Post-tensioning (PT) strands have been employed in a number of self-centering seismic force resisting systems as part of the restoring force mechanism which virtually eliminates residual building drifts following seismic loading. As a result of the PT strands large elastic deformation capability, they have been proven to work efficiently in these types of systems. Although typically designed to stay elastic during design basis earthquake events, strands may experience inelastic cyclic loading during extreme earthquakes. Furthermore, the yielding and fracture behavior of PT strand systems is central to the collapse behavior of self-centering systems. The loading conditions to which PT strands are typically subjected in prestressed/post-tensioned concrete applications are vastly dissimilar, and only limited research has explored the behavior of PT strands as subjected to inelastic cyclic loading. The testing program conducted to characterize the behavior of PT strand systems as they might be applied in self-centering applications incorporated more than fifty tests, including monotonic and cyclic tests to failure. Variations in the test configuration included strand obtained from two manufacturers, single-use and multiple-use anchorage systems, and variations in initial post-tensioning strand stress. Characteristics of the response that were investigated included seating losses, deformation capacity prior to initial fracture, additional deformation capacity after initial fracture, and the overall load-deformation behavior. Data was analyzed to provide recommendations for PT strand system usage in self-centering seismic force resisting systems. It was concluded that significant strength and ductility allow PT strand systems to consistently provide self-centering systems with reliable restoring force capability. / Master of Science

In the past two decades, there have been significant advances in the development of self-centering (SC) seismic force resisting systems. However, examples of SC systems used in practice are limited due to unusual field construction practices, high initial cost premiums and deformation incompatibility with the gravity framing. A self-centering beam moment frame (SCB-MF) has been developed that virtually eliminates residual drifts and concentrates the majority of structural damage in replaceable fuse elements. The SCB consists of a I-shaped steel beam augmented with a restoring force mechanism attached to the bottom flange and can be shop fabricated. Additionally, the SCB has been designed to eliminate the deformation incompatibility associated with the self-centering mechanism. The SCB-MF system is investigated and developed through analytical, computational, and experimental means. The first phase of the work involves the development of the SCB concepts and the experimental program on five two-thirds scale SCB specimens. Key parameters were varied to investigate their effect on global system hysteretic response and their effect on system components. These large-scale experiments validated the performance of the system, allowed the investigation of detailing and construction methods, provided information on the behavior of the individual components of the system. The experimental results also provided data to confirm and calibrate computational models that can capable of capturing the salient features of the SCB-MF response on global and component level. As a part of the second phase, a set of archetype buildings was designed using the self-centering beam moment frame (SCB-MF) to conduct a non-linear response history study. The study was conducted on a set of 9 archetype buildings. Four, twelve and twenty story frames, each with three levels of self-centering ratios representing partial and fully self-centering systems, were subjected to 44 ground motions scaled to two hazard levels. This study evaluated the performance of SCB-MFs in multi-story structures and investigated the probabilities of reaching limit states for earthquake events with varying recurrence period. The experimental and computational studies described in this dissertation demonstrate that the SCB-MF for steel-framed buildings can satisfy the performance goals of virtually eliminating residual drift and concentrating structural damage in replaceable fuses even during large earthquakes. / Ph. D.

This research introduces and develops a counter-intuitive seismic retrofit strategy, referred to as “Selective Weakening” (SW), for pre-1970s reinforced concrete (RC) frames with a particular emphasis on the upgrading of exterior beam-column joints. By focusing on increasing the displacement and ductility capacities of the beam-column joints, simple retrofit interventions such as selective weakening of the beam and external post-tensioning of the joint can change the local inelastic mechanism and result in improved global lateral and energy dissipation capacities. The thesis first presents an extensive review of the seismic vulnerability and assessment of pre-1970s RC frames. Following a review of the concepts of performance-based seismic retrofit and existing seismic retrofit solutions, a thorough conceptual development of the SW retrofit strategy and techniques is presented. A “local-to-global” design procedure for the design of SW retrofit is proposed. Based on the evaluation of the hierarchy of strength at a subassembly level, a capacity-design retrofit outcome can be achieved using various combinations of levels of beam-weakening and joint post-tensioning. Analytical tools for the assessment and design of the SW-retrofitted beam-column joints are developed and compared with the test results. Nine 2/3-scaled exterior joint subassemblies were tested under quasi-static cyclic loading to demonstrate the feasibility and effectiveness of SW retrofit for non-ductile unreinforced beam-column connections. Parameters considered in the tests included the presence of column lap-splice, slab and transverse beams, levels of post-tensioning forces and location of beam weakening. Extensive instrumentation and a rigorous testing regime allowed for a detailed experimental insight into the seismic behaviour of these as-built and retrofitted joints. Experimental-analytical comparisons highlighted some limitations of existing seismic assessment procedures and helped in developing and validating the SW retrofit design expressions. Interesting insights into the bond behaviour of the plain-round bars, joint shear cracking and post-tensioned joints were made based on the experimental results. To complement the experimental investigation, refined fracture-mechanic finite-element (FE) modelling of the beam-column joint subassemblies and non-linear dynamic time-history analyses of RC frames were carried out. Both the experimental and numerical results have shown the potential of SW retrofit to be a simple and structurally efficient structural rehabilitation strategy for non-ductile RC frames.

