New methodology for calculating damage variables evolution in Plastic Damage Model for RC structures

The behavior of reinforced concrete (RC) structures under severe demands, as strong ground motions, is highly complex; this is mainly due to joint operation of concrete and steel, with several coupled failure modes. Furthermore, given the increasing awareness and concern for the important seismic worldwide risk, new developments have arisen in earthquake engineering. Nonetheless, simplified numerical models are widely used (given their moderate computational cost), and many developments rely mainly on them. The authors have started a long-term research whose final objective is to provide, by using advanced numerical models, solid basis for these developments. Those models are based on continuum mechanics, and consider Plastic Damage Model to simulate concrete behavior. Within this context, this paper presents a new methodology to calculate damage variables evolution; the proposed approach is based in the Lubliner/Lee/Fenves formulation and provides closed-form expressions of the compressive and tensile damage variables in terms of the corresponding strains. This methodology does not require calibration with experimental results and incorporates a strategy to avoid mesh-sensitivity. A particular algorithm, suitable for implementation in Abaqus, is described. Mesh-insensitivity is validated in a simple tension example. Accuracy and reliability are verified by simulating a cyclic experiment on a plain concrete specimen. Two laboratory experiments consisting in pushing until failure two 2-D RC frames are simulated with the proposed approach to investigate its ability to reproduce actual monotonic behavior of RC structures; the obtained results are also compared with the aforementioned simplified models that are commonly employed in earthquake engineering.Postprint (published version

Numerical study on the relevance of columns hidden failure modes in the seismic capacity of non-ductile RC Frames

In simplified seismic structural analyses, not all the deterioration modes are adequately considered. This work discusses the relation among the hidden failure modes of columns of non-ductile reinforced concrete building frames and their global collapse mechanism. With this aim, a numerically efficient model is developed and implemented in OpenSEES. Two benchmark problems are analyzed with this model: the well-known Van Nuys Hotel and a prototype building designed for gravity loads only; in this last case, the results are compared with experiments on a one-third scale model. The obtained results confirm that simplified models grossly overestimate the building capacity.Peer ReviewedPostprint (author's final draft

Advanced computationally efficient modeling of RC structures nonlinear cyclic behavior

