DSpace Repository :: Browsing by Author "Hernández, Francisco"

Browsing by Author "Hernández, Francisco"

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33
(Springer New York LLC, 33) Astroza, Rodrigo; Hernández, Francisco; Díaz, Pablo; Gutierrez, Gonzalo; 33
A full scale five-story reinforced concrete building was built and tested on the Large High Performance Outdoor Shake Table (LHPOST) at the University of California, San Diego in 2012. The main objective of the test program was to study the seismic response of the structure and the nonstructural components (NSCs) and their dynamic interaction at different levels of seismic excitation. The building specimen was first tested base-isolated and then fixed at its base. In the fixed-base configuration, a suite of six earthquake motions of various intensities was applied to the building to progressively increase the seismic demand. In this paper, the modal parameters of the fixed-base building are identified using the input-output dynamic data recorded during the seismic tests. The deterministic-stochastic subspace identification method (DSI) is employed to estimate the variations of the modal properties of the building during the seismic tests by employing a short-time windowing approach. The changes of the modal parameters during the seismic motions are tracked, analyzed, and compared to those previously obtained from ambient vibrations and low-amplitude white noise base excitation tests. The identified natural frequencies and equivalent damping ratios of the building changed with the intensity of the input motions and damage in the structure, while the mode shapes are found to be insensitive to them.
33
(33, 33) Birrell, Matías; Astroza, Rodrigo; Murcia-Delso, Juan; Hernández, Francisco; Bazáez, Ramiro; 33
This paper proposes a simplified tri-linear model for the lateral force-displacement relationship of monolithic exterior shear keys in reinforced concrete bridges, failing in sliding shear. To allow for implementation compatibility with a widely used simplified model for shear keys failing in diagonal tension (DT), this model is presented in a tri-linear formulation that builds on past experimental data and expressions for the capacity of shear keys. Capacity points are adapted from previously published work on sliding shear (SS) and sliding friction (SF) failing shear keys, and displacement points are devised around available experimental data and numerical studies. These consider the effective stiffness of concrete members, dowel action in vertical steel reinforcement, and empirical observations from monolithic shear key tests. Model parameters are probabilistically characterized in a Bayesian parameter estimation framework, to incorporate experimental data. The calibrated expressions are then validated by comparing sampled predictions to experimental observations, and finally a single distribution for model parameters is suggested. The goal of this model is to provide a simplified formulation to be implemented alongside DT failure in time-history response simulations of structure-level finite element models of bridges, thus covering both major failure mechanisms in monolithic reinforced concrete shear keys.
33
(33, 33) Pinto, Francisco J.; Toledo, José; Birrell, Matías; Bazáez, Ramiro; Hernández, Francisco; Astroza, Rodrigo; 33
Bridges are essential structures in the logistic chain of countries, making it critical to design them to be as resilient as possible. One way to achieve this is through performance-based seismic design (PBSD), which involves using nonlinear Finite Element (FE) models to predict the response and potential damage of different structural components under earthquake excitations. Nonlinear FE models need accurate constitutive models of material and components. Among them, seismic bars and laminated elastomeric bearings play an important role in a bridge’s response to earthquakes; therefore, properly validated and calibrated models should be proposed. Only default parameter values from the early development of the constitutive models widely used by researchers and practitioners for these components tend to be used, and low identifiability of its governing parameters and the high cost of generating reliable experimental data have prevented a thorough probabilistic characterization of their model parameters. To address this issue, this study implements a Bayesian probabilistic framework using Sequential Monte Carlo (SMC) for updating the parameters of constitutive models of seismic bars and elastomeric bearings and proposes joint probability density functions (PDF) for the most influential parameters. The framework is based on actual data from comprehensive experimental campaigns. The PDFs are obtained from independent tests conducted on different seismic bars and elastomeric bearings, to then consolidate all the information in a single PDF for each modeling parameter by means of the conflation methodology, where the mean, coefficient of variation, and correlation between calibrated parameters are obtained for each bridge component. Finally, findings show that the incorporation of model parameter uncertainty through a probabilistic framework will allow for a more accurate prediction of the response of bridges under strong earthquakes.
