1,720,981 research outputs found
Evaluation of overstrength-based interaction checks for columns in steel moment frames
No full textA study is presented to examine the implications of this guidance, which disregards the column interaction check (including both axial force and moment) under overstrength seismic loads. A set of thirteen steel moment frames are designed using multiple rules that apply and disregard overstrength, drift, and cross-sectional compactness checks in various combinations. The frames are subjected to a suite of simulations including linear elastic, nonlinear static pushover, nonlinear response history, and continuum finite element simulations that are able to represent a range of physical behavior modes in the columns including interactive nonlinear geometric instabilities that could trigger loss of the load carrying capacity of the member. The simulations indicate no significant distinction between the seismic performance of steel moment resisting frames designed as per current code-based provisions (i.e., disregarding the column interaction check for overstrength seismic loads), and those designed with the use of the interaction check, with each providing acceptable response without failure. The simulations also indicate that design checks for drift and cross-sectional compactness play a significant role ensuring acceptable response, providing additional margin of safety beyond the member strength checks.RESSLA
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Experimental–Computational Framework for Fracture Risk Evaluation and Simulation of Welded Column Splices in pre-Northridge Steel Moment Frames
Get PDFThe 1994 Northridge earthquake in California exposed the susceptibility of welded connections in Steel Moment-Resisting Frames (SMRFs) to brittle fracture, resulting in a decade-long research effort that culminated in improved seismic design and construction practices for new steel moment frames. However, nearly three decades later, guidance for evaluating and retrofitting the connections in these existing SMRF buildings remains limited. Of particular concern are the Partial Joint Penetration (PJP) welded column splice connections in these buildings, which feature low flange penetration and low-toughness weld materials, making them susceptible to fracture with catastrophic consequences due to the potential loss of gravity load-carrying capacity. The absence of tools or guidelines often results in extraordinarily disruptive and expensive retrofits. Given the implications of both under- and over-estimating fracture risk, this dissertation establishes frameworks and tools for evaluating fracture risk and simulating the fracture response of PJP-welded column splices in pre-Northridge SMRFs.A probabilistic fracture mechanics-based framework is first developed to estimate the fracture fragility of these splices. The framework addresses shortcomings of previous research and performance assessment guidance that do not consider key mechanistic or statistical effects. The framework is implemented within a tool that automates the entire process, facilitating application in a professional setting. To demonstrate fracture in these splices and to provide experimental validation of the framework, an experimental program comprising 65 monotonic tensile tests on pull-plate specimens was conducted. These pull-plates were fabricated with weld metals, detailing, and procedures representative of pre-Northridge practice, with variations in weld penetration, flange configuration, plate width, preheating, and testing temperature. Ancillary tensile and Charpy V-Notch tests were conducted to characterize base and weld metal properties for fracture modeling. The experimental results reveal brittle fracture in most configurations, with fracture stresses substantially below the net section capacities. Existing approaches for estimating fracture probability are found to be conservative, whereas the probabilistic fracture mechanics framework, when stress-constraint effects are incorporated, closely reproduces the observed fragilities.To simulate the splice fracture and post-fracture response in structure-scale Non-Linear Response History Analysis (NLRHA), a novel fiber-based element – the Splice Fracture Element (SFE) – is formulated and implemented in OpenSees. The SFE incorporates several key features - representation of the loss of strength in any fiber at a fracture stress, the ability to simulate the loss of shear strength when the entire section is severed – a phenomenon not readily simulated in conventional fiber elements, and the ability to track the kinematics of the severed parts of the column to represent transfer of compressive stresses on contact. Demonstrative NLRHA of building with splices modeled using the SFE shows that the element can realistically simulate the physical response of splices and their interaction with the global structural response.Collectively, these advances establish an integrated experimental and computational foundation for evaluating the fracture risk and modeling fracture in pre-Northridge welded splices. The developed frameworks improve mechanistic understanding and provide practical tools to support risk evaluation and decision-making for the retrofit and performance assessment of existing steel moment frames
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ANALYTICAL MODELS FOR COLUMN BASE CONNECTIONS AND THEIR EFFECT IN SEISMIC PERFORMANCE OF STEEL MOMENT FRAMES
Get PDFAnalytical models for the characterization of the response of column base connections (i.e. exposed and embedded base plates) are developed and implemented in the Open System for Earthquake Engineering Simulation (OpenSees) software framework. These analytical models are validated against large-scale experimental programs conducted in different research institutions. The suggested analytical models are utilized to assess the performance of Steel Special Moment Resisting Frames (SMRFs) in the Performance Base Assessment/Design Framework. Specifically, this dissertation presents four studies: 1) A new hysteretic model formulation to capture the peculiar flag-shaped cyclic moment-rotation response of exposed base plates is proposed. This model is developed based on the physical processes involved in the hysteresis response. Five tests from a large scale experimental program at UC-Davis are used to calibrate the parameters of the proposed model. 2) A method is detailed to characterize the rotational stiffness of embedded column base connections. The method considers deformations of the components within the connection. The results are assessed against 9 tests from two experimental programs (ay UC-Davis and Brigham Young University). 3) The seismic demands on column base connections are investigated through a series of nonlinear time history simulations on 2-, 4-, 8-, and 12-story SMRFs. The 2- and 4- story frames feature exposed base plate type connections, whereas the 8-, and 12- story frames feature embedded connections. Results indicate that for exposed base plate connections, failure is likely to be controlled by the minimum axial compression accompanied by high moment. When embedded base connections are specified, the response is largely controlled by moment. 4) An approach to simulate the hysteretic and dissipative response of embedded base connections is described. This approach simulates embedded connections as an arrangement of two springs in parallel to reflect moment contributions due to horizontal and vertical bearing stresses. The resultant model is fit to five large-scale tests to calibrate its parameters.The intention of these four studies is to contribute to a deeper understanding and more accurate characterization of the response of column base connections of Steel Moment Frames subjected to seismic loads
