Browsing by Subject "Finite element analysis"
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Item A numerical study on mechanical properties of low-density two-dimensional networks of crosslinked long fibers(2023-04-03) Mane, Soham Manohar; Huang, Rui, doctor of civil and environmental engineering; Liechti, K. M.; Kyriakides, Stelios; Rausch, Manuel; Bonnecaze, RogerIn this dissertation, we study mechanical properties of low-density two-dimensional (2D) networks by finite element methods. Fiber-based materials are prevalent in nature and in engineering applications. To understand the relationship between the effective mechanical properties and the underlying microstructures, we consider a variety of periodic and random 2D networks of crosslinked long fibers. The linearly elastic properties of periodic 2D networks (e.g., square, triangular and Kagome) are well understood. However, for low-density networks, cooperative buckling of the fiber segments can take place at small strains, leading to nonlinear, anisotropic elastic behaviors. A transition from stretch to bending and then back to stretch dominated deformation is predicted for the Kagome and triangular networks. For random 2D networks, the stress-strain behavior is statistically isotropic and slightly nonlinear under uniaxial tension, dominated by stretch of the fibers aligned closely to the loading direction. Meanwhile, stochastic buckling occurs continuously in the random networks, leading to significant lateral contraction. Consequently, while the effective Young’s modulus follows a nearly linear scaling with respect to the relative density, the effective Poisson’s ratio exhibits a transition from stretch to bending dominated mode as the relative density decreases. The comparison between the periodic and random 2D networks highlights the profound effects of the network topology on the effective elastic properties. Furthermore, we study the strength of 2D periodic networks. First we present the elastic beam models to predict the effective tensile strength of the rotated square, triangular and Kagome networks. Next we conduct finite element analyses to simulate the damage initiation and progression in the periodic 2D networks assuming elastic-brittle fibers. For the Kagome networks subject to uniaxial tension in the y-direction, four different failure modes (including post-buckling modes) are predicted, depending on the relative density and the fiber strength. The elastic beam model does not consider the nonlinear elastic behavior due to buckling and thus generally overestimates the tensile strength. Moreover, for Kagome networks consisting of many unit cells, the effective tensile strength depends on the boundary conditions, and the presence of a crack-like defect could reduce the strength considerably.Item The adhesive interactions between graphene and substrates by blister tests(2015-12) Cao, Zhiyi, Ph.D.; Liechti, K. M.; Huang , Rui; Ravi-Chandar , K.; Lu, Nanshu; Akinwande, DejiA blister test and associated analysis was developed to characterize the interfacial adhesion between graphene and copper and silicon substrates to which it has been transferred. Chemical vapor deposition grown graphene had been transferred to a highly polished copper or silicon substrate from its seed foil. The graphene/photoresist or graphene/PDMS/photoresist composite film was pressurized with deionized water through a hole in the substrate and the deflection of the membrane was measured by a full field interference method. Different mixed-mode conditions were achieved by varying the thickness of the backing layers. The measured adhesion energy for the graphene/copper and graphene/silicon interfaces showed a strong dependence on the mode-mix. The deflection profiles were modeled by plate, membrane theory and finite element analysis. The variation of energy release rate with blister radius and thickness for graphene/copper and graphene/silicon interfaces were obtained. The traction-separation relations of the graphene/copper interface were determined in the modified blister tests. The blister profiles and normal crack opening displacements were measured by two synchronized camera. Cohesive zone models associated with traction-separation relations were developed to study the damage initiation and crack propagation under various mixed-mode conditions. It was determined that the maximum normal and shear strength, which governed damage initiation, was independent of the mode-mix. The softening parameter, which governed damage evolution, was also independent of the mode mix. The numerical solution for and experimental measurements of the pressure vs. blister radius and deflection, as well as NCOD were in good agreement. A model for the variation of traction-separation relations with mode-mix was developed based on an asperity shielding model. The delamination paths of the graphene/photoresist, and graphene/PDMS/photoresist samples and the quality of graphene after the blister test were confirmed by Raman spectroscopy. The use of pressure could provide a path to large-scale graphene transfer.Item Behavior of the shear studs in composite beams at elevated temperatures(2015-12) Dara, Sepehr; Engelhardt, Michael D.; Helwig, Todd A; Williamson, Eric B; Ghannoum, Wassim M; Ezekoye, Ofodike AIn order to improve the fire safety and at the same time to provide more economical design of composite floors in fire, it is important to understand the behavior of these systems under fire exposure. An important step needed to reach this goal is to better understand the behavior of shear studs in composite beams at elevated temperatures, which was the focus of this research study. Typically, corrugated metal decks are used in construction of composite beams. These decks act as formwork and provide reinforcement for the concrete. For this study, however, the corrugated deck was not included. Rather, this study focused on cases where there is a solid concrete slab over the steel beam. The purpose of this limitation was to first gain a thorough understanding of shear stud behavior under fire exposure for this simpler configuration. This study on shear stud behavior at elevated temperature in solid slabs