Papers by Dimitrios Rizos
2018 Joint Rail Conference
A ballast layer is used to facilitate drainage and load transferring in railroad track structure.... more A ballast layer is used to facilitate drainage and load transferring in railroad track structure. With tonnage accumulation, fine materials, such as coal dust, clay, locomotive sand, degraded ballast aggregate, and other small particles, will penetrate into the clean and uniformly graded ballast layer causing contamination, usually referred as fouling. Fouling is unfavorable to railroad track performance due to the reduced drainage and consequent engineering challenges including but not limited to mud pumping, excessive settlement, and reduced bearing capacity. Previous research has investigated the mechanical behavior of the fouled ballast in both the laboratory and the field environment. However, the fundamental mechanism that governs the manner in which the fouling materials are transported and accumulated in the ballast layer is not thoroughly understood. Researchers at the University of South Carolina have initiated the effort to investigate the fouling process in the ballast l...
The accurate and efficient, from a practical point of view, analysis of spherical tanks is the pr... more The accurate and efficient, from a practical point of view, analysis of spherical tanks is the primary motivation for this work. The proposed methodology implements: (a) the Boundary Element Method (BEM) for the simulation of the soil-structure interaction, (b) a discrete 2-DOF mechanical system for the approximate computation of the impedances of a massive, rigid, ring foundation, a usual feature of a spherical tank, and (c) a FEM simulation of the structure and fluid regions. The above modules are coupled via the pre-and post-processing capabilities of the widely used general purpose program ANSYS. The entire analysis is performed directly in time domain assuming linear elastic structure and soil region.
Boundary Elements XIII, 1991
A direct time domain Boundary Element Method (BEM) is applied to the groundwater flow problem in ... more A direct time domain Boundary Element Method (BEM) is applied to the groundwater flow problem in a 2-D unconfined coastal aquifer. A time marching numerical scheme and quasi-steady state approximations are utilized in the proposed solution. Numerical examples and a comparison study is presented.
Proceedings of the Sixth International Conference on Computational Structures Technology
ABSTRACT A detailed parametric study on the influence of soil-structure interaction in the dynami... more ABSTRACT A detailed parametric study on the influence of soil-structure interaction in the dynamic characteristics of usual multistory frame structures is presented. A finite element model Transient analysis, based on finite element model, is conducted for a typical 2-D, six-story, three-bay concrete frame. Numerical results obtained using fixed-base support conditions are compared to those corresponding to foundation-soil and foundation-soil-foundation interaction conditions. Soil-structure interaction is taken into consideration by employing discrete spring, dashpot, and virtual mass elements at the soil-foundation interface. The conclusions drawn from these analyses support the notion that, under certain conditions, soil-structure interaction should be considered in the analysis of usual multistory buildings and should not be limited to special, large scale structures only.
Computational Mechanics, 1998
A Boundary Element for 3-D elastodynamic analysis is introduced in detail. The method uses a new ... more A Boundary Element for 3-D elastodynamic analysis is introduced in detail. The method uses a new generation of the Stokes fundamental solutions that utilize the B-spline family of polynomials. The integration techniques of the boundary element kernels are also discussed for both the singular and non-singular cases. A number of numerical examples are presented for the validation of the proposed methodology.
2020 Joint Rail Conference
Continuous Welded Rail (CWR) practice is used in modern railroads to alleviate maintenance issues... more Continuous Welded Rail (CWR) practice is used in modern railroads to alleviate maintenance issues associated with joints and to improve ride quality. The absence of expansion joints, however, leads to long rail segments that are prone to the development of longitudinal thermal stresses that may cause track buckling, or rail pull-apart. A critical parameter in the susceptibility of the track to failure due to thermal loading is the Rail Neutral Temperature. This parameter is the temperature at which the rail is stress free. Rail stress management practices depend on the knowledge of the total net stress in the rail and the RNT. Current in-situ rail stress measurement techniques are destructive and disruptive of service. A new non contacting, nondestructive methodology is under development at the University of South Carolina for RNT and longitudinal stress measurements. The method is based on stereo vision image acquisition and Digital Image Correlation (DIC) for acquiring the full fi...
