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

Evaluation of the Rayleigh damping model for buildings

Illustration de la methode par le bâtiment de l'administration de l'Ecole Superieure Arvin endommage par le seisme du Comte de Kern de 1952 L'observation du comportement de batiments durant des seismes indique clairement l'importance de la flexibilite des dalles de planchers et de toitures Presentation d'une nouvelle methode analytique pour l'analyse dynamique de bâtiments a 1 ou 2 etages dont les planchers ont une flexibilite dans le plan La methode traite les planchers comme poutres en flexion et les murs comme poutres en cisaillement Formulation des equations du mouvement et des conditions limites des planchers et des murs et resolution de ces equations pour obtenir l'equation caracteristiques du systeme qui peut etre resolue numeriquement pour obtenir les frequences naturelles

Analytical models for low-rise buildings with flexible floor diaphragms

This paper studies the nature of uniqueness in the identification of building structural systems subjected to strong ground shaking. Characterization of the stiffness distribution in the structure from a knowledge of response of one of the floors to a base excitation is investigated. It is shown that uniqueness of the results, in the inverse problem, can be established by proper sensor location. At sensor locations where nonunique solutions are present, an upper bound on the number of such solutions has been presented. The degree of nonuniqueness is found to monotonically increase with increasing height of sensor in the building system from at most one, for a sensor located at the first floor level, to at most N! for a sensor located at the Nth floor of an N story structure.

Some Uniqueness Results Related to Building Structural Identification

L'analyse de vulnerabilite sismique du bâti existant necessite le releve de parametres determinant le comportement dynamique des structures. Dans les methodes existantes comme HAZUS ou RiskUE, ces informations sont collectees par expertise visuelle ou choisies parmi des valeurs forfaitaires. Meme dans le cas d'un diagnostic plus complet, les experts doivent se passer des plans de la structure, estimer la qualite des materiaux utilises, leur vieillissement voire leur endommagement. Or, les vibrations ambiantes des bâtiments et les parametres modaux (frequences propres, amortissements et deformees modales) qui peuvent en etre extraits incluent naturellement tous ces parametres dans leur domaine de fonctionnement lineaire elastique. L'objectif de ce travail est d'utiliser au mieux ces parametres modaux pour aider au diagnostic de vulnerabilite sismique. Un modele modal dynamique lineaire fonde sur les parametres modaux experimentaux est propose. La limite en deformation du premier endommagement a ete fixee pour differents types de bâtiments. Ce modele est notamment valide a l'aide d'enregistrements de seismes a l'Hotel de Ville de Grenoble, une structure instrumentee par le Reseau Accelerometrique Permanent (RAP). Par ailleurs, la courbe de fragilite correspondant a l'etat de dommage "Leger" est developpee en utilisant de nombreux mouvements du sol simules. Cette methodologie a ete appliquee a la ville de Grenoble ou des tests sous vibrations ambiantes ont ete menes sur 61 bâtiments. L'analyse modale a ete realisee grâce a la Frequency Domain Decomposition, une methode qui permet de decomposer les modes, meme s'ils sont proches. Une etude structurale de chaque type de bâtiment a ete menee a l'aide des parametres modaux experimentaux et les courbes de fragilite ont ete developpees suivant la methodologie proposee. Le risque sismique de la ville a egalement ete etudie a l'aide de scenarios de seismes, dont les resultats ont ete compares a ceux d'une methode d'analyse de vulnerabilite empirique.

Vulnérabilité Sismique de l'échelle du bâtiment à celle de la ville - Apport des techniques expérimentales in situ - Application à Grenoble

Two modern multistory buildings, one a five-story reinforced concrete building, the other a nine-story steel frame building, have been the subjects of an extensive series of dynamic tests The vibrations of the buildings were induced by means of synchronized vibration exciters The mathematical analysis needed in order to determine the stiffness and damping matrices from the experimentally determined modal properties of a structure has been developed

Three translational and one torsional mode of vibration of the reinforced concrete building were investigated in considerable detail The damping in each mode and the resonant frequency was determined under various levels of excitation Complete mode shapes were determined as well The measurements of the resonant frequencies show a well-defined nonlinearity that can be well explained from the hysteretic material properties The values of damping were for all modes approximately 276 with a tendency for the value of damping to increase with increasing force levels

A total of seven translational and three torsional modes of vibration of the nine-story steel frame building were investigated in detail A mode in which the floor slabs vibrate horizontally as free-free beams was excited as well The lowest translational modes in the two principal directions of the building had damping values of about 0516 The second lowest translational modes had damping values of approximately 10016 For both buildings the damping values are considerably less than the values usually mentioned in the literature Since most earlier tests used run-down tests rather than the steady-state tests used in the present work, comparison tests were run to explore possible differences in the test results It was concluded that run-down tests could easily overestimate the values of damping by several hundred per cent A new method for the measurements of natural periods of vibration of structures is proposed The new method has several important advantages over wind-excited vibration tests which have been used extensively in the past to measure the natural periods of vibration of structures

Niels Norby Nielsen

Papers

Dynamic response of multistory buildings