Browsing by Author "Dr. Mervyn Kowalsky, Committee Member"

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Behavior of FRP Repair/Strengthening Systems for Prestressed Concrete
(2006-08-23) Rosenboom, Owen Arthur; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member; Dr. Paul Zia, Committee Member; Dr. Sami Rizkalla, Committee Chair
This research study examines the behavior of prestressed concrete beams retrofitted with Fiber Reinforced Polymer (FRP) materials. Due to deficiencies in the built environment, engineers may be asked to retrofit or upgrade the capacity of an existing concrete structural member. This could be a result of new demands on the structure, or a repair of damage from an unforeseen event. Retrofits are possible using the traditional building materials of concrete and steel. The cross-section of the structural element can be increased, or steel plates can be bolted or adhesively affixed to the structure to increase capacity. Many of these techniques are costly, and some perform poorly under service conditions. The main benefit for using FRP materials for the strengthening of existing structures is the lightweight nature of the composite material, which makes the use of extensive scaffolding (required in the installation of steel plates) obsolete. The objectives of this research are twofold. First, the overall structural behavior of an FRP strengthened or repaired concrete beam is studied. Two different loading conditions are examined: extreme loading simulated by a monotonic load to failure, and fatigue loading designed to simulate service loads. The structural behavior of the system is evaluated under these different conditions, and an analytical model is presented which predicts the flexural behavior of the system assuming certain failure modes. The second objective of this research is to evaluate the bond behavior of an FRP strengthened reinforced or prestressed concrete flexural member. A database of experimental failures was constructed, and an analytical model is proposed which predicts the bond failure of the FRP strengthening system.
Influence of Residual Stress on the Initiation of Fatigue Cracks at Welded Piping Joints
(2004-05-21) Humphreys, Abigail Elaine; Dr. J. Michael Rigsbee, Committee Member; Dr. Mervyn Kowalsky, Committee Member; Dr. Tasnim Hassan, Committee Chair
Fatigue failures of small bore piping systems have historically occurred in nuclear power plants, resulting in unanticipated plant downtime and substantial financial loss. These failures have been reported with increasing frequency over the past 20 years and have motivated the research described in this thesis. Recent research at North Carolina State University (NCSU) pointed to the strain ratcheting response of the welded joint as a probable reason for fatigue failure and indicated that welding residual stresses might be responsible for inducing this strain ratcheting response. It was the primary objective in this investigation to determine what happens to residual stresses at the welded piping joint under the application of low-cycle fatigue loading and to understand how residual stresses induce strain ratcheting and thus affect the fatigue life of the welded joint. In order to achieve stated objectives, a systematic set of residual stress measurements and low-cycle fatigue tests was conducted. Initial residual stresses near the weld toe of six welded piping specimens were measured using the technique of x-ray diffraction. The specimens were then loaded in low-cycle fatigue to intermediate points in their fatigue lives and residual stresses were measured again. Strain response data near the weld toe was gathered throughout specimen fatigue life. Residual stress data and recorded strain responses in fatigue prompted conclusions concerning the role of residual stresses in inducing strain ratcheting which were verified by additional material level experiments. Initial residual stress measurements obtained from the welded specimens revealed that residual stresses near the weld toe—the location of fatigue crack initiation and final failure—were compressive in the overwhelming majority of measurement cases. It is widely accepted that compressive residual stresses are beneficial in fatigue. However, in this set of experiments, it was observed that compressive residual stresses induced tensile strain ratcheting and were thereby detrimental to fatigue life. Residual stress measurements obtained at intermediate points in specimen fatigue lives showed that residual stresses relaxed with fatigue cycles. The extent of relaxation at a point on the welded specimen in fatigue was dependent upon the amplitude of strain cycle experienced. At the location of maximum strain cycling in fatigue, complete relaxation of residual stresses was observed in all specimens. Strain response data gathered in the fatigue tests reiterated earlier findings at NCSU—positive axial strain ratcheting occurred at the top and bottom weld toes of all specimens subjected to displacement-controlled fatigue cycles. In light of residual stress relaxation and strain response data gathered in the welded specimen tests, it was anticipated that during residual stress relaxation, the prescribed fatigue interactions between multiaxial stresses exerted a reverse mean stress effect , which induced ratcheting response. In other words, the relaxation of the compressive residual stress in the axial direction induced a positive mean stress effect in the prescribed fatigue loading cycle. Consequently, tensile strain ratcheting in the presence of compressive residual stresses resulted. In order to gain insight into this newly observed phenomenon of reverse mean stress effect, the mechanism of multiaxial stress interaction was further investigated through additional laboratory experimentation and cyclic plasticity analysis at the material level. Results from this step more directly explain the new observation. This study reveals the mechanism of recently observed ratcheting fatigue failures of welded joints. Additional research is required to understand the failure mechanism in full and for incorporation of this mechanism in design methodology.
