Browsing by Subject "Finite element method"

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Analysis of the DPG method for the Poisson equation
(University of Minnesota. Institute for Mathematics and Its Applications, 2010-10) Demkowicz, L.; Gopalakrishnan, Jayadeep
Computational Multi-material Inverse Design of Soft Robotic Actuators via Nonlinear Functional Optimization
(2023-08) Awasthi, Chaitanya
The compliant nature of soft robotic actuators allows them to maneuver and interact with the environment in ways which are more adaptable and inherently safer compared to the traditional rigid body robots. However, this compliance makes it difficult to control their deformation as these actuators can potentially have infinite degrees of freedom. Controlling the deformation of a soft robotic actuator has potential applications in fields requiring precise control over the shape of the robot. Areas such as medical robotics can use the shape control of soft robots to gently treat aneurysms in humans or deliver medicines within the body, among other applications. However, given a known external loading, it is usually not possible to deform a soft robot into an arbitrary shape if it is fabricated using only a single material. Even if the robot is fabricated using multiple materials, there is a lack of "voxel-level" material tuning to allow for the generation of a soft robot that can take an arbitrary shape. A major contribution of this dissertation is the proposal of a new physics-based method for the computational design of soft hyperelastic robotic actuators to address this problem. The method takes as input an undeformed robot shape, a specified external load, and a user desired final shape. It then solves an inverse problem in design using nonlinear optimization subject to physics constraints. The nonlinear program is solved using a gradient-based interior-point method. Analytical gradients are computed for efficiency. The method outputs fields of material properties, at the level of individual voxels, which can be used to fabricate a soft robot. A body fabricated to match this material field is expected to deform into a user-desired shape, given the same external loading input. The inverse design method is tested for validity and robustness. The performance of the method is tested on several example cases in silico. Another key contribution of the present work is the development of an adaptive impedance controller that allows for a rigid robot (a single degree of freedom indenter) to safely interact with an unknown soft environment, such as a body tissue. The controller is mathematically proven to be asymptotically stable and the simulation results demonstrate the efficacy of the controller in achieving force tracking without the use of a force sensor. The results show that this force tracking is achieved through the asymptotic convergence of the estimated tissue parameters to the true tissue parameters.
A Computational Study of the I-35W Bridge Collapse
(Center for Transportation Studies, University of Minnesota, 2009-10) Liao, Minmao; Okazaki, Taichiro
Reported evidence suggests that failure of gusset plates initiated the collapse of the I-35W Bridge in Minneapolis, Minnesota. The particular gusset plates were at a panel point designated as U10. Therefore, a computational study was conducted on the condition of the U10 gusset plates at the time of bridge collapse. The primary objectives of this study were: (1) to evaluate previous research and existing design methods for gusset plate connections; (2) to examine the mechanical condition of the U10 gusset plates at the time of the collapse; (3) to examine possible scenarios that led to the collapse of the entire bridge; and (4) to identify research needs to improve the design methods for gusset plate connections. The forces delivered to panel point U10 were reproduced using available information of the bridge. The truss forces were introduced to detailed nonlinear, three-dimensional finite element models to calculate stress and strain states of the gusset plates. The results indicate that substantial portions of the U10 gusset plates were yielded at the time of collapse, confirming earlier findings from federal and state investigations. Insufficient strength of the gusset plate, along with weight increase due to past deck reconstruction and construction material and equipment staged on the day of collapse, were identified as the main contributing factors to the substantial yielding. It is important to note that, even with the weight increase, the gusset plates would not have yielded substantially if the gusset plates had adequate thickness. The results also suggest that the interaction of compression and shear played an important role in the gusset plate failure and should be addressed in gusset plate design. This interaction is not well understood based on available research.
Development and Simulation Software for Modelling Pavement Response at Mn/ROAD
(1994) Zhang, Zhonglan; Stolarski, Henryk K.; Newcomb, David E.
