SEISMIC ISOLATION TO MITIGATE EARTHQUAKE IMPACT

EARTHQUAKE DESTRUCTION IN POOR COUNTRIES

"The recent earthquakes have again emphasized the fact that the major loss of life in earthquakes happens when the event occurs in developing countries. Even in relatively moderate earthquakes in areas with poor housing, many people are killed by the collapse of brittle, heavy, unreinforced masonry or poorly constructed concrete buildings. Modern structural control technologies such as active control or energy dissipation devices can do little to alleviate this, but it is possible that seismic isolation could be adapted to improve the seismic resistance of poor housing and other buildings such as schools and hospitals in developing countries". (J.M.Kelly).

RELIEF OR PREVENTION ?

“More effective prevention strategies would save not only tens of billions of dollars, but save tens of thousands of lives. Funds currently spent on intervention and relief could be devoted to enhancing equitable and sustainable development instead, which would further reduce the risk for war and disaster. Building a culture of prevention is not easy. While the costs of prevention have to be paid in the present, its benefits lie in a distant future. Moreover, the benefits are not tangible; they are the disasters that did NOT happen.” (Kofi Annan, UN Secretary-General: “Introduction to Secretary-General’s Annual Report on the Work of the Organization of United Nations, 1999").

SEISMIC ISOLATION

Seismic isolation is a construction method for protecting buildings, in which the building and ground are separated by an isolation system to limit the transmission of vibrations through the building. It reduces the earthquake force and changes it to a slow vibration, so not only the building, but also everything inside is protected.

Seismic isolation provides numerous benefits:

Safety improvement: Damage to not only the building itself but also interior facilities are restricted and gas or water leaks are prevented, as are secondary accidents due to falling furniture and human lives are protected.

Maintenance of function: The function of buildings can be ensured even after a major earthquake and life can continue as normal.

Protection of property: The difficulties of repair, reinforcement, demolition and rebuilding were experienced after the Great Hanshin Earthquake in Japan. Seismic isolation reduces concern.

Improvement of relief: Fear of earthquake can be alleviated and psychological burden is reduced.Evacuation route is secured after an earthquake.

Economic effect of seismic isolation: Considering safety improvements for disaster during an earthquake and reduction of repair costs after an earthquake, seismic isolation can reduce life cycle cost.

Increased flexibility: Earthquake input to building can be considerably reduced while design flexibility is increased. Precast construction method simplifies the whole structure including junctions and reduces weight of members.

Coronado Bay Bridge - Base Isolated (approaches)

San Francisco City Hall - Base Isolated

Seismic Isolation in Japan

Seismic Isolation in the US

THE BASE ISOLATED LOW-INCOME RESIDENTIAL BUILDING OF RAPOLLA, POTENZA, ITALY

An experimental project of seismic tests on a real building constructed at Rapolla, Potenza (PPT presentation), in Southern Italy demonstrated the effectiveness of base isolation in protecting low-income dwellings. For the application in developing countries the use of rubber devices together with the cheap sliding isolators (Hybrid Isolation) can bring two major advantages. It allows preventing any damage in the structure and control the vibrations even during a strong earthquake, and at the same time could make this technology particularly suitable for an economically sustainable use.

This video shows the construction progress, the placement of the isolators between the foundation and the base of the building, and the sequence of the seismic tests perfomed. (Courtesy F. Braga, M. Laterza).

During a strong earthquake the structure can move independently from the foundations, and undergo no damage. (Courtesy F. Braga, M. Laterza).

Due to the high flexibility of rubber isolators the structure is able to accomodate large movements while still carrying the vertical loads. In this way the impact of the earthquake is reduced through slow high amplitude controlled vibrations. (Courtesy F. Braga, M. Laterza).

The sliding isolator allows a better control of the movement, while still carrying the vertical load. The simple manifacturing process (a steel plate sliding against a PTFE pad) makes it particularly convenient for applications in developing countries. (Courtesy F. Braga, M. Laterza).

Jack-machine used to push the structure. (Courtesy F. Braga, M. Laterza).

COMPUTER SIMULATION. The seismic vibrations are simulated making a "virtual" test. Modern software tools allow engineers to create the action of earthquakes on a computer model of the structure. The dynamic behavior of the real building can be reproduced with good accuracy. These tools are extremely useful in the design of a new building or for the analysis and retrofit of a deficient existing one. Seismic building codes provide the engineering practice with guidelines and assistance in the design and in the use of analysis tools and software.

Study of Nonlinear Response of Base-Isolated Building through Large-Scale Free-Vibration Tests and Nonlinear Analyses

ABSTRACT

Base isolation is a technique adopted with success for the seismic protection of structures worldwide. Since the commonly employed isolation devices often exhibit a nonlinear behavior, laboratory tests are always needed to support both the design and the research. Various researchers have carried out extensive experimental studies on single devices or simplified reduced-scale structural models. Nevertheless, only experimental investigations with large scale structures can provide complete information on the real dynamic interaction between the isolators and the structural or non-structural system components. A base-isolated residential building located in Rapolla, South Italy, was subjected to free vibration tests with initial displacement amplitudes up to the design displacement of 18 cm. The tests have provided information on the nonlinear dynamic behavior of the whole building system under the design earthquake. A switchable HDRB-Slider isolation device was used to allow testing two different isolation systems having different nonlinearity. The experimental results have shown a modal energy exchange effect due to the nonlinear coupling of the modal responses in presence of frictional devices. The experimental response was reproduced by time history analyses using a nonlinear three-dimensional finite element model. The numerical analyses have pointed out the need for good model parameters for isolation devices based on dynamic tests, and the importance of accurate nonlinear time history analyses for capturing the higher-mode response of the superstructure. Further analyses under seismic action have shown that simplified procedures in some codes which neglect the nonlinear modal coupling could considerably underestimate the story shears in a building isolated with frictional or hysteretic devices.

DOWNLOAD

UCSD, University of California San Diego website

Distinguished Seminar Series

Abstract- bio (PDF)

Full presentation (PDF), Distinguished Seminar series, Department of Structural Engineering, University of California San Diego

Michelangelo Laterza's homepage @ UNIBAS, University of Basilicata

REFERENCES

F. Braga, M. Laterza, Field testing of low-rise base isolated building, Engineering Structures, Vol./Iss. 26/11 (2004) pp. 1599-1610.

F. Braga, M. Faggella, R. Gigliotti, M. Laterza, 2005. “Nonlinear dynamic response of HDRB or Hybrid HDRB-Friction Sliders base isolation systems” – Bulletin of Earthquake Engineering, Vol 3 n. 3, Springer editor.

F. Braga, M. Faggella, R. Gigliotti, M. Laterza, 2005. “Effetti delle non-linearità costitutive dei sistemi di isolamento sismico e metodi semplificati per la valutazione della risposta” – Ingegneria Sismica, Year 2005 Vol.2, Patron editor.

F. Braga, M. Faggella, R. Gigliotti, M. Laterza, 2007. “La modellazione delle strutture isolate in presenza di sistemi di isolamento nonlineare e l’analisi dinamica nonlineare semplificata secondo l’OPC 3274/3431 (punto 10.7.6).” Italian National Congress A.N.I.D.I.S. "L'ingegneria Sismica in Italia", Pisa 2007.