Prior to 2005, the AASHTO LRFD Bridge Design Specifications also recommended simple equations for prestress losses. However, Tadros et al. (2003) showed that these calculations are not well-suited for high-strength concrete girders. Based on detailed measurements of seven bridge girders of various designs, it was shown that the pre-2005 AASHTO equations overestimated prestress losses by an average of 60%. One of the primary shortcomings of the AASHTO equations was that only the ultimate losses were calculated, rather than the time-dependent losses at various time intervals. Tadros et al. proposed equations to calculate the losses with respect to time and included adjustments to account for high-strength concrete. It was shown that these predictions as well as the PCI predictions were significantly more accurate for predicting prestress losses for high-strength girders than the pre-2005 AASHTO methods. The recommended equations were subsequently adopted by the AASHTO specifications beginning with the 2005-2006 Interim Revisions (AASHTO, 2006). A study by Byle et al. (1997) also showed that the pre-2005 AASHTO method overestimated losses for twelve girders constructed with high-strength concrete, although the margin was only 8%. They recommended an alternate set of time-dependent equations for prestress losses.
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The prestressing force in a containment building decreases with time due to the immediate and time- dependent reasons such as anchorage slip and creep of concrete. Prediction of the prestress losses is very important but difficult task because of many uncertainties about concrete. In-service inspection(ISI) is conducted by the regulatory guide to check the prestressing force in the tendon but it has its limitations. Therefore, a new supplementary method to predict the prestress losses is developed and verified in this paper. The concept of the method is to make test specimens which represent some parts of the containment building and compare the behaviours of the specimens under the prestressing condition with the analysis results. If the loss of prestressing force of specimen predicted by the analysis corresponds with the loss of load experimentally measured from the specimen, it can be concluded that the analysis method and input parameters used for the analysis of the specimen are reasonable. Therefore, the long-term loss of the prestressing force in containment building can be predicted with less uncertainties by utilizing the analysis method and parameters decided from the test specimens. Details about the design of the specimen, analysis method, and experimental results are included in this paper. Development of the test specimens will be very helpful to predict the loss of prestressing force together with ISI and maintain structural integrity of the containment building.
Abstract: Prestress losses assumed for bridge girder design and deﬂection analyses are dependent on the concrete modulus of elasticity (MOE). Most design speciﬁcations, such as the American Association of State Highways and Transportation Ofﬁcials (AASHTO) bridge speciﬁcations, contain a constant value for the MOE based on the unit weight of concrete and the concrete compressive strength at 28 days. It has been shown in the past that that the concrete MOE varies with the age of concrete. The purpose of this study was to evaluate the effect of a time-dependent and variable MOE on the prestress losses assumed for bridge girder design. For this purpose, three different variable MOE models from the literature were investigated: Dischinger (Der Bauingenieur 47/48(20):563–572, 1939a; Der Bauingenieur 5/6(20):53–63, 1939b; Der Bauingenieur, 21/22(20):286–437, 1939c), American Concrete Institute (ACI) 209 (Tech. Rep. ACI 209R-92, 1992) and CEB-FIP (CEB-FIP Model Code, 2010). A typical bridge layout for the Dallas, Texas, USA, area was assumed herein. A prestressed concrete beam design and analysis program from the Texas Department of Transportation (TxDOT) was utilized to determine the prestress losses. The values of the time dependent MOE and also speciﬁc prestress losses from each model were compared. The MOE predictions based on the ACI and the CEB-FIP models were close to each other; in long-term, they approach the constant AASHTO value. Dischinger’s model provides for higher MOE values. The elastic shortening and the long term losses from the variable MOE models are lower than that using a constant MOE up to deck casting time. In long term, the variable MOE-based losses approach that from the constant MOE predictions. The Dischinger model would result in more conservative girder design while the ACI and the CEB-FIP models would result in designs more consistent with the AASHTO approach.
The objective of the study was to validate the FE model and study the behaviour of curved box girder bridges, the details of the bridge models of the curved and straight box have been presented previously. To compare the box bridges models, the same modelling techniques were employed for both the straight and curved bridge models except for changing the radius of curvature for each case of the curved box girder. Direct stresses at cross sections the cases of straight and curved were obtained. The midspan stresses as recorded in table (3.1) represent the highest value of stresses where the comparison is made based on the stresses to understand their behaviour under self-weight, prestressed effects and both. It can be noted that in the straight box girder the stress distribution is symmetric from one end to the other (in left and right top slab and soffit) whereas in the curved box girder the stress profile is not symmetric due to the effects of torsion and warping. Table (3.2) represents reactions, torsion moments and prestress losses. Notes: All stresses in the tables are N/mm 2 and curvature angle in degrees.