This report presents the development of simulation software for modelling dynamically loaded pavement response. The analysis is carried out by employing the finite element method and by integrating the resulting discrete equations of motion through the central difference method. The lower pavement layers (base, subbase and subgrade) are assumed to be elasto-plastic and are described by using the flow theory of plasticity. The mapped infinite elements are used instead of viscous boundaries to mitigate the wave reflection from the boundaries of the model. The predicted pavement responses are compared with the experimental results obtained by a Falling-Weight Deflectometer (FWD). Dowel bar load transfer mechanism is also analyzed.
Development of Improved Test Rolling Methods for Roadway
(Minnesota Department of Transportation, 2008-02) Hambleton, J.; Drescher, Andrew
Test rolling is a quality assurance test in which penetration of the wheels of a heavy vehicle into subgrade soils is used as a measure of the adequacy of compaction. Current criteria for acceptable test roller penetration are empirical. Two theoretical approaches for modeling test rolling are developed. One is analytic and the other is numerical, based on the finite element code ABAQUS. Both approaches relate wheel penetration to wheel geometry, wheel load, and soil strength parameters (friction angle and cohesion). Elastic soil properties are included in the numerical simulations but play a secondary role. The models accommodate both rigid and flexible wheels. Homogeneous and layered soil structures are considered. Scaled laboratory tests support the theoretical predictions, although full validation requires execution of extensive full scale field testing. The models developed can be used to enhance interpretation of test rolling data and assess the effects of test roller modifications.
Effects of compressive and tensile fields on the load carrying capacity of headed anchors.
(2011-02) Piccinin, Roberto
The results of research initiated in the early 1980s led to the replacement of plasticity-based design guidelines for the load-carrying capacity (concrete breakout) of headed anchors embedded in concrete with those developed using fracture mechanics. While provisions are available in the design codes that account for the presence of tensile fields causing concrete cracking, no provisions are available for anchors embedded in prestressed concrete. This thesis presents the results of linear and nonlinear elastic fracture mechanics analyses of the progressive failure of headed anchors embedded in a concrete matrix under compressive or tensile prestress. In addition, and because of the complete lack of experimental evidence, the results of a relatively large experimental investigation of the behavior of headed anchors embedded in compressively prestressed concrete are presented. Discrete crack finite element models and experiments predict an increase (decrease) in load-carrying capacity and post-peak dissipated energy with increasing compressive (tensile) prestress for all the embedment depths investigated. For extremely shallow cases, in which the embedment depth is less than the (typical) maximum aggregate size of concrete, it is shown that deterministic continuum-based models are not applicable. Overall, the results show that there is very little difference between the linear elastic and nonlinear elastic fracture mechanics approaches, this implying that the concrete breakout strength is governed by the strongest possible size effect. In addition to providing analytical support to the existing design approaches for the capacity of headed anchors embedded in cracked concrete (under tension), the present work provides an experimentally and analytically based preliminary easy-to-use design formula for the concrete breakout capacity of headed anchors in compressively prestressed concrete.
Electrostatic Assist of Liquid Transfer in Printing Processes
(2018-07) Huang, Chung-Hsuan
Printing processes are being explored for the large-scale manufacture of electronic de- vices. Transfer of liquid from one surface to another plays a key role in most printing processes. During liquid transfer, a liquid bridge is formed and then undergoes sig- nificant extensional motion. Incomplete liquid transfer can produce defects that can be detrimental to device operation. One important printing process is gravure, which involves transfer of liquid from micron-scale cavities at high speeds. Electric fields are sometimes used to enhance liquid transfer, a technique known as electrostatic assist (ESA). However, its underlying physical mechanisms remain a mystery. This thesis uses a combination of theory and experiment to understand the fundamental mechanisms by which electrostatic forces influence liquid transfer. Liquid transfer without electric fields and cavities must be understood before study- ing the mechanism of ESA. We develop one-dimensional (1D) slender-jet and two- dimensional (2D) axisymmetric models of this phenomenon and compare the resulting predictions with previously published experimental data. At relatively low stretching speeds, predictions from both models of the amount of liquid transferred agree well with the experimental data. When the stretching speed is high enough, the models predict that each surface receives half the liquid, in agreement with experimental observations. For intermediate values of the stretching speed, predictions from each model can deviate substantially from the experimental data, which we speculate is due to the influence of surface defects that are not included in the models. The 1D and 2D model are modified to include electrostatic effects. The liquid be- haves like a perfect (non-conducting), or leaky dielectric (poorly conducting) material. The liquid is confined between two plates, with the top plate having a constant electro- static potential while the bottom plate is grounded. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface because it slows the surface-tension-driven breakup of the bridge, thereby allowing more time for the con- tact line to retract on the less-wettable surface. For leaky dielectrics, application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the interfacial charge. Experimental results confirm the enhancement of the amount of liquid transferred when the electric field is present, and the measured values are in good agreement with the predictions of the 1D perfect dielectric model. When one of the plate is replaced by a cavity, the presence of the cavity causes the contact line on the cavity wall to effectively pin and inhibits the liquid transfer. For perfect dielectrics, application of an electric field unpins the contact line on the cavity and leads to improvement of cavity emptying. For the leaky dielectrics, the presence of the surface charge does not further improve liquid transfer because of nearly zero electric tangential stress near the contact line on each surface.
