Civil & Environmental Engineering
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Item Stress-Controlled Versus Strain-Controlled Triaxial Testing of Sand(1994) Alqutri, Samir Ahmed; Goodings, Deborah J.; Civil Engineering; University of Maryland (College Park, Md); Digital Repository at the University of MarylandThe purpose of this research was to compare the strength characterizations of Mystic White Silica Sands using stress-controlled loading versus strain-controlled loading in a standard compression triaxial tests. To this end one hundred sixty-six tests were conducted involving two types of quartz sand, one fine MWSS45 and one medium coarse MWSS18 , tested at three low to intermediate confining stresses of 14 kN/m2, 28 kN/m2 and 55 kN/m2 with only one specimen diameter size of 71.1 mm. Of the one hundred sixty-six tests, eighty-six were stress-controlled tests and eighty were strain-controlled tests. All specimens were dry, but both loose and dense specimens were tested. The results were evaluated individually and as group. Comparison of the two types of loading tests were evaluated for repeatability, stress-strain characteristics and strength parameters. The plots show that stress-controlled loading in general gives more reproducible results with smoother. steeper stress-strain plot s and a larger average deviator stresses at failure than strain-controlled loading at all three levels of confining stresses for both sands. This results in somewhat larger values of Φ' . Stress-controlled specimens were stiffer and failed with a clear cut failure surface while strain-controlled specimens mostly barreled.Item SCALING OF SURFACE ENERGY FLUXES USING REMOTELY SENSED DATA(2001) French, Andrew Nichols; Brubaker, Kaye L.; Civil and Environmental Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Accurate estimates of evapotranspiration (ET) across multiple terrains would greatly ease challenges faced by hydrologists, climate modelers , and agronomists as they attempt to apply theoretical models to real-world situations. One ET estimation approach uses an energy balance model to interpret a combination of meteorological observations taken at the surface and data captured by remote sensors. However, results of this approach have not been accurate because of poor understanding of the relationship between surface energy flux and land cover heterogeneity, combined with limits in available resolution of remote sensors. The purpose of this study was to determine how land cover and image resolution affect ET estimates. Using remotely sensed data collected over El Reno, Oklahoma, during four days in June and July 1997, scale effects on the estimation of spatially distributed ET were investigated. Instantaneous estimates of latent and sensible heat flux were calculated using a two-source surface energy balance model driven by thermal infrared, visible- near infrared, and meteorological data. The heat flux estimates were verified by comparison to independent eddy-covariance observations. Outcomes of observations taken at coarser resolutions were simulated by aggregating remote sensor data and estimated surface energy balance components from the finest sensor resolution (12 meter) to hypothetical resolutions as coarse as one kilometer. Estimated surface energy flux components were found to be significantly dependent on observation scale. For example, average evaporative fraction varied from 0.79, using 12-m resolution data, to 0.93 , using 1-km resolution data. Resolution effects upon flux estimates were related to a measure of landscape heterogeneity known as operational scale, reflecting the size of dominant landscape features. Energy flux estimates based on data at resolutions less than 100 m and much greater than 400 m showed a scale dependent bias. But estimates derived from data taken at about 400-m resolution (the operational scale at E l Reno) were susceptible to large error due to mixing of surface types. The El Reno experiments show that accurate instantaneous estimates of ET require precise image alignment and image resolutions finer than landscape operational scale. These findings are valuable for the design of sensors and experiments to quantify spatially-varying hydrologic processes.Item FINITE ELEMENT ANALYSES OF PARTIALLY REINFORCED MASONRY SHEAR WALLS(1996) Love, Aaron Ray; Chang, Peter; Civil & Environmental Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Partially reinforced shear walls are used in regions of relatively low seismic risk. Nevertheless, these structures should be capable to resist some lateral motion. The purpose of this paper is to examine the behavior of in-plane cyclic load tests of typical partially-grouted masonry shear walls and the capability of FEM/I in simulating their response. FEM/I is a nonlinear finite element program originally developed for reinforced masonry shear walls with a uniform grid of orthogonal reinforcement at close spacing. FEM/I has successfully simulated the response of fully grouted uniformly distributed reinforced masonry walls [Ewing, 1987]. FEM/I uses a smeared property approach in which the reinforcing steel and masonry composite is modeled as a single material. The applicability of FEM/I to partially grouted partially reinforced masonry shear walls is measured by comparing FEM/I force-displacement cycles, peak lateral forces, strains, energy dissipation and crack patterns with those generated from the experimental tests conducted at the National Institute of Standards and Technology. Partially reinforced shear walls can be modeled in FEM/I by smearing the steel over the blocks which are grouted and reinforced. The