included both experiments and numerical simulations. The objective of the experimental test was to develop additional data on the load-slip behavior of shear studs in solid concrete slabs at elevated temperatures, and to compare the measured shear stud strength values with the limited test data and code provisions available in the literature. Two different specimen heating scenarios were introduced. One was meant to result in a temperature gradient in the specimen to simulate a fire condition. The other scenario was meant to result in a uniform temperature throughout the specimen for comparison purposes with the other scenario. One of the conclusions was that the shear stud strength and initial stiffness in the shear stud load-slip behavior have strong correlations with bottom of stud temperature, regardless of the heating scenario. Therefore, choosing the bottom of stud temperature as a reference temperature in predicting the shear stud ultimate strength and initial stiffness is reasonable. The objective of the numerical simulations was to develop a finite element (FE) model which can predict the thermal and mechanical behavior of shear studs in solid concrete slabs at elevated temperatures, and to validate the model against the experimental data. Different aspects of modeling the specimen using the general purpose finite element software, Abaqus, were discussed. Results of the analyses were compared with the experimental results of this study. Temperatures resulting from the heat-transfer analysis were found to be in a good agreement with experimental results at some locations in the specimen. However, at some other locations the difference between the experimental and FE results were more than 100 ºC. The existing level of uncertainty in the input data highly contributes to the errors in the temperature results, and emphasizes the difficulty that exists in heat transfer modeling. The load-slip curves found from FE analysis were presented for all the tests. The ultimate strength and the initial stiffness of the specimens were predicted well by the FE analyses. However, the slip capacity did not match between the experiments and FE analyses. Several parametric studies using the finite element model were conducted to investigate the sensitivity of the analysis results to various model parameters, both for heat transfer analysis and structural response analysis. The studied parameters included thermal conductivity of concrete, convective heat transfer coefficient, resultant emissivity, thermal joint conductance coefficient, Concrete Damaged Plasticity model parameters, steel stress-strain curves recommended by two different code provisions, and concrete tensile strength. The current gaps in our knowledge about these parameters were discussed.Item Design and analysis of the Hobby-Eberly Telescope Dark Energy Experiment bridge(2010-05) Worthington, Michael Scott; Nichols, Steven P.; Beno, Joseph H.A large structural weldment has been designed to serve as the new star tracker bridge for the Dark Energy Experiment upgrade to the Hobby-Eberly Telescope at McDonald Observatory. The modeling approach, analysis techniques and design details will be of interest to designers of large structures where stiffness is the primary design driver. The design includes detailed structural analysis using finite element models to maximize natural frequency response and limit deflections and light obscuration. Considerable fabrication challenges are overcome to allow integration of precision hardware required for positioning the corrector optics to a precision of less than 5 microns along the 4-meter travel range. This thesis provides detailed descriptions of the bridge geometry, analysis results and challenging fabrication issues.Item Design and hybrid additive manufacturing for electronic components that can withstand shock loading(2021-08-30) Gunsbury, Connor Thomas; Seepersad, Carolyn; Kovar, DesiderioA hybrid additive manufacturing approach is studied to demonstrate the feasibility of using a combination of negative stiffness elements (NSEs) and micro-cold spray (MCS) to produce electronic devices that can survive shock loading such as that experienced in an artillery launch. This work, supported by the Army Research Laboratory, explores how the design of NSEs can be tuned to dissipate as much energy as possible and how a primary building block of an electrical circuit—a resistive film—can be deposited onto a substrate with the MCS process. The NSE is designed to maximize amount of energy dissipated during the snap-through event using analytical equations built into a Microsoft Excel® tool, and the effectiveness of the optimized design is simulated in Abaqus® finite element software. The final design is predicted to mitigate impulses of up to 60,000 Gs by up to 80%. Tin oxide (SnO₂), commonly used in resistor components, is chosen for the resistive film. Suitable MCS processing parameters for SnO₂ are determined, and powder characterization is carried out to validate that the powder diameter is in a range that can deposit films and that the mixer is effective at breaking up agglomerates. Resistive elements consisting of SnO₂ films produced with the MCS process and silver pads for contact measurements are written on alumina substrates.Item Development of three-dimensional finite element software for curved plate girder and tub girder bridges during construction(2017-12) Biju-Duval, Paul, Ph. D.; Helwig, Todd Aaron, 1965-; Williamson, Eric B., 1968-; Bayrak, Oguzhan; Clayton, Patricia M; Engelhardt, Michael D; Landis, Chad MBecause of its ability to be easily shaped, steel is an attractive material for curved girders. Plate girder and tub girder bridges, for example, are often the preferred solution for direct connectors in highway networks. This flexibility in fabrication, however, presents challenges for structural engineers because of the difficulties associated with accounting for combined bending and torsion with curved geometry. The potential presence of skewed supports is a further source of complexity. In fact, no commercial structural engineering program currently addresses the evaluation of plate girder and tub girder bridges while