No description available.

<p>Seismic isolation is a seismic design philosophy that aims to reduce the demand on structures as opposed to increasing their capacity to endure forces. Seismic isolation can be achieved by placing isolating bearings with relatively low stiffness compared to the structure itself beneath the superstructure. This low stiffness layer increases the structural period, shifting the structure into a period range of low seismic energy content.</p> <p>The objectives of this research were to investigate the dynamic properties, durability and limitations of stable unbonded fibre reinforced elastomeric isolator (SU-FREI) bearings. Vertical compression testing indicated the bearings possessed adequate vertical stiffness. Due to lack of bonding at the bearing interface surfaces rollover deformation was observed to occur during lateral cyclic testing. This response behaviour was found to result in advantageous effective lateral stiffness and damping properties. The bearings maintained stability during rollout testing while serviceability and fatigue testing both conformed to code specified test specimen adequacy limitations. Experimental shake table testing showed that the isolated structure behaved essentially as a rigid body during testing. Test results showed that a SU‐FREI isolation system significantly reduced the seismic demand on the structure.</p> <p>Modelling of the bearings dynamic properties was completed using a bilinear model and a backbone curve model. Both models showed adequate results in predicting experimental peak responses. A simplified design spectrum analysis was presented and used to model the structure in four Canadian cities. This design spectrum analysis approach showed adequate capabilities in predicting peak response values, such that the method could be used in preliminary analysis and design of isolated structures.</p> / Master of Applied Science (MASc)

<p dir="ltr">Post-tensioned concrete is a popular structural system in modern bridge design, allowing for longer bridge spans and the construction of curved, spliced, and segmental girders. When designed and constructed properly, post-tensioned concrete can offer improved durability over reinforced concrete as it is more resistant to cracking. However, corrosion of prestressing strands can compromise the integrity of post-tensioned construction. Grouting has been the primary method of providing corrosion protection to the prestressing strands. However, grouting of tendons presents numerous limitations and results in multiple problems in practice. As a result, there has been increased interest in the use of unbonded tendons. Corrosion protection can still be achieved in unbonded tendons through the use of flexible fillers such as grease and microcrystalline wax. Limited research has been conducted on unbonded tendons, and no known research exists regarding the behavior of unbonded tendons subject to torsion. The objective of this research was to investigate the torsional behavior and strength of unbonded post-tensioned bridge girders. The scope of work included testing six specimens to investigate the difference in behavior between bonded and unbonded tendons as well as the influence of internal versus external ducts. The influence of transverse reinforcement was also considered.</p>

No description available.

