to locate actually those exposed structural elements which undergo first elasto-plastic deformations.
To confirm the numerical verification of a fully base isolated construction by experimenta-tion, a model of a standard construction has been chosen and exposed to an excitation corresponding to the rnost intensive earthquakes known yet.
This standard construction, presented in exploded view in figure 5, shows a clear static and dynamic conception. The ground-plane has a punctual symmetry and the design is monol-ithic, compact, and box- and honeycomb-like. It guarantees a strong rigid construction. Again, the superstructure is called D, the isolators C and the foundation B. The storey slabs D.1, the }oof D.2, the core D.3 with the staircase, the inner and exterior walls D.4 and the-columns D.5 contribute all to the inner bracing of the construction.
This standard building is representative for a tower-like building.
In applying the proposed technical and analytical means to this building, the feasibility has been established of a structure meeting the requirements of the Integral Earthquake Protection.
Thus, it has been proven by an extended numerical and experimental verification that it is pos-sible in practice to create a hole of eigenfrequencies covering the range of resonance of the seismic design spectrum.
: .. `1 _ _ _ _ , . _ _
Claims (11)
- PATENT CLAIMS
I CLAIM:
In a body, integrally protected against damage from motions transmitted through the ground, by mechanical isolators which 3-dimensionally yieldingly connect the body to a base, the base being rigidly linked to a surrounding formation, the body having a set of lower, fundamental eigenfrequencies and a set of upper eigenfrequencies, the improve-ment comprising: means providing the combined body and isolators with a hole of eigen-frequencies which lies between the highest of the six fundamental eigenfrequencies and the lowest of the upper eigenfrequencies of the combined body and isolators, the hole of eigenfrequencies being further selected so that it falls within the resonance range of the design response spectrum; whereby the body is devoid of eigenfrequencies within the resonance range of the design spectrum and is thereby isolated from motions correspond-ing high amplitude motions transmitted through the ground. - 2. A body according to claim 1 wherein the means is formed so that either one of the lowest two fundamental eigenfrequencies of the combined body and isolators is no more than about 40 % of the highest fundamental eigenfrequency thereof.
- 3. A body constructed according to claim 1 wherein the means is formed so that the highest fundamental eigenfrequency of the combined body and isolators is not substantially more than 1.6 Hz and the lowest upper eigenfrequency thereof is not substantially less than 6.0 Hz.
- 4, A body according to claim 1 wherein the body comprises a box like structure having load carrying and substantially continuous exterior walls.
- 5. A body according to claim 4 including the inner support elements acting as interior brac-ing elements of the box like structure.
- 6. A body according to claim 5 wherein the inner support elements include generally hor-izontally oriented, vertically spaced apart slabs extending between the walls.
- 7. A body according to claim 6 wherein the inner support elements include generally verti-cally oriented load supporting members disposed between the slabs.
- 8. A body according to claim 1 wherein the base comprises a continuous slab extending over substantially the entire plan configuration of the body.
- 9. A body according to claim 1 wherein the formation comprises a firm subsoil surrounding the base.
- 10. A body according to claim 1 wherein both the formation and the body each have a vertical stiffness, and wherein the isolators have a vertical stiffnesss which is at most one-sixth the vertical stiffness of either one of the body and the formation.
- 11. A body according to claim 1 wherein the body comprises one of a building, a machine and an electrical substation.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH498480A CH625302A5 (en) | 1980-06-27 | 1980-06-27 | |
| CH4984/80-5 | 1980-06-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1160650A true CA1160650A (en) | 1984-01-17 |
Family
ID=4285931
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000380290A Expired CA1160650A (en) | 1980-06-27 | 1981-06-22 | System for protecting a body from motions transmitted through the ground |
Country Status (9)
| Country | Link |
|---|---|
| JP (1) | JPS5740035A (en) |
| BR (1) | BR8104067A (en) |
| CA (1) | CA1160650A (en) |
| CH (1) | CH625302A5 (en) |
| DE (1) | DE3121045C2 (en) |
| FR (1) | FR2507657B1 (en) |
| GR (1) | GR75710B (en) |
| MX (1) | MX152987A (en) |
| NZ (1) | NZ197523A (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2750440B1 (en) * | 1996-06-26 | 1999-04-23 | Jarret | DEVICE FOR POSITIONING AT LEAST ONE FIXED POINT IN A CIVIL ENGINEERING WORK AND APPLICATION TO SUCH WORKS |
| CN110206055B (en) * | 2019-06-12 | 2021-07-20 | 乐昌市第三建筑工程有限公司 | Building foundation structure capable of coping with seismic wave resonance |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CH450675A (en) * | 1966-01-20 | 1968-01-31 | Hubacher Carl | Support of structures to protect them in the event of vibrations in their foundations |
| US3748800A (en) * | 1971-04-22 | 1973-07-31 | R Glicksberg | Earthquake-insulation foundations |
| CA1010912A (en) * | 1973-08-10 | 1977-05-24 | Chiyoda Chemical Engineering And Construction Co. Ltd. | Earthquake isolating and vibration absorbing equipment for structures |
| IN145684B (en) * | 1975-07-01 | 1979-04-21 | Spie Batignolles | |
| CH590387A5 (en) * | 1977-02-22 | 1977-08-15 | Zwahlen Robert |
-
1980
- 1980-06-27 CH CH498480A patent/CH625302A5/de not_active IP Right Cessation
-
1981
- 1981-05-27 DE DE3121045A patent/DE3121045C2/en not_active Expired
- 1981-06-16 FR FR8111866A patent/FR2507657B1/fr not_active Expired
- 1981-06-22 CA CA000380290A patent/CA1160650A/en not_active Expired
- 1981-06-24 GR GR65329A patent/GR75710B/el unknown
- 1981-06-25 NZ NZ197523A patent/NZ197523A/en unknown
- 1981-06-26 JP JP56098491A patent/JPS5740035A/en active Pending
- 1981-06-26 BR BR8104067A patent/BR8104067A/en unknown
- 1981-06-26 MX MX188025A patent/MX152987A/en unknown
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| Publication number | Publication date |
|---|---|
| FR2507657B1 (en) | 1986-04-25 |
| CH625302A5 (en) | 1981-09-15 |
| MX152987A (en) | 1986-07-14 |
| DE3121045C2 (en) | 1986-08-07 |
| DE3121045A1 (en) | 1982-02-25 |
| FR2507657A1 (en) | 1982-12-17 |
| BR8104067A (en) | 1982-03-16 |
| NZ197523A (en) | 1985-07-12 |
| GR75710B (en) | 1984-08-02 |
| JPS5740035A (en) | 1982-03-05 |
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