Search

Pretensioned concrete elements are commonly fabricated with strands 12.7 or 15.2 mm (0.5 or 0.6 in.) in diameter; however, the industry has seen interest in using larger-diameter strands in recent years. The use of larger-diameter strands results in greater transverse tensile stresses within the girder end regions, which may increase cracking at the time of prestress transfer. Such cracks may continue to grow during service and cause durability concerns. Moreover, increased damage around the strands and at the web-flange interfaces may lead to unconventional failuremechanisms, such as anchorage-induced or horizontal shear failures. This paper introduces amodeling approach for investigating the performance of pretensioned girders fabricated with strands 17.8 mm (0.7 in.) in diameter from prestress transfer until failure under shearcritical loading. Data from seven full-scale prestress transfer tests and 10 load tests showed that the model can capture the transfer lengths, end-region stresses, and cracking at prestress transfer as well as load-deflection response and failure modes. Subsequently, three remedial endregion reinforcement details were investigated using the validated approach to examine their efficacy in controlling end-region cracks and stresses. [ABSTRACT FROM AUTHOR]

Abstract This work focuses on entangled wire materials as an option for use between segments of a novel self-centring bridge pier inspired from the human spine mechanism to increase energy dissipation capability of the pier in rocking. A comprehensive set of free-decay vibration tests was conducted on small-scale columns with and without entangled wire materials. Wooden blocks are used as vertebrae with entangled wire materials as intervertebral disks. The whole system is tied together using a pre-tensioned tendon. Dynamic properties of columns (i.e. frequency and damping ratio) were then identified and compared. It is found that the use of entangled wire materials significantly increases the energy dissipation capacity of the system during rocking. This finding is very encouraging for future use of entangled wire materials composite systems in large-scale testing of the proposed rocking column, while their shear and axial stiffness needs be improved to reduce large shear and axial deformations. [ABSTRACT FROM AUTHOR]

Copyright of Hermenéutica is the property of Hermeneutic - Revista Digital de Arte, Critica y Filosofia and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract A novel friction-based wedge anchor system was developed experimentally and numerically for the prestressed carbon fiber reinforced polymer (CFRP) plates with several commercial widths and thicknesses. A simple analytical model was first developed to investigate the CFRP compression distribution. Both static and fatigue tests were then conducted to determine the anchor performance. The effects of the barrel and wedge thickness, the wedge shape and contact surface (corrosion and cross knurling) were investigated experimentally on the anchor behavior. The design parameters of the new anchor were recommended through the pull-off experiments, and this anchor can be capable of not only anchoring at least 90% of the CFRP plate specified strength of 2000 MPa statically, but also maintaining the stress level of 900 MPa under a fatigue stress range of 100 MPa. Both the simulation of the CFRP stress distribution and a sensitivity analysis were also conducted on finite element numerical models. The numerical analysis shows that the barrel flexural deformation occurs across the CFRP width, and leads to the bimodal stress distribution across the CFRP plate width. The stress sensitivity results show the stress distribution is significantly sensitive to the wedge shape and contact surface, the bolt pretension and the CFRP width. [ABSTRACT FROM AUTHOR]

Bond behavior between corroded steel strand and concrete in pretensioned prestressed concrete beams was investigated experimentally in this study. Nine pretensioned beams were tested to study the effects of corrosion on global force-slip response, bond strength, and failure mode. Experimental results showed that strand corrosion loss less than 5.25% improved the bond stiffness. Strand corrosion significantly decreased the bond stiffness when the corrosion loss was greater than 8.08%. The ultimate bond stress didn't degrade with corrosion progression until the corrosion loss was greater than 4.4%. The failure mode of the beam changed from concrete crushing to strand fracture when the corrosion loss and crack width exceeded 5.25% and 0.31 mm (0.013 in.), respectively. Compared with existing data on tensile degradation behavior of corroded steel prestressing strands, corrosion deteriorated the tension strength of strands more significantly than the bond strength for corrosion loss less than 9.24%. [ABSTRACT FROM AUTHOR]

Transfer length is a significant parameter in the design of pretensioned concrete members. This study reports the measured transfer length of 15.2 mm (0.6 in.) prestressing strands. Twenty-four pretensioned concrete beams were cast using conventional concrete. The compressive strength at release ranged from 27 to 65 MPa (4000 to 9500 psi). Transfer lengths were determined by measuring concrete surface strains and using the average maximum strain (AMS) method. A new technique was developed to smooth the measured concrete surface strains. The measured transfer lengths were compared to the predicted transfer lengths using ACI 318-14 and AASHTO-LRFD equations. The results indicate that the proposed smoothing technique provides better results than the current threepoint moving average technique. The current code equations are conservative to predict transfer length for pretensioned concrete members having concrete compressive strength at release greater than 27 MPa (4000 psi). [ABSTRACT FROM AUTHOR]

Over the past 20 years, many long-span cable-stayed bridges with asymmetric spans have been constructed, and the counterweight is always used to balance the self-weight of the main span. This paper presents an optimization method to determine the cable pre-tension forces in long-span cable-stayed bridges considering the counterweight. This method includes: finite element (FE) model, formulation of the optimization problem and optimization algorithm. FE model is established considering the geometrical nonlinearity. The optimization problem is formulated with the objective of minimum weighted total bending energy. In addition, the constraints for the cable pre-tension forces, the bending moment of the girder and the tower, the load of the counterweight, the bearing reactions of the transition piers and auxiliary piers are all implemented in the optimization model. The optimization algorithm solves the optimization problem through the variable-step search along each design variable including the cable pre-tension forces, the load and the range of the counterweight. The efficiency and the accuracy of the proposed method are demonstrated by an application example and the results exhibit the importance of considering counterweight in the design of asymmetric cable-stayed bridges. [ABSTRACT FROM AUTHOR]

