Browsing by Author "Dr. Sami Rizkalla, Committee Member"

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Analytical Evaluation of Concrete Penetration Modeling Techniques
(2010-02-22) Bush, Blake Marshall; Dr. Sami Rizkalla, Committee Member; Dr. Murthy Guddati, Committee Member; Dr. Emmett Sumner, Committee Chair
The resistance of concrete targets to penetration of high speed projectiles is a topic of high value in the national security and catastrophic design fields. Many methods have been developed to effectively analyze these types of problems. Currently there are a number of numerical codes and constitutive models used to analyze concrete impact and penetration with new methods developed continuously. This research evaluates the accuracy of four analysis codes and five concrete constitutive models. Two Lagrangian analysis programs, EPIC and LS-DYNA, as well as an Eulerian code, CTH, are compared in this work. A developmental version of the Material Point Method is also evaluated in order to study the effectiveness of Arbitrary Lagrangian Eulerian (ALE) modeling methods for concrete impact and penetration. The concrete models evaluated in this research include Holmquist Johnson Cook, Brittle Failure Kinetics, Osborn, Karagozian and Case, and Drucker-Prager. The modeling programs and constitutive models are evaluated by comparing simulation results to a series of concrete impact and penetration experiments. The experimental test data, provided by Sandia National Laboratories, comprises concrete targets of two compressive strengths (3.3 and 5.7 ksi) and two thicknesses and projectiles of two nose shapes. Extensive material testing of the experimental concrete is used to calibrate the constitutive models in each analysis package. An additional parametric study investigates the influence of experimental variables on the most promising analytical configuration. Observations from this research show that the EPIC and LS-DYNA analysis codes are currently best suited for concrete impact and penetration problems. Both codes contain features which allow for realistic modeling and produce accurate results for the experimental impact tests. Recommendations for improving analysis methods specific to concrete impact and penetration are presented.
Composites from Natural fibers
(2008-07-23) Ichhaporia, Pratik; Dr Jagannadham Kasichainula, Committee Member; Dr Donald Shiffler, Committee Member; Dr. Sami Rizkalla, Committee Member; Dr. Behnam Pourdeyhimi, Committee Co-Chair; Dr William Oxenham, Committee Co-Chair
Composites are widely used in our day to day life. Due to their low weight and ability to be tailored for specific end use they have gained a considerable ground in the high performance applications, such as aerospace and automobile industry. However, the use of polymers that can be recycled when used with carbon and other niche fibers renders the composite non-recyclable. This has become a major issue as the landfills are filling up at a faster pace along with the need for going green due to global warming. To tackle these issues research in recent years has been focused on substituting olefin(polypropylene, polyethylene, etc) based composites with biodegradable ones. In order to achieve the goal of recyclable composites, natural fibers surfaced as the fiber of choice for reinforcing composites. The overhead for using natural fibers is their cleaning and processing needs. The current project uses hemp, a natural fiber, as a reinforcing media for producing a biodegradable polymer. The novel aspect is in terms of processing, in the elimination of carding, a cost and time intensive process. The web coming from the opening and cleaning line are needle-punched to provide integrity before being used as reinforcement. After considering various materials for use in a matrix, a soy protein isolate and poly lactic acid(PLA) were selected, because the soy protein isolate used as matrix material can be dissolved in water. In order to form composites the water has to be completely evaporated from the hemp and soy protein isolate mixture. The cost for evaporating water became a hindering factor, along with poor composite properties such as porosity and brittleness. Thus poly lactic acid(PLA) was selected as material of choice for using as matrix. PLA in fiber form was blended with hemp fibers during processing. The hemp-PLA web produced were hot pressed in order to form composites. Various parameters studied to derive performance metrics were blend ratio of fibers and processing temperatures. The composites were analyzed for various mechanical properties like tensile, and flexural properties. Composites were fabricated from epoxy vinyl ester based resin reinforced with hemp fiber webs in order to compare their properties with the biodegradable composites formed from hemp-PLA. The hemp-PLA composites though not as superior as the hemp-epoxy vinyl ester composite are made from renewable resources and can be used for non-structural applications and may provide superior thermal and acoustic insulation.
Influence of Confinement Plates on the Seismic Performance of Reinforced Clay Brick Masonry Walls
(2002-08-21) Durham, Adrian Semaj; Dr. Sami Rizkalla, Committee Member; Dr. James Nau, Committee Member; Dr. Meryvn Kowalsky, Committee Chair
This thesis focuses on the behavior of clay masonry walls subjected to cyclic racking loading. It proposes that the seismic performance of clay masonry walls can be substantially improved if the section is adequately confined in the extreme compression zone at the toe of the wall to delay crushing of the masonry unit. This is accomplished by placing a 3.2mm thick galvanized steel plate in the mortar joint, of successive courses, in the plastic hinge region of the wall. The objective is investigated by conducting seven tests on full-scale clay masonry walls with various longitudinal and confining reinforcing ratios under seismic excitation. The results presented in this thesis show that adequately confining the grout of the clay masonry walls in the plastic hinge region may lead to substantially favorable seismic performance.