ungrouted blocks can be modeled as reinforced blocks with a reinforcement ratio of zero. This approach was shown to be adequate when the displacements and cracks were small. As the cracks increase in size, the smeared property assumption can no longer adequately represent the wall• s geometry and it's property. The result is a poor prediction of both local and global behavior at large displacements. The ratio of the lateral loads at the first major event (FME) demonstrate a good relationship in the forces generated by FEM/I for each wall with the exception Wall I. Results from Walls 3, 5, 9 and 11 exhibit FEM/I was able to predict the lateral load adequately up to the FME. The ratio of the lateral load up to the FME ranges from 0.9 - 1.3. After the occurrence of the FME, FEM/I overpredicts the lateral load considerably. In each of the finite element analyses, FEM/I overestimated the peak strength of the masonry specimens. The FEM/I models for Wall 11 and Wall 3 produced the best prediction of the peak strength. The difference for these two walls in the FEM/I predicted maximum lateral load and experimental data were 31 o/o and 41 o/o, respectively. Individual force displacement cycles are plotted at the various stages in the displacement history. FEM/I performs fairly well in predicting the force displacement response of the experiment. Walls 3, 5, 9 and 11 exhibit a good force displacement relationship for the first half of their displacement history until the development of major cracks. Wall 7 corresponded well with the experiment during the initial stages (Cycles 1 -17) of its displacement history. FEM/I did not produce good results in representing the cracking pattern generated by the experimental study. The inability of modeling the crack pattern is also shown in the differences in the plots for the amount of energy dissipated. FEM/I did reasonably well in the prediction of yielding in the vertical reinforcement. Local stress and strain of masonry predicted by FEM/I did not match the experimental data.Item Stress Intensity Factors for Structural Steel I-beams(1996) Feng, Daqing; Albrecht, Pedro; Sanford, Robert J.; Civil and Environmental Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The application of fracture mechanics to highway steel bridges has been hampered by a lack of stress intensity factor (SIF) solutions for cracks in I-beams. Previous work cannot provide satisfactory solutions. In this study, the finite element analysis method was used to develop accurate SIFs for two-tip and three-tip cracks in I-beams under tension or bending. Cracked I-beams were modeled with eight-node shell elements, with the web and flanges being fully joined along the junction lines. The region around the crack tips, singularity quarter-point elements were used. To ensure accurate and converging solutions, mesh patterns around the crack tips were studied. Also, different methods of extracting SIFs from FEA results were discussed based on benchmark problem studies. Governing parameters for cracked I-beams were determined. For two-tip web cracks, the SIFs are functions of applied stress, crack length, eccentricity, and flange-to-web cross-sectional area ratio. For three-tip cracks in web and flange, the SIFs are functions of applied stress, web and flange crack lengths, and flange-to-web cross-sectional area ratio. The flange-to-web area ratio describes the constraining effect of the flange on the web crack of a two-tip cracked I-beam; the interaction forces between web and flanges greatly affect SIFs for a three-tip cracked I-beam. The SIFs were calculated based on a total of 2,106 FEAs performed for a wide range of the parameters. The results were fitted with equations for ready use by practicing engineers. An example illustrates the calculation of SIFs for a three-tip crack in a composite steel-concrete beam of a steel bridge.Item Design of Rigid Overlays for Airfield Pavements(1987) Rollings, Raymond Sydney; Witczak, Matthew W.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)Existing rigid overlay pavement design methods are empirical and use a specified level of cracking as the defined failure condition. The existing empirical designs are based on tests run thirty years ago, and current analytical models provide greatly improved abilities to examine the overlay pavement structure. Emphasis by many agencies on life cycle cost analysis and more sophisticated maintenance and rehabilitation strategies require methods of predicting pavement performance rather than simply developing safe designs. A layered elastic analytical model was selected to evaluate stresses from applied loads in the pavement structure. Pavement performance was measured in terms of a Structural Condition Index which related the type, degree, and severity of pavement cracking and spalling on a scale of 0 to 100. Models were developed to represent the effect of cracking in base slabs under the overlay, to account for fatigue damage of previous traffic on the base pavement, and to account for the effects of substandard load transfer at slab joints. The predicted performance of overlays and pavements using this analysis was checked against the results of full-scale accelerated traffic tests conducted by the Corps of Engineers and against current overlay design methods and was found to provide reasonable agreement. This methodology using the layered elastic analytical model and analysis of fatigue and cracking in the base slab provides a method of predicting pavement and overlay deterioration in terms of a Structural Condition Index.Item Using MODIS Satellite Images to Confirm Distributed Snowmelt