modeling them to the full extent of their three-dimensional configuration. Most engineers, for example, use a two-dimensional bridge representation, which is often accurate for typical design of a complete bridge but may also be unconservative in many cases. The few programs that allow a full three-dimensional representation require extensive knowledge of finite element theory as well as significant time to model any complex structure. This dissertation presents the assumptions, methodology and calculations involved in the programming of a new structural engineering program designed to assess the behavior and stability or curved plate girder and tub girder bridges during erection or deck placement. It then illustrates the capabilities of the program for various structural systems subjected to a variety of loads, from self-weight to wind and temperature loads. In addition to a linear elastic analysis, multiple types of analysis are offered to the engineer: a geometrically nonlinear analysis provides a more accurate behavior for flexible systems, a linearized buckling analysis yields an upper bound evaluation of the stability of the structure, while a modal dynamic analysis estimates the free vibration modes of that structure.Item Direct shear behavior of composite concrete girders with tall haunches(2023-03-24) Rutenberg, Shay P.; Williamson, Eric B., 1968-Composite bridges constructed using prestressed concrete (PSC) girders and concrete slabs may require haunches—a region of concrete that connects the bridge deck and girders—to maintain a uniform deck thickness despite camber, cross-slope, construction errors, and perhaps other reasons. The Texas Department of Transportation (TxDOT) provides structural design guidance for a limited range of haunch heights, though not for tall haunches that exceed 3.5 in. Gravity loads on composite bridges induce flexure in the concrete slab and PSC girders. The overall strength and stiffness of the bridge deck system depend upon the properties of the composite connection between the reinforced concrete (RC) deck and PSC girders, which relies on shear transfer at the interface between these components. Push-out tests idealize this shear transfer behavior by isolating a part of the deck and girders and then subjecting the interface to direct shear loading. The shear transfer parameters between the PSC girders and deck include reinforcement detailing, friction, and cohesion. Experimental tests at the Ferguson Structural Engineering Laboratory (FSEL) examine the effects of design variables such as haunch height, material properties, and reinforcement detailing on the shear capacity and failure modes of composite push-out test specimens. Parametric studies (validated with the experimental tests) examine the effects of a wider array of design variables than possible with experimental tests alone. The parametric studies conducted for this project rely on commercial finite element analysis (FEA) software to create three-dimensional, symmetric models of the experimental push-out test specimens. The results from this project show that the capacities of composite bridge decks with PSC girders and cast-in-place slabs depend on the haunch depth, concrete compressive strength, surface roughness, and reinforcement detailing at the shear transfer interface. Changes to the reinforcement spacing, size, and type show limited influence on the strength of these specimens but can influence their ductility. Tapering a specimen’s haunch increases its haunch width and capacity compared to a specimen without a tapered haunch. Specimen strength increases as a function of girder concrete compressive strength but is not significantly influenced by the haunch concrete compressive strength. Validation efforts indicate that PSC girder specimens' shear strengths and failure modes depend on the friction and cohesion properties between the concrete surfaces at the composite interface. Parametric studies demonstrate that the strength of concrete girder bridges with composite decks in direct shear are not sensitive to variations in haunch height, so the current TxDOT provisions reasonably neglect this variable. Composite concrete girder bridges may experience brittle debonding failure at the composite interface when subjected to direct shear, necessitating the use of a shear strength reduction factor. Future research can investigate ways to increase composite friction, cohesion, and haunch width as methods to improve the ductility and strength of composite concrete bridges.Item DPG methods for nonlinear fiber optics(2018-06-13) Nagaraj, Sriram; Demkowicz, Leszek; Caffarelli , Luis A; Engquist, Bjorn; Bui-Thanh, Tan; Simmons, Christopher; Babuska, Ivo MIn recent years, the Discontinuous Petrov-Galerkin (DPG) method has been the subject of significant study. It comes with a collection of desirable properties, including uniform/mesh independent stability, localizable test norms via broken test spaces, and a canonical error indicator that is incorporated as part of the solution. In this work, the DPG method is applied to problems arising in fiber optics. Accurate modeling of wave propagation in nonlinear media is an important task in fiber optics applications. Nonlinear Maxwell equations in the context of optical fibers have been studied extensively in the past. Analysis of these intensity-dependent nonlinearities are based on several simplifying approximations which result in a nonlinear Schrodinger (NLS) type equation. The Schrodinger equation from a spacetime DPG perspective is discussed. In particular, a 2nd order L² stable ultraweak formulation of the Schrodinger equation is constructed by introducing the notion of an auxiliary boundary operator. This theoretical device requires an operator-specific conforming element to develop optimal convergence rates. Numerical studies show how, modulo (expected) roundoff issues, the theoretical convergence rates are delivered. Next, the use of the DPG method in modeling and simulating optical fiber laser amplifiers with nonlinear Raman gain is studied. In this application, the interaction of two time harmonic electromagnetic fields (the signal and pump fields) governed by two weakly