In post-tensioned (PT) bridges, prestressing steel tendons are the major load carrying components. These tendons consist of strands, ducts, and cementitious grout that fill the interstitial space between the strands and ducts. However, inspections on PT bridges have reported the presence of voids, moisture, and chlorides inside grouted ducts as the major cause of accelerated corrosion of strands. Corrosion of the strands has resulted in PT bridge failures in Europe and tendon failures in the United States. As most of the PT bridges have high importance measures and the consequences of failure are significant, it is important to maintain high levels of safety and serviceability for these bridges. To meet this goal, bridge management authorities are in dire need of tools to quantify the long-term performance of these bridges. Time-variant structural reliability models can be useful tools to quantify the long-term performance of PT bridges. This doctoral dissertation presents the following results obtained from a comprehensive experimental and analytical program on the performance of PT bridges. 1) Electrochemical characteristics of PT systems 2) Probabilistic models for tension capacity of PT strands and wires exposed to various void and environmental conditions 3) Time-variant structural reliability models (based on bending moment and stress limit states) for PT bridges 4) Time-variant strength and service reliabilities of a typical PT bridge experiencing HS20 and HL93 loading conditions and different exposure conditions for a period of 75 years The experimental program included exposure of strand specimens to wet-dry and continuous-atmospheric conditions. These strand specimens were fabricated to mimic void and/or grout-air-strand (GAS) conditions inside the tendons. It was found that the GAS interface plays a major role in strand corrosion. The GAS interfaces that are typically located in the anchorage zones of harped PT girders or vertical PT columns can cause aggressive strand corrosion. At these locations, if voids are present and the environment is relatively dry, then limited corrosion of the strands occurs. However, if the presence of high relative humidity or uncontaminated and chloride-contaminated water exists at these interfaces, then corrosion activity can be high. The strands were exposed for a period of 12, 16, and 21 months, after which the remaining tension capacity was determined. The analytical program included the development of probabilistic strand capacity models (based on the experimental data) and the structural reliability models. The timevariant tension capacity predicted using the developed probabilistic models were reasonably consistent with the tendon failures observed in PT bridges in Florida and Virginia. The strength reliability model was developed based on the moment capacity and demand at midspan. Service reliability model was developed based on the allowable and applied stresses at midspan. Using these models, the time-variant strength and service reliabilities of a typical PT bridge were determined based on a set of pre-defined constant and random parameters representing void, material, exposure, prestress, structural loading, and other conditions. The strength and service reliabilities of PT bridges exposed to aggressive environmental conditions can drop below the recommended values at relatively young ages. In addition, under similar conditions the service reliability drops at a faster rate than the strength reliability.

The Texas Department of Transportation (TxDOT) has completed the design of a signature bridge in Fort Worth, TX. The proposed structure is comprised of precast, post-tensioned concrete network arches. The arches will be cast on their sides and then rotated into the vertical orientation. Concerns exist about the durability and stability of the arches during stressing, handling, and transportation. The rotation process in particular represents a critical period in the life of the arches. A monitoring system was proposed to track stresses in the arches throughout the construction operations. The primary goals of the project are to install vibrating wire gages (VWGs) in the arches prior to casting to monitor the performance of the arches until the bridge is completed. The instrumentation will be used to provide real-time feedback to TxDOT and the contractor during stressing, handling, and bridge construction. This thesis focuses on the results of a preliminary laboratory study conducted in support of the instrumentation initiative. The purpose of the study was two-fold: to establish the capabilities and limitations of the VWGs and to study the buckling behavior of slender concrete elements with unbonded post-tensioning. More than sixty axial load tests were performed on two slender concrete specimens instrumented with VWGs. Observations are made on the accuracy and reliability of the VWGs. In general, the VWGs were found to be both accurate and reliable in measuring structural parameters and reporting trends in behavior, even at low loads. Some apparent errors were identified, but these were attributed to testing inconsistencies and scale factors rather than to gage error. Observations were also made on the buckling behavior of the elements under a variety of axial loading configurations. The effects of the engagement of the tensioned strand with the duct had a significant impact on the behavior. Strand engagement was shown to increase the buckling capacity of the members through stiffening action, but did not necessarily eliminate the risk of instability. Both the gage resolution study and the stability tests are expected to significantly enhance the ability of the research team to support the arch construction operations. / text