Under severe seismic excitation, structural behavior of buildings and other constructions is highly complex. It involves, among other issues, soil-structure interaction, large strains and displacements, damage, plasticity, and near-collapse behavior. Moreover, in reinforced concrete structures, there are several coupled degradation and failure modes: cracking, crushing and spalling of concrete, yielding and pull-out of tensioned reinforcement, yielding and buckling of compressed reinforcement. Furthermore, another circumstance makes the situation more alarming: given the increasing awareness and concern on the huge worldwide seismic risk, earthquake engineering has experienced in last years substantial advances. New design and analysis strategies have been proposed, leading to relevant developments. These developments rely on extensive testing and numerical simulation mainly based on oversimplified models referred in this work as structural component-based models, as a result of their moderate computational cost. Therefore, there is a strong need of verifying the reliability of the new developments by comparison with analyses performed using more advanced simulation tools and with experiments. This work is organized in two parts. First part presents an accurate model, while the second part deals with a more simplified model, although highly computational efficient. First part. This research clarifies the aforementioned issues by developing a new continuum mechanics-based model for simulating the monotonic and cyclic behavior of reinforced concrete structures. The developed model combines a new methodology for calculating the damage variables in Concrete Plastic Damage Models "CPDM", and a new approach to integrate CPDM with a 3-D interface bond-slip model developed by other researchers. A new scheme to implement the interface model in a continuum FEM model of regions with crossing reinforcement bars is also presented in this research. Mesh-insensitivity, accuracy and reliability of the proposed model are verified by simulating several monotonic and cyclic tests; the obtained results are compared with experimental ones, satisfactory agreement has been accomplished. Second part. The developed model is the First Part is compared with simplified structural component-based models that are commonly used in earthquake engineering; results has shown the superiority of the proposed model to predict the actual behavior of highly damaged RC elements and frames, capturing strength reduction, stiffness degradation and pinching phenomena. However, some of the structural component-based models have shown an acceptable performance considering the law computationally cost in comparison with the advanced continuum mechanics-based model. After this conclusion, this part presents a numerical study on the relation among the non-simulated deterioration modes of the elements in non-ductile RC frames and their final capacity. A structural component-based model has been developed for simulating the nonlinear dynamic behavior of non-ductile reinforced structures, accounting for flexure, shear and axial deterioration modes. The developed model is numerically efficient, thus being suitable for day use in earthquake engineering. The capacity of the developed model is verified by simulating the nonlinear dynamic behavior of an existing non-ductile building and the prototype building. Obtained results shows that the developed model, despite its moderate computational cost, detects and reproduces accurately the nonlinear dynamic behavior of non-ductile RC structures, as well, capturing the deterioration modes that are blind to the simplified models. Comparison with results from more simplified models highlights the importance of hidden failure modes in the behavior of each element and in the overall collapse mechanisms. The relation between the non-simulated failure modes and the so-called "Structural Resurrection" is addressedEl comportamiento estructural de edificios y otras construcciones bajo severas excitaciones sísmicas es muy complejo e implica temas como, la interacción suelo-estructura, grandes esfuerzos y desplazamientos, daños, plasticidad y el comportamiento de la estructura cerca del colapso. Por otra parte, en estructuras de hormigón armado, existen varios modos de fallo y de degradación: agrietamiento, aplastamiento y desprendimiento del hormigón, plastificación y extracción de las armaduras traccionadas y plastificación y pandeo de las armaduras comprimidas. Además, otras circunstancias hacen que la situación sea más alarmante: dada la creciente conciencia y preocupación por el enorme riesgo sísmico mundial, la ingeniería sísmica ha experimentado en los últimos años avances sustanciales, para lo cual se han propuesto nuevas estrategias de análisis y diseño, lo que conduce a desarrollos relevantes. Estos desarrollos se basan en pruebas y simulaciones numéricas basadas principalmente en modelos simplificados referidos en este trabajo como modelo basados en la estructura, resultando un costo computacional moderado. Por lo tanto, existen una gran necesidad de verificar la fiabilidad de los nuevos desarrollos en comparación con los análisis realizados utilizando herramientas de simulación más avanzadas y con ensayos. Este trabajo se organiza en dos partes; en la primera se describe un modelo preciso basado en la mecánica del medio continuo y en la segunda se presenta otro modelo más simplificado basado en los componentes de la estructura. Primera parte. En esta parte se desarrolla un nuevo modelo basado en la mecánica del medio continuo para simular el comportamiento monotónico y cíclico de estructuras de hormigón armado. El modelo desarrollado combina una nueva metodología para el cálculo de las variables del daño en el Modelo de Daño Plástico del Hormigón “CPDM”, y un nuevo enfoque para integrar el CPDM con un modelo de interface de 3-D desarrollado en otra investigación. También se presenta un nuevo esquema para implementar la interfaz del modelo en un modelo FEM continuo de regiones con armaduras que se cruzan en varias direcciones. La precisión, la fiabilidad y la insensibilidad a la malla del modelo propuesto se verifican simulando varias pruebas incrementales y cíclicas; los resultados obtenidos se comparan con experimentales, lográndose un ajuste satisfactorio. Segunda parte. El modelo desarrollado in el Primer Parte ha sido comparado con modelos simplificados basados en los componentes estructurales de uso común en la ingeniería sísmica, los resultados mostraron la superioridad del modelo pro-puesto para predecir el comportamiento real de los elementos y pórticos RC altamente dañados, capturando la reducción de la resistencia, la degradación de la rigidez y el efecto pinzamiento (“pinching”). Sin embargo, algunos de los modelos basados en componentes estructurales han mostrado un desempeño aceptable teniendo en cuenta el costo computacional de la ley en comparación con el modelo avanzado basado en la mecánica del medio continuo. Con de esta conclusión, este parte de este trabajo presenta un estudio numérico sobre la relación entre los modos de deterioro no-simulados de pórticos de hormigón sin ductilidad y su capacidad última. Se ha desarrollado un modelo avanzado basado en los componentes de la estructura para simular el comportamiento dinámico no lineal de las estructuras sin ductilidad, teniendo en cuenta los modos de deterioro de flexión, corte y axial. El modelo desarrollado es numéricamente eficiente, siendo pues adecuado para el uso profesional en ingeniería sísmica. La capacidad del modelo desarrollado se verifica mediante la simulación del comportamiento dinámico no lineal de un edificio no dúctil existente y del edificio prototipo.Postprint (published version