Hybridizable discontinuous Galerkin method for nonlinear elasticity
(2017-11) SHEN, JIGUANG
Hybridizable discontinuous Galerkin method for nonlinear elasticity
Liquid-film coating on rotating discrete objects
(2018-01) Li, Weihua
The flow of liquid films on discrete objects is encountered in coating processes for a wide range of products such as biomedical devices, automobiles, and food. Describing the shape of liquid films as they flow over discrete objects is a challenging task due to the large number of forces at play. These include gravitational, inertial, viscous, surface-tension, and centrifugal forces, and the complex interplay among them may lead to the growth of instabilities that degrade the quality of the final product. Motivated by the need to improve fundamental understanding of coating flows on discrete objects, we pick cylinders that rotate about their horizontal axes as model discrete objects and investigate four model problems highly relevant to industrial coating processes for rotating discrete objects. In each model problem, the interplay among all the forces is systematically examined to reveal the critical conditions for which a smooth coating can be obtained. For coating of surfactant-laden liquids on rotating cylinders, we applied lubrication theory to derive coupled nonlinear evolution equations to describe the variation of the film thickness and surfactant concentration as a function of time, the angular coordinate, and the axial coordinate. In the absence of gravitational effects, linear stability analysis reveals that surfactant-induced Marangoni stresses suppress the growth rate of instabilities driven by centrifugal effects and hinder the leveling of perturbations to the film thickness in both the angular and axial directions. When gravitational effects are present, Marangoni stresses lower the critical rotation rate needed to cause a liquid lobe to form and rotate in the angular direction. These stresses also lead to faster damping of this lobe, giving rise to a more axisymmetric coating. With the growth of axial instabilities at long times, Marangoni stresses significantly weaken the stabilizing effect of surface-tension forces, which are found to be responsible for keeping the coating axially uniform in a stable speed window. In addition, Marangoni stresses tend to reduce the spacing between droplets that form at low rotation rates, and suppress the growth rate of rings that form at high rotation grates. Flow visualization experiments yield observations that are qualitatively consistent with our simulation results. For cylinders with complex surface geometries (i.e., topographically patterned cylinders and elliptical cylinders), the Galerkin finite-element method is used to solve the Stokes equations, augmented with a term accounting for centrifugal forces, in a rotating frame of reference. For rapidly rotating cylinders where gravitational forces are negligible, surface-tension forces tend to drive liquid to the low-surface-curvature areas (e.g., pattern troughs) leading to the formation of liquid pools, while centrifugal forces tend to drive liquid in the opposite direction, giving rise to liquid droplets. The number of droplets or pools at steady state depends on the rotation rate, strength of surface tension, pattern frequency, and cylinder aspect ratio. When gravitational forces become significant, it is possible to obtain a coating that closely conforms to the cylinder surface in the patterned-cylinder case. With an increase in the pattern amplitude, recirculation regions start to form inside the troughs, which may strongly influence mixing, mass transport, and heat transport. These reciprocation regions can appear and vanish as the cylinder rotates due to the variation of gravitational forces around the cylinder surface. In the elliptical-cylinder case, simulation results show that smaller aspect ratio corresponds to less liquid that can be supported on the cylinder and also larger gradients in film thickness. A suitably chosen time-dependent rotation rate can greatly improve coating smoothness relative to the constant-rotation-rate case. For cylinders with sufficiently small aspect ratio, film rupture and liquid shedding may occur over the cylinder tips, so simultaneous drying and rotation along with the introduction of Marangoni stresses will likely be especially important for obtaining a smooth coating.