Fracture Propagation Modeling in Civil Steel Structures at Different Length-Scales
Get PDFWithin the civil structural community, nonlinear time history analysis has become a ubiquitous tool to evaluate the structural performance of steel structures when subjected to extreme loadings such as earthquakes, blast, and strong winds. Extreme limit states such as structural instability, local bulking, plastic stress/strain localization in critical regions and structural components can be reliably simulated using current available analysis methods. However, the existing methods cannot reliably model fracture—an extreme limit state which may precipitate structural failure and collapse. Henceforth, on both the structural component and system level, researchers and engineers typically implement a capacity check evaluation approach in which a fracture toughness demand index, calculated based on the predicted continuum stress and strain fields, is checked against a material toughness parameter. Conservatively equating end-of-life (e.g., ultimate failure) to fracture initiation rather than to the onset of unstable crack propagation (e.g., cleavage), such approach disregards the remaining inherit capacity of the steel structure or components—as evident in recent large-scale experimental studies in which the steel components often sustained significant amount of stable ductile crack growth prior to ultimate failure. Clearly, a holistic framework or tool to reliably simulate crack propagation in concert with the global analysis of steel components enables a more realistic assessment of the structural performance of steel structures and designs because it captures the complex interactions between the overall structural response and advancing crack front. Depending on many factors such as the existing numerical tools and associated computational cost, nature of the crack propagation, and the size scale, some numerical frameworks may be more appropriate than others for modeling crack propagation in steel structures. Motivated by this, the scope of this project entails modeling crack propagation in steel structures on three different scales: continuum level, structural component level, and structural frame level. At the continuum level, a novel computational framework is developed and implemented to simulate ductile fracture initiation and propagation. This framework incorporates a local micromechanistic continuum damage model into a cohesive zone model; the continuum damage model predicts fracture initiation, whereas the cohesive zone model simulates the physical process of crack growth and propagation. The framework has been demonstrated to give reliable results (i.e., mesh-convergent agreement between test data and simulations using a single set of model parameters) using test data from CNT and CT specimens. At the structural component level, the framework successfully simulates crack propagation in test specimens that are meant to imitate practical structural design details such as the bolted connections and the reduced-beam-section (RBS) specimen under monotonic loading. Ideally, on the structural frame level, the established framework may be applied to model fracture propagations in key structural components throughout the frame. However, the high computational cost renders such approach impractical. Clearly, a phenomenological frame-element based model is more appropriate. Such model is developed to simulate post-fracture response of welded column splices. The novel model is informed by fracture-mechanics based estimates of splice strength and reproduces phenomena such as gapping and re-seating that occurs in the splices after fracture. Specifically, within the framework of Performance Based Earthquake Engineering, the effects of column splice fracture on the seismic performance of steel moment frames are assessed. It is concluded that due to the rocking phenomenon (e.g., rocking of the top stories above a story with fractured column splices), splice fractures auspiciously affect the dynamic response. Additionally, the phenomenology of splice fracturing throughout the structural system are investigated
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FRACTURE TOUGHNESS DEMANDS IN SEISMIC MOMENT FRAME CONNECTIONS OF BOX COLUMNS
Get PDFWelded connections between beams and built-up box columns in steel moment frames require the attachment of a continuity plate to the inside of the box column using complete joint penetration welds. Electroslag Welding (ESW) is often used to weld one of the sides of this continuity plate to the inside box column. These welds are susceptible to fracture owing to decreased material toughness and the installation of containment plates that create a notch condition. Finite element based fracture mechanics simulations are presented to examine the effect of various design and detailing parameters on the fracture toughness demands in these connections. The results indicate that the use of improved bevel details in the containment plate, and the use of a Reduced Beam Section (RBS) connection strongly mitigate fracture toughness demands, whereas eccentricity in the weld (with respect to the continuity plate) and mismatch between the beam and column flange widths exacerbate fracture toughness demands. The beam and column size, as well as weld access hole details are determined to have only a modest effect on the fracture toughness demands. Simulations also conducted to examine column wall thickness limits beyond which continuity plates may not be required. These suggest that using similar limits for wide-flanged columns may not provide adequate performance. Limitations of the study are presented, along with directions for future work
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A 3-D Frame Element to Simulate Inelastic Lateral, Local and Torsional Buckling in Wide-Flanged Steel Members Subjected to Monotonic and Cyclic Loadings