This paper focuses on the modelling of the anchorage zone of a pretensioned girder. The finite element software Abaqus was used to create a 3D non-linear finite element model (FEM). This analysis was performed on a full-scale pretensioned girder with end blocks and with various types of strand modelling, where accurate contact properties between prestressing steel and concrete are essential. The model has been validated by comparing the numerical strain results with the strain measurements on a full-scale girder with end blocks, which was produced in a precast concrete plant. At the same time, a parametric study was executed to determine the value of the coefficient of friction between the strands and the concrete and to examine the sensitivity of the input parameters of the model. The results have indicated that an accurate determination of the concrete properties at the time of release is very important. Furthermore, the Hoyer effect is analysed. It is found that the radial expansion of the strand in the finite element model is in good agreement with the theoretically calculated expansion of the strand which demonstrates the accuracy of the finite element model. In addition, it is shown that the Hoyer effect influences the force transfer of two adjacent strands. The aim of this research is to investigate the potential of an alternative FE model of a pretensioned girder based on strand to concrete frictional behaviour, rather than the commonly used models based on an assumed shear stress distribution along the transfer length. [ABSTRACT FROM AUTHOR]

Development length of prestressing strands is a significant parameter in the design of pretensioned concrete members. The adequacy of the code equations used to predict the development length needs to be verified when a new material or design parameter is introduced for precast, prestressed concrete applications. This study develops a simple and reliable technique to quantify the development length based on the premise of the slip theory, which proposes the free-end slip (FES) as an indicator of the development length. Twenty-five pretensioned concrete beams were cast with 15 mm diameter strands. FES was measured at both beam ends at prestress transfer. Development length was determined by performing bending tests at different embedment lengths. The experimental results were synthesised to revise the slip theory. The applicability of the revised-slip theory was validated by collected experimental data of pretensioned concrete members using various types of concrete, strand diameters and release techniques. [ABSTRACT FROM AUTHOR]

Time evolution of prestress loss and bond transfer length holds vital information concerning long-term performance of pretensioned prestressed concrete. In this paper, it is proposed to utilize long-term measurements of strand draw-in to extract this time-varying information, which could be more effectively obtained than concrete or strand surface strain measurements. Theoretical investigations combined with numerical studies of experimental data are carried out for this purpose. This study builds on Guyon’s boundary value problem (BVP) model which quantifies draw-in for instantaneous elastic response. A one-dimensional linear viscoelastic standard solid model is employed to model the creep of concrete, which is a simple but effective model of the time-dependent response of concrete. Guyon’s BVP model is generalized to include time dependence and then combined with an existing initial value problem (IVP) model for post-tensioned concrete leading to a new mixed model for time-dependent prestress loss and bond transfer in pretensioned concrete. Analysis is undertaken in this study to (i) quantify long-term prestress loss and bond-transfer behaviors by applying the proposed model and directly utilizing measured draw-in time history data, (ii) validate the proposed quantitative analysis by using scaled pretensioned concrete beams with different types of aggregates and strengths of concrete - among other factors, and (iii) offer discussion of future work. For example, introducing concrete drying shrinkage into the proposed simple model seems promising and is necessary to provide an improved representation of the time-dependent behavior of prestressed concrete. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to examine the effect of strand configuration on the behavior of precast, pretensioned concrete I-girders by considering design load components for highway bridges as dead loads and equivalent lane loads. Under these loads, bridge girder's bottom flange is exposed to tensile stresses. To minimize or eliminate the tensile stresses, compressive stresses are induced in pre-stressed concrete with strands. Determination of strand configuration is important as well as number of strands because it affects stress distribution and displacement of bridge girders. One of the typical precast I-girder with 90 cm height is considered in this study. To determine strand configuration effects, eighteen I-girders with the same crosssection, effective span length and material properties but different strand configuration are selected as an application. Equal prestressing force is applied all strands simultaneously. Three dimensional finite element (FE) models of girder are constituted using ANSYS software. Result of beam theory is used to verify the modeling techniques. At the end of the study, numerically identified stress distribution and displacement for Igirders compared with each other. It is seen that proper strand configuration is effective to reducing stresses and displacements of pre-stressed I-girder. [ABSTRACT FROM AUTHOR]

A study was conducted to determine the effect of prestressing steel indentation types and concrete release strength on development length and flexural capacity of pretensioned members. Thirteen different indented 5.32 mm (0.209 in.) diameter prestressing wires were used to cast prisms with a square cross section (3.5 x 3.5 in. [88.9 x 88.9 mm]). Different concrete release strengths of 3500, 4500, and 6000 psi (24.13, 31.03, and 41.37 MPa) were obtained to test the effect of concrete release strengths on the development length. All prisms were tested in three-point bending at different spans to estimate development length. Lastly, a design equation was developed based on experimental data for prediction of development length for 5.32 mm (0.209 in.) diameter indented prestressing wires. Experimental results indicated that indentation type has a large impact on development length. Tests on specimens with different concrete release strengths indicated that the concrete compressive strength at detensioning had less effect on long-term moment capacity of sections using prestressing wires with deep indent patterns. [ABSTRACT FROM AUTHOR]

strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series "A" were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series "B" were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material. [ABSTRACT FROM AUTHOR]

