Rubén Boroschek

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.

The October 15, 1997 Punitaqui earthquake (Mw=7.1): a destructive event within the subducting Nazca plate in central Chile

Abstract The 1943 Illapel seismic gap, central Chile (30–32°S), was partially reactivated in 1997–1998 by two distinct seismic clusters. On July 1997, a swarm of offshore earthquakes occurred on the northern part of the gap, along the coupled zone between Nazca and South American plates. Most of the focal mechanisms computed for these earthquakes show thrust faulting solutions. The July 1997 swarm was followed on October 15, 1997 by the Punitaqui main event (Mw=7.1), which destroyed the majority of adobe constructions in Punitaqui village and its environs. The main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault. This event is located inland at 68-km depth and it is assumed to be within the oceanic subducted plate, as are most of the more destructive Chilean seismic events. Aftershocks occurred mainly to the north of the Punitaqui mainshock location, in the central-eastern part of the Illapel seismic gap, but at shallower depths, with the two largest showing thrust focal mechanisms. The seismicity since 1964 has been relocated with a master event technique and a Joint Hypocenter Determination (JHD) algorithm, using teleseismic and regional data, along with aftershock data recorded by a temporary local seismic network and strong motion stations. These data show that the 1997 seismic clusters occurred at zones within the Illapel gap where low seismicity was observed during the considered time period. The analysis of P and T axis directions along the subduction zone, using the Harvard Centroid Moment Tensor solutions since 1977, shows that the oceanic slab is in a downdip extensional regime. In contrast, the Punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate, which becomes subhorizontal at depths of about 100 km. Analog strong motion data of the Punitaqui main event show that the greatest accelerations are on the horizontal components. The highest amplitude spectra of the acceleration is in the frequency band 2.5–10 Hz, in agreement with the energy band responsible for the collapsed adobe constructions. The isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock, the poor quality of adobe houses and probably local site amplification effects.

Strong Ground Motion Attributes of the 2010 Mw 8.8 Maule, Chile, Earthquake

The Mw 8.8 Maule, Chile, earthquake produced 31 usable strong motion recordings from currently accessible arrays over a rupture distance range of 30 to 700 km. Site conditions range from firm rock to soft soil but are most often competent soil (NEHRP Category C or C/D). Most of the data were recorded on analogue instruments, which was digitized and processed with low- and high-cut filters designed to maximize the usable frequency range of the signals. The stations closest to the fault plane do not exhibit evidence of ground motion polarization from rupture directivity. Response spectra of nearby recordings on firm ground and soft soil indicate pronounced site effects, including several cases of resonance at site periods. A prior GMPE for interface subduction events captures well the distance scaling and dispersion of the data, but under-predicts the overall ground motion level, perhaps due to too-weak magnitude scaling.

Experimental verification of basic analytical assumptions used in the analysis of structural wall buildings

The observed response of medium high-rise buildings during the latest earthquakes in the USA, Chile, Mexico and Japan have indicated that buildings with structural walls or dual systems of frames and walls behave considerably better during strong shaking. A series of strong motion and ambient vibration records have been obtained in a 22-stories high structural wall Chilean building. Dynamic properties and response characteristics are identified using parametric and nonparametric system identification techniques. A strong coupling of translational modes and very low damping values are observed during medium intensity seismic events and ambient excitations. Typical values for critical damping ratios are between 1 and 2%. Rocking is observed as an appreciable effect on the overall displacements of the structure. Torsional effects are not important in the observed seismic response but they become relatively important during low level ambient excitations. The basic response characteristics identified are compared with those of a three-dimensional model. The model was determined using typical consulting office assumptions. The agreement between model and experimental records is good for global dynamic parameters, and with further adjustment, seismic response can be modeled with a good degree of accuracy validating a series of modeling assumptions.

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.