Load Rating Assessment of Three Slab-Span Bridges Over Shingle Creek
(Minnesota Department of Transportation, 2022-08) Hill, Kendall A.; Dymond, Benjamin Z.; Hedegaard, Brock D.; Linderman, Lauren E.
Three slab-span bridges crossing Shingle Creek in Brooklyn Center, Minnesota, have poor American Association of State Highway and Transportation Officials (AASHTO) load rating factors for certain truck configurations. Characterization of load distribution is useful for determining the load rating of bridges, but results in the literature have shown that the AASHTO code results in conservative load rating factors. The focus of this study was to determine if the load rating of the three concrete slab-span bridges was conservative and could be improved using results from live load testing and finite element analysis. Field testing used a suite of instrumentation that included displacement transducers, strain gauges, accelerometers, and tiltmeters. A three-dimensional solid-element finite element model was used to determine an expected range of behaviors and corroborate the field data regarding how load distributed when placed near and away from a barrier. In addition, a method for developing a simple plate model of slab span bridges was developed considering in-situ material properties and effects of secondary elements such as barriers. Results indicated that the AASHTO load rating was conservative, and an improved rating factor could be obtained considering the field test data and computational modeling results.
Mechanistic Modeling of Unbonded Concrete Overlay Pavements
(Minnesota Department of Transportation, 2012-01) Ballarini, Roberto; Liao, Minmao
An unbonded concrete overlay (UBCO) system is a Portland cement concrete (PCC) overlay that is separated from an existing PCC pavement by an asphalt concrete (AC) interlayer. Current UBCO design procedures are based on empirical equations or highly simplified mechanistic models. To overcome the limitations, fracture mechanics concepts, specifically the finite element method-based cohesive zone model (CZM), are introduced in this research as a new paradigm for analyzing UBCOs with the ultimate goal of establishing a more rational design procedure. To illustrate the advantages of a fracture mechanics-based approach to design, specific attention is paid to but one type of failure associated with pavement structures: reflection cracking. The design against reflection cracking approach relies on a load-carrying capacity equivalency between the designed UBCO and a reference newly designed single layer PCC pavement. An illustrative fracture mechanics-based design procedure for UBCOs is developed and proposed by a large number of crack propagation simulations of both the UBCO composite and the reference single layer pavement. Preliminary comparisons of the results with field observations suggest that the fracture mechanics paradigm offers promise for improved design of UBCOs against reflection cracking and other potential loading conditions that could be analyzed using nonlinear fracture mechanics models. It is recommended that an experimental program be established to assess the accuracy of the model predictions, and additional experiments and three-dimensional fracture mechanics simulations be considered to provide additional insights as to whether UBCOs can be “thinned-up”.