This article experimentally studied the sliding behaviour of high-strength bolted connections. A total of 14 specimens varying in types of hole and bolt diameters were designed and tested under ambient temperature and target temperature of 130°C and 200°C, respectively. The experimental results show that the pretension in the bolts increases with the increase in temperature below 200°C. After 200°C, the pretension reverses and starts to decrease. It was found that the reverse temperature depended on the relative thermal expansion of the steel plates and bolts. The model of temperature-dependent thermal expansion coefficients given by Australian standard AS 4100 is in good agreement with the test results. The friction coefficient increases with the increase in temperature. The friction coefficients of connections with slot holes increase by 12% at 200°C compared with that at ambient temperature. At the same temperature, the friction coefficients of connections with slot holes are found to be 12% lower than that of connections with standard holes. [ABSTRACT FROM AUTHOR]

Corner supported modular system possess promising ability for high compression bearing which is suitable for high rise modular buildings. The economical approach to transfer forces and moments is to develop a more rigid connection between modules. This study investigated the seismic performance of a framed modular system with pretension assembled intermodular connections. A quasi-static loading test was performed on a full-scale pretension assembled framed modular system, with a loading process based on different earthquake design levels in China. Then, a simplified finite element modelling method was proposed to simulate the assembled compositions, and the effectiveness was verified with test data. Results showed that the pretension assembled connection had comparatively rigid connecting behavior. The modular frame had acceptable strength and stiffness within fortified intensity shake earthquake levels, and it satisfied ductility requirements without much structural failure. The frame exhibited self-centering deformation restoration after loading, and the strength degradation was mostly due to bonding loss within modular columns. Simulation results indicated that the pretension assembled framed modular system had different internal stress distribution from the traditional frame because of assembled compositions. [ABSTRACT FROM AUTHOR]

Minimum radii of curvature of internal bonded tendons Due to the introduction of EAD 160004 as new basis for issuing European Technical Assessments concerning post-tensioning systems for prestressing of structures, minimum radii of curvature for deviation of internal bonded tendons have been defined on the European level for the first time. This definition differs significantly from the minimum radii of curvature permitted in Germany so far. In addition to explain some of the background, the paper shows a relationship between the existing and the new minimum radii by means of a differentiating comparison. Using a suitable value of the allowable transverse pressure under the most loaded prestressing strand, it is possible to convert the previous radii into the new format on the basis of an explicit acceptance criterion. If the limiting value for the radius of curvature of 2.50 m is observed, the bending stress due to deviation of the prestressing strand is not decisive. [ABSTRACT FROM AUTHOR]

Pre-tensioned, spun, high-strength concrete (PHC) piles are the most commonly used type of pile in South Korea. Approximately 60% of the pile's strength is used in the design bearing capacity, and the rest is simply residing in the ground. Increasing the ground bearing capacity is crucial to reduce waste of the pile strength and to improve efficiency. Extended end (Ext) piles are a new kind of composite pile that can overcome the weakness of PHC piles. This study investigates the behaviour of Ext piles. Through field testing, it is confirmed that the bearing capacity of Ext piles is better than PHC piles by about 35% to 50%. Based on the study findings, the Ext pile design reduces the number of piles by around 38% compared to the PHC pile design through application in a selected construction site. The increased bearing capacity of Ext piles affects both work duration and project cost, which are 25% and 14% decreased, respectively. [ABSTRACT FROM AUTHOR]

The use of 18-mm prestressing strands is advantageous in the construction of long-span bridges. Transmission length is a significant parameter in the design of pretensioned concrete members. This study provides a unique set of experimental data to validate the applicability of using the fib Model Code 2010 to predict the transmission length and strand draw-in of 18-mm strands. Twelve pretensioned concrete beams were cast with high-strength concrete. Transmission lengths were measured at prestress transfer and at 28-day of age. The experimental results indicated the lower and upper bounds of transmission length specified by the code adequately predict the measured values. [ABSTRACT FROM AUTHOR]

Defective design and construction became a reason of a damage of a complex structure of a heliport. Shrinkage and temperature made additional problems. Reinforced concrete frames, pretensioned double T beams and reinforced concrete slab on the top create the structure. The frames support the beams and the slab strengthens the last elements. Since it was unclear what initiated and developed the damages, modelling of the composite structure helped to solve the problem. The paper presents the damages of the structure, construction process and environmental conditions which were essential in the evaluation of damages and their causes. The major part is devoted to presentation of modelling of the structure behaviour during construction and under dead loads, shrinkage and temperature. Due to the complexity of the problem nonlinear analysis was conducted. The analysis included staged construction process which required stress redistribution. It was found that destruction was initiated in the area where the structure was faultily designed. Since the environmental temperature and the shrinkage were not considered in the design additional damages appeared. They were enhanced with insufficient area of designed reinforcement and even less built in. [ABSTRACT FROM AUTHOR]

