Do directionality effects influence expected damage? A case study of the 2017 central Mexico earthquake

We analyze the case of a building that collapsed in a multifamily complex of Tlalpan borough in Mexico City during the 19 September 2017 Central Mexico earthquake. Despite having similar materials and similar structural and geometric properties, this was the only building that collapsed in the complex. A structural analysis of the building and a study of the soils’ predominant periods indicated that resonance effects, if any, would not be significant. However, phenomena related to the anomalous performance of buildings in dense urban areas such as geological soil, soil–structure interaction, and soil–city interaction effects were also investigated. A detailed analysis of the directionality of seismic actions recorded at nearby accelerometric stations and of the azimuths of sound and damaged buildings indicates that directionality effects were responsible for the collapse of the building. Subsequently, a set of 58, two-component acceleration records of the earthquake was used to perform a thorough directionality analysis. The results were then compared with the foreseen uniform hazard response spectra and the design spectra in the city. Seismic actions in the city due to this earthquake were stronger than those corresponding to the uniform hazard response spectra. In addition, although design spectra have been significantly improved in the new 2017 Mexican seismic regulations, they were exceeded in 11 of 58 analyzed spectra. In 4 of these 11 cases, the design spectra were exceeded due to directionality effects. These results confirm the necessity of considering directionality effects in damage assessments, strong-motion prediction equations, and design regulations.Peer ReviewedPostprint (author's final draft

Intensidad de Arias y duración significativa en análisis dinámico de estructuras

El uso de procedimientos y metodologías estandarizadas para realizar análisis dinámico no lineales (ADNL) está en aumento. En este tipo de análisis, varios de los criterios pueden ser definidos de forma arbitraria o seleccionados a criterio del experto, tal es el caso de la selección de los acelerogramas a ser utilizados como dato de partida para el ADNL. Es evidente que una selección inadecuada de los parámetros de entrada, incluyendo las acciones sísmicas, generaría resultados poco realistas y sin sentido físico. En este sentido, el presente estudio tiene como objetivo analizar la influencia de la intensidad de Arias (IA) y la duración significativa (Da5-95) en la selección de registros de aceleración compatibles con un espectro de diseño objetivo para la realización de ADNL. De forma particular, se estudia la variación del daño estructural de un edificio a partir de cuatro acelerogramas, con diferentes IA y Da5-95, ajustados al mismo espectro de diseño. Los resultados demuestran diferencias significativas en el daño esperado. Los registros con mayor intensidad de Arias y duración significativa produjeron un daño estructural mayor a pesar de disponer de espectros de respuesta equivalentes. Se concluye que es relevante considerar estos parámetros sísmicos en la selección de acelerogramas para su uso en ADNLPeer ReviewedPostprint (published version

Dynamic soil-structure interaction analyses considering directionality effects

The paper investigates directionality effects of ground motions in the context of dynamic soil-structure interaction (DSSI) analyses. The problem addressed corresponds to a nonlinear soil deposit, overlaying firm ground, where the input motion was derived from an acceleration time history recorded at a rock outcrop. A simplified procedure is proposed to incorporate directionality effects. The main objective is to identify in advance the incidence angle producing the maximum response of a structure for a given earthquake. Results from the simplified procedure were evaluated by comparison with what is called here the complete rotational approach, where the behaviour of the structure, as a function of the incidence angle of the input motion, is derived through a large number of nonlinear dynamic soil-structure interaction analyses. The obtained results show the importance of considering directionality effects in DSSI analyses. The maximum response of the system was reasonably captured with the simplified approach.Peer ReviewedPostprint (author's final draft

On the equal displacement aproximation for mid-rise reinforced concrete buildings

The equal displacement approximation is a well-known procedure for estimating the non-linear behavior of structures subjected to earthquake ground motions. This procedure plays a significant role in current seismic design, since it constitutes the basic assumption for defining strength reduction factors. In this paper, calculation of the performance point based on this rule is used to estimate engineering demand parameters such as those obtained by advanced probabilistic non-linear dynamic analysis, NLDA. We present a modification to the classic approach, to improve the predictability of the equal displacement rule. Uncertainties in seismic action and structural properties are considered. Mid-rise reinforced concrete buildings will be used as a testbed. To obtain a representative sample of buildings for statistical analysis, we describe the development through implementation of a numerical tool for calculating probabilistic NLDA. This tool, which is expected to evolve into interoperable software for assessing the seismic risk of structures, is developed within the framework of the KaIROS project. The results presented in this paper could be used to estimate the seismic risk of structures in a very simplified manner.Postprint (published version