Model reduction framework in space and time for the Generalized single step single solve family of algorithms
(2017-11) Deokar, Rohit
This thesis presents the developments in the field of model order reduction framework in space and time for the Generalized single step single solve (GSSSS) family of algorithms. The GSSSS framework has been developed in the past two decades as a unified theory encompassing all the computationally competitive time integration schemes for first and second order systems over the past 50 years. Using the underlying versatility of the GSSSS framework, a novel model order reduction procedure in space is proposed to eliminate spurious high frequency participation in dynamical systems. Spurious high frequency participation are vestiges of numerical discretization and often pose serious numerical issues degrading solution accuracy. Numerically dissipative schemes which were originally proposed to deal with these high frequency participation lose energy over time and damp out the physics in the system. The proposed method for elimination of high frequency participation deals with this very problem by combining the advantages of the energy conserving and numerically dissipative algorithms through projection techniques. The DAE (iIntegration) framework which was recently proposed, extends the GSSSS family of algorithms to constrained mechanical systems (DAEs) while preserving the optimal properties that are desired from time integration schemes. This thesis extends the proposed model reduction methodology in space to the GSSSS DAE framework thereby reducing the computational complexity which can otherwise be daunting for constrained subdomain systems with subcycling. In addition, the so called "Finite element in time" framework for the GSSSS family of algorithms is developed using the weighted residual methodology. Based on the finite element in time methodology, a novel general purpose a posteriori error estimator for first and second order systems under the umbrella of GSSSS family of algorithms is proposed to foster adaptive time stepping. The applicability of the proposed estimator to several existing time integration algorithms including the well known schemes like the Newmark method, HHT-Alpha, Classical midpoint rule, Crank Nicolson and in addition, new algorithms and designs as well is demonstrated with single and multi-degree of freedom, linear and nonlinear dynamical problems. Lastly, model reduction in space and time through the so called staggered space-time MOR procedure is proposed which aims at refining the discretizations in space while employing a reduced dimension in time. Conversely, a reduced dimension in space is used to improve the discretization in time and the process is performed in an iterative fashion
Modeling and control of cadmium zinc telluride grown via an electro-dynamic gradient freeze furnace.
(2007-12) Lun, Lisa San
In this thesis, numerical models are used to study the effect of novel processing methods to grow bulk, single crystal cadmium zinc telluride (CZT) in a vertical Bridgman (VB) furnace. Additionally, we investigate new mathematical algorithms for improved solving capability of equations that describe such crystal growth systems. A two-dimensional crystal growth model for the simulation of bulk crystal growth in a VB system is presented. This model consists of conservation equations for coupled continuum level transport of heat, mass, and momentum. Thermodynamic relations associated with phase change are also included. The Galerkin finite element method is used to discretize the spatial portion of the governing equations. The resulting sets of nonlinear algebraic equations are solved using Newton's method. Novel processing methods that are not practical to attempt in experiments are investigated using numerical modeling. A two-dimensional, planar, crystal growth model is used to explore the effect of ampoule tilting on zinc segregation in a CZT crystal. Tilting is shown to improve lateral segregation. We also analyze the use of closed-loop control to improve the macroscopic melt-crystal interface shape during growth by changing the furnace temperature gradient. Targeted closed-loop control on the temperature gradient adjacent to the solid only gave the best results and unexpectedly produced a favorable convex shape. A multi-scale crystal growth model is developed by coupling pre-existing codes, one which specializes in modeling the complex crystal growth process and the other which specializes in modeling the heat transfer effects in a furnace. Previously, a coupling algorithm based on the Gauss-Seidel method was used but it converged unreliably [136, 196]. Here, we use an Approximate Block Newton approach where we approximate Newton's method used to solve the two separate models as if they were one monolithic model. A Schur complement formulation is employed to solve the free-boundary problem associated with melt crystal growth systems. With this form, the difficult interface location part of the problem is mapped away from the equations governing transport. We assess the behavior of this method using two-dimensional simulations, but the goal is to improve solvability of three-dimensional problems.
Modeling and Monitoring the Long-Term Behavior of Post-Tensioned Concrete Bridges
(Minnesota Department of Transportation, 2014-11) French, Catherine E.W.; Shield, Carol K.; Hedegaard, Brock D.
The time-dependent and temperature-dependent behavior of post-tensioned concrete bridges were investigated through a case study of the St. Anthony Falls Bridge, consisting of laboratory testing of concrete time-dependent behaviors (i.e., creep and shrinkage), examination of data from the in situ instrumented bridge, and time-dependent finite element models. Laboratory results for creep and shrinkage were measured for 3.5 years after casting, and the data were best predicted by the 1978 CEB/FIP Model Code provisions. To compare the in situ readings to constant-temperature finite element models, the time-dependent behavior was extracted from the measurements using linear regression. The creep and shrinkage rates of the in situ bridge were found to depend on temperature. An adjusted age using the Arrhenius equation was used to account for the interactions between temperature and time-dependent behavior in the measured data. Results from the time-dependent finite element models incorporating the full construction sequence revealed that the 1990 CEB/FIP Model Code and ACI-209 models best predicted the in situ behavior. Finite element analysis also revealed that problems associated with excessive deflections or development of tension over the lifetime of the bridge would be unlikely. The interactions between temperature and time-dependent behavior were further investigated using a simplified finite element model, which indicated that vertical deflections and stresses can be affected by the cyclic application of thermal gradients. The findings from this study were used to develop an anomaly detection routine for the linear potentiometer data, which was successfully used to identify short-term and long-term artificial anomalies in the data.
