SUMMARY An understanding of the precise nature of the non -linear response of structures is essential for future improvement of earthquake resistant design procedures. This paper presents a summary of observations of dynamic behaviour which were made on two typical modern buildings during a period of about ten years. These structures underwent numerous tests and experienced three strong earthquake ground motions . The data presented should prove useful for calibration of parameters in theoretical non-linear models. For buildings having an apparent soft -spring-type non -linearity, a partial or complete recovery of the structural stiffness appears to occur following the large strains created by strong ground shaking . The rate and extent of this recovery appear to depend strongly on the strain levels throughout the excitation.

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Time and amplitude dependent response of structures

The Millikan Library Building, a nine-story reinforced concrete shear-wall structure at the California Institute of Technology, was tested dynamically by means of two eccentric mass vibration generators located on the ninth floor. The response levels ranged up to a maximum acceleration of 0:02 g. The natural periods of vibration, the mode shapes and the energy dissipation were measured for the first and second E-W translational modes, the N-S fundamental mode and the first torsional mode. Soil-structure interaction was investigated by measuring foundation motion and nearby soil surface movements during resonent vibrations in the N-S and E-W fundamental modes. Other tests included “man-excited” vibrations at low stress levels and a measurement of resonance of an air handling unit on the roof, which was found to magnify the roof response by a factor of 8.5.

The measured fundamental periods were short compared to typical framed structures of this height, 0.50 sec in the N-S direction, 0.66 in the E-W direction and 0.46 in torsion. These values increased roughly 3 per cent over the range of testing. The energy dissipation as measured by a viscous damping factor, varied between 0.70 and 2.00 per cent of critical. This large variation over the testing range indicates that tests at higher stresses are needed to determine the energy dissipation expected during the response to strong earthquake motions.

The soil-structure interaction measurements showed that the building responded very nearly as if fixed at the foundation; rocking contributed less than 1 per cent to the total roof motions of the structure and foundation translation about 2 per cent. Although negligible as far as the building motion is concerned, the results demonstrate the possibility of performing full-scale soil-structure interaction experiments.

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Vibration and soil-structure interaction tests of a nine-story reinforced concrete building

This paper outlines the methods that have been used to determine the frequencies and modes of vibration of multistory buildings from their wind-induced vibrations. Three buildings of ten, thirty-eight and forty-seven stories were investigated. A simple theoretical model was used to calculate the frequencies of vibration of the buildings; the model was based on the assumption that there was no joint rotation in the building frames. A comparison of the theoretical and measured values of the frequencies showed that this simple model was a realistic representation of only the smaller building. It is concluded that a model that includes joint rotation would be more realistic for the taller buildings. Auto-correlation and power spectrum analysis of the vibration records were used to obtain an estimate of the damping characteristics of the buildings. The values obtained were 1 to 3 per cent of the critical amount of damping.

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Wind-induced vibrations and building modes

