Predicting Time-Dependent Deformations in Prestressed Concrete Girders

A prestressed concrete girder camber prediction program, which utilizes a time-step approach, was improved to a new version including modifications, new features, and the fib Model Code 2010 material prediction model. The original version of the software was developed by Claire E. Schrantz in 2008. A few crucial corrections were included by Brandon R. Johnson in 2012. In the new version, a user is able to obtain predicted strains and stresses at designated depth, define actual and design compressive strengths, import variables from and export output data to a spreadsheet for further analysis. The camber prediction software includes four concrete modulus of elasticity (MOE) development models: two-point MOE model (uses MOE test results from two ages), AASHTO LRFD, ACI 209R-92, and fib MC 2010. Creep and shrinkage prediction models include AASHTO LRFD, ACI 209R-92, and fib MC 2010. Experimental data were collected from four previous research projects at Auburn University. This part of the study was used to verify the application’s capability of predicting time-dependent strain, curvature, and deflection. Measurements of strain and camber values were obtained from full-scale prestressed girders constructed with vibrated concrete (VC) or self-consolidating concrete (SCC). Camber and strain measurements of nineteen AASHTO BT-54 girders, fourteen AASHTO BT-72 girders, and six AASHTO Type I girders were collected. iii Measured camber data from twelve 15 in. deep T-Beams were also used in this study. Compressive strength of the concrete mixtures at 28 days varied between 6300 and 13,600 psi. Time-dependent properties and deflections were predicted by using the new version of the camber prediction software. The two-point MOE model in combination with various creep and shrinkage prediction models was chosen for predictions. Predictions were compared with the collected data from the fifty one girders to evaluate the accuracy of the creep and shrinkage prediction models. The comparison of time-dependent responses showed that camber growth, which is affected by creep, is overestimated for high-strength girders on average and include both over- and underpredictions for moderate-strength girders. ACI 209 predicts creep most accurately for high-strength girders, and AASHTO LRFD predicts creep most accurately for moderate-strength girders. Shrinkage predictions, which have an effect on prestress losses, are overestimated for high-strength girders and have mixed distribution of estimations for moderate-strength girder

Physics-Based Earthquake Ground Shaking Scenarios in Large Urban Areas

With the ongoing progress of computing power made available not only by large supercomputer facilities but also by relatively common workstations and desktops, physics-based source-to-site 3D numerical simulations of seismic ground motion will likely become the leading and most reliable tool to construct ground shaking scenarios from future earthquakes. This paper aims at providing an overview of recent progress on this subject, by taking advantage of the experience gained during a recent research contract between Politecnico di Milano, Italy, and Munich RE, Germany, with the objective to construct ground shaking scenarios from hypothetical earthquakes in large urban areas worldwide. Within this contract, the SPEED computer code was developed, based on a spectral element formulation enhanced by the Discontinuous Galerkin approach to treat non-conforming meshes. After illustrating the SPEED code, different case studies are overviewed, while the construction of shaking scenarios in the Po river Plain, Italy, is considered in more detail. Referring, in fact, to this case study, the comparison with strong motion records allows one to derive some interesting considerations on the pros and on the present limitations of such approach

Élaboration de Stratégies de Sélection de Signaux Accélérométriques pour le Calcul du Comportement des Structures

