Browsing by Author "Dr. Amir Mirmiran, Committee Chair"

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Behavior of FRP-Concrete Beam-Columns under Cyclic Loading
(2003-07-02) Shao, Yutian; Dr. Amir Mirmiran, Committee Chair; Dr. Sami H. Rizkalla, Committee Member; Dr. Mervyn J. Kowalsky, Committee Member; Dr. Eric C. Klang, Committee Member
Use of concrete-filled fiber-reinforced polymers (FRP) tubes (CFFT) for columns and piles has been studied extensively over the last decade. The focus, however, has been exclusively on the monotonic behavior of CFFT. An issue that has received little attention is the implications of using CFFT in seismic regions. Survey of damaged structures in recent earthquakes indicates that catastrophic failure of an entire structure may result from failure of few columns in a chain action. Since it may not be economical to design columns to respond to earthquake loads in their elastic range, dissipation of energy by post-elastic deformation is desired. Although, FRP materials are known for their linear elastic behavior, some FRP systems may exhibit non-linearity due to their laminate architecture and inter-laminar shear. Also, confinement of concrete core in CFFT improves its ductility. This study was carried out to evaluate the cyclic behavior of CFFT beam-columns, and determine whether non-linearity of FRP or confinement of concrete can provide seismic performance comparable to reinforced concrete (RC) columns or concrete-filled steel tubes (CFST). The experimental work consisted of cyclic loading and unloading of FRP-wrapped concrete cylinders and FRP coupons, and reverse cyclic loading of CFFT beam-columns under constant axial load. Some measures of hysteretic performance, including cumulative energy dissipation, ductility and pinching effect were used to evaluate the cyclic response of tested CFFT beam-columns. The study resulted in a cyclic model for FRP-confined concrete in compression, and cyclic models for linear and non-linear FRP materials in tension and compression. A fiber element model was employed to predict the cyclic behavior of CFFT beam-columns. A parametric study was carried out on the cyclic behavior of CFFT beam-columns, and to compare the hysteretic response of CFFT beam-columns with those of RC and CFST members. The two types of CFFT beam-columns tested under this study represented two different failure modes; a brittle compression failure for the over-reinforced white tube specimens with thick FRP tube and with majority of the fibers in the longitudinal direction, and a ductile tension failure for the under-reinforced yellow tube specimens with thin FRP tubes and off-axis fibers. The study showed feasibility of designing ductile CFFT members for seismic applications comparable to RC members. Significant ductility can result from the inter-laminar shear in the FRP tube. Moderate amounts of internal steel reinforcement can further improve the performance of CFFT members. Adding internal steel can be ineffective and may lead to premature failure. Slender CFFT members have less capacity than their short stocky counterparts. However, they are less susceptible to pinching effect and premature shear failure.
Prestressed FRP Tubular Deck System
(2003-04-18) Wu, Zhenhua; Dr.James M. Nau,, Committee Member; Dr. Sami H. Rizkalla,, Committee Member; Dr. Amir Mirmiran, Committee Chair
An experimental and analytical study was undertaken to assess the behavior of a new FRP deck system. The deck consists of a series of pultruted FRP tubes, laid side by side on existing stringers, perpendicular to the direction of traffic. The tubes are then post-tensioned at mid-point between the stringers in the direction of traffic. The experimental work consisted of seven FRP tubular specimens crushed on their sides, eleven FRP decks in static bending, and four FRP decks in fatigue bending. The analytical work consisted of modeling of crushing test for a single FRP tube and multiple unbounded FRP tube specimens, modeling of static flexural test for a bonded and an unbounded FRP deck panel, and load rating of the floor system for the bridge with the installed FRP deck. The study showed feasibility of the new deck system for bridges with limited truck traffic and closely spaced stringers, where lack of panel action is not a concern. Failure mode, stiffness and capacity of the deck system are all functions of the FRP material properties and tube size, span length, interface bond and prestress level. In general, longer span decks fail in bending, whereas shorter span decks suffer from local shear failure due to stress concentrations at the corner of the tubes most adjacent to the applied load or the support. The deck system has some redundancy and reserved strength built into it by means of prestressing strand or bar. Short span decks are susceptible to early fatigue failure. The finite element analysis was shown to provide a good simulation of the FRP deck system. The floor system in the bridge was rated for 30 tons and 19 tons at the operating and inventory levels, respectively.
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.