coupled nonlinear Maxwell equations results in the amplification phenomenon. A novel Raman gain model for describing the phenomenon is proposed and an ultra weak DPG formulation is used for the discretization of the proposed model. The nonlinearity is handled by using simple iterations between the two systems. DPG implementation of a perfectly matched layer (PML) at the exit end of the fiber is essential in this model, as is the use of sum factorization for element computations. The presented results show that the signal field indeed gains power along the fiber, thereby justifying the use of the model. Auxiliary results presented in this dissertation include the construction of DPG Fortin operators for 2nd order problems.Item Estimation of beam prestress by deflection and strain measurements(2012-08) An, JinWoo; Tassoulas, John Lambros; Manuel, LanceLaboratory test of reinforced and prestressed concrete structures have been used widely to explore the behavior of reinforced and prestressed concrete components and structures; Such tests are often time-consuming and costly. However, numerical models have been shown to compare favorably with experiments. Thus, computations are viewed nowadays as efficient alternatives to tests, time-wise and cost-wise. In the research reported in this thesis, finite-element model were used in a study of pretressed structural components in order to correlate levels of pretension with deflection and strain measurements. The two main objectives were to develop a suitable finite element model of prestressed concrete beams and to forecast beam prestension on the basis of deformations resulting from specified simple load, e.g., a uniformly distributed transverse load. A commercial finite-element analysis package (ANSYS 12) was used to set up, use and evaluate the computational model. Furthermore, a finite-difference model was employed in order to ascertain the validity of ANSYS results by comparison with engineering beam theory taking into account the applied pretension. This study demonstrates the potential usefulness of deflection and strain measurements as indicators of the pretension applied or remaining in prestressed concrete beams.Item Experimental and analytical investigation of panel zone behavior in steel moment frames(2017-05) Shin, Sungyeob; Engelhardt, Michael D.; Tassoulas, John L; Helwig, Todd A; Ghannoum, Wassim M; Kyriakides, SteliosSteel moment resisting frames (MRFs) are one of the most commonly used lateral force resisting system for steel building construction located in regions of high seismic risk. Although steel moment frames were studied extensively following the 1994 Northridge Earthquake, one critical design issue is not yet completely understood: the role of the panel zone. Recent U.S. building codes have significantly increased the required shear strength of the panel zone for seismic applications. This leads to increased column sizes or the need for costly doubler plates. A number of past research studies showed that shear yielding of panel zones results in highly ductile behavior and provides a major contribution to frame deformation as an excellent source of energy dissipation. However, these same studies suggested that large panel zone shear deformation can cause high strain concentrations near the beam flange groove welds and can cause premature fracture at the joint. The overall goal of this research is to provide additional data to help answer the question: how much panel zone participation should be permitted in the inelastic seismic response of a steel moment frame? Despite a number of past studies on this issue, there are sharply conflicting views of how panel zones should be treated in design, both within the research community as well as within the building regulatory community. At the crux of the disagreements are concerns regarding fracture induced by panel zone yielding. There appears to be broad agreement that panel zone yielding is a highly ductile process. However, there is broad disagreement on the role that panel zone yielding plays in joint fracture. While there have been a number of past experimental studies on moment frame subassemblies with weak panel zones, the availability of data on large-scale specimens constructed using current US connection detailing and welding practices is very scarce. Further, the experimental database lacks critical data on panel zone behavior in deep columns and for columns subjected to significant axial forces. The primary focus of this dissertation is to provide much needed large-scale experimental data on the cyclic loading performance of steel moment frame joints with weak panel zones. The experimental study was supplemented by finite element analysis of the test specimens, intended to provide additional insights into the response of the test specimens. Cyclic loading tests were conducted on ten large-scale interior steel moment connections to study the seismic performance of the connections. The key variables for the tests were: panel zone strength; beam and column size; beam-to-column connection detail; and column axial stress. Nine of the ten test specimens performed well and met the acceptance criteria of 0.04 radian story drift angle for special moment frames in the current AISC Seismic Provisions. One specimen failed by fracture of the column flange just prior to achieving 0.04 radian story drift angle. In the experimental program, the weak panel zone specimens showed excellent performance, developing drift capacities as large as or larger than the strong panel zone specimens. The weak panel zone specimens exhibited less beam buckling and therefore less strength degradation than the strong panel zone specimens. Shear yielding of the panel zones in these specimens showed highly ductile behavior with stable hysteretic loops. The panel zone was an excellent energy dissipater. The results of this test program showed that acceptable performance of moment frame joints can be achieved over a range of panel zone strengths, varying from very weak to very strong. The experimental results also show that joints with weak panel zones can achieve very large inter-story drift angles prior to fracture. These experimental