Dynamic monitoring of a mid-rise building by real-aperture radar interferometer: advantages and limitations

In this paper, remote and in situ techniques to estimate the dynamic response of a building to ambient vibration are reported: data acquired through a real-aperture radar (RAR) interferometer and conventional accelerometers are analyzed. A five-story reinforced concrete housing building, which was damaged during the May 11th 2011 Lorca (Spain) earthquake, is used as a case study. The building was monitored using both types of instruments. The dynamic properties of the building are estimated first taking acceleration measurements using a set of 10 high-precision accelerometers installed on the roof of the building. Further, the displacement–time histories, recorded with the RAR device pointing to a corner of the building, are analyzed. Then, the ability and shortcomings of RAR measurements to deal with the fundamental frequencies of vibration of the structure are investigated. The advantages and limitations of from-inside (accelerometric) and from-outside (RAR) measurements are highlighted and discussed. A relevant conclusion is that, after strong earthquakes, RAR may be an interesting and useful tool, as it allows surveying the structural response of mid-rise buildings remotely, without the need to enter the structures, which may be dangerous for inspectors or technicians in cases of severely damaged buildings. Given that the instrumented building suffered significant damage, the ability of these kinds of measurements to detect damage is also discussed.This research has been partially funded by the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government and by the European Regional Development Fund (ERDF) of the European Union (EU) through projects with references CGL2015-65913-P (MINECO/ERDF, EU) and EFA158/16/POCRISC (INTERREG/POCTEFA. EU). R.E.A. holds a PhD fellowship from the Consejo Nacional de Ciencia y Tecnología (CONACyT) in México. L.A.P. is supported by a Ph.D. scholarship grant from the Institute for the Training and Development of Human Resources (IFARHU) and the Government of Panama’s National Secretariat of Science, Technology, and Innovation (SENACYT).Peer ReviewedPostprint (published version

Remote ambient vibration measurements with real aperture radar to estimate buildings dynamic properties

The use of sensors to obtain the dynamic properties of structures by means of ambient vibrations has become a common practice. However, conventional and more extensively used methods require the installation of sensors, which should be in contact with the structure. These methods could be even risky in case of structures weakened by a hazardous event, for which the risk of collapse is unknown. In this study, two main subjects are analyzed and discussed. The former is related to the use of the Real-Aperture Radar (RAR) as a non-invasive, remote sensing interferometric method to measure the vibration response to ambient noise of buildings. As a test bed, two high-rise regular buildings with reinforced concrete frames (8-story) and steel frames (12-story), located in Barcelona, Spain, are analyzed. Notice that the RAR device is a sensor using the principle of interferometry to measure displacements. This type of sensors has exhibited a good performance in urban environments at different atmospheric, meteorological and lighting conditions, compared to other types of remote sensors (e.g. Laser Doppler). Another advantage is that RAR devices have a very good resolution and they can operate at great distances from the object to be measured. However, so far, RAR is rarely used in buildings, being still a poorly studied technique. The latter subject is related to the optimal post-processing of the acquired data. At this respect, the Power Spectral Density (PSD) signal-processing technique is used to process the signals acquired. From this analysis, the predominant periods of the analyzed buildings were obtained and then they were compared with those periods obtained from measurements of accelerometers located inside the structures. The results indicate that the use of RAR can be a reliable alternative to estimate the structural periods in a relatively simple and non-invasive manner.Postprint (published version

