To achieve thinner and longer floor slabs, rapid construction, and tight control of inservice deflections, modern concrete structures increasingly use high-strength, posttensioned prestressing steel as reinforcement. The resulting structures are called posttensioned (PT) concrete. Post-tensioned concrete slabs are widely believed to benefit from ‘inherent fire endurance.’ This belief is based largely on results from a series of standard fire tests performed on simply-supported specimens some five decades ago. Such tests are of debatable credibility; they do not capture the true structural behaviour of real buildings in real fires, nor do they reflect modern PT concrete construction materials or optimization methods. This thesis seeks to develop a more complete understanding of the structural and thermal response of modern prestressing steel and PT concrete slabs, particularly those with unbonded prestressing steel conditions, to high temperature, in an effort to steer current practice and future research towards the development of defensible, performance-based, safe fire designs. An exhaustive literature review of previous experimentation and real case studies of fire exposed PT concrete structures is presented to address whether current code guidance is adequate. Both bonded and unbonded prestressing steel configurations are considered, and research needs are identified. For unbonded prestressing steel in a localised fire, the review shows that the interaction between thermal relaxation and plastic deformation could result in tendon failure and loss of tensile reinforcement to the concrete, earlier than predicted by available design guidance. Since prestressing steel runs continuously in unbonded PT slabs, local damage to prestressing steel will affect the integrity of adjacent bays in a building. In the event that no bonded steel reinforcement is provided (as permitted by some design codes) a PT slab could lose tensile reinforcement across multiple bays; even those remote from fire. Using existing literature and design guidance, preliminary simplified modelling is presented to illustrate the stress-temperature-time interactions for stressed, unbonded prestressing steel under localised heating. This exercise showed that the observed behaviour cannot be rationally described by the existing design guidance. The high temperature mechanical properties of modern prestressing steel are subsequently considered in detail, both experimentally and analytically. Tests are presented on prestressing steel specimens under constant axial stress at high temperature using a high resolution digital image correlation (DIC) technique to accurately measure deformations. A novel, accurate analytical model of the stresstemperature- time dependent deformation of prestressing steel is developed and validated for both transient and steady-state conditions. Modern prestressing steel behaviour is then compared to its historical prestressing steel counterparts, showing significant differences at high temperature. Attention then turns to other structural actions of a real PT concrete structure (e.g. thermal bowing, restraint, concrete stiffness loss, continuity, spalling, slab splitting etc.) all of which also play inter-related roles influencing a PT slab’s response in fire. A series of three non-standard structural fire experiments on heavily instrumented, continuous, restrained PT concrete slabs under representative sustained service loads were conducted in an effort to better understand the response of PT concrete structures to localised heating. To the author’s knowledge this is the first time a continuous PT slab which includes axial, vertical and rotational restraint has been studied at high temperature, particularly under localised heating. The structural response of all three tests indicates a complex deflection trend in heating and in cooling which differs considerably from the response of a simply supported slab in a standard fire test. Deflection trends in the continuous slab tests were due to a combination of thermal expansion and plastic damage. The test data will enable future efforts to validate computational models which account for the requisite complexities. Overall, the research presented herein shows that some of the design guidance for modern PT concrete slabs is inadequate and should be updated. The high temperature deformation of prestressing steel under localised heating, as would be expected in a real fire, should be considered, since uniform heating of simplysupported elements is both unrealistic and unconservative with respect to tensile rupture of prestressing steel tendons. The most obvious impact of this finding would be to increase the minimum concrete covers required for unbonded PT construction, and to require adequate amounts of bonded steel reinforcement to allow load shedding to the bonded steel at high temperature in the event that the prestressing steel fails or is severely damaged by fire.

Unbonded post-tensioned concrete slabs have been widely used in Canada and the United States since the 1960s, as they allow increased span-to-depth ratios and excellent control of deflections compared to non-prestressed reinforced concrete flexural members. The satisfactory fire performance of unbonded post-tensioned concrete slabs in North America was established by a series of standard fire tests performed in the United States during the 1960s. However, there is a paucity of data on the effect of elevated temperatures on cold-drawn prestressing steel, both in terms of post-fire residual mechanical properties and high-temperature stress relaxation, which can lead to significant prestress loss both during and after a fire. A detailed and comprehensive literature review is presented that provides background on the residual mechanical properties of prestressing steel, as well as on the creep-relaxation behaviour experienced at elevated temperatures under stress. The results of two test series are discussed; the first examining the effects of elevated temperatures on the residual mechanical properties of prestressing steel exposed to elevated temperatures. The second test series examines the irrecoverable and significant loss of prestress force that results from steel relaxation and other thermal effects experienced during heating. A preliminary analytical model is presented, capable of predicting the change in prestress force experienced by a stressed strand under transient heating. The model is then compared with experimental elevated temperature relaxation data. Finally, the analytical model developed and residual mechanical properties obtained through experimentation are used along with a pre-existing finite difference heat transfer model (developed for concrete slabs) to examine the effect of elevated temperature exposure on the residual flexural capacity of a typical unbonded post-tensioned example slab. Several parameters, such as heated length and concrete cover, are examined using the example structure. From this it was observed that, after one hour of exposure to a standard fire (ASTM E119), significant losses in effective prestress and moment capacity occurred even with the appropriate amount of concrete cover. This is a finding which is of the utmost practical importance to engineers engaged in the evaluation of fire damaged unbonded post-tensioned structures. / Thesis (Master, Civil Engineering) -- Queen's University, 2007-12-18 17:15:17.521 / Natural Sciences and Engineering Research Council of Canada, and the Department of Civil Engineering at Queen’s University