The introduction of Fibre Reinforced Polymers (FRP) to the civil engineering market in the late 1980s resulted in the emergence of a range of new tools for rehabilitating and strengthening concrete structures. Strengthening using FRPs is typically accomplished using non-prestressed externally bonded FRPs. The technical and economic benefits of such strengthening could be further increased by prestressing the FRPs, especially when dealing with concrete structures. Prestressing concrete structures suppresses the appearance and growth of cracks in the serviceability limit state. This in turn increases the structure’s stiffness and resistance to degradation. Prestressing also increases the structure’s yield load but does not change its failure load relative to that of an analogous non-prestressed structure, provided that all other parameters are kept constant. In 2004, a pilot study was carried out at the Luleå University of Technology (LTU) to investigate the scope for using unbonded Carbon Fibre Reinforced Polymer (CFRP) strengthening systems, particularly those involving prestressing. In the early stages of this project, a number of difficulties were encountered in anchoring the CFRP rods to concrete structures: the conical wedge anchorages that were used tended to either cause premature failure of the rods or allowed the rod to slip out of the anchorage. It was therefore decided to study the mechanisms at work within these anchorages in more detail. The goal of the project was to develop a small, practical, reliable, and userfriendly anchorage for use in unbonded external CFRP strengthening systems. On the basis of a thorough literature review, which is described in Paper 1, it was concluded that despite the difficulties encountered, the conical wedge anchorages used with steel reinforcing rods were the most promising starting point for the design of a new anchorage for use with CFRPs. Importantly, the conical wedge anchorage can be made small in size and easy to mount while retaining a high degree of versatility; this is not true of bonded, sleeve, and clamping anchorages. Analytical and numerical models were used to investigate the distribution of radial stress within these highly pressurized anchorages. Paper 2 describes an evaluation of the capability of three types of models - an analytical axisymmetric model based on the thick-walled-cylinder-theory and two Finite Element (FE) models, one axisymmetric and one three-dimensional - to predict the behaviour of a conical wedge anchorage. It was concluded that the axisymmetric models were incapable of modelling the stress distribution within the anchorage with sufficient accuracy, and so 3D FE models were used exclusively in subsequent studies. Paper 3 describes the development of a new anchorage for CFRP rods. The design process involved conducting pull-out studies on a series of prototypes, in conjunction with computational studies using a basic FE model, to identify and understand the prototypes’ failure modes. Between the computational data and experimental results, a good understanding of the factors affecting the interaction between the CFRP rod and the anchorage was obtained. The new anchorage design employs a one-piece wedge which effectively incorporates the three wedges and the inner sleeve from more conventional wedge anchorages into a single unit. This increases the reliability and user-friendliness of the anchorage because it eliminates the need to check the alignment of individual wedges. The new design has been patented; the published Swedish patent is included in the thesis as Paper 6. The newly-developed anchorage was then incorporated into a prestressing system and its performance was evaluated using a series of test beams. In parallel with the planning of these tests, a series of pull-out tests was conducted using the new anchorage. The strain measurements obtained in these experiments were compared to predictions made using a new, more advanced FE model, and used to refine the design of the new anchorage. Paper 4 describes this new FE model, the most important parameters affecting anchorage behaviour, and the final anchorage design. Paper 5 focuses on the possibilities provided by the new anchorage. Tests were performed using seven three meter long concrete beams prestressed with external unbonded CFRP tendons. One beam was unstrengthened; the other six were strengthened in different ways, with different prestressing forces, initial tendon depths, and with or without the use of a midspan deviator for the tendons. The results of these tests were compared to those obtained using otherwise identical beams prestressed with steel tendons and to the predictions of an analytical beam model developed for use with steel tendons. These tests showed that the prestressing works as intended and that the behaviour of beams prestressed with external unbonded CFRP tendons is fully comparable to that of beams prestressed with steel tendons. It was also found that the predictions of the analytical model were in good agreement with experimental observations, although there were some differences between the measured and predicted tendon stresses. The development of a functional anchorage represents a fulfilment of the objectives laid out at the start of this project, and represents an important step towards the practical use of prestressed unbonded external CFRP tendons in strengthening concrete structures. However, a number of outstanding questions remain to be addressed. Little is known about the safety of this kind of system, and the benefits of using CFRP tendons should be quantified. Furthermore, there are a number of potential technical issues that must be addressed. These include the risk of creep-rupture in the CFRP, the effects of thermal contraction and expansion on the anchorage, and the scalability of the anchorage as the tendon diameter is increased. Finally, the long-term behaviour of the anchorage and prestressing system should be investigated. / I och med introduktionen av fiberkompositer i byggbranschen under slutet av 80-talet har en rad nya verktyg för förstärkning och underhåll av betongkonstruktioner utvecklats. Förstärkning har oftast utförts med pålimmade kompositer utan förspänning. För att ytterligare öka verkningsgraden, både den tekniska och ekonomiska, kan förspänning vara en möjlighet. Särskilt för betongkonstruktioner. Förspänning av en betongkonstruktion medför att man i bruksgränstillståndet begränsar uppkomsten av sprickor och deras storlek. Det ger i sin tur en ökad styvhet hos konstruktionen. Därutöver höjs lasten för när det slakarmerade stålet flyter. I jämförelse med ospända konstruktioner är dock brottlasten densamma, så länge övriga parametrar behålls. Under 2004 genomfördes en pilotstudie vid Luleå tekniska universitet (LTU) för att undersöka framtida möjligheter och utmaningar med förspända, icke vidhäftande kolfiberkompositkablar. I det läget upptäcktes svårigheter att förankra kompositkabeln mot betongen. De koniska killås som användes orsakade antingen brott på kabeln redan vid låga belastningar eller glidning hos kabeln, som omöjliggjorde fullgod kraftöverföring. Ett beslut togs då att tills vidare fokusera på förankringen och genomföra en mer ingående studie kring denna. Som mål sattes upp att arbetet skulle resultera i en liten, tillförlitlig och användarvänlig förankring. Den skulle sen i en förlängning kunna användas för att slutföra pilotstudien och därefter i större tillämpningar. Trots de förhållandevis nedslående resultaten från pilotförsöken visade den grundliga litteraturstudien som presenteras i Artikel 1 att koniska killås trots allt verkar vara den mest lovande typen av förankring för kolfiberkablar. Den bör därför användas som utgångspunkt för fortsatt utveckling. I motsats till vidhäftande, hyls och klämmande förankringar kan killåset göras litet, lätt att montera och också användas i många praktiska tillämpningar. För att undersöka hur de höga radiella tryckspänningarna i ett sådant killås fördelas är olika former av beräkningsmodeller nödvändiga verktyg. I Artikel 2 jämförs tre olika modeller med avseende på hur väl de kan beskriva komplexiteten hos ett koniskt killås. Det är dels en analytisk axisymmetrisk modell, som också härleds i artikeln, dels en axisymmetrisk Finita Element (FE) modell och dels en 3D FE modell. Undersökningen visade att ingen av de axisymmetriska modellerna har kapacitet nog att tillförlitligt modellera killåset. I fortsatta undersökningar har därför endast 3D FE använts. Resultaten från en enkel FE modell ligger också, tillsammans med tidiga laboratorieförsök, som grund för Artikel 3. Däri beskrivs hur ett nytt killås via prototyper och nya lösningar utvecklats, och hur arbetet för att få fram det nya låset också gett en bättre förståelse för interaktionen mellan kolfiberkompositkabel och lås. Som avslutning presenteras en innovativ design där de tre kilarna och den inre hylsan sammanfogats till en enhet. Med den nyutvecklade designen blir förankringen såväl mer tillförlitlig som användarvänlig. Alla kilar har då redan från början rätt position i förhållande till varandra. Den utvecklade förankringslösningen har också lett fram till ett beviljat svenskt patent, bifogat i avhandlingen som Artikel 6. Efter utvecklingen av den nya förankringen var nästa steg i de uppsatta målen implementering av densamma i ett förspänningssystem och nya balkförsök i konstruktionslabbet. Parallellt med planeringen för balkförsöken pågick ett arbete med att ytterligare förbättra låsdesignen. Bland annat användes en mer detaljerad FE modell som sedan jämfördes med mätningar från en ny serie med dragprov. Den nya FE modellen tillsammans med en utvärdering av viktiga parametrar och den slutliga förankringsdesignen presenteras i Artikel 4. Artikel 5 sammanfattar och avslutar forskningsstudien med en testserie om sju stycken, tre meter långa, betongbalkar förspända med utanpåliggande kolfiberkompositstavar. En av balkarna provades utan förstärkning. Förstärkningen hos de övriga varierades med avseende på förspänningsgrad, förspänningens effektiva höjd och användandet av deviator vid balkmitt. Resultaten har jämförts mellan de provade balkarna, med identiska balkar förspända med stålkablar samt med en analytisk modell utvecklad för förspänning med stålkablar. Från resultaten kan utläsas att förspänningen fungerar bra och att beteendet hos balkarna förspända med utanpåliggande kolfiberkablar är fullt jämförbart med det hos balkarna förspända med stålkablar. Likaså visar jämförelsen med de modellerade beteendena på god överensstämmelse, även om vissa skillnader finns mellan uppmätta och modellerade spänningar i kolfiberkabeln. Med målen för forskningen uppfyllda och en ny fungerande förankring framtagen så har vägen till praktiska tillämpningar kortats betydligt, ändå finns några frågetecken kvar att räta ut. Ett är säkerheten hos den här typen av system och nyttan av att använda kolfiberkomposit istället för stål. Innan systemet används i praktiken bör därför följande frågeställningar belysas: Risk för krypbrott i kolfiberarmeringen, inverkan av temperaturförändringar (och temperaturrörelser) i förankringen samt eventuella storlekseffekter vid förankring av kablar med större diametrar. De här frågorna tillsammans med långtidsförsök på förankringen och förspänningssystemet bör ses som viktiga framtida forskningsfrågor. / Godkänd; 2011; 20110128 (ysko); DISPUTATION Ämnesområde: Konstruktionsteknik/Structural Engineering Opponent: PhD Chris Burgoyne, Dep of Engineering, University of Cambridge, UK Ordförande: Professor Björn Täljsten, Institutionen för samhällsbyggnad och naturresurser, Luleå tekniska universitet Tid: Fredag den 18 februari 2011, kl 10.00 Plats: F1031, Luleå tekniska universitet