Experimental Mechanics
Digital Image Correlation (DIC) is widely used for remote and non-destructive structural health e... more Digital Image Correlation (DIC) is widely used for remote and non-destructive structural health evaluation of infrastructure. Current DIC applications are limited to relatively small areas of structures and require the use of stationary stereo vision camera systems that are not easy to transfer and deploy in remote areas. The enclosed work describes the development and validation of an Unmanned Aircraft System (UAS, commonly known as drone) with an onboard stereo-vision system capable of acquiring, storing and transmitting images for analysis to obtain full-field, three-dimensional displacement and strain measurements. The UAS equipped with a StereoDIC system has been developed and tested in the lab. The drone system, named DroneDIC, autonomously hovers in front of a prestressed railroad tie under pressure and DIC data are collected. A stationary DIC system is used in parallel to collect data for the railroad tie. We compare the data to validate the readings from the DroneDIC system. We present the analysis of the results obtained by both systems. Our study shows that the results we obtain from the DroneDIC system are similar to the ones gathered from the stationary DIC system. This work serves as a proof of concept for the successful integration of DIC and drone technologies into the DroneDIC system. DroneDIC combines the high accuracy inspection capabilities of traditional stationary DIC systems with the mobility offered by drone platforms. This is a major step towards autonomous DIC inspection in portions of a structure where access is difficult via conventional methods.
This work presents a direct time domain methodology for Fluid-Structure Interaction analysis betw... more This work presents a direct time domain methodology for Fluid-Structure Interaction analysis between linearized fluids and rigid bodies. To this end, a direct time domain Boundary Element formulation pertaining to 3-D wave propagation in acoustic media is developed. The method is based on higher order B-Spline fundamental solutions developed for scalar wave propagation in 3-D infinite media and uses higher order spatial discretization. Integration of singularities appearing in the fundamental solutions is treated numerically through triangle coordinates and a successive mapping technique. This method is used within the B-Spline Impulse Response framework for the calculation of the time history of the response of the acoustic medium and the calculation of the hydrodynamic forces on the wetted surface of the fluid-structure interface. The proposed B-Spline BEM is stable as compared to the relevant Retarded Potential formulations reported in the literature. Although this method is non-...
2018 Joint Rail Conference
Concrete ties have become a promising alternative to timber ties for freight lines with increased... more Concrete ties have become a promising alternative to timber ties for freight lines with increased curvature, high annual traffic, and large axle loads. They are also widely adopted in passenger lines. High strength (HS) concrete is the material of choice in the fabrication of prestressed concrete railroad ties. The higher strength of the concrete is directly related to higher values of the Elastic Modulus, thus increasing the rigidity of the material. The combination of increased strength, rigidity, and the material brittleness may lead to the development of high amplitude stresses with high gradients, which appears to be a common underlying cause of premature cracking and deterioration observed in some concrete ties. Realizing the current issues associated with the performance of concrete ties and recalling the findings of an almost fifteen-year-old research conducted at the University of South Carolina (USC), a hypothesis was formulated that there is a potential benefit in introdu...