Due to the unbonded nature of tendons to the slab within Unbonded Post Tensioned (UPT) concrete structures, tendon stress relaxation under heating affects all regions of the slab spanned by the tendon; not just in the locality of the fire. The numerical modelling of bonded and unbonded post tensioned concrete structures in fire has been performed to some degree, notably by Bailey and Ellobody. The consideration of elevated temperature creep to the relaxation of tendon prestress however, has not been considered. This thesis attempts to incorporate a uniaxial creep strain rate function of stress and temperature into the commercial FE software package Abaqus, compatible for use within the in-built multiaxial metal plasticity constitutive framework. What follows is a validation study of the Harmathy’s uniaxial creep strain accumulation function via the modelling of stress relaxation in isolated, tensioned and heated prestressing steel tendons, against experimental data. From here, UPT concrete slab models are analysed whilst exposed to a standard fire temperature-time curve and subsequently allowed to cool. Tendon prestress relaxation and resulting UPT concrete slab deflection is compared, where tendon creep is explicitly modelled, as opposed to implicitly covered by Eurocode 2 determined temperature dependent stress-strain curves. Following this, a large scale continuous one-way spanning UPT concrete structural model is developed to consider global structural behaviour resulting from localised fire, where realistic boundary conditions such as beam rotation and deflection are permitted. The ignorance of explicit elevated temperature creep consideration, in prestressing steel tendons, is commonly justified through the implicit accountability stated within Eurocode 2 temperature dependent stress-strain curves. This however is not completely true; Eurocode 2 states implicit accountability only holds should the tendon be heating at a rate within the bounds of 2⁰C/min to 50⁰C/min. Where only heating of a UPT concrete slab is considered, evidence from this thesis suggests Eurocode 2 determined stress-strain curves can implicitly account for accumulated creep strain up to limited temperatures. Prestressing steel tendons are however embedded within a concrete slab through which thermal gradients build up during fire. This means heat transfer can continue to the tendon, increasing its temperature postfire at an ever decreasing rate until it reaches its peak. Should post-fire cooling behaviour not be considered, continued tendon heating and subsequent creep strain accumulation will be ignored. Further, during the transition from heating to cooling within the tendon, it will be exposed to elevated temperatures with a rate of change below 2⁰C/min, whereby Eurocode 2, as stated cannot implicitly account for creep. It is shown, a significant degree of subsequent relaxation of prestress, UPT concrete slab deflection and concrete damage in hogging can occur during this phase of postfire behaviour, where the tendon temperature peaks during its transition from heating to cooling. In order to justify non consideration of creep, it should be shown tendon temperature will remain suitably low throughout the entire heating-cooling regime to which the UPT concrete slab is exposed. This must be achieved through adequate specification of minimum concrete cover to tendons to limit tendon temperature exposure for a given parametric fire curve duration, including the potential continued rise post-fire. Evidence within this thesis identifies 350⁰C as a critical temperature whereby the explicit consideration of tendon creep does not significantly increase predicted prestress relaxation and subsequent UPT concrete slab deformation, compared to implicit creep consideration from Eurocode 2. The manufacturing standard to which prestressing steel tendon strands are produced has been shown experimentally by Gales to significantly influence their susceptibility to elevated temperature creep. This is reflected by Gales determining differing creep parameters as a function of stress for incorporation in Harmathy’s uniaxial creep strain function. Modelled prestress relaxation of isolated, tensioned and heated tendons within this thesis is therefore significantly reduced when tendons are manufactured to a yield stress of 1860MPa according to the BS 5896 standard, as opposed to the ASTM A416 standard. As a result Eurocode 2 determined stress-strain curves implicitly account for accumulated creep strain during heating, at 10⁰C per minute, up to approximately 400⁰C for grade 1860 ASTM A416 manufactured tendons and 500⁰C for grade 1860 BS 5896 standard tendons. The aforementioned critical temperature of 350⁰C does not in actuality apply to necessary explicit creep consideration for UPT concrete slabs modelled with grade 1860 BS 5896 standard tendons. This temperature however remains a design temperature limit, owing to the potential onset of microstructural recrystallization beyond 400⁰C and the associated degradation of mechanical properties that coincides. The reasons for such differing elevated temperature creep and stress relaxation behaviour between the two manufacturing standards of prestressing steel wires and strands has been postulated within this thesis to be due to differing chemical compositions. This relates specifically to large relative differences of phosphorus and sulphur found in wires manufactures to each standard as tested by Gales.