An innovative strategy for the analysis and design of the anchorage zones of pre-tensioned, concrete girders is presented. In this approach, the bond behaviour of the prestress strands is first characterised in a small-sized beam model. A new relation between the slip and radial strain of the prestress strand is introduced and used together with the radial stress–strain relation resulting from a thick-walled cylinder model to establish the bond-slip behaviour at the steel–concrete interface. This bond behaviour is implemented in a numerical model and validated via a comparison of the computed transfer length with the results of two experimental campaigns. Next, the bond-slip relation of the small-scale model is applied in full-scale models of pre-tensioned, concrete girders to derive the stress distribution in the anchorage zones. The non-linear material behaviour of concrete is taken into account and a comparison of the numerical results with full-scale experimental data is made. An acceptable agreement is achieved between the experimental results and the numerical calculations regarding the bond behaviour and transfer lengths as well as the crack patterns and the stress values in the reinforcement bars. This efficient modelling approach allows for a full analysis of the anchorage zone based solely on the geometrical and material properties known at the design stage. [ABSTRACT FROM AUTHOR]

Pretensioned spun high-strength concrete (PHC) piles are increasingly used to improve highway foundations with large pile spacing and short pile length without splicing. In this paper, a series of field tests comprising cone penetration tests (CPTs), piling, and static pile load tests were performed to investigate the load transfer mechanism of such PHC piles, the load transfer mechanisms of which are different from that of long piles with splices used in structural foundations. The variation of axial strain along PHC piles was monitored during piling and load tests using fiber Bragg grating (FBG) sensing technology and conventional vibrating wire (VW) strain gauges. The results show the superiority of FBG sensing technology in terms of reliability of data and feasibility of measurement under fast changing axial strain. The experimental data also show significant variation in the shaft and tip resistance during piling and the pile-soil strength limit state of the load tests. [ABSTRACT FROM AUTHOR]

A new concept was developed to accelerate the construction of highway bridge bents, to improve their seismic resilience, and to make them more durable. On-site construction activities are accelerated by precasting the columns and cap beams and by using socket connections at the bottoms of the columns and grouted connections at the top. Unbonded pretensioning of the columns increases both the bridge's seismic resilience (through improved recentering) and its durability (through reduced cracking), whereas precasting improves quality control. Previous researchers have incorporated unbonded posttensioning into structural systems to reduce postearthquake residual displacements, but pretensioning is preferable in terms of construction convenience, speed, and quality control. Epoxy coating also adds corrosion resistance to the pretensioned strands. Two cantilever concrete subassemblies, representing the top and bottom connections for a column, were subjected to constant axial load and cyclic lateral loading. The tests demonstrated that the new system recenters better than does a comparable RC column. Furthermore, the inelastic action was restricted to the columns, thereby validating the capacity protection of the connection regions. [ABSTRACT FROM AUTHOR]

This paper compares the prestress losses as calculated by four different design codes; British standard CP110, Comite Europeen du Beton 70 and 78, American Concrete Institute 77 and the Prestressed Concrete Institute method (PCI). The comparison is done by determining the total losses which take place in a rectangular prestressed concrete beam for both pretensioning and post-tensioning systems. The results show that total losses calculated for the post-tensioning method are higher than those calculated for the pre-tensioning method, which is not the usual case. It seems that the PCI method may be required for special structures or for simply supported slender members which may be sensitive to small changes in deflections. However, for non-special structures, or where actual losses have little effect on the design, it is better to compute losses by the ACI method because it is simple and does take into considerations interactions between the various sources of losses. However, it is not possible to conclude which method gives the more accurate prediction of shrinkage and creep without direct co-relation to realistic insitu data. [ABSTRACT FROM AUTHOR]

The corrosion-induced bond degradation between corroded steel strands and concrete is investigated in the present study. Ten pull-out specimens with different corrosion levels were tested to study corrosion's effects on concrete splitting, strand twisting, failure mode, and global force-displacement behavior. The local bond characteristics are discussed and derived based on the test results. Experimental results show that the bond behavior depends on the corrosion loss level and does not have a monotonic trend. Strand corrosion had beneficial effects on the bond strength when the corrosion loss is less than 6.24% in the current study. The corrosion loss significantly decreased the bond strength when the corrosion loss is greater than 9.26% or the corrosion-induced crack is wider than 0.67 mm. The strand will be pulled out instead of fracturing when the corrosion loss exceeds 8%. Corrosion affects the bond strength more significantly than the tension strength for severe corrosion conditions. A simplified model is proposed to predict the residual bond strength between corroded strands and concrete. The proposed model's predictions are validated with experimental data. Several conclusions and suggestions for future work are drawn based on the conducted study. [ABSTRACT FROM AUTHOR]

Failures of strand-concrete bonds have been widely observed in load tests of precast-pretensioned concrete I girders. Through a review of 22 test programs, 15 terminologies were identified to describe failures associated with bond loss. In many cases, previous researchers used different terms to describe the same failure behavior. In response to the wide-ranging and sometimes inconsistent terminologies used in the literature, this technical note makes two contributions. First, the different types of failure that involve strand-concrete bond loss are characterized, and a consistent labeling scheme is proposed. The 15 labels given in the referenced test programs are condensed into four primary behaviors. Second, a flowchart is presented for assisting future researchers in characterizing and labeling bond-loss failures. Decision points in the flowchart are based on a synthesis of the reviewed test programs, which included 120 unique load tests having some type of bond-loss failure. [ABSTRACT FROM AUTHOR]

This paper analyzes the influences of important structural design parameters on the wind-induced response of cross-rope suspension tower-line. A finite element model of cross-rope suspension tower-line system is established, and the dynamic time-history analysis with harmonic wave superposition method is conducted. The two important structural design parameters such as initial guy pretension and sag-span ratio of suspension-rope are studied, as well as their influences on the three wind-induced vibration responses such as tensile force on guys, the reaction force on mast supports, and the along-wind displacement of the mast top; the results show that the value of sag-span ratio of suspension-rope should not be less than 1/9 and the value of guy pretension should be less than 30% of its design bearing capacity. On this occasion, the tension in guys and compression in masts would be maintained in smaller values, which can lead to a much more reasonable structure. [ABSTRACT FROM AUTHOR]