Suitability of seismic isolation for buildings founded on soft soil. Case study of a RC building in Shanghai

Base (seismic) isolation is a promising technology for seismic protection of buildings and other constructions. Nowadays, it is accepted that such a technique is efficient and reliable; however, it has two major limitations: soft foundation soil, and tall buildings. The first issue restrains the seismic isolation spreading, given that soft soil is frequent in densely populated areas, and usually such a soil type concentrates the highest seismicity levels. This paper aims to contribute to demonstrating that base isolation, if properly implemented, can be suitable for soft soil. A representative case study is analyzed: a 6-story reinforced concrete (RC) building with base isolation that has recently been built in Shanghai. Since the building is founded on soft soil, concern regarding base isolation suitability arose; even the Chinese design code does not recommend this solution for soft soil. To clarify this issue, non-linear time-history analyses are carried out for a number of natural and artificial seismic inputs that represent the site seismicity; the superstructure behavior is linear, while nonlinearities are concentrated in the isolation layer. The adequacy of base isolation is assessed in the superstructure (in terms of reduction of interstory drift, absolute acceleration and shear force) and in the isolation layer (in terms of axial force, torsion angle and shear strain). The relevance of soil–structure interaction is discussed. The behavior when the mechanical parameters of the isolation units have experienced important changes is also analyzed. The major conclusion is that base isolation of ordinary mid-height RC buildings founded on soft soil can perform satisfactorily in medium seismicity regions.Peer ReviewedPostprint (published version

Importance of non-simulated failure modes in incremental dynamic analysis (IDA) of non-ductile RC frames

Nonlinear dynamic analysis of reinforced concrete structures requires a thorough understanding of behavior of structural elements for all levels of performance, especially collapse. Main reason is that reinforced concrete is a mixed material having complicated nonlinear behavior, each element having several coupled degradation modes that influence the entire structure and can change its collapse mechanism. Therefore, using oversimplified models can lead to misleading results. This paper presents a numerical study on the relation among the non-simulated failure modes of the structural elements of non-ductile RC frames and their collapse mechanism; this study reports on the so-calledPostprint (published version

Numerical study on the relevance of columns hidden failure modes in the seismic capacity of non-ductile RC Frames

In simplified seismic structural analyses, not all the deterioration modes are adequately considered. This work discusses the relation among the hidden failure modes of columns of non-ductile reinforced concrete building frames and their global collapse mechanism. With this aim, a numerically efficient model is developed and implemented in OpenSEES. Two benchmark problems are analyzed with this model: the well-known Van Nuys Hotel and a prototype building designed for gravity loads only; in this last case, the results are compared with experiments on a one-third scale model. The obtained results confirm that simplified models grossly overestimate the building capacity.Peer Reviewe

Nonlinear time-history analysis of a base-isolated RC building in Shanghai founded on soft soil