results suggest that current U.S. code requirements for panel zone strength merit reevaluation. Finite element models were developed of the test specimens, including a global model using shell elements and a local joint model using solid elements. The overall load deflection response of the test specimens predicted by the global models showed good agreement with the experimental results. Local joint models were developed to examine the potential for ductile fracture initiation using the Rupture Index. These studies suggested that the weak panel zone specimens should be far more prone to fracture than the strong panel zone specimens. This prediction, however, was inconsistent with the experimental observations. This suggest that using the Rupture Index as an indicator of the potential for ductile fracture initiation when comparing differing moment frame joint design options, as has been done by previous researchers, should be approached with caution.Item Extreme energy absorption : the design, modeling, and testing of negative stiffness metamaterial inclusions(2013-08) Klatt, Timothy Daniel; Seepersad, Carolyn; Haberman, Michael R. (Michael Richard), 1977-A persistent challenge in the design of composite materials is the ability to fabricate materials that simultaneously display high stiffness and high loss factors for the creation of structural elements capable of passively suppressing vibro-acoustic energy. Relevant recent research has shown that it is possible to produce composite materials whose macroscopic mechanical stiffness and loss properties surpass those of conventional composites through the addition of trace amounts of materials displaying negative stiffness (NS) induced by phase transformation [R. S. Lakes, et al., Nature, 410, pp. 565-567, (2001)]. The present work investigates the ability to elicit NS behavior without employing physical phenomena such as inherent nonlinear material behavior (e.g., phase change or plastic deformation) or dynamic effects, but rather the controlled buckling of small-scale structural elements, metamaterials, embedded in a continuous viscoelastic matrix. To illustrate the effect of these buckled elements, a nonlinear hierarchical multiscale material model is derived which estimates the macroscopic stiffness and loss of a composite material containing pre-strained microscale structured inclusions. The nonlinear multiscale model is then utilized in a set-based hierarchical design approach to explore the design space over a wide range of inclusion geometries. Finally, prototype NS inclusions are fabricated using an additive manufacturing technique and tested to determine quasi-static inclusion stiffness which is compared with analytical predictions.Item Finite element analysis of doubler plate attachment details and load paths in continuity plates for steel moment frames(2012-05) Donkada, Shravya; Engelhardt, Michael D.; Helwig, ToddThis thesis presents results of research aimed at developing an improved understanding of the behavior of column panel zones reinforced with doubler plates in seismic resistant steel moment frames. A primary goal of the research was to develop data to support the development of improved design guidelines for welding doubler plates to columns, with and without the presence of continuity plates. The research addressed several issues and questions related to welding and detailing of doubler plates. This included evaluation of the effects of welding the top and bottom of the doubler plate in addition to the vertical edges, the effects of extending the doubler plate beyond the panel zone, and the impact of welding a continuity plate to a doubler plate. These issues were investigated through detailed finite element models of a simplified representation of the panel zone region, subjected to monotonic loading. The results of the research suggest that, in general, there is little benefit in welding the top and bottom edges of a doubler plate if the vertical edges are welded, particularly in terms of overall panel zone strength and stiffness. However, the top and bottom welds provide some benefit in reducing stresses on the vertical welds. The results also suggest that extending the doubler plate above and below the panel zone has little benefit for heavy columns of shallow depth, such as the W14x398 considered in this analysis. However, extending the doubler plate did result in approximately a 10-percent increase in panel zone strength for deeper columns, such as the W40x264 considered in this analysis. Finally, the results showed that welding a continuity plate directly to a doubler plate had no adverse effects on the doubler plate in terms of increased forces or stresses. Interestingly, welding the continuity plate to the doubler plate simply changed the load path for transfer of load from the beam flange to the column web and doubler plate, but did not change the stresses in the doubler plate. Further research is needed to validate these findings for more accurate representations of the panel zone region of the column and for cyclic loading.Item Finite element analysis of Eccentrically Braced Frame shear links with reduced web sections(2019-12-06) Alp, Yucel; Engelhardt, Michael D.This thesis presents the results of a finite element study of the behavior of the shear links with reduced web sections subjected to cyclic and monotonic loading protocols, for use in seismic-resistant steel Eccentrically Braced Frames (EBFs). The capacity design philosophy for EBFs relies on the principle that the link, where inelastic deformation is intended to occur in an earthquake, must be the weakest element in the frame. One approach to satisfying capacity design requirements of EBFs is to decrease the strength of the links. Removing some portion of the link web is one method to reduce the shear capacity of the link. Twenty-one finite element models were analyzed under cyclic and monotonic loading protocols. Finite element models were based on a W18x40 shear link, without web holes, tested in an actual experiment. Modeling techniques and material definitions were validated using the same experimental findings. The finite element shear link models with various hole