Purpose: Precise pre-tensioning protocol of the graft has not been determined in anterior cruciate ligament (ACL) reconstruction. The aims of this study are to measure the human autologous graft elongation, and to reveal what portion of the graft elongated greater after pre-tensioning in ACL reconstruction.Methods: Twenty-four hamstring tendon grafts which were harvested from patients were included. A continuous load of 150 N was applied to the graft twice for 30 seconds each (150 N-1 minute), and the same loading was repeatedly applied (150 N-2 minute). The amount of elongation of the tendon portion (Length T) and the stitched portion (Length S) were measured after each pre-tensioning.Results: Length S gradually increased by 1.57 ± 0.67 mm after the 150 N-1 minute pre-tensioning and by 2.12 ± 0.76 mm after the 150 N-2 minute pre-tensioning, respectively. Length T was not significantly elongated after 150 N-1 min (p = 0.66) and 150 N-2 min (p = 0.59).Conclusions: Graft elongation of the approximately 2 mm was observed, particularly in the stitched portion. It is necessary for a surgeon to focus on careful removal of slack from each stitch during suturing. [ABSTRACT FROM AUTHOR]

This paper proposes a new pretension simulation algorithm and examines the pretension feasibility for a new type of cable dome: a negative Gaussian curvature cable dome. First, the paper gives a pretension simulation algorithm based on the fuzzy relationship between the prestress and internal force. Second, a negative Gaussian curvature cable dome is proposed and the corresponding experimental model is designed and pretensioned according to the simulation results from the proposed simulation algorithm. The results indicate that the pretension simulation algorithm is efficient and accurate for simulating the pretension process and guiding the practical pretension. And the results also confirm that the negative Gaussian curvature cable dome can be pretensioned successfully, thus it can be constructed as a new type of cable dome. [ABSTRACT FROM AUTHOR]

A composite beam theory is developed to study the pretensioned concrete structures. This theoretical development defines the longitudinal interaction that occurs between the prestressing tendon and concrete under normal service condition. The transfer length and prestress loss due to slip, elastic shortening as well as the prestress gain due to external loads are solved with closed form solutions. Validation is provided and comparisons are made between the present and conventional approaches. It is found that there is excellent agreement between the predicted and tested concrete strain. The transfer length results calculated by design provisions are close to the predicted upper bound but test results can be anywhere between the predicted lower and upper bounds. Current immediate prestress losses formulas may result in overestimation and the degree of overestimation is dependent on prestressing tendon eccentricity. Overall, it is demonstrated that the approach presented in this study improves the accuracy and facilitates a better understanding of prestressed concrete mechanics while maintaining concise closed form solutions. [ABSTRACT FROM AUTHOR]

Prestressed (PS) strands are widely used to develop prestressed concrete (PSC) and are stronger than other types of structural steel used in the construction industry. Because of their high strength, PS strands are more effective for use in structures than other materials. Recently, high-strength 2,400-MPa PS strands have been developed and are being considered as an alternative to conventional 1,860-MPa PS strands. However, there have been no reports of research or proposed regulations for pretensioned 2,400-MPa strands in PSC. In this study, 28 PSC beams were fabricated by using 2,400-MPa PS strands, and the transfer length of the PSC was measured. Experiments were performed to evaluate several parameters such as the compressive strength of concrete, steel-fiber volume ratio, and stirrup reinforcement along with different de-tensioning methods in order to determine their effects on the transfer length. In addition, the transfer length with the 2,400-MPa PS strands was measured and compared with the transfer length predicted by previously proposed models. [ABSTRACT FROM AUTHOR]

A feasible method of pre-tension was proposed to improve the sensing performances of carbon fibre reinforced polymer (CFRP) composites through reducing the misalignment and winding of carbon fibres. The CFRP composites consisted of high strength carbon fibres as active elements, epoxy resin as a matrix and sensing electrodes. Firstly, the sensing model was established based on the conductivity analysis of the CFRP composites. Then, a series of experiments were carried out to study the strain sensing linearity and stability under cyclic loading. The experiments showed that the linearity of the CFRP composites increases with increasing pre-tension amplitudes, and that a stable and linear sensing behavior can be obtained when the pre-tension amplitude is larger than 200 με. The ΔR/R 0 -strain curves can be linearly fitted with a correlation coefficient larger than 0.985. It was revealed that the loading manner also has some influence on the sensing behavior of the CFRP composite sensors. The gauge factors for an intermittent cyclic loading (range: 2.85–3.37) are larger than those for a continuous cyclic loading (range: 1.89–2.16). Furthermore, the long term experimental results reveal that the electrical behavior of the as-researched composites are characterized by excellent long-term linearity and recoverability. [ABSTRACT FROM AUTHOR]