Engineering mechanics, 1996
Concrete ties have become a promising alternative to timber ties for freight lines with increased... more Concrete ties have become a promising alternative to timber ties for freight lines with increased curvature, high annual traffic, and large axle loads. They are also widely adopted in passenger lines. High strength (HS) concrete is the material of choice in the fabrication of prestressed concrete railroad ties. The higher strength of the concrete is directly related to higher values of the Elastic Modulus, thus increasing the rigidity of the material. The combination of increased strength, rigidity, and the material brittleness may lead to the development of high amplitude stresses with high gradients, which appears to be a common underlying cause of premature cracking and deterioration observed in some concrete ties. Realizing the current issues associated with the performance of concrete ties and recalling the findings of an almost fifteen-year-old research conducted at the University of South Carolina (USC), a hypothesis was formulated that there is a potential benefit in introducing weathered granite aggregates into mix designs for railroad concrete ties. A high strength, yet lower rigidity, concrete will reduce the amplitude of the stress field and equally important, will regularize the stress field providing for a smoother load distribution that will diffuse stress concentrations. Consequently, the High Strength Reduced Modulus (HSRM) concrete improves the cracking resistance and fatigue performance, thus extending the life of the tie. A comprehensive research program has been conducted at USC to identify the benefits of using HSRM in concrete ties. The research is based on experimental investigations and computer simulations at the material, component and structural member levels. This work presents the details of the computer simulation studies that pertain to center binding conditions. Three-dimensional nonlinear Finite Element (FE) models have been developed for the HSRM and the “Standard” concrete ties. Nonlinear material models based on damaged plasticity are implemented. The concrete-steel bond interface is also modeled and discussed. Validation of these models is conducted through comparisons with laboratory testing of prestressed concrete prisms, and it has shown excellent accuracy. Subsequently, a study related to center binding conditions in a tangent track have been conducted. These studies showed that the HSRM concrete tie outperformed the Standard concrete tie in these benchmark tests by (i) showing smoother stress distribution, (ii) delaying the initiation of cracks and (iii) failing at higher ultimate loads. The analysis results are discussed and future recommendations presented.
Currently the South Carolina Department of Transportation employs a detail of a plain pile embedm... more Currently the South Carolina Department of Transportation employs a detail of a plain pile embedment for the connection between precast prestressed piles and cast-in-place bent caps. This connection has proved beneficial in terms of time and cost associated with construction, and has been previously investigated albeit in a limited capacity. As such an improved understanding of the behavior of the connection is warranted. The University of South Carolina has concluded a research program focusing on the behavior and design of this specific connection detail. South Carolina contains the highest level of seismicity along the east coast of the United States. The state relies heavily on the transportation infrastructure and a high level of confidence in bridge design practices is essential. This research began with a focus on the evaluation and understanding of the current detail employed in the connection between precast prestressed piles and cast-in-place bent caps. The work continues ...
The accurate and efficient, from a practical point of view, analysis of spherical tanks is the pr... more The accurate and efficient, from a practical point of view, analysis of spherical tanks is the primary motivation for this work. The proposed methodology implements: (a) the Boundary Element Method (BEM) for the simulation of the soil-structure interaction, (b) a discrete 2-DOF mechanical system for the approximate computation of the impedances of a massive, rigid, ring foundation, a usual feature of a spherical tank, and (c) a FEM simulation of the structure and fluid regions. The above modules are coupled via the pre-and post-processing capabilities of the widely used general purpose program ANSYS. The entire analysis is performed directly in time domain assuming linear elastic structure and soil region
Computing in Civil Engineering, 1994
A new interactive computer code is presented in this work for the calculation of artificial earth... more A new interactive computer code is presented in this work for the calculation of artificial earthquake records that comply with any arbitrarily specified target response spectra or power spectral density requirements. It does not create new time histories but modifies, on the basis of a deterministic approach, existing records to fit specific requirements. In addition, among its other features, it performs quadratic baseline correction and calculates correlation factors for up to three earthquake records.
Continuous Welded Rail (CWR) practice is used in modern railroads to alleviate maintenance issues... more Continuous Welded Rail (CWR) practice is used in modern railroads to alleviate maintenance issues associated with joints and to improve ride quality. The absence of expansion joints, however, leads to long rail segments that are prone to the development of longitudinal thermal stresses that may cause track buckling, or rail pull-apart. A critical parameter in the susceptibility of the track to failure due to thermal loading is the Rail Neutral Temperature. This parameter is the temperature at which the rail is stress free. Rail stress management practices depend on the knowledge of the total net stress in the rail and the RNT. Current in-situ rail stress measurement techniques are destructive and disruptive of service. A new non contacting, nondestructive methodology is under development at the University of South Carolina for RNT and longitudinal stress measurements. The method is based on stereo vision image acquisition and Digital Image Correlation (DIC) for acquiring the full fi...