The observed variability is very large among natural earthquake records, which are not consolidated in the engineering applications due to the cost and the duration. In the current practice with the nonlinear dynamic analysis, the input variability is minimized, yet without clear indications of its consequences on the output seismic behavior of structures. The study, herein, aims at quantifying the impact of ground motion selection with large variability on the distribution of engineering demand parameters (EDPs) by investigating the following questions:What is the level of variability in natural and modified ground motions?What is the impact of input variability on the EDPs of various structural types?For a given earthquake scenario, target spectra are defined by ground motion prediction equations (GMPEs). Four ground motion modification and selection methods such as (1) the unscaled earthquake records, (2) the linearly scaled real records, (3) the loosely matched spectrum waveforms, and (4) the tightly matched waveforms are utilized. The tests on the EDPs are performed on a record basis to quantify the natural variability in unscaled earthquake records and the relative changes triggered by the ground motion modifications.Each dataset is composed by five accelerograms; the response spectrum compatible selection is then performed by considering the impact of set variability. The intraset variability relates to the spectral amplitude dispersion in a given set, and the interset variability relates to the existence of multiple sets compatible with the target.The tests on the EDPs are performed on a record basis to quantify the natural variability in unscaled earthquake records and the relative changes triggered by the ground motion modifications. The distributions of EDPs obtained by the modified ground motions are compared to the observed distribution by the unscaled earthquake records as a function of ground motion prediction equations, objective of structural analysis, and structural models.This thesis demonstrates that a single ground motion set, commonly used in the practice, is not sufficient to obtain an assuring level of the EDPs regardless of the GMSM methods, which is due to the record and set variability. The unscaled real records compatible with the scenario are discussed to be the most realistic option to use in the nonlinear dynamic analyses, and the ‘best’ ground motion modification method is demonstrated to be based on the EDP, the objective of the seismic analysis, and the structural model. It is pointed out that the choice of a GMPE can provoke significant differences in the ground motion characteristics and the EDPs, and it can overshadow the differences in the EDPs obtained by the GMSM methods.Les signaux accélérométriques enregistrés lors de l’occurrence d’un événement sismique est très large présentent une forte variabilité, par conséquent ils ne sont pas utilisé dans les analyse dynamiques de tenue sismique des structures. En effet, l’utilisation des accélérogrammes réels, pour les analyses dynamiques non linéaires, s’avère couteuse en termes de temps de calcul. La pratique courante prévoit la minimisation (voir suppression) de telle variabilité, mais les conséquences d’une telle opération sur la réponse des structures ne sont pas clairement indiquées. L’étude ci-présente a pour scope la quantification de l’impact des méthodes de sélection qui gardent la variabilité du signal sur les résultats de l’analyse de la réponse des structures (exprimée en termes d’engineering demand parameters EDPs). En particulier les questions suivantes seront investiguées :Quel est le niveau de variabilité des accélérogrammes réels et comment ce niveau est modifié par les techniques couramment utilisées ?Quelle est l’impact de la variabilité sur la réponse de plusieurs types de structures ?Pour un scénario sismique donné, un spectre cible est défini à partir de plusieurs équation de prédiction du mouvement sismique, sélection parmi celles disponibles en littérature. Les accélérogrammes sont sélectionnés à partir de quatre familles d’accélérogrammes, chacune relative à une méthode de modification : réels (enregistrés); mise à l’échelle (multiplication, par un facteur) ; calés aux spectres cibles avec large tolérance ; calés aux spectres cibles dans une plage de tolérance étroite.Chaque jeu de signaux est composé de cinq accélérogrammes et la sélection des signaux est faite en tenant compte de deux sources de variabilité : la variabilité au sein de chaque jeu de données (intraset), et la variabilité entre les différents jeux de données (interset) tous compatibles avec le même spectre cible. Les tests sur les EDPs menés sur les signaux accélérométriques réels mènent à la quantification de la variabilité naturelle (pour le scénario considéré). Les analyses basées sur les signaux réels sont utilisés comme benchmark afin d’évaluer non seulement de combien la distribution des EDPs (en termes de valeur moyenne et variabilité) est réduite par les différentes méthodes testées, mais aussi d’évaluer l’impact des choix de l’équation de prédiction du mouvement, des plages de tolérance, du nombre d’accélérogrammes constituant chaque jeu, du nombre de jeux, de le scope de l’analyse structurale et le modèle de structure.Ce travaille nous conduit à conclure que un seul jeu d’accélérogramme, tel qu’utilisé dans la pratique courante, est insuffisant pour assurer le niveau d’EDPs indépendamment de la méthode de modification utilisés, cela est lié à la variabilité des signaux et entre les jeux d’accélérogrammes. Les signaux réels, compatibles avec le spectre définis pour le scénario sismique, are l’option plus réaliste pour l’analyse dynamique non-linéaire ; si une méthode de modification du signal est nécessaire, la plus adaptées dépend du scope de l’analyse spectrale et du modèle. Le choix de l’équation de prédiction du mouvement sismique utilisée pour définir le spectre cible impacte significativement les caractéristiques des mouvements sismiques et des EDPs. Cette observation ne dépend pas de la stratégie de de modification du signal

Coupled Soil-Structure Interaction Effects of Symmetric and Asymmetric Buildings In Urban Regions

<p>This thesis deals with the response of idealized building clusters during earthquakes, their effect on the ground motion, and how individual buildings interact with the soil and with each other. We simulate the ground motion during the 1994 Northridge earthquake and focus on the coupled response of multiple simplified symmetric and asymmetric building models located within the San Fernando Valley and the Simi Valley. We use the Domain Reduction Method (DRM) in order to perform these simulations efficiently while recurrently modifying the models without having to redo the entire simulation every time. Numerical results show that the soil-structure interaction (SSI) effects vary with the number and dynamic properties of the buildings, their separation, and the impedance with respect to the soil. These effects appear as: (i) an increased spatial variability of the ground motion; and (ii) significant reductions in the buildings’ base motion at high frequencies, changes in the higher natural frequencies of the building-foundation systems and variations in the roof displacement, with respect to those of the corresponding rigid-base and single SSI models. Torsional coupling of the asymmetric structures combined with SSI effects are also investigated, and results, in comparison with the symmetric structures, are given.</p

The Influence of Building Clusters on the Variability of the Ground Motion During Earthquakes

Spatial variability and ground motion uncertainty during earthquakes can significantly influ-ence both our interpretation of seismic data and the behavior of structures and infrastructure systems, especially those susceptible to differential motions, or those that benefit from more diffuse wave-fields. Spatial variations typically observed in ground motions are mostly the consequence of wave interferences, refraction, scattering and other phenomena resulting from the three-dimensional nature of the crust, the surface topography, site conditions, and heterogeneities in the transmitting media. Also influential but regularly ignored is the presence of the built environment, especially in the case of densely urbanized regions. We are interest-ed in investigating the extent to which the presence of building-foundation systems can modify earthquake ground motions and contribute to their variability. We present preliminary results from a series of three-dimensional simulations using a finite element software for seis-mic wave propagation problems, with and without the presence of simplified building (block) models. We explore the level of influence exerted by the built environment on the ground motion through comparisons between the simulations with building models and equivalent simulations without them. This is the initial step of a project in which we seek to identify param-eters that can serve as proxies to characterize site-city interaction effects

Coupled Soil-Structure Interaction Effects of Building Clusters during Earthquakes

This study addresses the responses of idealized building clusters during earthquakes, their effects on ground motion, and the ways individual buildings interact with the soil and with each other. We simulate the ground motion during the 1994 Northridge earthquake and focus on the coupled responses of multiple simplified building models located within the San Fernando Valley. Numerical results show that the soil-structure interaction (SSI) effects vary with the number and dynamic properties of the buildings, their separation, and their impedance with respect to the soil. These effects appear as: (i) an increased spatial variability of the ground motion; and (ii) significant reductions in the buildings’ base motion at high frequencies, changes in the higher natural frequencies of the building-foundation systems, and variations in the roof displacement, with respect to those of the corresponding rigid-base and single SSI models.</jats:p