33
(33, 33) Arellano, Benjamín; Hernández, Francisco; Massone, Leonardo M.; Astroza, Rodrigo; Soto, Pedro; Contreras, Nicolás; Garrido, Bastián; 33
A novel system identification technique called Mod-? (var) is formulated, implemented, and validated to study the seismic response of a 3D R/C shear wall building during the 2010 central Chilean mega-earthquake (Mw = 8.8). The Mod-?(var) approach is an evolution of Least-Square modal system identification techniques, where modal parameters are adjusted through small data windows to fit seismic data in the frequency and time domains. This technique offers several advantages over traditional methods, including the estimation of time-variant dynamic properties during seismic events and the reliable assessment of continuous nonlinear modal responses. As a result, the Mod-?(var) approach allows for determining empirical response spectra related to each seismic input. The technique can also compute the local response of measured and unmeasured floors as the product between the nonlinear modal responses (obtained from the Mod-?(var) approach) and the normalized seismic mode shapes for all building floors, which can be estimated from ambient vibration data. Finally, the seismic floor deformations can be imposed on a FEM to determine relevant engineering quantities such as inter-story drift, inter-story forces, and local demand of structural elements (e.g., drift ratios, curvatures, internal forces, stresses, strains, etc.).
33
(33, 33) Hernández, Francisco; Astroza, Rodrigo; Beltrán, Juan Felipe; Zhang, Xihong; Mercado, Vicente; 33
An energy dissipation mechanism made of a cable-pulleys system placed in series with a spring-damper device (fluid viscous) is experimentally studied. The system aims to provide high damping ratios for all the structural modes by using a unique spring-damper device to dissipate the seismic energy of the entire structure (and all its structural modes). Shake table tests and pull-back tests are carried out on a scaled five-story structure to compare the dissipation capabilities provided by the proposed system. Therefore, the same structure is tested under different configurations that included: i) the structure itself without any energy mitigation device, ii) the structure with viscous dampers installed on each story, iii) the structure with the proposed cable-pulleys and the spring-damper system, and iv) the structure with the cable-pulleys system but without any dissipation device. The experimental results showed that the structure with the proposed system exhibits a highly nonlinear response (mainly explained by the cable-pulleys interaction) evidenced by the significant change of the structure's dynamic properties during the time. The Short-Time Transfer Function plots show that the structure's natural frequencies change significantly when the cable-pulleys system is included. Complementarily, a novel time-variant system identification approach, termed Mod-?(var), is proposed, which allows estimating the time-variant evolution of the structure's dynamic properties during seismic tests (natural frequencies, damping ratios, and mode shapes). Moreover, the Mod-?(var) approach also enables computing relevant engineering quantities such as the empirical response spectrum from experimental data. It is found that the analyzed energy dissipation system provides high damping ratios (>10%) for all the structural modes, allowing reducing the seismic demands in terms of the empirical response spectrum, inter-story drifts, inter-story shear forces, peak accelerations, and Housner Intensities at each floor.