THIS LITERATURE SURVEY AND STATE-OF-THE-ART STUDY ENCOMPASSES A BROAD AREA OF THE STRUCTURAL ENGINEERING FIELD; DEFLECTIONS OF BUILDING STRUCTURES OTHER TYPES OF STRUCTURES SUCH AS TOWERS, BRIDGES, ETC ARE BRIEFLY MENTIONED BUT PRIMARILY IN THE CONTEXT OF INFORMATION THAT MAY BE APPLICABLE TO BUILDING STRUCTURES THE REPORT WAS ASSEMBLED CONSIDERING BUILDING STRUCTURE DEFLECTION IN A SEQUENCE THAT WOULD BE USED WHEN CONSIDERING THE SOLUTION OF A STRUCTURAL DEFLECTION PROBLEM, IE, SEPARATION OF STATIC AND DYNAMIC DEFLECTIONS AND THEN CONSIDERING EACH OF THESE AS A THREE COMPONENT PROBLEM OF LOAD -- STRUCTURAL CHARACTERISTICS -- RESPONSE (DEFLECTION) THIS SEQUENCE IS DEVELOPED AND DEFINED FOR THE RESPECTIVE COMPONENTS THROUGHOUT THE REPORT ALSO THE INTERACTIONS OF MAJOR STRUCTURAL DEFLECTIONS WITH BUILDING STRUCTURES SUBSYSTEMS AND HUMAN OCCUPANTS IS REVIEWED IN THIS REPORT AN ATTEMPT IS MADE TO IDENTIFY SERVICEABILITY CRITERIA FOR THE PRESENT PRACTICE OF BUILDING CONSTRUCTION, TO DETERMINE THE THEORETICAL AND EXPERIMENTAL BASIS FOR THESE REQUIREMENTS AND TO EVALUATE THE RANGE OF APPLICABILITY OF THE EXISTING CRITERIA PRIMARY EMPHASIS IS ON THE IDENTIFICATION AND DOCUMENTATION OF EXISTING SERVICEABILITY PROVISIONS THE BIBLIOGRAPHICAL LISTINGS ARE NOT NECESSARILY A LISTING OF ALL AVAILABLE LITERATURE BUT RATHER A LISTING OF WHAT WAS CONSIDERED BY THE AUTHORS TO BE THE MOST PERTINENT THIS REPORT IS BROAD IN SCOPE AND COVERS THE AREAS OF ANALYSIS, DESIGN AND EXPERIMENTATION /AUTHORS/

Structural deflections : a literature and state-of-the-art survey

Coburn et Spence (2002) rappellent qu'au cours du dernier siecle, le cout des seismes ramene a la valeur de l'annee 2000 est de l'ordre de mille milliard de dollars. Rapporte par an, ils constatent que cette valeur augmente au cours du XXeme siecle, essentiellement a cause de l'augmentation et de la concentration des populations dans les grands centres urbains exposes a un alea sismique fort. Des agglomerations de plus en plus grandes, exposees aux seismes et constituees d'un habitat heterogene et de qualite tres variable, sont donc les ingredients qui positionnent le milieu urbain a l'etape la plus critique de l'evaluation du risque sismique. Cette position est renforcee du fait que de nombreuses villes gigantesques (mega-cities) exposees se trouvent, pour des raisons historiques et sociologiques, sur des remplissages sedimentaires majeurs, facteur aggravant de l'alea sismique local. Le bâti existant va egalement presenter une heterogeneite forte, consequence de l'evolution de ses caracteristiques structurales au cours du temps, renforcant l'idee d'une ville vulnerable et fragile. Enfin, tandis que le risque sismique consiste bien souvent a aborder separement l'alea de la vulnerabilite, il n'est pas surprenant que l'urbain puisse modifier, perturber, voir meme contaminer le mouvement sismique incident, en particulier lorsque l'habitat est dense et massif, ce qui a l'aube du 21eme siecle semble etre l'evolution majeure des grandes concentrations urbaines. Au cours de cet expose, nous aborderons la sismologie urbaine en essayant de comprendre sur des resultats experimentaux recents la facon de reagir d'un milieu urbain a une sollicitation sismique. Deux echelles d'espace seront discutees: la reponse de la ville en montrant comment celle-ci peut a la fois subir le seisme (vulnerabilite sismique) et contribuer de facon significative au mouvement du sol (interaction site-ville); et la reponse d'une structure simple en evaluant sa capacite a se deformer sous une sollicitation sismique (analyse modale operationnelle) et a predire son integrite a la fin de la secousse (courbes de fragilite et structural health monitoring).

Sismologie Urbaine: Aléa local, dynamique des structures, interaction site-ville et vulnérabilité urbaine.

It is shown that the inertia force obtained by a man moving his body back and forth in synchronism with the natural period of vibration of a large structure is sufficient to build up a measurable amplitude of motion. By recording such structural vibrations versus time, the natural period and damping of several of the lower modes of vibration can be determined. The amplitudes of motion set up in this way are for many structures significantly larger than can be obtained from wind excitation, which has been used in the past for the measurement of the period of the fundamental mode.

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N. N. Nielsen

Papers

A new method for the measurement of the natural periods of buildings