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

Bei dem Prozess einer Wasserstoffinduzierten Spannungsrisskorrosion (H-SpRK) handelt es sich um einen zeitabhängigen Vorgang, der zu einer Reduzierung der Duktilität und Widerstandskraft des Spannstahls führt und daraus resultierend ein sprödes und schlagartiges Versagen eines Spannbetonbauwerkes zur Folge haben kann. Der Prozess selbst und insbesondere auch die ihn beeinflussenden Parameter sind für die im Bauwesen verwendeten hochfesten Spannstähle älterer Produktion weitestgehend unerforscht. Die Relevanz für bestehende Bauwerke ist jedoch nicht zuletzt durch einzelne dokumentierte und untersuchte Schadensfälle nachgewiesen. Ziel der vorliegenden Arbeit war es, die zur Verfügung stehenden Bauwerksuntersuchungen aus den zurückliegenden ca. 10 Jahren statistisch zu analysieren und auszuwerten. Auf dieser Basis war ein Berechnungsmodell wahrscheinlichkeitstheoretischer Basis zu entwickeln, mit dem die Gefahr eines spröden Bauwerksversagens für das Gesamttragwerk beurteilt und hinsichtlich der Auswirkungen auf das einzuhaltende Sicherheitsniveau bewertet werden kann. Es wurden insgesamt 31 Bauwerksuntersuchungen statistisch ausgewertet und beurteilt. Die zur Verfügung stehenden Daten wurden analysiert und hinsichtlich der Prüfqualität sowie der Quantität der Proben bewertet. Dabei war festzustellen, dass aufgrund fehlender konkreter Vorgaben im Regelwerk eine sehr heterogene Datenbasis vorliegt. Nicht alle Untersuchungsergebnisse konnten in die weitere Auswertung einbezogen werden. Die in ausreichender Datenqualität und Datenumfang geeigneten Untersuchungen wurden hinsichtlich ihrer Relevanz für den Prozess einer H-SpRK analysiert und die Auswirkungen einzelner Parameter bewertet. Im Ergebnis der materialtechnischen Untersuchungen und statistischen Auswertung der Bauwerksuntersuchungen wurde ein Berechnungsmodell vorgestellt, das den gleichzeitigen Ausfall von Spannstahl in allen Bereichen des Bauwerkes berücksichtigt. Zusätzlich zum Standardverfahren des beschriebenen Vorgehens wird das Modell um den Ansatz eines korrelierten Spannstahlausfalls erweitert. Außerdem wird für Bauwerke mit einem statisch unbestimmten Anteil der Vorspannung sowie für Konstruktionen mit gestaffelter Spannstahlbewehrung die Anwendung des Verfahrens konkretisiert. Neben der Erstellung des Berechnungsmodells wurden Vorschläge zu Vorgaben für die Bauwerksprüfung vorgestellt. Dazu zählen insbesondere die Festlegung von Prüfintervallen sowie einheitliche Vorgaben zu den verwendeten Prüfmethoden und -verfahren. Bezüglich der Festlegung von Mindestumfängen von Proben wird zwischen bestehenden und auch weiter zu nutzenden Bauwerken einerseits sowie Probennahmen im Rahmen von Rückbaumaßnahmen unterschieden.

Doctor of Philosophy / Department of Industrial & Manufacturing Systems Engineering / Chih-Hang John Wu / Quality is a result of product design and production control. Product design must maximize the ability to function across variations in production and environment. Production control must monitor and maintain the key design characteristics necessary for the intended function. Failure to do so results in premature part failure and increased costs. This has occurred in the production of modern cross ties. By designing quality into the product and production process, performance is maximized. This research presents a methodology for incorporating quality into the product design and production process. For product design, a relationship between product performance and design parameters is established by modeling techniques. These models provide a means to redesign the product to maximize performance and to understand the sensitivity of the design to fluctuation in production and environment. These models also establish the key design parameters that are critical for sustaining quality. For production, a method of monitoring the key design parameters is presented that provides an affordable means of automated inspection. Automated inspection removes operator error from the inspection process and allows for greater sampling rates to be achieved. The methodology presented allows for a potential of 100% inspection to be achieved with minimal impact to production costs. The research is applied to the analysis and quality control of pre-stressing steel reinforcement for concrete cross-ties. This application provides an opportunity to test and verify the research findings on a real world problem. Novel automated 3D spatial analysis algorithms are presented. This research furthers the state of the art of performing Geometrical Dimensioning and Tolerancing (GD&T). A cost effective method of non-contact surface profiling was developed with high resolution and high density surface profiles. The combined research findings present a methodology of achieving quality by design.