Modeling of Concentrated High Intensity Electric Field (CHIEF) and Its Comparison with Other Non-thermal Liquid Food Pasteurization Technologies
(2015-12) Peng, Peng
Non-thermal preservations of food have received rising attention due to the increase concern of environmental sustainability and the demand of safer food with improved nutritional functionalities. High pressure and electric field treatment are two non-thermal food treatment strategies that have been widely studied. Some representatives of non-thermal technologies that utilize high-pressure and electric field to pasteurize food products include High hydrostatic pressure (HHP), high-pressure homogenization (HPH), and pulsed electric field (PEF). These non-thermal technologies, together with concentrated high intensity electric field (CHIEF) are studied and compared in this thesis research. This study used finite element (FEM) and computational fluid dynamics (CFD) methods to model and simulate the fluid flow, electric field distribution and temperature rise in CHIEF reactor. The simulation was confirmed to be valid by comparing it with experimental results. The model built in this study showed that the performance of CHIEF system was influenced by a set of intrinsic and extrinsic parameters. This model could be used to control and set variables in further optimization of the CHIEF system. Each of the non-thermal technologies discussed in this study has its advantages and unique field of use. HHP, dynamic high-pressure treatment and PEF are relatively mature technologies, while CHIEF system is an innovative and promising non-thermal method that can potentially be used as alternative to PEF.
Plastic analysis of processes involving material-object interaction.
(2010-06) Hambleton, James Paul
The dissertation presents theoretical models for processes involving continuous failure and displacement of material via contact with a rigid object. Focus is mainly on processes relating to material-wheel interaction, which are central in a number of engineering applications (e.g., vehicle mobility and metal rolling). The analysis considers indentation, transient rolling, and steady-state rolling on an elastic-perfectly plastic or a rigid-perfectly plastic cohesive-frictional material. Mechanics-based models are developed using two separate approaches. The first is based on comprehensive numerical simulation using the Finite Element Method (FEM), which enables rigorous analysis of the three-dimensional deformation occurring for narrow wheels. The second approach is analytic and formulated by considering the entire process of deformation as a sequence of incipient plastic flow problems. Using the theoretical models, the relationships between dimensionless variables are quantitatively assessed. It is further shown that the predictions display reasonable agreement with experimental data from two types of small-scale indentation and rolling tests: one aimed at measuring the force-penetration relationship for indenting and rolling wheels and the second type concentrated on measuring the incremental displacement field at wheel midplane using Particle Image Velocimetry (PIV). The models and experiments provide insights into how indentation and rolling processes are influenced by three-dimensional effects, non-associativity, localization, and the presence of displaced material.
A review of unified a posteriori finite element error control
(University of Minnesota. Institute for Mathematics and Its Applications, 2010-10) Carstensen, C.; Eigel, M.; Löbhard, C.; Hoppe, Ronald H.W.
Stretching and slipping liquid bridges: liquid transfer in industrial printing.