Daño sísmico y propiedades modales en estructuras de edificación

The time required to inspect damaged structures after an earthquake has a direct impact on the social and economic consequences of the event, which can be catastrophic. Developing tools to aid the inspection task is of great interest to reduce the impacts of seismic events and increase the resilience of cities. In this work we study the basis of a methodology for structural damage diagnosis, which contributes to make faster and more objective decisions about the safety and reoccupation of buildings after an earthquake. It is proposed to estimate the damage in buildings from the change in their dynamic properties. This concept is well established, but its use lacks an explicit and practical purpose of damage assessment. In this regard, we explore new remote sensing techniques for operational modal analysis. In the first part of this thesis, we study feasibility of estimating modal properties in different building types by means of an innovative remote sensing technique, based on Real Aperture Radar (RAR). To improve the quality of the measurements, novel signal analysis and data fusion techniques are explored. The results are contrasted with those obtained from conventional in-contact sensing techniques. The second part of the thesis focuses on the evaluation of structural damage and its relationship with the modal properties of the structure. Specifically, the increase of the fundamental period induced by stiffness degradation due to structural damage, is investigated. Finally, the main advantages and disadvantages of the different measurement techniques are discussed, as well as the increase of the structural period as a damage indicator. A relevant conclusion is that RAR technology can become a very useful tool after an earthquake. This technique allows the identification of modal parameters, not only of tall and flexible structures, but also of low- to medium-rise buildings, without the need to access the structure. Therefore, the time and cost of evaluations can be reduced, in addition to removing the risk of entering a damaged structure. It is worth mentioning that these tools would not only be useful in the post-earthquake emergency phase, where speed of assessment is critical, but also in the subsequent stages of damage assessment and for other purposes, such as structural health monitoring and numerical model calibration.En la gestión de una emergencia post-sísmica el tiempo requerido para inspeccionar las estructuras dañadas tiene una repercusión directa en las consecuencias sociales y económicas del sismo, las cuales pueden llegar a ser catastróficas. Desarrollar herramientas que faciliten la tarea de inspección resulta de gran interés para reducir los impactos de los eventos sísmicos e incrementar la resiliencia de las ciudades. En este trabajo se estudian las bases de una metodología de diagnóstico de daño estructural, la cual contribuya a tomar decisiones más rápidas y objetivas sobre la seguridad y reocupación de las edificaciones después de un sismo. Se propone estimar el daño en las edificaciones a partir del cambio en las propiedades dinámicas de las mismas. Este concepto está bien establecido, pero su uso carece de un propósito explícito y práctico de evaluación del daño. En este sentido, se hace uso del análisis modal operacional con nuevas técnicas de medición sin contacto. En la primera parte de esta tesis se estudia la viabilidad de estimar las propiedades modales en distintas tipologías de edificios por medio de una innovadora técnica de teledetección, basada en el Radar de Apertura Real (RAR). Para mejorar la calidad de las mediciones, se exploran técnicas novedosas de análisis de señales y fusión de datos. Los resultados son contrastados con los obtenidos de técnicas convencionales de medición con contacto. La segunda parte se centra en la evaluación de daño estructural y su relación con las propiedades modales de la estructura. Específicamente, se investiga el incremento del periodo fundamental en un edificio de tipología común en España, inducido por la degradación de rigidez debida al daño estructural. Finalmente se discuten las principales ventajas y limitaciones de las distintas técnicas de medición, así como del incremento de periodo estructural como indicador de daño. Una conclusión relevante es que la tecnología RAR puede llegar a ser una herramienta de gran utilidad después de un terremoto. Esta técnica permite la identificación de parámetros modales, no solo de estructuras altas y flexibles, sino también de edificaciones más rígidas de mediana altura, sin la necesidad de acceder a la estructura. Por lo tanto, se puede reducir el tiempo y costo de las evaluaciones, además de eliminar el riesgo que representa entrar en una estructura dañada. Cabe mencionar que estas herramientas no solo serían útiles en la fase de emergencia post-sismo, donde la rapidez de la evaluación es fundamental, sino también en las etapas siguientes de evaluación de daño y con otros propósitos, tales como el monitoreo de la salud estructural y la calibración de modelos numéricos.Postprint (published version

Case study of a heavily damaged building during the 2016 MW 7.8 Ecuador earthquake: directionality effects in seismic actions and damage assessment