An Investigation of Bridge Deck Overhang Falsework Systems Installed onto Modified Bulb Tee Girders
(2007-04-27) Lackey, Paul Ellis; Dr. Sami Rizkalla, Committee Member; Dr. James Nau, Committee Member; Dr. Emmett Sumner, Committee Chair
Bulb Tee girders provide a practical, efficient means of spanning the large distances required in today's bridge designs by utilizing large moments of inertia to withstand the massive moments created in the spans. A bulb tee girder possesses a wider, thinner top flange than conventional precast concrete cross-sections. Previous research suggests the thin top flange of an exterior bulb tee girder in the bridge deck overhang falsework system to fail in punching shear and concrete bearing/spalling at premature loads. Meadow Burke Products, Inc. has manufactured an innovative falsework hanger for use with thin flange girders such as the bulb tee. The hanger utilizes a bearing plate to distribute the vertical loads on the top flange. Experimental full scale testing of the innovative hanger on a bulb tee girder was completed as the first phase of this study. The hangers were installed onto a 63 in. North Carolina Department of Transportation (NCDOT) Modified Bulb Tee (MBT) girder and loaded by a hydraulic load cylinder at a 45 degree angle. The behavior and ultimate strength of the hanger and top flange of the girder were recorded. Phase two of the experimental study consisted of testing the overhang falsework system with support brackets attached to the Meadow Burke hangers. The support brackets were loaded vertically utilizing hydraulic load cylinders. The load distribution behavior of the bracket and ultimate loading capacity of the system were recorded. Both phases of the experimental study concluded that a punching shear failure will occur in the top flange of the girder at a consistent loading. Subsequent analytical studies were carried out in an attempt to predict and emulate the results seen in the experimental testing. The analytical study consisted of finite element models and ACI code provisional analysis for punching shear. The finite element models developed are able to accurately predict the behavior and ultimate strength of the system. ACI code for punching shear does not accurately predict the punching shear capacity of the MBT girder flange.
Shear Response and Bending Fatigue Behavior of Concrete-filled Fiber Reinforced Polymer Tubes
(2004-11-29) Ahmad, Iftekhar; Dr. Eric Klang, Committee Member; Dr. Amir Mirmiran, Committee Chair; Dr. James M. Nau, Committee Member; Dr. Sami Rizkalla, Committee Member
Recent field applications and research findings have demonstrated the effectiveness of concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) as an efficient and promising hybrid system for designing main components such as pier columns, girders and piles for a bridge system. The vision was to provide a cost-competitive unified system composed of FRP/concrete hybrid members, which may act as a viable alternative to conventional reinforced and prestressed concrete structural systems. To achieve their broad-based implementation in civil infrastructure, understanding of their behavior and developing analytical tools under full spectrum of primary and secondary load demands are essential. Response characterizations under primary load demands namely, axial compression, flexural and axial-flexural, and seismic loadings have already been reported. However, investigations under primary shear and secondary fatigue load demands remain to be addressed. The present study consists of two phases. In the first phase, an experimental and analytical investigation was undertaken to characterize the behavior of a CFFT beam. Study on shear was primarily focused on the deep beam behavior. Comparisons of behavior of deep, short and slender beams were also highlighted. A strut-and-tie model approach, pertinent to analysis of deep reinforced and prestressed concrete members, was proposed to predict the shear strength of deep CFFT beams. Prediction showed good agreement with test results. It was concluded that shear failure mode is only critical for beams with shear span less than their depth. In the second phase, a detailed study on flexural fatigue behavior and modeling was undertaken. The main objective was to evaluate the performance of beams under four basic criteria; i) damage accumulation ii) stiffness degradation, iii) number of cycles to failure, and iv) reserve bending strength. Effects of laminate fiber architecture, reinforcement index, load range, and end restraint on the fatigue response of CFFT beams were addressed. A fiber element was developed, capable of simulating sectional strain profile and moment curvature at any given time or number of cycles under single and two stages of loading. The model can also predict deflections at mid-span, and can analyze the reserve bending response of a fatigued CFFT beam. Parametric study revealed that flexural fatigue performance of CFFT beams could be enhanced by increasing reinforcement index and the effective elastic modulus in the longitudinal direction.
Web Crippling Strength of Sigma-Shaped Cold-Formed Steel Studs Subjected to Axial Load
(2006-10-16) Boylan, Matthew Aaron; Dr. Emmett Sumner, Committee Chair; Dr. Sami Rizkalla, Committee Member; Dr. James Nau, Committee Member
Load-bearing light steel framing (LSF) systems have gained good acceptance to the low to mid-rise construction market in the U.S. in recent years. This construction market covers a wide range of building usage, including apartment and office buildings, hotels, and schools. For years, standard C-shaped metal studs have been the only option for designers and contractors when selecting a cross-section for load bearing studs. As design loads for the studs get larger with heavier floor systems or at lower levels of mid-rise buildings, designers have been required to either use multiple (built-up) C-shaped studs or switch to structural steel members. An alternative to the standard shapes, although seldom found in the U.S., is the sigma-shaped section, but this shape is primarily used as a roof purlin in Europe. The Steel Network, Inc. of Raleigh, North Carolina recognized the potential of this section for use within a LSF system and developed the SigmaStud. A testing program was developed for the SigmaStud and testing was conducted at North Carolina State University's Constructed Facilities Laboratory (CFL). Presented are the results of a series of tests to evaluate the web crippling behavior of a new sigma-shaped metal stud when subjected to lateral load in addition to axial load. Also presented is an analytical study that results in proposed modifications to the AISI web crippling equation to account for sigma-shaped sections with or without axial load.