Get PDFThe emergence of performance-based engineering for extreme loads, such as earthquakes, has highlighted the need for accurate model-based simulations to predict structural collapse. In steel moment frame structures, the loss of load-carrying capacity is often caused by Interactive Buckling (IB), a wide range of combinations of local, flexural, and lateral torsional buckling. Accurately capturing these behaviors, especially during post-buckling state, is essential. However, the existing numerical techniques are either too computationally expensive or overly simplistic. This dissertation proposes a novel 3-dimensional (3-D) displacement-based Torsional Fiber Element (TFE) model to address these issues. The TFE model captures lateral and torsional buckling by incorporating accurate St. Venant and warping torsion through an enriched deformation field, while local buckling is represented using a phenomenological multiaxial softening material model. As part of collaborative research, development of a multiaxial Plate Local Buckling (PLB) constitutive model to simulate phenomenology in local buckling, supplements the TFE. Additionally, an Updated Lagrangian (UL) nonlinear geometric reference system is introduced to account for geometric instabilities. The integrated TFE-PLB model is implemented within the open-source finite element platform OpenSees. The model is validated against benchmark responses including experimental and continuum finite element results. The methodology successfully replicates a range of IB behaviors in wide-flange steel moment frame members under various loading conditions. This research provides a protocol for users to implement these modeling techniques and outlines the conclusions and limitations of the developed approach
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Seismic Performance of Dissipative, Biaxially loaded and Embedded Column Base Connections
Get PDFThis dissertation investigates the seismic response of Column Base Connections in Steel Moment Frames, a critical structural component used to transfer forces from the steel superstructure to the supporting concrete foundation. The research presented is intended to highlight and address various unresolved issues on the seismic performance of column base connections, and develop methods and criteria for their design resulting in economical and reliable connections.Column bases are arguably the most important connections in steel structures, transferring forces from the entire building to the foundation. A variety of details are commonly used for these connections ranging from exposed type with anchor rods, to embedded type. Current design/construction practices for base connections result in major conservatism in material requirements (deeper embedments or large anchor rods) and other inefficiencies stemming from the following: (1) Column base connections are generally capacity-designed to be stronger than the adjoining column presuming that the connections will be less ductile than the column, (2) Embedded Column Base connections have been designed for years without direct experimental support, such that current methods assume them to be similar to coupling beams in shear walls; disregarding many physical mechanisms that contribute to the strength of the actual connections. These include, the effect of reinforcement welded/attached to the column, and the beneficial effect of a slab-on-grade that overtops exposed-type connections, resulting in a shallowly embedded (blockout) connection. Consideration of these effects has the potential to greatly reduce costs by decreasing required embedment depth, minimizing other detailing (such as heavy anchor rods), and reducing logistical challenges.Four studies are presented, investigating (1) the seismic response of dissipative exposed-type base connections, (2) the strength characterization of biaxially-loaded column bases, (3) the seismic performance of blockout column base connection, as well as (4) embedded-type base connection with reinforcement attachments. The first study presents full-scale tests on Exposed Column Base Plate Connections with ductile anchors, with the aim to examine the seismic performance of these connections for their prospective use as dissipative/weak bases. These connections feature upset thread anchor rods, providing a stretch length over which inelastic deformations may be distributed. The tested specimens (with varying parameters) survived, with no anchor rod failure, the application of two back-to-back lateral deformation protocols (each to drift amplitudes of 5%), followed by additional cycles to 6.5% drift amplitude. Complementary line element-based and continuum finite-element simulations are conducted to examine to what extent the experimentally observed response may be generalized to untested configurations. The second study demonstrates a new method to characterize the internal stress distribution and anchor rod forces in Exposed Column Base Plate Connections subjected to biaxial bending and axial compression. The method is based on and validated against finite element simulations and available experimental data. The method is demonstrated to predict anchor force with good accuracy across a range of configurations (including column and base plate size) and loadings (level of axial force and bending angle).The third study presents full-scale experiments for shallowly embedded “Blockout” base connections to investigate the effect of additional strength and stiffness provided by an overtopping slab cast over the conventional exposed base connection. The connections are subjected to combinations of axial compression and cyclic lateral deformations. Significant increases in both stiffness and strength, with stable and ductile hysteretic response are noted. Results from this study are synthesized with results from previous studies on similar connections to propose a strength model and evaluate previously proposed stiffness models.The final study involves large-scale tests on embedded column base connections with attached reinforcement, and analyzes the effect of common reinforcement detailing for connections under axial compression and cyclic lateral deformation representative of seismic loading. It is observed that the introduction of horizontal reinforcement attached to the embedded column flanges reduces the strength and stiffness of such connections, as compared to cases where no reinforcement is attached. This is because, the horizontal reinforcement introduces a tension field in the concrete area above the uplifting region of the embedded plate, resulting in a reduced vertical resistance and an overall net reduction in strength. Based on observations, a method is developed to predict the strength of Embedded Column Base connections with various detailing features. The method shows good agreement with test data when compared with available strength models. Implications on design, detailed analysis and discussion of limitations of each study are provided
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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