The goal of the splice girder research project 0-6652 funded by the Texas Department of Transportation is to utilize the full potential of splicing prestressed TX girders continuously. The TX girder family of beams is cost effective alone due to their simple, repetitive fabrication, but to truly optimize their potential would be to span several beams together as one continuous unit. The weight and length restrictions allowed by trucks or barges limit the prestressed beam lengths. Therefore, splicing together prestressed beams becomes the solution to the transporting obstacle. As a result, the prestressed girders will be more competitive to other bridge types such as steel I-girders, steel trapezoidal girders, cable-stayed bridges, and concrete segmental bridges. In fact, a prestressed/post-tensioned concrete bridge is preferred over steel designs in highly corrosive environments such as the coast or in snow regions where de-icing chemicals are used. In comparison, to a segmental box girder bridge, the post-tensioned prestressed bridge has reduced complexity due to fewer segments and the number of reduced joints susceptible to corrosion. The issue that arises with splicing prestressed beams is that in the process of connecting them together an opening must be made to install the post-tensioning (PT) steel strands. The openings are created by installing several steel or plastic circular ducts into the web region. Since the post-tensioning results in a reduction of the concrete web region, a modification is necessary to the shear capacity equation. The experimental study performed at the Ferguson Structural Engineering Laboratory consisted of fabricating and testing two full-scale prestressed Tx46 girders. One girder contained a plastic post-tensioning duct with grout and steel strands installed in the web region. The other beam was a standard Tx46 beam fabricated without a duct. Both beams had a reinforced concrete deck installed with an overhang to model an actual bridge section. Furthermore, the purpose of the standard beam was to serve as a direct comparison to the beam with a duct and determine the actual reduction in shear capacity. The research and findings will include the impact of the plastic duct in the Tx46 compared to the control beam. The failure loads of the test specimens will be compared to the current 2012 AASHTO code predictions for shear design. Also, revisions to the AASHTO code will be recommended if necessary. The primary goal of this research was to improve the design and detailing of the skewed end-blocks commonly used in these beams. As U-beams had been in service for several decades without incident, it was anticipated that there would be little need for change in the design, and the findings of the research would involve a slight tweaking to improve the overall performance. / text

Unbonded post-tensioned (UPT) flat plate concrete slabs have seen widespread use in multi-storey office and condominium buildings since the 1960s. The popularity of these systems can be attributed to various economic and structural benefits, including reductions in slab thickness, storey height, building mass, and excellent deflection control over large spans. The “inherent fire resistance” of these systems is often quoted as a key additional benefit as compared with competing structural systems. Such statements are apparently based largely on satisfactory results from large scale standard fire resistance tests performed on UPT slabs during the 1960s and on experience from real fires in UPT buildings. However, much remains unknown about the true structural behaviour of continuous multiple bay UPT slabs in real building fires. For instance, relatively little data exist on the effects of elevated temperature on cold drawn prestressing steel under realistic, sustained service stress levels. The primary objective of this thesis is to provide a greater understanding of the high-temperature performance (predominantly related to prestress relaxation) of prestressing steel used in UPT flat plate slabs. A computational model is developed, extending previous research by others, to predict transient high temperature stress relaxation (i.e., prestress loss) for a tendon in a typical UPT multiple span flat plate concrete slab under transient heating and cooling. The computational model is validated by comparison against a series of novel high temperature experiments on locally-heated, stressed, and restrained prestressing tendons with realistic as-built configurations. Reasonable agreement between measured and predicted prestress losses is observed, although some refinement of the model’s input parameters may be required. Test data also indicate that the most crucial fire scenario on a UPT concrete slab may be localized heating rather than a global, fully developed fire. The model is subsequently used to predict the capacity in flexure and punching shear of a UPT flat plate structure under various spatial and temporal heating regimes. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate, robust concrete cover for structural fire safety. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-09-24 18:27:25.559