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No description available.

This master‘s thesis deals with the behaviour of strengthened reinforced concrete beams. The aim was to evaluate different types of strengthening in comparison not only with each other, but also with the reference beam. Three reinforced concrete beams were fabricated for the purpose of the thesis. The first served as a reference beam, the second was strengthened with carbon lamella glued to the lower edge of the reinforced concrete beam, and the last one was strengthened with unbonded post-tensioning system. Subsequently, they were all loaded by a four-point bending. An evaluation was then carried out which shows that the two strengthened reinforced concreate beams resist the load much better than the reference beam. In the conclusion, different utilization possibilities of both methods are listed, as well as the disadvantages that must be taken into account when designing.

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This thesis examines the development of a superstructure for a slab-on-girder wood-concrete composite highway bridge. Wood-concrete composite bridges have existed since the 1930's. Historically, they have been limited to spans of less than 10 m. Renewed research interest over the past two decades has shown great potential for longer span capabilities. Through composite action and suitable detailing, improvements in strength, stiffness, and durability can be achieved versus conventional wood bridges. The bridge makes use of a slender ultra-high performance fibre-reinforced concrete (UHPFRC) deck made partially-composite in longitudinal bending with glued-laminated wood girders. Longitudinal external unbonded post-tensioning is utilized to increase span capabilities. Prefabrication using double-T modules minimizes the need for cast-in-place concrete on-site. Durability is realized through the highly impermeable deck slab that protects the girders from moisture. Results show that the system can span up to 30 m while achieving span-to-depth ratios equivalent or better than competing slab-on-girder bridges.