A new bridge system has been developed to (1) reduce on-site construction time by using precast components, (2) eliminate major earthquake damage by utilizing column rocking and confinement of the column ends with a steel tube, and (3) maintain the system functionality after a strong earthquake by minimizing residual drift through the use of pretensioned strands in the columns. Furthermore, it uses only conventional materials. This paper compares the shaking table performance of a quarter-scale, two-pan bridge constructed through the use of the new system with that of a conventional cast-in-place bridge with similar geometry tested in 2005. The new bridge system was constructed in approximately 20% of the time needed for the conventional cast-in-place system. In tests, the conventional bridge suffered major concrete cracking and spalling, whereas in the new system, damage to the concrete was only cosmetic. In the conventional bridge, the longitudinal bars buckled, and both the longitudinal and spiral reinforcement fractured, whereas in the new system, the damage to the reinforcement was limited to longitudinal bar fracture, and that occurred only under excitations much larger than the design-level motion. Furthermore, bar fracture in the new system could be delayed by increasing the unbonded length of the bars. The residual drift of the new system was essentially zero for all motions, whereas one of the exterior bents of the conventional bridge was so badly damaged and out of plumb that some of the supplemental mass on the bridge had to be removed, and testing was stopped shortly thereafter. [ABSTRACT FROM AUTHOR]

Bonding a composite patch to a cracked steel section is an efficient technique to reinforce cracked members or to delay fatigue crack growth in the structural elements. In this paper, a numerical study was performed to highlight the effect of bonded prestressed composite patches on the fracture parameters, such as crack tip opening displacement and the plastic zone. A three-dimensional finite element model of the double sided strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of modes I and II loading conditions. The influence of the pretension level, patch stiffness ratio, and fiber orientation on the crack tip opening displacement and plastic zone are also investigated. The introduction of a compressive stress by pretensioning of the composite patch prior to bonding produces a significant reduction in the crack tip opening displacement and plastic zone. This pretensioning reduces the crack tip driving force and subsequently reduces the fatigue crack growth rate, which increases the fatigue life of the structural member. Furthermore, the increase in the patch stiffness ratio reduces the amount of plastic zone and opening displacement at the crack tip. The effectiveness of the fiber orientation in the composite patch depends primarily on the loading directions and crack inclination angle. [ABSTRACT FROM AUTHOR]

The pretension deviation caused by two kinds of geometrical errors, manufacturing error of member length and installation deviation of anchoring joint, is investigated for a novel crescent-shaped tensile system applied as the canopy structure in Yueqing Stadium, Zhejiang, China. The deviation of anchoring joint is transformed into the length error of radial cable, and the member length error is assumed to yield Gaussian distribution. An analytical expression, which reflects the relationship between the member length errors and pretension deviations, is deduced. A different choice of active cables leads to different sensitivity matrix in this analytical expression, which indicates theoretically that the effect for controlling pretension deviation varies with the tensioning scheme adopted in the construction. A cable truss is employed as an illustrative example to investigate the validity of this method. According to the limit values of the two kinds of geometrical errors specified in China standards, the structural pretension deviations under five tensioning schemes are calculated for the crescent-shaped tensile canopy structure. The pretension sensitivity of each member category to the geometrical errors is analyzed by comparing the relative pretension deviations. The control effects on pretension deviations are also compared for these five tensioning schemes. The results indicate that distinct pretension deviations exist in the structure if only controlling the lengths of all members or the tension forces of ring cables during construction. Stretching the upper and lower radial cables and also controlling their tension forces can prominently decrease the structural pretension deviations. However, the pretension deviations of suspenders cannot be effectively controlled for any of the five tensioning schemes. [ABSTRACT FROM AUTHOR]

This study describes a self-tensioning system in which the gravitational loads acting on a horizontal structural element are automatically converted to a posttensioning force on that component. The self-tensioning effect has a variable intensity, constantly adjusted depending on the applied service loads. The self-tensioning is eccentrically applied over the cross section, and it generates a negative moment that compensates the deformations due to the gravitational loads. The system can be utilized in beams, slabs, and structural framings of different materials and can be implemented using different mechanical and hydraulic solutions. The study describes the operation of a mechanical solution for the self-tensioning system and analyzes its behavior in large-span timber floor framings. When combined with conventional pretensioning, the self-tensioning system notably improves the strength and deformation behavior and permits a design of timber floor framings with a total height of 0.03 times the span length, achieving relative deflections below 1/1,000 of the span for the service loads of the structure. [ABSTRACT FROM AUTHOR]

The use of bolted sleeve joints has been proposed for assembling pultruded glass fiber-reinforced polymer (GFRP) tubular profiles into space lattice shell structures. Such joint configurations may introduce a semi-rigid end condition and further affect the capacity of the connecting members in compression. Three batches of specimens assembled with pultruded GFRP profiles of different lengths and bolted sleeve joints at both ends were prepared and tested under static axial compression. A detailed three-dimensional finite element model considering bolt geometry, contact behavior, bolt pretension, initial geometric imperfection, and failure criterion for fiber-reinforced polymer (FRP) composites was developed and validated with experimental results showing good comparisons. It was found that the bolted sleeve joints exhibited semi-rigid behavior and that the failure modes and the effective length factor were dependent on member slenderness. The relationship between effective length factor and member slenderness was derived based on finite element analysis and then used in conjunction with existing FRP column design equations to predict the member capacity for structural members with bolted sleeve joint end conditions. [ABSTRACT FROM AUTHOR]