(2011-08) Dodds, Shawn
Liquid bridges with moving contact lines are found in a variety of settings, such as capillary feeders and high-speed printing processes. Despite this relevance, studies on liquid bridges often assume that the contact lines remain pinned in place during stretching. While this may be the case for some applications, contact line motion is \emph{desirable} in printing processes so that the amount of liquid transferred can be maximized. In this thesis we study several model problems to improve our understanding of how moving contact lines alter the dynamics of liquid bridges. We use the finite element method to study the stretching of a liquid bridge between either two flat plates or a flat plate and a cavity. For axisymmetric bridges we find that while the wettability of the two surfaces is a key factor in controlling liquid transfer between two flat plates, the presence of a cavity leads to fundamentally different bridge dynamics. This is due to the pinning of the contact line on the cavity wall, which leads to a decrease in the amount of liquid transferred to the flat plate. We find that the presence of inertia aids in cavity emptying by forcing the interface further into the cavity. However, this increase in emptying can be offset by an increased tendency for the production of satellite drops as the flat plate is made more wettable. To study non-axisymmetric flows we solve the Navier-Stokes equations in three dimensions. We find that when the stretching motion is asymmetric the liquid remains evenly distributed after breakup, so long as the two plates are not accelerating relative to each other. If the bridge shape is not initially cylindrical we find that the ability of the bridge to maintain its initial shape after breakup depends on the friction between the contact line and the solid. Finally, we use flow visualization to observe the stretching of liquid bridges both with and without small air bubbles. We find that while the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force the bubbles selectively toward the least hydrophillic plate confining the bridge.
Wakota Bridge Thermal Monitoring Program Part I: Analysis and Monitoring Plan
(Minnesota Department of Transportation, 2013-05) Scheevel, Christopher J.; Morris, Krista M.; Schultz, Arturo E.
In this work, a common refined design method is evaluated with respect to a recently constructed bridge. Two finite element models of the Wakota Bridge in South St. Paul, Minnesota, were produced, one using a design level program (SAP2000) and the other using a research level program (ABAQUS). These models were verified with respect to each other using linearly elastic materials and were found to behave similarly. After this verification, an arbitrary temperature load was applied to each model and the refined design method was evaluated for accuracy of reduced section properties with respect to the more descriptive progressive cracking solution simulated by ABAQUS. The refined design method was employed using two, four, and six stiffness segments at which stiffness is evaluated along the height of the pier walls. It was seen that accuracy increased as the number of stiffness segments increased and that four segments seemed to balance accuracy and time-commitment by the engineer adequately. A staged construction model of the Wakota Bridge was also built, using the design level program, which incorporates all time-dependent effects of the construction sequence as well as locked-in forces. A pile analysis was performed and appropriate rotational springs were found for Foundations 2 and 3. A simplified method for the determination of the rotational springs is discussed, and a range of effective lengths was found for use with this procedure. The staged construction model is used for field data correlation in Part two of this report. The staged construction model was also used to evaluate the different design options as described in the AASHTO LRFD. The two options given for accounting for reduced section properties were evaluated and compared. The refined analysis option and gross section option were compared for the Wakota Bridge and are shown to correlate to within about 10%. The two temperature application methods (Procedure A and B in the AASHTO LRFD) were also compared. As expected, Procedure B produced much larger design moments than that of Procedure A.
Wakota Bridge Thermal Monitoring Program Part II: Data Analysis and Model Comparison
(Minnesota Department of Transportation, 2013-05) Morris, Krista M.; Schultz, Arturo E.
In this work, a common refined design method is evaluated with respect to a recently constructed bridge. Two finite element models of the Wakota Bridge in South St. Paul, Minnesota were produced using a design level program (SAP2000). These models were analyzed and their results compared to the data collected from the bridge. The second half of this study concerned the comparison of the collected field data with the values produced by evaluating the design-level finite element models previously created in Phase I of the project, and calibrating these models to provide an accurate prediction of the future behavior of the bridge. This was done by calculating changes in axial force and moment from strain data collected from the Wakota Bridge and changing various parameters within the design level model (DLM) in order to calibrate the models to the field data. The model using the refined design method was shown to correlate to the superstructure field data to within 2 percent, while between 13 percent and 35 percent correlation was seen between the model deploying the gross section method and the field data. The pier behavior predicted by the two models showed much less correlation to the field data. After calibration, it was possible to predict the general trend of the pier behavior, but the values of changes in moment did not correspond to the field data. This was especially true in Pier 4. Further consideration of the model parameters is necessary to fully calibrate the models. The two temperature application methods (Procedure A and B in the AASHTO LRFD) were also compared. The internal concrete temperature ranges measured in the field were much closer to the range specified in Procedure A.

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