Mauricio Sarrazin

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.

Effect of SMA Braces in a Steel Frame Building

Pull-back and shaking table test results on a simple model of a three-storey structure that includes shape memory alloys (SMA) copper-based dampers are presented and discussed. The model corresponds to a rigid-framed steel structure and the dampers to austenite CuAlBe wires inserted as bracing at each story. The inclusion of the dissipators in the structure increases the percentage of critical damping from 0.59% for the bare case to 5.95% for the braced system. At the same time, the structural stiffness increases making the first fundamental frequency change from 2.5–3.7 Hz (0.4–0.27s). The net effect of these two factors is a 30–60% reduction of peak relative displacements compared to the ones obtained without dissipation devices when the structure is subjected to earthquake records. Depending on records frequency contents, a reduction of the peak accelerations to near 58% also can be obtained. Additionally, a crude nonlinear analytical model has been studied that can predict the earthquake responses reasonably well.

Optimal Control of Accelerations in a Base-Isolated Building using Magneto-Rheological Dampers and Genetic Algorithms

This article presents a numerical study aimed at improving effectiveness of the isolation system of an actual building by adding magneto-rheological (MR) dampers that act in parallel to the existing rubber bearings (RB). The building itself is modeled with uniaxial elastic elements. Additional elements that include the RBs and the MR dampers are added at the base of the building and two different genetic algorithms are used to optimize operation of the MR dampers. Maximum acceleration and relative displacement at the top of the building are taken as the variables to be minimized. Records of destructive earthquakes are used as input. A comparison is made between the building responses with RB and the one with the additional control system.

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.

Axisymmetric shells for non-axisymmetric loads: an exact conical element approach

Abstract A program to solve axisymmetric shell structures for non-axisymmetric loading is presented. The structure is divided into conical elements. Exact classical shell equations are used for each element. The displacements and external loads are then developed in Fourier series in the hoop direction, and the resulting diffential equations are solved by means of analytical functions of the potential series type. Circular beams connected at elements interface are also considered. A computer program that can be used for a wide variety of problems is then developed. It is applied, as an example, to a ball mill, used normally in the mining industry. The method showed an excellent behaviour in different examples. No more than 20 elements were needed in the potential series to get a relative error of less than 10 −3 .