33
(33, 33) Pinto, Francisco; Astroza, Rodrigo; Bazáez, Ramiro; Hernández, Francisco; Navarro, Nibaldo; 33
Deterioration mechanisms, such as chloride-induced corrosion, affect bridges in aggressive environments, making them more vulnerable to extreme events like earthquakes. Although many studies have assessed the impact of chloride-induced corrosion on the seismic vulnerability of reinforced concrete (RC) highway bridges, several gaps still need to be addressed. Accordingly, this research primarily focuses on evaluating the seismic performance of RC highway bridges in aggressive environments susceptible to chloride-induced corrosion and earthquakes. To achieve this, a probabilistic framework is employed, which incorporates uncertainties associated with corrosion progression, seismic events, and the impact of different modeling approaches for boundary condition. The framework considers the time-dependent effects of corrosion on the physical and material properties of steel and concrete in bridge columns. Monte Carlo Simulations (MCS), probabilistic seismic hazard analysis (PSHA), and record selection strategies are utilized to address the uncertainties in corrosion and seismic demand. Nonlinear dynamic models and multiple stripe analysis (MSA) are employed to obtain fragility surfaces and curves for main bridge components and the entire system under different boundary conditions. The study focuses on a five-span highway bridge with simply-supported prestressed concrete I-girder and RC multi-column bents located in Chile. The results reveal that elastomeric bearings are the most vulnerable components, exhibiting varying vulnerability levels under different corrosion exposure and boundary conditions. Abutments, although the second most susceptible, are unaffected by corrosion uncertainty in terms of seismic fragility. Bridge columns are identified as the third most vulnerable components, with the probability of exceeding slight damage state consistently increasing with more prolonged corrosion exposure. It is noted that only flexure failure mode in column is analyzed and possible shifting to shear or flexure-shear modes is not accounted for. The findings of this study underscore the exceptional resilience of Chilean highway bridge columns to seismic demand and corrosion uncertainties, contrasting with the situation in US regions where columns are more vunerable. Additionally, the study indicates that Soil-Structure Interaction (SSI) tends to reduce bridge vulnerability under combined corrosion and earthquake effects. The insights obtained from this study and the proposed framework can inform the development of maintenance programs based on bridge performance expectations and enhance the seismic resilience of bridge systems worldwide.
33
(33, 33) Pinto, Francisco; Astroza, Rodrigo; Pizarro, Alonso; Bazáez, Ramiro; Hernández, Francisco; 33
Bridges are vital infrastructures but face compounding risks from scouring and seismic events. Although recent studies have explored these individual and overlapping hazards, they often utilize deterministic approaches and focus on specific bridge types in limited geographical areas. This study addresses these gaps by introducing a probabilistic framework for assessing the seismic performance of multi-span reinforced concrete (RC) highway bridges in the presence of scour-related uncertainties. A time-dependent scour model, informed by a Bayesian approach and literature-based uncertainties, is integrated into the framework. Probabilistic Seismic Hazard Analysis (PSHA) and record selection strategies are employed to define the seismic demand probabilistically. Monte Carlo simulation (MCS) is utilized to propagate combined uncertainties during the analysis, considering different scour scenarios defined according to the discharge peak condition of the flood event at the specific location of the bridge. In the last step of the probabilistic framework, fragility analyses are conducted on individual components and the entire system by means of nonlinear dynamic models and multiple stripe analysis (MSA). An actual five-span bridge with simply supported prestressed concrete girders, located in Chile, is used as application example of the proposed approach, considering measured hydrological data and seismic hazard of the real location of the bridge. The study identifies distinct vulnerabilities across various bridge components under scour conditions. Elastomeric bearings are particularly susceptible in scenarios characterized by low scour depths (low discharge peak condition). Conversely, abutments display heightened vulnerabilities as scour depth increases (medium discharge peak condition). Piles also exhibit notable vulnerability, escalating in scenarios with great scour depth (medium discharge peak condition). Notably, piles tend to be more susceptible than columns to scour effects, especially in bents featuring shorter columns. These disparities in vulnerability are influenced by the bridge design philosophy, as well as the synergistic effects of seismic forces and varying scour depths. System fragility curves further elucidate that the likelihood of exceeding specific damage thresholds varies according to different scour scenarios, with the deeper scour depth scenario, displaying the higher vulnerabilities.
Application of elastic metamaterials/meta-structures in civil engineering: a review
Contreras, Nicolás; Zhang, Xihong; Hao, Hong; Hernández, Francisco
Bayesian inference for calibration and validation of uniaxial reinforcing steel models
Birrell, Matias; Astroza, Rodrigo; Restrepo, José I.; Loftizadeh, Koorosh; Carreño, Rodrigo; Bazáez, Ramiro; Hernández, Francisco
Time variant system identification of superstructures of base-isolated buildings
Hernández, Francisco; Díaz, Pablo; Astroza, Rodrigo; Ochoa-Cornejo, Felipe; Zhang, Xihong

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