A 6-story RC building with base isolation using rubber bearings and viscous dampers has been recently built in Shanghai. Since the building is founded on soft soil, concern regarding base isolation suitability arose; even the Chinese design code does not recommend this solution for soft soil. To clarify this issue, nonlinear time-history analyses are carried out for a number of natural and artificial seismic inputs that represent the site seismicity, accounting for the soil conditions. The relevance of soil-structure interaction is discussed and some simulations are performed. Adequacy of base isolation is assessed both in the superstructure and the isolation layer. In the superstructure, appropriateness is judged in terms of reduction of interstory drift, absolute acceleration and shear force. In isolators, correctness is evaluated in terms of axial force, torsion angle and lateral displacement; prescriptions of Chinese and European regulations are considered. The major conclusion is that base isolation of ordinary mid-height RC buildings founded on soft soil can perform satisfactorily in medium seismicity regions.Postprint (published version

Numerical simulation of RC frame testing with damaged plasticity model

This study aims to investigate the capacity of simplified and advanced numerical models to reproduce the nonlinear monotonic behavior of reinforced concrete framed structures. The considered simplified models are commonly used in earthquake engineering and are based either on concentrated or distributed plasticity. The considered advanced models are based on continuum mechanics and use the Damaged Plasticity Model for concrete simulation. The simplified and advanced models are used to simulate pushing experiments of a 2D RC frame. The results from the models are compared with the experiments results. That comparison shows the higher ability of the continuum mechanics models to represent the actual behavior of the frame until the collapse; the simplified models lack of the capacity to represent accurately all the phases of the behavior. This paper includes an explanation of the damage plasticity models and their implementation in the FEM package ABAQUS. A strategy to avoid the mesh sensitivity in the advanced numerical simulation of reinforced concrete structures is also described.Postprint (published version

Nonlinear time-history analysis of a base-isolated RC building in Shanghai founded on soft soil

A 6-story RC building with base isolation using rubber bearings and viscous dampers has been recently built in Shanghai. Since the building is founded on soft soil, concern regarding base isolation suitability arose; even the Chinese design code does not recommend this solution for soft soil. To clarify this issue, nonlinear time-history analyses are carried out for a number of natural and artificial seismic inputs that represent the site seismicity, accounting for the soil conditions. The relevance of soil-structure interaction is discussed and some simulations are performed. Adequacy of base isolation is assessed both in the superstructure and the isolation layer. In the superstructure, appropriateness is judged in terms of reduction of interstory drift, absolute acceleration and shear force. In isolators, correctness is evaluated in terms of axial force, torsion angle and lateral displacement; prescriptions of Chinese and European regulations are considered. The major conclusion is that base isolation of ordinary mid-height RC buildings founded on soft soil can perform satisfactorily in medium seismicity regions

New methodology for calculating damage variables evolution in Plastic Damage Model for RC structures

The behavior of reinforced concrete (RC) structures under severe demands, as strong ground motions, is highly complex; this is mainly due to joint operation of concrete and steel, with several coupled failure modes. Furthermore, given the increasing awareness and concern for the important seismic worldwide risk, new developments have arisen in earthquake engineering. Nonetheless, simplified numerical models are widely used (given their moderate computational cost), and many developments rely mainly on them. The authors have started a long-term research whose final objective is to provide, by using advanced numerical models, solid basis for these developments. Those models are based on continuum mechanics, and consider Plastic Damage Model to simulate concrete behavior. Within this context, this paper presents a new methodology to calculate damage variables evolution; the proposed approach is based in the Lubliner/Lee/Fenves formulation and provides closed-form expressions of the compressive and tensile damage variables in terms of the corresponding strains. This methodology does not require calibration with experimental results and incorporates a strategy to avoid mesh-sensitivity. A particular algorithm, suitable for implementation in Abaqus, is described. Mesh-insensitivity is validated in a simple tension example. Accuracy and reliability are verified by simulating a cyclic experiment on a plain concrete specimen. Two laboratory experiments consisting in pushing until failure two 2-D RC frames are simulated with the proposed approach to investigate its ability to reproduce actual monotonic behavior of RC structures; the obtained results are also compared with the aforementioned simplified models that are commonly employed in earthquake engineering