Hugo Bachmann

On the Seismic Vulnerability of Existing Buildings: A Case Study of the City of Basel

In order to assess the seismic risk for Switzerland, and particularly for the city of Basel, the seismic vulnerability of the existing buildings needs to be evaluated. Since no major damaging earthquake has occurred in Switzerland in recent times, vulnerability functions from observed damage patterns are not available. A simple evaluation method based on engineering models of the building structures suitable for the evaluation of a larger number of buildings is therefore proposed. The method is based on a nonlinear static approach acknowledging the importance of the nonlinear deformation capacity of the buildings subjected to seismic action. Eighty-seven residential buildings in a small target area in Basel were evaluated. The results are vulnerability functions that express the expected damage as a function of the spectral displacement. In order to extrapolate the results to other residential areas of the town, building classes were defined for which the vulnerability is presented in a probabilistic form that can be used directly for earthquake scenario projects.

TUNED VIBRATION ABSORBERS FOR "LIVELY" STRUCTURES

In the first part of this paper, the theoretical background and a design procedure for tuned vibration absorbers are presented. In the second part, practical experiences with two footbridges and a ...

ON THE SEISMIC VULNERABILITY OF EXISTING UNREINFORCED MASONRY BUILDINGS

A large part of the building population in Switzerland is made of unreinforced masonry. For the assessment of the seismic risk the evaluation of the seismic vulnerability of existing unreinforced masonry buildings is therefore crucial. In this paper a method to evaluate existing buildings, which was developed for the earthquake scenario project for Switzerland, is briefly introduced and discussed in more detail for unreinforced masonry buildings. The method is based on a non-linear static approach where the seismic demand on the building is compared with the capacity of the building. In-plane and out-of-plane behaviour are considered. Comparisons with test results from model buildings show that the proposed method suitably forecasts the capacity of a building. Finally, a numerical example of the application of the method to a building in the city of Basel is given.

Influence of Shear and Bond on Rotational Capacity of Reinforced Concrete Beams

A considerable amount of experimental and theoretical research into the plastic behaviour of statically indeterminate structures has been carried out during the last 20 years. The application of the simple plastic analysis to steel structures has been shown to be valid. In statically indeterminate reinforced concrete structures, however, the rotational capacity of plastic hinges may be very small and as a result theoretical ultimate load can not be reached.

Nonuniform earthquake input for arch dam–foundation interaction

Abstract Wave scattering and dam–foundation interaction are important aspects of a realistic earthquake analysis of arch dams. In the first part of this paper it is shown how the motion obtained from a two-dimensional scattering analysis can be used as an input for a three-dimensional dam–foundation analysis. In the second part, a method for calculating the scattered motion is explained. The scattered motion is obtained via the two-dimensional dynamic stiffness matrix. The dynamic stiffness matrix for an out-of-plane motion is calculated by the Complementary-Domain Method (CDM). Some examples are presented to verify the method and to show the influence of the scattering of the seismic ground motion.

VIBRATION UPGRADING OF GYMNASIA, DANCE HALLS AND FOOTBRIDGES

Rhythmical human body motions can cause strong vibrations of structures. Possible remedial measures are frequency tuning, limiting amplitudes and use of tuned mass dampers. Case studies of vibration upgrading of gymnasia, dance halls and two footbridges illustrate the practical use of the fundamental principles. A reinforced concrete footbridge with a hollow box section and a width of about 4 m was designed as a continuous beam with six sections of regular span lengths of 15.5 m, and the bridge was supported by roughly 7 m high single columns with a cross-section of 65 x 30 cm. Dynamic tests were performed by the Swiss Federal Institute of Technology at Lausanne after unacceptable horizontal vibrations occurred in both the transverse and the longitudinal directions. As a result the bridge was upgraded and the horizontal frequency increased by strengthening the foundation slab and the columns. The second bridge, a 2.5 m wide steel footbridge and cyclist bridge designed as a continuous beam with 4 spans, exhibited strong vibrations under walking and running motions and was found to have a fundamental frequency of 2.46 Hz without any substantial live load. This bridge was upgraded by installing two tuned mass dampers in the middle of the 2.5 m span.

DURCH MENSCHEN VERURSACHTE DYNAMISCHE LASTEN UND DEREN AUSWIRKUNGEN AUF BALKENTRAGWERKE

Dynamische Kraefte aus rhythmischen menschlichen Koerperbewegungen werden oft unterschaetzt. Eine Bemessung von Tragwerken auf statische Lasten allein kann daher zu einer erheblichen Schwingungsanfaelligkeit fuehren. Dies gilt insbesondere fuer Fussgaengerbruecken und bei Turnhallentraegern, die haeufig vorgespannt werden. Dadurch sind sie schlanker und weisen eine geringere Daempfung auf als entsprechende Stahlbetontragwerke. Bei teilweiser Vorspannung ergeben sich bei gut verteilten feinen Rissen bessere Daempfungseigenschaften als bei voller Vorspannung ohne Risse. In Versuchen am Institut fuer Baustatik der ETH Zuerich wurden die zeitlichen Verlaeufe der dynamischen Kraft beim Gehen, Laufen und Huepfen einer Einzelperson mit Hilfe einer Kraftmessplatte bestimmt und anschliessend die Amplitudenkoeffizienten der ersten drei Harmonischen der Fourier-Zerlegung ermittelt. Ausserdem wurde der Einfluss solcher Einwirkungen auf einen 19 m langen Fussgaengerbrueckentraeger und einen 14 m langen Turnhallentraeger experimentell untersucht. Dabei entstanden Beschleunigungen bis zu 1,5 g, was zum Abheben der Traeger von den Lagern fuehrte. Der Einsatz eines Schwingungstilgers am Fussgaengerbrueckentraeger bewirkte fuer Huepfen eine Reduktion der Schwingungen um den Faktor 7.

Einfluss der Rissbildung auf die dynamischen Eigenschaften von Leichtbeton- und Betonbalken

Seit langerer Zeit ist bekannt, dass die dynamischen Eigenschaften von Stahlbetontragwerken durch die Rissbildung verandert werden. In Berechnungen wird meist angenommen, dass im Risszustand nebst kleineren Eigenfrequenzen eine erheblich grossere Dampfung als im ungerissenen Zustand vorhanden sei. Die in diesem Artikel dargestellten Forschungsergebnisse zeigen jedoch, dass durch die Rissbildung die Dampfung nach anfanglichem Anstieg auf sehr kleine Werte absinken kann. Demgegenuber ergeben sich durch die Verwendung von Leichtbeton anstelle von Beton keine erheblichen Unterschiede im dynamischen Verhalten der Tragwerke.

Seismic Upgrading of a Fire-Brigade Building by Base Isolation in Basel, Switzerland

Recently, the main building of the professional fire brigade of the city and canton Basel-Stadt built in the year 1943 was placed on special seismic elastomeric bearings and sliding bearings. This is the first such seismic upgrading of an existing building, north of the European Alps. Compared to conventional strengthening, the building costs were significantly smaller, and no temporary transfer of the fire-brigade troops and operational facilities to an expensive provisional building was necessary. Moreover, the resulting seismic safety is considerably higher, and the vulnerability to a severe earthquake is essentially smaller than in the case of conventional strengthening. Therefore, softening instead of strengthening was a much better upgrading strategy.

Resume et Recommandations

Les dalles minces comprimees ou tendues des profils en Te et des sections evidees sont frequemment soumises a des sollicitations combinees d’effort tranchant longitudinal et de flexion transversale qui s’ajoutent a l’effort de compression ou de traction longitudinal.