patterns, locations, and percent web removal were then analyzed to investigate the effect of removing material from the web on the behavior of the shear link in terms of strength and rotation capacity. This study suggests that shear links with reduced web sections provide a potentially viable method to reduce link shear strength.Item Finite element analysis of steel moment frame joints with doubler plates and continuity plates(2016-08) Cheng, Yu-Fang, M.S. in Engineering; Engelhardt, Michael D.; Helwig, Todd AThis thesis presents the results of a finite element study of the behavior of the panel zone region in beam-column joints in seismic resistant steel moment frames, with a focus on the interaction between continuity plates and extended doubler plates. Several recent studies have examined this same issue, but utilized a simplified finite element model of the panel zone region that did not include a complete representation of the beams attached to the column.. The previous studies concluded that welding a continuity plate to a doubler plate produced no apparent detrimental effects in the doubler plate. However, there is still a question on whether welding a continuity plate to a thin doubler plate may have a detrimental effect on the beam flanges, as the doubler plate may negatively impact the effectiveness of the continuity plate in reducing stress and strain concentrations in the beam flanges. As an extension to these previous research, the primary goal of this research was to examine the conclusions of these previous studies and to determine if any these previous conclusions might be altered once the entire beam and beam-to-column connection is included in the model. The research involved parametric finite element studies that included a full and very detailed representation of the beam and beam-to-column connection, using a welded unreinforced flange – welded web (WUF-W) connection The development and validation of modeling techniques used for this research is described, along with the results of the extensive series of parametric studies. The results suggest that welding continuity plates to thin doubler plate, as thin as 3/16-inch, does not significantly reduce the effectiveness of the continuity plates, in terms of controlling stress and strain concentrations in the beam flange. This study on full beam-column subassemblies showed essentially the same conclusion as that reported by previous studies.Item Finite element analysis of wood shoring towers used in Urban Search and Rescue(2012-12) Blair, Robert Stevenson; Wheat, Dan L.; Engelhardt, Michael DThis thesis focuses on the finite element modeling and analysis of wood shoring towers used by Urban Search and Rescue (US&R) teams during emergency response situations. These shores are constructed on site to provide temporary stabilization to a damaged structure. A high demand exists for experimental testing of the performance of these shores under non-ideal loading conditions, and for possible design modifications that could improve their overall behavior. To respond to this need, a total of thirteen vertical shores of the type laced post (LP) and plywood laced post (PLP) were constructed and tested at the Ferguson Structural Engineering Laboratory (FSEL) in Austin, Texas. The tests conducted on these shores aimed to investigate their performance under purely vertical load as well as various combinations of vertical and lateral loads. Finite element models for eight of the shores tested at FSEL were built and analyzed in Abaqus to compare the computed results with the actual linear elastic response of the shores. Material properties for the posts in each shore were obtained through further material testing at the conclusion of each shore test. Shore members were assumed to be isotropic. Solid elements were used to model each member, and Cartesian connector elements with a predefined nonlinear stiffness were used to model each nail. In general, the vertical load-displacement response computed from Abaqus exhibited good agreement with the laboratory results for the linear elastic range. The same general modeling scheme was then used to make design changes to the original shores based on observations gained during testing as well as modeling. Each design change was modeled, analyzed, and then compared with the computed results from the original shore design as well as the other design changes. The basis for evaluating the effectiveness of a given shore design involved comparing the bending moment diagrams for each post and the maximum first story nail slips (connector displacements). Recommendations were made for improved shore designs to be verified by experimental testing.Item Localization instabilities in pseudoelastic NiTi tubes under multiaxial stress states(2022-07-01) Kazinakis, Karlos Thomas Leonidas; Kyriakides, S.; Landis, Chad M.; Kovar, Desiderio; Liechti, Kenneth M.; Ravi-Chandar, KrishnaswamyNearly equiatomic NiTi has a unique property called pseudoelasticity in that strain of several percent is recoverable at room temperatures. This characteristic is attainable due to solid-state transformations between the austenite and martensite phases. It is well established that the transformation in tension is associated with localization during the loading/unloading stress plateaus of its hysteresis. By contrast, the transformation in compression is essentially homogeneous and occurs at much higher stresses and lower strains. Recently conducted biaxial experiments on NiTi tubes revealed, in addition to tension/compression asymmetry, an inherent anisotropic behavior. The interaction of these material nonlinearities with geometric instabilities results in challenging structural problems and the need for adept constitutive models is vital. Hence, a J₂-type kinematic hardening model was developed by our group, which incorporates asymmetry and is now extended to include anisotropy. Two numerical studies of NiTi tubular structures incorporate this framework within a finite element analysis aiming to reproduce the experimental responses and the transformation-induced strain patterns. The first problem investigates the buckling and collapse of a thin-walled NiTi tube under pure