This diploma thesis deals with the static review of structure for a football pitch. It is crosswise prestressed reinforced sliding plate with variable cross section. Assessment of the elements was done according to limit states. Internal forces from the load effects were calculated using the SW SciaEngineer 2015.1. The elements were assessed by SW Idea StatiCa 6. The main outputs of this static calculation are drawings of the shape and drawings of the reinforcement.

Bei dem Prozess einer Wasserstoffinduzierten Spannungsrisskorrosion (H-SpRK) handelt es sich um einen zeitabhängigen Vorgang, der zu einer Reduzierung der Duktilität und Widerstandskraft des Spannstahls führt und daraus resultierend ein sprödes und schlagartiges Versagen eines Spannbetonbauwerkes zur Folge haben kann. Der Prozess selbst und insbesondere auch die ihn beeinflussenden Parameter sind für die im Bauwesen verwendeten hochfesten Spannstähle älterer Produktion weitestgehend unerforscht. Die Relevanz für bestehende Bauwerke ist jedoch nicht zuletzt durch einzelne dokumentierte und untersuchte Schadensfälle nachgewiesen. Ziel der vorliegenden Arbeit war es, die zur Verfügung stehenden Bauwerksuntersuchungen aus den zurückliegenden ca. 10 Jahren statistisch zu analysieren und auszuwerten. Auf dieser Basis war ein Berechnungsmodell wahrscheinlichkeitstheoretischer Basis zu entwickeln, mit dem die Gefahr eines spröden Bauwerksversagens für das Gesamttragwerk beurteilt und hinsichtlich der Auswirkungen auf das einzuhaltende Sicherheitsniveau bewertet werden kann. Es wurden insgesamt 31 Bauwerksuntersuchungen statistisch ausgewertet und beurteilt. Die zur Verfügung stehenden Daten wurden analysiert und hinsichtlich der Prüfqualität sowie der Quantität der Proben bewertet. Dabei war festzustellen, dass aufgrund fehlender konkreter Vorgaben im Regelwerk eine sehr heterogene Datenbasis vorliegt. Nicht alle Untersuchungsergebnisse konnten in die weitere Auswertung einbezogen werden. Die in ausreichender Datenqualität und Datenumfang geeigneten Untersuchungen wurden hinsichtlich ihrer Relevanz für den Prozess einer H-SpRK analysiert und die Auswirkungen einzelner Parameter bewertet. Im Ergebnis der materialtechnischen Untersuchungen und statistischen Auswertung der Bauwerksuntersuchungen wurde ein Berechnungsmodell vorgestellt, das den gleichzeitigen Ausfall von Spannstahl in allen Bereichen des Bauwerkes berücksichtigt. Zusätzlich zum Standardverfahren des beschriebenen Vorgehens wird das Modell um den Ansatz eines korrelierten Spannstahlausfalls erweitert. Außerdem wird für Bauwerke mit einem statisch unbestimmten Anteil der Vorspannung sowie für Konstruktionen mit gestaffelter Spannstahlbewehrung die Anwendung des Verfahrens konkretisiert. Neben der Erstellung des Berechnungsmodells wurden Vorschläge zu Vorgaben für die Bauwerksprüfung vorgestellt. Dazu zählen insbesondere die Festlegung von Prüfintervallen sowie einheitliche Vorgaben zu den verwendeten Prüfmethoden und -verfahren. Bezüglich der Festlegung von Mindestumfängen von Proben wird zwischen bestehenden und auch weiter zu nutzenden Bauwerken einerseits sowie Probennahmen im Rahmen von Rückbaumaßnahmen unterschieden.

The objective of this master's thesis is a design of suspension footbridge. The footbridge is suspended construction of stress ribbon by one span. The brick deck is supported on the edges and it is kept in a parabolic arc. The suspension cables are kept at two inclined levels. The model lengthways is implemented in the program ANSYS, by non-linear solution. The model crosswise is implemented in the program SCIA ENGINEER 2011. The design is according to relevant current standart.

The objective of this master´s thesis is a design of suspension footbridge across the Elbe river. The footbridge is suspended construction by three spans. The brick deck is supported on the outer edges and it is kept in a parabolic arc. The suspension cables are kept at two inclined levels. The model is implemented in the program ANSYS. The solution is non-linear. The designe is according to the europien standart.