[Abstract]Over the last few decades, there has been a rapid increase in the volume and weight of heavy vehicles using national road networks. More than half of the bridges around the world are over forty years old. The deterioration of these existing bridges due to increased traffic loading, progressive structural aging, and reinforcement corrosion from severe environmental conditions has become a major problem in most countries. Several techniques have been used to strengthen these structures around the world. External post-tensioning is one of the widely used strengthening techniques in many countries due to its advantages over other methods. Furthermore, flexural strengthening using external post-tensioning has become a well established technique over the past few decades. However, when external post-tensioning is used to strengthen shear damaged reinforced concrete members, unlike flexural damage, the efficiency is significantly reduced by existing shear cracks.This research study was carried out to investigate the behaviour of reinforced concrete beams with existing shear cracks when strengthened by external means. The study consists of two parts: experimental investigations of reinforced concrete beams with different parameters and numerical analysis of reinforced concrete beams usingsimplified theoretical formulation and finite element modelling.To study the behaviour of shear damaged concrete beams, two different strengthening techniques, namely external post-tensioning and external clamping, were used. In addition to the strengthening, the effect of cracks on the behaviour of reinforced concrete beams was investigated by repairing such cracks using epoxy resin injection. Experimental results showed that existing shear cracks have a substantial effect on the member capacity when strengthened by external posttensioning. Although there are concerns about the practical applications of externalclamping, the experimental results suggest that external clamping could be a more effective technique than external post-tensioning to reduce the effect of existing shear cracks on the behaviour of concrete beams. Furthermore, proper repair of the shear cracks could significantly reduce their impact.In the numerical analysis, a simplified mathematical approach was developed to estimate the capacity of shear damaged reinforced concrete beam by expanding themodified compression field theory (MCFT). In addition to the simplified theoretical formulation, a finite element model was developed using the commercial finite element package, Abaqus. Comparison between the predicted behaviour using finite element analysis (FEA) and the experimental data illustrated that the developed finite element model could be used as a reliable tool to estimate the capacity of shear damaged reinforced concrete beams. A parametric study was conducted to investigate the effect of different parameters such as concrete strength, amount of shear reinforcement and crack width, using the developed finite element model. From the numerical study, it was concluded that the simplified approach developedin this study can be used as a reliable and conservative technique to predict the member capacity of a cracked reinforced concrete beam strengthened by external means. Furthermore, the parametric study showed that crack width is the most sensitive parameter that affects the capacity of a cracked beam strengthened by external post-tensioning.Based on this research study it can be concluded that existing shear cracks have a substantial effect on the behaviour of reinforced concrete beams strengthened byexternal post-tensioning. The simplified mathematical approach developed in this study can be used to estimate the capacity of such beams.

This thesis investigates a parametric study of a flat plate floor system designed using post-tensioning. The load balanced by the post-tensioning, the slab depth, and the strength of concrete were varied to create the parametric study of a hotel/condominium grid layout. In order to perform the parametric study, research was conducted on the development of post-tensioning, methods of analysis for two-way slab design, and post-tensioning methods of analysis. Design was conducted by hand through a series of Excel spreadsheets and compared to results found using the computer analysis program, ADAPT-PT. The designs found in the parametric study were then used to perform a cost analysis across ten cities in the United States: Atlanta, Boston, Chicago, Denver, Houston, Los Angeles, Miami, Phoenix, San Francisco, and Seattle. The designs from the hand analysis and the ADAPT-PT model provided similar results for the post-tensioning, and both methods provide an adequate design. The use of ADAPT-PT is recommended because of its ease of use and quick calculation capabilities. The designs of the hand analysis were quantified and along with unit prices gathered from contractors and suppliers the cost analysis found that the design with 100% of the dead load balanced provided the least expensive solution for all the cities, and the design using a 6000 psi strength concrete provide the most expensive solution for all cities. The least expensive slab design was $9.02 per square foot in Atlanta, Georgia, and the most expensive slab design was $24.96 per square foot in Miami, Florida. A more rigorous parametric study in the future may provide a better optimization for the hotel/condominium slab investigated as the parametric study of this thesis found costs which varied by less than 10% between the most expensive and least expensive slabs in the ten cities.

O presente trabalho faz um estudo da representação da protensão em estruturas de barras através de um Conjunto de Cargas Concentradas Equivalentes para determinação dos esforços solicitantes e dos deslocamentos, gerados pela protensão. O trabalho aborda a conceituação de protensão, forças de desvio e perdas imediatas de protensão. Na sequência discute-se alguns métodos para determinação de esforços de protensão, inclusive para o caso de peças hiperestáticas, como por exemplo o método dos esforços solicitantes iniciais e o da carga distribuída equivalente. A seguir discute-se o algoritmo em estudo - Conjunto de Cargas Concentradas Equivalentes, CCCE (também conhecido como Método da Força Variável), suas vantagens e aplicações. Na parte final compara-se, através de exemplos, a aplicabilidade e precisão do CCCE com alguns dos métodos mais tradicionais citados anteriormente assim como as vantagens e desvantagens de cada um deles. / This work is a study about the representation of the prestressing through a CELG (Concentrated Equivalent Loads Group) in order to determine the internal forces and displacements in prestressed structures, due to prestressing. This study considers the concept of prestressing, deviation forces and immediate loss of prestressing. Furthermore some alternative methods to determine forces of prestressing are discussed including the case of hiperestatic structures e.g. initial forces and equivalent distributed loads. Next, the studied algorithm is discussed - CELG, (also known as Variable Force Method), its advantages and uses. Finally the use and precision of CELG is compared to some of the most traditional methods quoted beforehand and also its advantages and disadvantages.