Model Results in a Small Arctic Watershed(2009) Choy, David F.; Brubaker, Kaye; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Environmental analysts face the problem of obtaining distributed measurements to evaluate the performance of models with increasingly small spatiotemporal resolution. While U.S. government agencies readily provide both measurement products and data tools for the study of global change occurring over entire seasons and across continental areas, analysts need access to the low-level data that provides the basis for global products. Finally, analysts need to consider sensor errors inherent in low-level products that are accounted for in global, composite products. Hydrologists using tools for managing low-level snow swath measurements, in particular, must consider how measurements are affected by sensor errors like snow-cloud confusion and sensor errors due to low ground illumination at night. This thesis aims to explore the use of remotely sensed snow maps to confirm a time series of model maps. Specifically, snow covered area (SCA) measurements remotely sensed by the National Aeronautics and Space Administration (NASA) are used to confirm SCA predictions modeled by the United States Agriculture Department (USDA). The measurements come from the two Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard near-polar, sun-synchronous satellites named Aqua and Terra. The USDA calls the model TOPMODEL-Based Land-Atmosphere Transfer Scheme (TOPLATS). The Upper Kuparuk River Watershed (UKRW) on the North Slope of Alaska acts as the case study location. To meet the map-comparison goal, the Kappa statistic, Kappa statistic variants, and probability density functions expressing measurement uncertainty in discrete scenes all evaluate the ability of MODIS measurements to confirm the accuracy of TOPLATS model maps. Data management objectives to make measured data accessible and comparable to the model output comprise a supporting goal. Results show that individual composite statistics, like the proportion of agreement between two maps, can easily obscure spatiotemporally distributed confirmation information without additional statistics and side-by-side images of measurement maps and model maps. These tools show some promise for using MODIS to confirm model predictions of snowmelt that occur across less than 150 km2 and less than a few days, however, clouds and malfunctioning sensors limit such use.Item Modeling of Consolidation by Household for Emergency Evacuation Events(2009) Liu, Ke; Lovell, David J; Chang, Peter C; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Evacuation studies have grown in importance over the years as a number of recent emergencies, natural and man-made, have raised the general level of awareness about public responses to the threat or actual occurrence of disasters. An accurate prediction of the rates of evacuation and estimate of the time required to clear a risk area are important planning tools that can mitigate the consequences of an emergency situation. Traditional evacuation models are predicated on the assumption that everyone would seek the quickest or shortest route to safety, given a life-threatening situation. Observations, however, show that a large percentage of the population does not seek the quickest route to safety. Parents may move toward dangers to pick up their children from schools. Persons at work may go back home to pick up dependent family members, pets, and personal effects before evacuation begins in earnest. Incorrect assumptions of evacuee behaviors could lead to measures that negatively impact the traffic flow during evacuation. One effective method to evaluate different evacuation strategies is the use of simulation. Most established simulation models, however, are not built to take the underlying drivers' social behavior into considerations. In this study, we develop a computerized tool for modeling evacuation dynamics with household consolidation, and then incorporate it into a traffic-simulation software platform. This tool will allow a percentage of the population to consolidate as a family before they evacuate. After that, a study is conducted to explore the consolidation by household in a network under various demand levels. A mathematical model is presented to capture the underlying relationships among the network components. Next, the traffic volumes entering and leaving the network are investigated to highlight some recommendations about the appropriate implementation of contraflow or staged evacuation strategies. To help decision makers have a better understanding of the evacuation traffic patterns, this study also examined the influences from spatio-temporal information such as the information dissemination delay, the evacuees' preparedness time, the numbers and locations of shelters in a network, and demographical information like the number of vehicles in a family. The proposed research will allow planners to study more realistically the effects of evacuation strategies. The results of studying such household by consolidation behavior are (1) evacuation times are significantly longer compared to the assumption of evacuees taking the shortest route away from danger in low/average demands; (2) with heavy demand, low consolidation rates can produce long evacuation times due to the rapid development of congestion at the network exits; (3) with heavy demand, high consolidation rates could delay the turning point to reverse the inbound lanes to outbound in a contraflow operation; (4) the sequencing of converting inbound lanes to outbound in a contraflow operation should start at the outermost