Transfer length is a significant parameter in the design of pretensioned concrete members. The estimation of transfer length is typically achieved by measuring concrete strain or strand slip at the member ends. The end slip method is simple and requires little effort, but relies on an empirical formula with an undetermined coefficient (α), which relates to the bond stress distribution.Many studies have proposed appropriate a coefficients; however, the variability in estimating the a coefficient exists between studies depending on the chosen empirical fit. This study gives an estimation of the a coefficient using α probabilistic approach called the maximum likelihood (ML) method. Approximately 600 data points of transfer lengths and strand end slips were measured for 25 pretensioned concrete beams. Strand end slips were measured at release and at 3, 5, 7, 14, and 28 days. Concrete strains were measured at the same period of time using a detachable mechanical strain (DEMEC) gauge to determine the transfer lengths. An estimated a coefficient of 2.61 maximized the likelihood of the observed sample of the measured transfer lengths and end slips. [ABSTRACT FROM AUTHOR]

In pretensioned concrete members, prestress is introduced by the bond mechanism between prestressing tendon and surrounding concrete. Therefore, to secure the intended level of effective prestress in the tendon, sufficient bond stresses between the prestressing tendon and the concrete should be developed at release, for which a certain length from the end of the pretensioned concrete member is required, and this required distance is defined as the transfer length of the prestressing tendon. In this study, the prestress introduction mechanism between concrete and prestressing tendon was mathematically formulated based on thick-walled cylinder theory (TWCT). On this basis, an analytical model for estimating the transfer length was presented. The proposed model was also verified through comparison with test results collected from the literature. It was confirmed that the proposed model can accurately evaluate the effects of influential factors - such as diameter of prestressing tendon, compressive strength of concrete, concrete cover thickness and magnitude of initial prestress - on the transfer lengths of prestressing tendons in various types of pretensioned concrete member. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Higher Education is the property of Canadian Society for the Study of Higher Education and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A comprehensive experimental database containing 237 specimens from 27 different research studies was assembled during a larger research effort undertaken at the University of Texas at Austin to investigate three different short-term loss and four different longterm loss estimation procedures. The assembled database was filtered to eliminate specimens with incomplete information or those not adequately representing those found in the field (as a result of either being too small or overstressed at release). The specimens contained within this filtered database offered a diversity of characteristics well representative of beams found in bridges. This database was used to evaluate several different short- and longterm loss estimation procedures. The results of these evaluations can help to aid the designer in selection of an appropriate procedure for estimating prestress loss. [ABSTRACT FROM AUTHOR]

The use of 0.7 in. (17.8 mm), Grade 270 (1860) prestressing strands has advantages over 0.5 and 0.6 in. (12.7 and 15.22 mm) strands. This study provides design guidelines for estimating transfer length of 0.7 in. (17.8 mm) strands. Sixteen pretensioned concrete beams using a single prestressing strand were fabricated with conventional concrete and self-consolidating concrete. The concrete strengths at 1 day ranged from 5.9 to 9.2 ksi (40.7 to 63.4 MPa). Transfer lengths were determined using concrete surface strains along with the Average Maximum Strain Method. Initial strand end slips were also measured for predicting transfer length at release using an empirical formula. Experimental results indicated ACI 318 and AASHTO specifications are applicable for estimating transfer length of 0.7 in. (17.8 mm) strands at release and at 28 days. The results also showed that a coefficient of 2.32 was the most appropriate value for estimating transfer lengths at release from initial strand end slips. [ABSTRACT FROM AUTHOR]

A pretension design method based on the projection principle is proposed for space deployable mesh reflector antennas. First, the equilibrium tension of the plane cable net structure is obtained by a force equilibrium method. Then, the obtained cable tensions of the plane cable net structure are projected to the spatial cable net reflector to obtain the spatial cable tensions. Finally, the implication relationship between the plane cable tensions and the tension distributions of front and back cable nets is revealed. A general analytical expression is deduced for the axisymmetric reflector, and an optimization model is established for the offset reflector. Some hoop truss deployable mesh antennas are examined as examples of pretension design. The uniform cable tensions and high surface accuracy of reflectors are obtained to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Studies on bending properties of cables have long history but those researches are mainly around electrical conductors with very limit experimental discussions on structural cables. One approach on dynamic problems are through understanding changes in bending stiffness and utilized the varying bending rigidity into beam theory or beam finite element analysis. Therefore, a serial of quasi-static bending tests on semi-parallel wire cables, stranded ropes and steel spiral strands (Galfan) were performed to understand the cable bending response, with considering different factors like end conditions and the present and magnitude of pretension. The effective flexural stiffness of those structural cables were then determined and compared. Test results indicated that the unstressed cables under bending had an elastic–plastic like force–displacement response, which was analogous to an elastic–plastic metallic beam. Galfan strands showed better wire bounding behavior and cooperative bending due to different winding method. Welded end measurements can add integrity to the cable but this strengthening effect is weak and limited. Flexural response of the cable under a constant tension force was more like a linear relation with sag deflection. Pretension can increase the cable flexural stiffness, and this strengthening effect increased with the pretension level but decrease with the cable dimension. A simplified evaluation method of effective flexural stiffness was also proposed. [ABSTRACT FROM AUTHOR]

In pretensioned concrete members, strand bond is necessary for transferring prestress force from the prestressing strands to the adjacent concrete. This paper proposes a new bond stress-slip model for 0.6 in. (15.2 mm), Grade 270 prestressing strands. The proposed model is incorporated into a nonlinear second-order differential equation of strand slip along the transfer zone to investigate strand behavior. The transfer length at release estimated by the analytical method is verified using experimental data from 19 pretensioned concrete beams collected from the literature and 45 beams cast at the University of Arkansas. Three parametric studies were conducted. The results indicated that: 1) the transfer length is shortened as concrete compressive strength increases; 2) pullout force is an important factor affecting the transfer length; and 3) the transfer length predicted by ACI 318 is conservative for concrete having compressive strength of 4 ksi (26.7 MPa) or greater at release. [ABSTRACT FROM AUTHOR]