This presentation covers the casting and testing of three full-scale AASHTO Type III lightweight ... more This presentation covers the casting and testing of three full-scale AASHTO Type III lightweight concrete bridge girders. The girders were loaded according to the ACI CLT load test method. Two of the girders were of lightweight self-consolidating concrete (LWSCC) while the third was of lightweight high early strength concrete (HESLC). One of the LWSCC girders was tested in fatigue to 2 million cycles prior to loading the specimen statically to failure. As a general statement the girders exhibited good behavior in terms of both ultimate capacity and displacement capacity. Results related to girder performance include transfer length, camber, prestress losses, end slip, and ultimate capacity in flexure. Material properties including compressive strength, splitting tensile strength, and modulus of elasticity are also discussed.
The U.S. Army maintains a number of “line of communication” bridge types in order to ensure criti... more The U.S. Army maintains a number of “line of communication” bridge types in order to ensure critical sustainment is uninterrupted along military supply routes. These bridges are considered to be “fixed” or “semi-permanent” and the designs are classified as standard (panel) and non-standard, e.g. Army Facilities Components Systems (AFCS). The AFCS bridges consist of a series of simply supported spans of lengths up to 60 ft while panel bridges are suitable for spans up to 200 ft. Both types are designed for military vehicles that do not meet current requirements, especially for the longer span designs. In addition, the volume and weights of these materials, and the need to build intermediate piers when longer gaps or heavier loads are encountered, often preclude their rapid deployment, and thus their use. A research study that was recently completed at the University of South Carolina in collaboration with the Engineer Research and Development Center (ERDC) of the U.S. Army Corps of E...
High performance concrete (HPC) is generally defined as a concrete with improved durability and/o... more High performance concrete (HPC) is generally defined as a concrete with improved durability and/or higher structural capacity as compared to ordinary portland cement concrete. The modulus of elasticity of the HPC (Class E) used for bridge decks in South Carolina is found to be very low. This has been attributed to the coarse aggregate used. The objective of this work was to quantify the effects of coarse aggregate on the modulus of elasticity of HPC using Class E concrete and develop a model for predicting the modulus of elasticity based on known physical or mechanical characteristics of the constituent materials. A series of laboratory tests and analytical and numerical investigations were conducted. The laboratory tests pertained to the experimental determination of the elastic modulus of HPC using coarse aggregates from various sources. Analytical and numerical models were calibrated based on experimental measurements and were used in some of the parametric studies. Results of th...
The research presented here is a comparative study of the static and fatigue performance of reinf... more The research presented here is a comparative study of the static and fatigue performance of reinforced concrete beams retrofit with three different commercially available methods of flexural strengthening using Carbon Fiber Reinforced Polymer (CFRP) materials, i.e.,: Conventional Adhesive Applied (CAA), Near Surface Mounted (NSM), and Powder Actuated Fastened (PAF). Both small-scale and full-scale beam were investigated. The small-scale beams spanned 4,572 mm (180 in.) and measured 254 mm (10 in.) deep and 152 mm (6 in.) wide. Ten small-scale beams were investigated. Nine of the beams were strengthened with CFRP composites and the remaining specimen was used as a control specimen. Of the ten specimens, six strengthened specimens were tested under cyclic loading conditions. Four specimens, three retrofit and one control, were tested monotonically to failure. Results from the small-scale experimental study indicated that all three of the methods of Fiber Reinforced Polymer (FRP) appli...
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Papers by Dimitrios Rizos