Tilt Motion Effects on the Double-Time Integration of Linear Accelerometers: An Experimental Approach

There is an important need for seismologists and engineers to determine seismic displacement including its quasi-static components. The existence of commercial low-cost and low-noise instruments has prompted researchers to try to derive displacement-time histories from acceleration records by using different integration techniques. In this article we show that an important reason for the failure of the integration process is the presence, in translational acceleration records, of tilt effects or rotational components of motions. The importance of these effects in seismic records has been considered in analytical studies before. In this work we show, using controlled experimental tests on a six-component shake table, that small tilts generate a distortion of the translational acceleration records that limits their validity, especially for low-frequency components. We show that knowledge of the complete history of tilt is necessary to obtain an accurate displacement after a double-time integration and that the knowledge of the initial and final tilt magnitudes is not sufficient to obtain the true displacements. Hence, it is crucial, as a minimum, to record at the same place and at the same time the complete time history of the three rotational and the three translational components or to have a non-rotation-sensitive instrument to calculate properly the translational ground displacement, including its permanent components.

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.

High Damping Rubber Model for Energy Dissipating Devices

This work presents the results of a study carried out to characterize the mechanical response of a high damping rubber to be used in designing and constructing energy dissipating devices and base isolators for controling strong vibrations in civil engineering structures. A new parametric model of the elastomer is proposed to be employed in the design procedure and structural analysis of passive controlled structures. The parameters of the model are calibrated using experimental data obtained from tests on rubber specimens under different loading paths. The main dissipating energy mechanisms of the rubber are identified. The proposed model is able to reproduce those main mechanisms as well as geometric second order effects such as tension stiffening due to the effect of axial strains in the response. The response predicted by the proposed model is compared with that obtained from experimental tests and from the Kelvin and plasticity models.

Modal parameter variations due to earthquakes of different intensities

This article presents the modal parameter variations during 55 low to high intensity earthquakes in a 22 story shear wall building located in Chile. Some of the earthquakes produced moderate damage in the structure. The building has been instrumented since 1997 with a network of 12 uniaxial accelerometers distributed on 4 different levels. Modal parameters are obtained through parametric Multiple Input - Multiple Output identification techniques. For the identified modal frequencies, decreasing values with respect to motion amplitudes are observed, in a range between 4% and 35% for the seismic records studied. For the modal damping ratios, increasing values with increasing motion amplitude are observed, with variation ranging from 50%, for the first translational modes, to over 120% for higher modes. For medium and low level earthquakes this variations disappear after the strong shaking has ended. This article also presents a preliminary analysis of the seismic records on the building during the Mw=8.8 earthquake occurred in Chile in 2010. Structural Damage was observed on the building and modal parameters changed significantly. During this seismic event, modal frequencies decreased up to 35% for the first translational modes. After the earthquake, modal frequencies had an 18% permanent decrease on average from initial values.

Lessons from the 2010 Chile Earthquake for Performance Based Design and Code Development

The February 27, 2010 Mw 8.8 Maule earthquake in Chile generated MM VII intensity or higher, and PGA = 0.3 g or higher on a 100 km wide by 600 km long corridor, as well as a tsunami . Notwithstanding the large area affected by strong shaking, where about eight million people live, there were only 521 casualties . Approximately a third of the casualties were due to the tsunami alone. Nearly the rest were due to the collapse of non-engineered low-rise dwellings. Eight people only died in modern buildings. The number of severely damaged tall buildings , most likely requiring demolition or heavy structural intervention, has been estimated at around 50 out of 2,000 buildings. A large proportion of the structural damage in tall buildings concentrated in buildings 10 or less years old supported on intermediate or soft soils. Taking into account the total building stock exposure and its damage, and the total population exposure and its losses, this earthquake showed that the local engineering practices are effective at preventing loss of life. However, the disproportionate concentration of structural damage in newly built buildings, the collapse of three buildings, and widespread damage to nonstructural components and systems prompted the government to revise current design practice, in part because current societal expectations are different from expected performance tacitly or explicitly stated in the local design codes.