An experimental study was conducted in which 30 full-scale, precast, pretensioned bridge girders were constructed and instrumented with the intention of investigating prestress loss. Several different precast beam fabrication plants were used to investigate the influence of different concrete materials and construction techniques. The constructed girders were conditioned in several different climates for up to 3 years. During this period, prestress loss was measured by using vibrating wire gauges (VWG) embedded in test specimens. Following the conditioning period, the girders were flexural service load-tested to quantify the prestress loss at the time of testing and in turn verify the losses measured using VWGs. Prestress losses were found to be heavily influenced by concrete stiffness, which was dependent on coarse aggregate type and quantity. The measured short- and long-term prestress losses were compared to those determined using several different estimation procedures, suggested by ACI Committee 423. [ABSTRACT FROM AUTHOR]

Longitudinal and transverse vibrations of a pretensioned current-carrying nanowire subjected to a suddenly exerted three-dimensional magnetic field are of concern. Using an elastic string model by considering the surface effect, the equations of motion of the nanostructure are established. By employing admissible mode shapes and Laplace transform approach, an analytical solution is proposed to determine the nanowire's elastic field. The explicit expressions of the longitudinal and transverse displacements of the nanowire are derived. The conditions that lead to the dynamic instability of the nanostructure are also discussed. The influences of the initial tensile force, electric current, and the components of the magnetic field on the dynamic displacements are comprehensively addressed. [ABSTRACT FROM AUTHOR]

Cable-stayed bridges have been developing rapidly in the last decade and have become one of the most popular types of long-span bridges. One of the important issues in the design and analysis of cable-stayed bridges is determining the pre-tensioning cable forces that optimize the structural performance of the bridge. Appropriate pre-tensioning cable forces improve the damaging effect of unbalanced loading due to the deck dead load. Because the cable-stayed structure is a highly undetermined system, there is no unique solution for directly calculating the initial cable forces. Numerous studies have been conducted on the specification of cable pre-tensioning forces for cable-stayed bridges. However, most of the proposed methods are limited in their ability to optimize the structural performance. This paper presents an effective multi-constraint optimization strategy for cable-stayed bridges based on the application of an inverse problem through unit load method (ULM). The proposed method results in less stresses in the bridge members, more stability and a shorter simulation time than the existing approaches. The finite element (FE) model of the Tatara Bridge in Japan is considered in this study. The results show that the proposed method successfully restricts the pylon displacement and establishes a uniform deck moment distribution in the simulated cable-stayed bridge; thus, it might be a useful tool for designing other long-span cable-stayed bridges. [ABSTRACT FROM AUTHOR]

Prestressed concrete wing-shaped members are frequently used in the construction of long-span roofs in precast concrete buildings. Roofs spanning more than 30 m (100 ft) are built by assembling a number of these thin-walled members. To optimize their design and take advantage of their potential load-carrying capacity, the transfer of large forces from the pretensioned strands to the concrete is critical. Information on the evolution of these forces and of the ensuing deformations is necessary, not only in the long run, but also immediately after concrete casting and during the first few weeks. The Department of Civil and Environmental Engineering of the University of Perugia in Italy and a major prefabrication firm of central Italy have completed an experimental program on pretensioned wing-shaped members to investigate the evolution of prestressing stresses and of camber during the different stages of concrete hardening. The result allowed the careful evaluation of the prestress losses that occur in the first three weeks during concrete placement and curing to emphasize the influence of the temperature history on the development of prestress losses and to calibrate suitable parameters for the prediction of camber growth. Numerical comparisons based on Eurocode 2 are also performed to check the reliability of the numerical procedure adopted by the authors and to ascertain whether using self-consolidating concrete (as in the manufacture of the specimens tested in this project) makes any sizeable difference with respect to the predictions based on Eurocode 2, which refers to ordinary concrete. [ABSTRACT FROM AUTHOR]

The results of in situ tests carried out on unreinforced and reinforced poor rubble masonry full-scale walls in the earthquake prone Messina (Italy) area are presented and discussed. This experimental research was aimed at the assessment of the in plane shear behaviour of ancient masonry strengthened with an innovative system for masonry and retrofitting of reinforced concrete element constituted by three-dimensional pre-tensioned stainless steel ribbons. A comparison between different strengthening configurations was made in order to characterise the behaviour of masonry panels under shear-compression load, focusing attention on the diagonal cracking failure mode. The effectiveness of retrofitting method in enhancing strength and ductility of poor rubble masonry walls is proved by comparison of the response of the unreinforced and reinforced structural systems. [ABSTRACT FROM AUTHOR]

Thin membranes are used in the spacecraft industry as extremely lightweight structural components. They need to be stiffened, usually by applying discrete forces, and this increases their susceptibility to wrinkling in regions where high tensile stresses develop. We consider a regular polygonal membrane uniformly loaded at its corners by equal forces and we prevent wrinkle formation by trimming the edges of the polygon into very gentle curves. We confirm this performance through simple physical experiments using Kapton, a typical membrane material and, using computational analysis, we show how the distribution of compressive stresses, responsible for causing wrinkles, dissipates following trimming. Finally, we accurately predict the required level of trimming for any number of sides of polygon using a simple, linear model, which invokes a plate-bending analogy. [ABSTRACT FROM AUTHOR]