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Application of prestressing in plate-like members significantly improves their performance enabling longer spans with thinner sections and less material. Furthermore, prestressing encourages the use of precast plate members leading to speedy construction and cost-effective, high-quality products. As for any other prestressed member, the effective prestress force plays a crucial role in the performance of these structures. Prestress force reduces with time due to prestress losses due to creep and shrinkage of concrete, relaxation of steel etc. and more severely due to defects and damages of the prestressing system. Excessive reduction of effective prestress can lead to poor performance or failure of the structure. Unavailability of a direct measurement method of effective prestress force and impossible visual inspection of tendons are common disadvantages of most prestressed structures which highlight the need for an indirect evaluation method.
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John J. Roller, Barney T. Martin, Henry G. Russell, Robert N. Bruce, Jr. (1993): -This paper performs a design and analytical test on bulb-tee girder with and without deck slab to evaluate flexure and shear strength & to determine in-service long-term behaviour under full design dead load by utilizing high strength concrete for bridge girders. Results of this experimental evaluation indicate that use of high strength concrete for prestressed bridge girders is both feasible and beneficial. This paper describes the details and results of the girder tests conducted testing on high strength concrete bridge bulb-tee girders. About two Girders had been tested for flexure and shear with and without deck slab. The design of girders had been done as per the AASHTO standards. Design of girders results the average compressive strength of three girders was 63.6 MPa were observed. Girder 3 was simply supported and subjected to a constant load approximating the in-service design dead load for a duration of 18 months to perform long term test. After design it was transported and deck slab had been casted using partially shored construction. The girders have been instrumented to measure concrete strains, camber and prestress losses and whittemore gauge had been used to determine transfer length, to provide an indication of concrete strains & measurement of concrete surface strains respectively. The material properties were adopted as per ASTM & AASHTO Standard Specifications for Highway Bridges for Girder Concrete, Prestressing Strand & Deck Slab Concrete. The design results in Material Properties, Transfer Length, Prestress Losses & Girder Camber is obtained using the AASHTO standards. The flexure and shear test were performed on the two girders using „hydraulic jacks‟ results in cracks gives cracking moment by using calculations. It observes measured concrete modulus of elasticity for the three girders correlated with values calculated using AASHTO. Transfer lengths examines from concrete surface strain measurements was better as expected. Concrete strains have been measured at a concrete age of 28 days indicated that prestress losses were significantly less than the losses & also, Study gives measured flexural and shear properties met or exceeded values calculated using provisions from the AASHTO Standards and measured material properties and prestressed losses. Data observed for the deck slab concrete surface (concrete topping) strain gauges indicated that the full width of the deck slab was effective throughout the flexural test. Long-term test of Girder 3 had been completed in September 1993.
One of the main sources of losses in the distribution system is the copper losses in power overhead lines and cables. Furthermore, unbalanced loading is another factor that can contribute to the line losses, where if one of the phases has more load than the other two, the losses will be larger than that if these phases are balanced. Temperature rise introduces significant increase of power consumption, where the power loading can increase by 3.75 % for 1 °C temperature rise. For the rainy day with higher humidity and lower temperature, a negative correlation among the power consumption was also found. On the other hand, the temperature change has less effect to feeder power losses because transformer losses dramatically contribute more in the power losses.
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(Received 4 April 2013; revised manuscript received 12 July 2013; published 28 August 2013) Analytical and numerical calculations are presented for the mechanical response of fiber networks in a state of axisymmetric prestress, in the limit where geometric nonlinearities such as fiber rotation are negligible. This allows us to focus on the anisotropy deriving purely from the nonlinear force-extension curves of individual fibers. The number of independent elastic coefficients for isotropic, axisymmetric, and fully anisotropic networks are enumerated before deriving expressions for the response to a locally applied force that can be tested against, e.g., microrheology experiments. Localized forces can generate anisotropy away from the point of application, so numerical integration of nonlinear continuum equations is employed to determine the stress field, and induced mechanical anisotropy, at points located directly behind and in front of a force monopole. Results are presented for the wormlike chain model in normalized forms, allowing them to be easily mapped to a range of systems. Finally, the relevance of these findings to naturally occurring systems and directions for future investigation are discussed.
