María O. Moroni

Dynamic characteristics of a long span seismic isolated bridge

The central zone of Chile is located in a high seismic risk area with occurrence of magnitude 7 earthquakes every 10 years and magnitude 8 events every 90 years. This paper deals with the mechanical and dynamic characteristics of a 383 m long seismic isolated bridge located in that area. The tests carried out on rubber components, isolators and the bridge structure are presented. Some simple structural models that show good correlation with measured response are also addressed. A strong motion permanent network installed on the bridge allowed the recording of ambient-traffic vibrations as well as seismic motions. Seismic records show the beneficial effect of the isolation in the horizontal direction, but important amplification occurs in the vertical direction for relatively high frequency components.

Experiments on a base-isolated building in Santiago, Chile

An experimental four-story building, supported on high damping rubber isolators, was constructed in Santiago, Chile in 1992. The building and a conventional twin, standing nearby, are instrumented with a local network of digital accelerometers. At least 24 earthquakes of different intensities have been registered in the past three years by the recording system. Data was also obtained at the isolated building for pull-back testing in an effort to evaluate the constitutive characteristics of the bearings. This paper describes the bearing verification testing program as well as the experiments carried out at the isolated building. It also contains the analysis of the records obtained for actual earthquakes, for both the isolated and the conventional building, using parametric and non-parametric identification techniques. For the earthquake records obtained, the reduction in the maximum acceleration at the roof level for the isolated building, as compared to the conventional one, varies from 1 to 3.5 times, depending on the level of the maximum ground acceleration and the characteristics of the earthquake motions.

Damping characteristics of a CuZnAlNi shape memory alloy

Abstract In this paper the mechanical behavior of copper-based SMA bars and their potential use for dissipating seismic energy is examined. In particular, results obtained from cyclic tests under tension–compression loading of martensitic CuZnAlNi shape memory alloy bars, with two different processing histories (hot rolled or extrusion), are reported as a function of strain amplitude, grain size and frequency. The stress–strain loops were asymmetric with respect to the central point and rather stable, especially for strains up to 2%. The effective module of elasticity diminishes significantly with strain. From hysteretic cycles, an equivalent damping of 12% was obtained for the larger strains imposed. The maximum tensile strain attained before fracture was 3.5%. Frequency, in the range of interest for seismic applications, had only a small influence on the damping values. For the conditions studied in this research, the bars produced from both processing histories presented similar mechanical properties. Fracture, when it occurred, was due to tensile action and it was rather brittle and intergranular.

Behavior of Instrumented Base-Isolated Structures during the 27 February 2010 Chile Earthquake

Several buildings and bridges with base isolation were in the area struck by the 27 February 2010, Mw 8.8 Maule earthquake. A building and two bridges that were instrumented with networks of accelerometers registered the quake; these records are analyzed here. Results of this kind are very valuable because a strong motion earthquake of large magnitude occurs only from time to time, and these rather new types of structures had not been exposed to such severe action. The isolated building is a four-story reinforced concrete and confined masonry structure located in the city of Santiago. The bridges are: (a) a 368m long continuous bridge located in Viña del Mar and (b) one section of a Santiago metropolitan train viaduct. Results show important reductions in horizontal accelerations, especially at the roof level of the building (to nearly 20% of the twin fixed-base case), and in the longitudinal direction of the bridges.

Seismic behavior of chilean bridges with seismic protection devices

Several bridges with seismic protection systems have been built in the last decade inChile. Accelerometer networks have been installed in some of them. Seismic records have been obtained by these instruments for moderate and severe earthquakes. In this paper, records registered atMargaMargaBridge,AmolanasBridgeand Metro Viaduct are analyzed, and analytical models of the structures are developed. The parameters used in the models are adjusted using actual records in order to obtain the best fit between models and measured structural responses, in particular, frequencies and maximum displacement. The validated models are then used to estimate maximum stresses in principal elements and to compare them with nominal values given by design codes.

Analytical Evaluation of Various Passive Isolation Systems on Seismic Performance of a Low-Cost Chilean Masonry House

In this study, a multi-degree-of-freedom numerical model is created to represent the elastic behavior of a typical Chilean two-family, low-cost, confined masonry house. Ambient vibration testing was performed to determine the fundamental natural frequencies of the structure in the longitudinal and transverse directions through a non-parametric system identification technique. Based on the ambient vibration data, an estimation of equivalent viscous damping of the structure is made using the bandwidth method. The stiffness values are estimated using the natural frequencies measured from the ambient vibration testing. A number of passive isolation devices are considered as potential isolation systems for the structure. Each device is numerically modeled using fuzzy logic. The fuzzy models are created using experimental and analytical data to predict the force exerted on the structure by the device. The passive isolation systems considered include friction pendulum systems, high-damping rubber bearings, and two hybrid systems involving use of high-damping rubber bearings and shape memory alloy wire. Seismic performance of the structure in the longitudinal direction, augmented with each of the isolation systems mentioned above is compared with the performance of the traditionally-constructed structure using several performance indices. CONFINED MASONRY CONSTRUCTION FIGURE 1. CONFINED MASONRY HOUSE Many structures in Chile, including low-cost housing structures, are constructed of confined masonry. Confined masonry is comprised of masonry walls that are confined by reinforced concrete columns and beams. The masonry walls are the main load-bearing elements, as they transmit both gravity loads and lateral loads to the foundation [Murty, Brzev, Faidon, Comartin, and Irfanoglu, 2006]. Performance of a confined masonry structure subjected to earthquake excitation depends significantly on design of the structure and the quality of its construction.