links and work inward, due to extra bi-directional traffic on the network engaged in consolidation activities; (5) information delays and evacuees' preparedness as a family, coupled with the family consolidation behavior, are important parameters to the evacuation performance; (6) information on demographics and geography also has an important impact on the network evacuation efficiency and evacuees' social behaviors; more specifically, the evacuation performance is very sensitive to the number of shelters in the network.Item Crushed Returned Concrete Aggregate in New Concrete: Characterization, Performance, Modeling, Specification, and Application(2009) Kim, Haejin; Goulias, Dimitrios; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Every year roughly 2% to 10% of the estimated 455 million cubic yards of ready mixed concrete produced in the USA (est. 2006) is returned to the concrete plant. The crushed returned concrete aggregate (CCA) is obtained from crushing the returned concrete that was discharged at the concrete pant and left for a period of time before crushing. It is estimated that about 60% of all returned concrete is managed with this manner by the concrete plant according to the national ready mixed concrete association report. But the reuse of the returned concrete aggregate is very much limited so that most of the returned concrete aggregate has been diverted to the landfill. The main obstacle to limit the use of the returned concrete aggregate is the current type of prescriptive specifications by controlling the concrete composition, which limits the ability to optimize concrete mixtures for performance. The CCA aggregate has useful aggregate properties among which it is free of any contamination. Thus, CCA aggregate is distinguished from other recycled concrete aggregate (RCA) that comes out of existing old structures with high contamination from many years of exposure during the service life. The objective of this research was to develop technical data that will support the use of the CCA aggregates from the returned concrete by the ready mixed concrete industry. Three CCA aggregates at three strength levels were characterized. Thereafter, the virgin coarse/fine aggregates and the three CCA aggregates were used with various amounts to prepare concrete mixtures so as to investigate the effect on the fresh and harden concrete properties. The second objective of this research was to develop the performance models of harden concrete properties. The harden concrete properties of a selected number of mixtures containing CCA aggregates were used for the modeling of compressive strength, drying shrinkage, elastic modulus, and rapid chloride ion penetrability. This analysis was instrumental for a better understanding of how the CCA aggregates affect the harden concrete properties. The fine CCA aggregates were further investigated for their potential use as internal curing agent due to their unique aggregate properties (i.e. low specific gravity and high water absorption capacity). Those two properties are crucial factors for the internal curing. The fine CCA aggregates were used with mortar mixtures to evaluate the strength and autogenous shrinkage behavior along with the lightweight fine aggregate. This new approach can promote the use of CCA aggregate in a specialized application. Another objective of this study was to demonstrate the advantages of using a performance based specification. An example of an experimental case study was used for both conventional and CCA based concrete for comparing performance and prescriptive specifications.Item Behavior and Analysis of an Instrumented Slab Bridge(2009) Jeong, Sungki; Fu, Chung C.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Because of quick construction and cost effectiveness, adjacent precast, prestressed box girder bridges have been used nowadays more often for short-span bridges, and the standardization of this modular bridge is highly desired. Maryland intends to revise its current practice of using tie-rods for the transverse post-tensioning in slab bridge design. The new design of using high strength rods will provide a more tightly integrated modular slab bridge system with higher post-tensioning forces. With the new design, the Maryland State Highway Administration is highly interested in the performance of the new design, especially compared with the old design. This thesis presents the procedure of test, live load test results and analysis results in association with the finite-element model simulated in a newly-built bridge.Item DETERMINING THE RELATIONSHIPS AMONG AIRPORT OPERATIONAL PERFORMANCE AREAS AND OTHER AIRPORT CHARACTERISTICS(2009) Chan, Kennis Yuen Man; Lovell, David J; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, a methodology is proposed to investigate pair-wise relationships between different types of airport operational performance variables. The methodology represents a fundamental contribution for comparing airport performance between different air traffic management systems. Considerable attention is paid to analyzing the most appropriate techniques in an effort to produce the most reliable results. Additionally, a method to display the results in a simple and clear way is also suggested to allow users to understand the results visually. The key variables obtained from the proposed methodology not only serve as building blocks for developing models to answer a variety of air traffic questions, which allow policy makers to make decisions on allocating resources wisely, but also can be used as an evaluation tool to assist the FAA in selecting candidate projects.