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In order to attain the capability with RCB, prestressed concrete concept is typically employed for prevention of containment failure. Through prestressing, concrete in RCB can be subjected to compression in advance so that cracks in RCB can be prevented even under design load or pressure which induces tensile stress in concrete. Among prestressing methods, post-tensioning is applied on RCB since RCB has cylindrical and spherical shape for wall and dome, respectively. When concrete structure as like RCB is subjected to post- tensioning, one of the most important things on design is to evaluate effective prestress force. However, it is not simple to evaluate effective prestress force because prestress force can be significantly affected by lots of parameters, such as anchorage slip, frictions between prestress tendons and sheath, time dependent behaviour of concrete, tendon relaxation, and so on . In addition, friction coefficients generally show a wide range of differences even through code provisions [4-7].
Figure 1 shows a drawing of the experimental set-up. An aluminium beam (a 1-D simplification of a solar array panel) is clamped at its left side by a steel fixing structure, by which it is connected to the head expander of the shaker. The physical and geometrical properties of the aluminium beam are given in Table 1. Underneath the right end of the aluminium beam a small M55 carbon beam is visible (a representation of a snubber element). This carbon beam is clamped at its right end by another steel fixing structure, by which it also is connected to the head expander of the shaker. The height of the carbon beam can be adjusted, which makes it possible to consider a system with backlash (as depicted in Figure 2), with prestress or with flush (backlash = prestress = 0). In the text below the term ‘panel’ refers to the aluminium beam and the term ‘snubber’ refers to the carbon beam.
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Abstract: Prestress loss evaluation in prestressed strand is essential for prestressed structures. However, the sensors installed outside the duct can only measure the total prestress loss. The sensors attached on strand inside the duct also have several problems, such as inadequate durability in an aggressive environment, vulnerable damage at tensioning and so on. This paper proposes a new installation method for long-gauge fiber Bragg grating (LFBG) sensor to prevent accidental damage. Then the itemized prestress losses were determined in each stage of the pre-tensioning and post-tensioning according to the LFBG measurements. We verified the applicability of the LFBG sensors for prestress monitoring and the accuracy of the proposed prestress loss calculation method during pre-tensioning and post-tensioning. In the pre-tensioning case, the calculated prestress losses had less deviation from the true losses than those obtained from foil-strain gauges, and the durability of the LFBG sensors was better than foil-strain gauges, whereas in post-tensioning case, the calculated prestress losses were close to those derived from theoretical predictions. Finally, we monitored prestress variation in the strand for 90 days. The itemized prestress losses at each stages of post-tensioning were obtained by the proposed calculation method to show the prospect of the LFBG sensors in practical evaluation.
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Figure 7 illustrates the allocation of the time capacity. In this example one month is the time frame, so both systems have the same amount of time capacity available. The used capacity differs slightly because System 2 uses extra shifts ordered by the management to produce all orders. System 1 has an extreme lack of orders and auxiliary time where no products are produced. System 2 is fully occupied by orders, but the auxiliary time is also high. It seems that the operators use this time code for losses. But nevertheless the focus of this method is to reduce losses, which are marked with a burst in the diagram. To improve the data quality, it is useful to automate the time measurement or change the behavior of the operators.
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A idealized section for the anchorage zone of a pretensioned beam is illustrated in Fig. 1. Steel is treated as a solid cylinder and concrete as a surrounding hollow cylinder with a radial thickness equal to the smaller cover ( c ) or the effective cover ( c ) defined by Uijl (1995). The outer surface of the notional concrete cylinder is assumed to behave as a free surface. The original radius of the steel cylinder is equal to (the radius of the unstressed steel). The concrete cylinder has an outer radius of and an inner radius of which is the radius of the stressed steel because the concrete is cast after the tendon is stressed first in the pretensioned members. Due to Poisson’s ratio, is always less than . At prestress transfer, the steel shortens and swells so a pressure develops at the interface. Using thick cylinder theory, the expressions for stresses, strains and displacements of an element can be derived by considering equilibrium and compatibility, and also imposing boundary conditions.