Models for Prediction of Surface Vibrations from Pile Driving Records
(2006-12-08) Robinson, Brent Ross; Dr. Mohammed Gabr, Committee Chair; Dr. Roy Borden, Committee Member; Dr. Mervyn Kowalsky, Committee Member
This study compares high strain dynamic testing measurements taken near the top of a driven pile to peak particle velocities on the ground surface and sound levels detected in the air some distance from the pile during driving. Based on a sample of installation records from 16 piles driven at the Marquette Interchange Project in Milwaukee, Wisconsin, a series of peak particle velocity plots versus distance, energy and scaled distance were created using traditional horizontal distance and rated hammer energy. These plots were modified using the seismic distance, the diesel hammer potential energy from the calculated stroke, and the energy transferred to the pile top. Incorporating these measurements tended to reduce some of the scatter in the data. More importantly, it was also discovered that components of peak particle velocity in the ground can be well correlated to the total pile resistance measured by dynamic testing. A plot of total resistance versus depth often independently yields the same shape curve as a plot of at least one component of peak particle velocity versus depth. A simple mathematical attenuation model is proposed as an initial step toward utilizing this relationship to predict at least one component of ground motions. Measured peak overpressure (noise) in the air correlated less directly to the quantities measured on the pile, but a conservative and simple mathematical model can still be proposed based on the dynamic testing-measured velocity near the pile top and idealized sound generation and attenuation theories.
Strengthening of Steel Structures with High Modulus Carbon Fiber Reinforced Polymer (CFRP) Materials
(2005-06-15) Schnerch, David; Dr. Sami Rizkalla, Committee Chair; Dr. Emmett Sumner, Committee Member; Dr. Mervyn Kowalsky, Committee Member; Dr. Kara Peters, Committee Member
Transportation departments and the telecommunications industry are currently demanding cost-effective rehabilitation and/or strengthening techniques for steel structures, including bridges and monopole towers. Rehabilitation is often required due to cross-section losses resulting from corrosion damage and strengthening may be required due to changes in the use of a structure. Current strengthening techniques, have several disadvantages including their cost, need to match the surface configuration of the existing structure, poor fatigue performance and the need for ongoing maintenance due to continued corrosion attack. The current research program makes use of new high modulus types of carbon fiber for strengthening steel structures. The experimental program was developed in four phases. These phases included the selection of suitable resins and adhesives for bonding the CFRP sheets and strips to the steel, characterization the bond to the steel through testing of the development length, performing large-scale tests on strengthened steel monopole towers and also determining the behavior of strengthened steel-concrete composite beams that are typical of bridge structures. The result of the experimental program was the demonstration of sizeable strength and stiffness increases for the steel structures, strengthened with the developed system. Analytical work has also been completed to predict these strength and stiffness increases as well as to determine the bond stresses to ensure the avoidance of a debonding failure, which is detrimental to the effective use of the high modulus CFRP material.