bending. The analysis captures the moment-end rotation response and the distinct diamond patterns that develop demonstrating how their interaction with ovalization leads to buckling and collapse. Parametric sensitivity studies illustrate the roles of the diameter-to-thickness ratio, geometric imperfections, and some key aspects of the model to the stability of the structure. The second problem examines thin-walled NiTi tubes under combined axial force and internal pressure. The simulations reproduce well the stress-average strain responses and the transformation stress loci, while for hoop dominant stress paths the extents of the transformation strains are somewhat overpredicted. The evolution of localization in the form of high or low strain helical bands, the variation of helix angles with the stress ratio, and the dissipated energy compare favorably. The hardening response and essentially homogeneous deformation exhibited in the neighborhood of the equibiaxial stress state is reproduced, but with reduced hardening and weak deformation patterns. The special case of equibiaxial tension is studied further as it highlights the effects of asymmetry and anisotropy in the constitutive model on structural behaviorsItem Multi-resolution modeling of the mitral valve : a novel computational pipeline for patient-specific simulations of valve repair(2019-02-01) Khalighi, Amir Hossein; Sacks, Michael S.; Barr, Ronald E; Bogard, David G; Neptune, Richard R; Gorman, Robert CThe mitral valve (MV) is the left atrio-ventricular heart valve that regulates blood flow direction during the cardiac cycle. Among the four heart valves, MV is the most problematic one, with MV-related pathologies directly afflicting 5% of the population in the industrialized world. Over the past 25 years, computational simulations of the MV based on biomechanical models have gained significant credibility in understanding valve function and improving surgical treatments. However, MV models with proven predictive power have yet to be developed on a patient-specific basis from clinical imaging data. The main challenge is that ultrasound, which is the prevailing imaging modality in the clinic, struggles to capture the full MV shape and its fine-scale geometric details. Thus, computational modeling of the MV for clinical applications first requires overcoming the obstacle that complete MV models cannot be developed directly from clinical images. In this Ph.D. project, we tackled this challenge through a detailed anatomical analysis of the MV constituents to better understand the comprising components of the MV apparatus and their impact on the MV modeling. This knowledge was then used to systematically identify the key characteristic of predictive MV modeling, build patient-specific models, and perform simulations of the MV repair. Remarkably, we established a framework to build faithful computational models of the MV for predictive surgical simulations based only on the information that can be acquired in the clinic and prior to the operation.Item Performance of suction caissons with a small aspect ratio(2013-12) Chen, Ching-Hsiang, active 2013; Gilbert, Robert B. (Robert Bruce), 1965-Suction caissons with a smaller aspect (length to diameter) ratio are increasingly used for supporting offshore structures, such as wind turbines and oil and gas production facilities. The design of these stubbier foundations is usually governed by lateral loads from wind, waves, or currents. It is desired to have more physical understanding of the behavior of less slender suction caissons under cyclic lateral loading condition and to have robust design tools for analyzing these laterally loaded caissons. In this study, one-g model tests with 1:25 and 1:50 suction can foundation scale models with an aspect ratio of one are conducted in five different soil profiles: normally consolidated clay, overconsolidated clay, loose siliceous sand, cemented siliceous sand, and cemented calcareous sand. This test program involves monitoring settlements, lateral displacements (walking), tilt, lateral load and pore water pressures in the suction can during two-way cyclic lateral loading at one, three and five degrees of rotation. The model foundations are monitored during installation, axial load tests, and pullout tests. In one and two-degree (±0.5 and ±1 degree) rotation tests, the suction can does not have significant walking or settlement in all the five soil profiles after 1000 load cycles. However, more significant walking or settlement may occur at extreme conditions such as the 5-degree (±2.5 degrees) rotation tests. Gaps between the foundation wall and the soil may also form in these extreme conditions in overconsolidated clay, cemented siliceous sand, and cemented calcareous sand. Plastic limit analysis, finite element analysis, and finite difference analysis are used to evaluate the laterally loaded suction can in clay. The plastic limit analysis originally developed for more slender suction caissons appears to predict a lateral capacity close to the measured short-term static capacity of the caisson with an aspect ratio of one when undisturbed undrained shear strength of soil is used. However, this plastic limit model underestimates the long-term cyclic lateral load capacity of the caisson when the remolded undrained shear strength was used. The finite element model developed in this study can simulate the development and effect of a gap between the foundation and surrounding soil as observed in the experiments in overconsolidated clay. The lateral load-displacement response predicted by this finite element model matches well with the experimental data. Finally, finite difference analysis for a rigid caisson with lateral and rotational springs was developed by fitting the lateral load-displacement response of the suction can in clay. The calibrated p-y curves for rigid caisson are significantly stiffer and have higher ultimate resistance than the p-y curves recommended by API which is consistent with other studies. This finite difference model provides an efficient approach to analyze a laterally loaded caisson with a small aspect ratio in clay.Item Predicting the behavior of horizontally