In this work, the directionality effects during the MW 7.8 earthquake, which occurred in Muisne (Ecuador) on 16 April 2016, were analyzed under two perspectives. The first one deals with the influence of these effects on seismic intensity measures (IMs), while the second refers to the assessment of the expected damage of a specific building located in Manta city, Ecuador, as a function of its azimuthal orientation. The records of strong motion in 21 accelerometric stations were used to analyze directionality in seismic actions. At the closest station to the epicenter (RRup = 20 km), the peak ground acceleration was 1380 cm/s2 (EW component of the APED station). A detailed study of the response spectra ratifies the importance of directionality and confirms the need to consider these effects in seismic hazard studies. Differences between IMs values that consider the directionality and those obtained from the as-recorded accelerograms are significant and they agree with studies carried out in other regions. Concerning the variation of the expected damage with respect to the building orientation, a reinforced concrete building, which was seriously affected by the earthquake, was taken as a case study. For this analysis, the accelerograms recorded at a nearby station and detailed structural documentation were used. The ETABS software was used for the structural analysis. Modal and pushover analyses were performed, obtaining capacity curves and capacity spectra in the two main axes of the building. Two advanced methods for damage assessment were used to obtain fragility and mean damage state curves. The performance points were obtained through the linear equivalent approximation. This allows estimation and analysis of the expected mean damage state and the probability of complete damage as functions of the building orientation. Results show that the actual probability of complete damage is close to 60%. This fact is mainly due to the greater severity of the seismic action in one of the two main axes of the building. The results are in accordance with the damage produced by the earthquake in the building and confirm the need to consider the directionality effects in damage and seismic risk assessments.This research has been partially funded by the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government and by the European Regional Development Fund (ERDF) of the European Union (EU) through projects with references CGL2015-65913-P (MINECO/ERDF, EU) and EFA158/16/POCRISC (INTERREG/POCTEFA. EU). R.E.A. holds a PhD fellowship from the Consejo Nacional de Ciencia y Tecnología (CONACyT) in México.Peer ReviewedPostprint (published version

Two complex high rise buildings case studies equipped with SLB seismic devices

Seismic protection devices such as energy dissipators have been used in structures for many years. However, it has not been until the last years that computers allowed assessing the full advantages of these devices in large structures by means of nonlinear dynamic analysis. Previous analysis/design techniques for these devices were based on simplified linear approaches such as considering an equivalent increment to the global damping. In this paper we evaluate the performance of the Shear Link Bozzo (SLB) seismic dissipators of two real high-rise buildings, located in a seismic prone area. In the first case study we compare the performance of the bare structure to a similar protection but using Buckle Restrained Braces and the bare structure. In the second case study, we show that a potential rebar optimization can be achieved using SLB devices without compromising the structural performance and satisfying the code requirements in high-rise buildings. The use of SLB devices showed not only economic advantages but also achieve a better performance compared to the bare structure and the equipped with BRBs.Postprint (published version

A simplified approach to account for directionality effects on 2D dynamic soil-structure interaction analysis

In this study, a simplified approach to account for directionality effects on twodimensional (2D) dynamic soil-structure interaction analyses (DSSI) is proposed. When an earthquake is recorded, the direction with the maximum intensities rarely corresponds with the orientation of accelerometers and, therefore, the maximum response of a system generally takes place at an intermediate unknown orientation. A straightforward approach is to perform DSSI analyses with a number of linear combinations of the horizontal as-recorded components to obtain the peak response parameters for each orientation. Nevertheless, DSSI analyses are computationally expensive especially when the nonlinear behavior of the soil is considered. Here, three hypotheses are evaluated for predicting the angle that generates the maximum response of a building. By determining this angle in advance, only one DSSI analysis would be required to obtain the maximum response of a system for a given earthquake. Here, a particular case is analyzed where a ground motion recorded at a rock outcrop is used to derive the input motion for a finite element model, including a soil deposit and a surface structure. A series of 2D dynamic finite element analyses were performed to assess the proposed approach where input motions were obtained from the linear combination of the horizontal as-recorded components of the 1976 Friuli earthquake. The obtained results show the importance of considering directionality effects in DSSI analyses. The maximum response of the system was reasonably captured with the orientation obtained through the simplified approach.This research has been partially funded by the Ministry of Economy and Competitiveness (MINECO) of the Spanish Government and by the European Regional Development Fund (FEDER) of the Eu-ropean Union (UE) through the project reference CGL2015-65913-P (MINECO/FEDER, UE). Alva RE, the first author of the study, is the holder of a PhD fellowship from the Centro Nacional de Ciencia y Tecnología (CONACyT) in México.Postprint (published version