Energy losses can be divided into active power losses and reactive power losses. Active power losses cause by the resistance of lines, and reactive power losses cause by the reactive elements. Normally, active power losses have more attention, as they reduce the efficiency of power distribution to the consumer. However, the reactive power losses are no less important. This is due to the fact that the reactive power flows must be maintained at certain values in order to maintain the voltage at the proper level. Total active and reactive power losses in the distribution system can be calculated using the expression:
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Based on the foregoing characteristics, no frictional loss of prestress occurs in the pretensioned concrete, which is one of the beneficial aspects when compared to post- tensioned concrete (Huang and Kang 2018). On the other hand, precast concrete structures have recently become increasingly popular worldwide due to a number of advantages that can be expected from systematic quality control during fabrication and assembly of precast mem- bers at a construction site. Accelerated and safe construc- tion, high quality and durability, aesthetics, and other features of precast concrete overcome the drawbacks of conventional cast-in-place concrete structures. The pre- cast concrete can be efficiently combined with preten- sion method in a large plant yard appropriate for mass production.
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The Flyback converter based power supply with multiple regulated outputs, reduced standby power consumption and high voltage protection best suited for smart meters is proposed. Typical losses in conventional flyback like startup resistor losses, snubber losses and feedback circuit losses are minimized .Incorporating a green flyback controller which enters into burst mode of operation at light/no load switching loss is reduced by 30%.In short standby power consumption of the smart meter is reduced to less than 600mW which is well within the limits of various energy compliances.
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The design of losses for multiclass prediction has received recent attention (Zhang, 2004; Hill and Doucet, 2007; Tewari and Bartlett, 2007; Liu, 2007; Santos-Rodr´ıguez et al., 2009; Zou et al., 2008; Zhang et al., 2009) although none of these papers developed the connection to proper losses, and most restrict consideration to margin losses (which imply certain symmetry conditions). Zou et al. (2005) proposed a multiclass generalisation of “admissible losses” (their name for classification calibration) for multiclass margin classification. Liu (2007) considered several multiclass generalisations of hinge loss (suitable for multiclass SVMs) and showed some of them were and others were not Fisher consistent, and when they were not it was shown how the training algorithm could be modified to make the losses behave consistently. Shi et al. (2010) have investigated the relationship between classification calibration of multiclass losses and losses for structure prediction, and have proposed an extension of classification calibration which they call parametric consistency, which attempts to take account of the function class used (classification calibration is, like all the results in this paper, concerned with behaviour per point; in practice one typically optimises over restricted classes of functions). Multiclass losses have also been considered in the development of multiclass boosting (e.g. Zhu et al., 2009; Mukherjee and Schapire, 2013; Wu and Lange, 2010).
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This paper summarizes a project concerning the prestress level in Swedish reactor containments initiated in year 2002. The overall purpose of this work was to clarify the long-term behaviour and find models to verify the status for large prestressing systems. The work is mainly based on measured tendon force from Swedish in-service inspections and includes three parts. (1) The first part focuses on the technique of measuring tendon force; especially problems involved with friction between tendon and duct are discussed. (2) In the second part results from tendon force measurements available at Swedish power plants are discussed. The different parameters that could influence the measured loss of prestress are evaluated and the loss of prestress is modelled and compared with the result from force measurements. (3) Does the measured tendon force meet the requirements? The third part of the work intend to answer this question by using a reliability-based method.
One half of the ICS is modelled to exploit the symmetricity of the structure. The ICS is connected to the 5.5m thick massive raft at the base. Hence, complete fixity has been assumed at the base of the ICS. The ICS is subjected to two types of loading i.e., constant load consisting of prestress and self weight of the structure and variable load consisting of internal pressure. The temperature load corresponding to the accident condition has not been considered in this study. The internal pressure, representing the accident pressure needs to be applied gradually. Hence, special provision is made in the FE programme to increment only the internal pressure.
* In conventional prestressing, the prestressing tendons were provided in longitudinal direction. Most probably the tendons were placed along girders. In case of long span bridges, the prestressing will be done by using end blocks and anchorage were provided on the ends of each segment. So large space will needed in between each segments. This will increases the prestress loss and also it limits the length of the bridge section.