curved I-girders during construction(2010-08) Stith, Jason Clarence; Williamson, Eric B., 1968-; Helwig, Todd Aaron, 1965-; Frank, Karl H.; Engelhardt, Michael D.; Liechti, Kenneth M.The majority of a bridge designer’s time is spent ensuring strength and serviceability limit states are satisfied for the completed structure under various dead and live loads. Anecdotally, the profession has done an admirable job designing safe bridges, but engineering the construction process by which bridges get built plays a lesser role in the design offices. The result of this oversight is the complete collapse of a few large bridges as well as numerous other serviceability failures during construction. According to the available literature there have been only a few attempts to monitor a full-scale bridge in the field during the entire construction process. Another challenge for engineers is the lack of analysis tools available which predict the behavior of the bridge during the intermediate construction phases. During construction, partial bracing is present and the boundary conditions can vary significantly from the final bridge configuration. The challenge is magnified for complex bridge geometries such as curved bridges or bridges with skewed supports. To address some of the concerns facing engineers a three span curved steel I-girder bridge was monitored throughout the entire construction process. Field studies collected data on the girder lifting behavior, partially constructed behavior, and concrete deck placement behavior. Additional analytical studies followed using the field measurements to verify the finite element models. Finally, conclusions drawn from the physical and analytical testing were utilized to derive equations that predicted behavior, and analysis tools were developed to provide engineers with solutions to a wide range of construction related problems. This dissertation describes the development of two design tools, UT Lift and UT Bridge. UT Lift is a macro-enabled Excel spreadsheet that predicts the behavior of curved I-girders during lifting. The derivation of the equations necessary to accomplish these calculations and the implementation are described in this dissertation. UT Bridge is a PC-based, user-friendly, 3-D finite element program for I-girder bridges. The basic design philosophy of UT Bridge aims to allow an engineer to take the information readily available in a set of bridge drawings and easily input the necessary information into the program. A straight or curved I-girder bridge with any number of girders or spans can then be analyzed with a robust finite element analysis for either the erection sequence or the concrete deck placement. The development of UT Bridge as well as the necessary element formulations is provided in this dissertation.Item Probabilistic assessment of the seismic performance of earth slopes using computational simulation(2020-08-11) Cho, Youngkyu; Rathje, Ellen M.; Cox, Brady R.; El Mohtar, Chadi; Gilbert, Robert B.; Dawson, Clint N.Earthquake-induced permanent slope displacement has been the main damage measure used in evaluating the seismic performance of earth slopes and various predictive models for this displacement have been proposed. However, these predictive models are mostly based on displacements computed using sliding block analysis, although nonlinear finite element or finite difference simulations are becoming the preferred method to evaluate the performance of slopes. This research aims at developing predictive models for slope displacement based on nonlinear finite element simulations, and demonstrating how these predictive models can be used in probabilistic assessments of slope displacement. These methodological developments are demonstrated first using a single slope geometry representative of a site-specific analysis and then generic predictive models are established using a range of slope geometries. These generic displacement models are developed through both classical and artificial neural network (ANN) regression. Toward these goals, this research comprises the following three sections. Nonlinear finite element analyses are performed for a soft clay slope using a suite of 105 input motions and the computed displacements are used to develop slope-specific displacement prediction models that utilize different ground motion intensity measures. The efficiency and proficiency of the displacement models using different combinations of intensity measures are assessed. These displacement models are used to compute probabilistic hazard curves of the permanent displacement, which represent the annual frequency of exceedance for a range of displacement levels. The computed hazard curves provide insight into the range of epistemic uncertainty associated with different displacement models. A large set of nonlinear finite element simulations are performed on 40 slope models each subjected to more than 1000 input motions. A generic predictive model for displacement is derived from the computed displacements using classical regression techniques. The predictive model characterizes the slope in terms of its yield acceleration (ky) and the natural period of the sliding mass (Ts), and characterizes the input motion in terms of its peak ground velocity (PGV). The displacement variability is partitioned into the between-slope component, which represents the variability associated different slope models, and the within-slope component, which represents the variability due to different input ground motions. Lastly, the database of slope displacements used in the classical regression are used to develop an artificial neural network (ANN) predictive model for displacement. ANN models allow researchers to investigate complex interactions between independent and dependent variables without specifying any restrictions on the functional form. The developed ANN moderately improves the displacement prediction relative to the classical regression model, although without the need of a complex functional form. The ANN displacement model is presented as a simplified mathematical expression that can be easily implemented into deterministic or probabilistic assessments of slope performance.