Prestressing Ratio Research Articles

프리스트레스트 경량 콘크리트 보의 휨 거동에 대한 부분 프리스트레싱비와 유효 프리스트레스의 영향

이 연구에서는 상부 2점 집중하중을 받는 프리텐션 경량 콘크리트 보의 휨 거동을 부분 프리스트레싱 비와 긴장재의 유효 프리스트레스에 따라 평가하였다. 절건비중 1,770 kg/㎥의 경량 콘크리트 설계강도는 35 MPa이었으며, 항복강도 383 MPa의 일반 이형철근과 인장강도 2,040 MPa의 3연선을 각각 주 인장철근과 프리스트레싱 긴장재로 사용하였다. 실험 결과, 프리텐션 경량 콘크리트 보의 휨 내력은 부분 프리스트레싱 비의 증가와 함께 증가하지만 긴장재의 유효 프리스트레스에는 거의 영향을 받지 않았다. 동일 휨 보강지수에서 프리텐션 경량 콘크리트 보의 무차원 휨 내력은 Harajli and Naaman 및 Bennet에 의해 실험된 프리텐션 보통중량 콘크리트 보와 비슷한 수준이었다. 한편 프리텐션 경량 콘크리트 보의 변위 연성비는 부분 프리스트레싱 비의 감소와 함께 그리고 유효 프리스트레스의 증가와 함께 증가하였다. 프리텐션 경량 콘크리트 보의 하중-변위 관계는 비선형 2차원 해석모델에 의해 적절하게 평가될 수 있었다. 또한 프리텐션 경량 콘크리트 보의 휨 균열 내력 및 최대 휨 내력은 각각 탄성이론 및 ACI 318-08의 등가응력블록과 긴장재의 응력평가 식을 이용하여 안전측에서 예측될 수 있었다.

Flexural Capacity of Concrete Beams Prestressed with Carbon Fiber Reinforced Polymer (CFRP) Tendons

Fiber reinforced polymer (FRP), particularly those incorporating carbon fiber (CFRP), has high strength, high stiffness-to-weight ratio and high resistance to corrosion, which shows potential for use as prestressing tendons in corrosive environment. However, concrete beams prestressed with FRP tendons have showed brittle flexural failure due to the elastic rupture of FRP tendons. In order to improve the ductility, a combination of bonded and/or unbonded prestressing tendons was used. Nine prestressed concrete beams were tested up to failure to study the effect of bonded and unbonded FRP tendons on their flexural capacity. Three factors were taken into consideration; the bonding condition of CFRP tendons, the location of CFRP tendons and the prestressing ratio. Also an analytical investigation was carried out to extend some flexural capacity calculation equations to this beam type. The results of the experimental showed that under the same condition, the carrying capacity of concrete beam prestressed with bonded FRP tendons was 20% higher than that of internal unbonded prestressed beam, and was 40% higher than that of external unbonded prestressed beam without deviators. By combination of bonded and unbonded FRP tendons, the ductility of prestressed concrete beams can be improved.

Flexural ductility of prestressed concrete beams with unbonded tendons

Based on a numerical method to analyse the full-range behaviour of prestressed concrete beams with unbonded tendons, parametric studies are carried out to investigate the influence of 11 parameters on the curvature ductility of unbonded prestressed concrete (UPC) beams. It is found that, among various parameters studied, the depth to prestressing tendons, depth to non-prestressed tension steel, partial prestressing ratio, yield strength of non-prestressed tension steel and concrete compressive strength have substantial effects on the curvature ductility. Although the curvature ductility of UPC beams is affected by a large number of factors, rather simple equations can be formulated for reasonably accurate estimation of curvature ductility. Conversion factors are introduced to cope with the difference in partial safety factors, shapes of equivalent stress blocks and the equations to predict the ultimate tendon stress in BS8110, EC2 and ACI318. The same equations can also be used to provide conservative estimates of ductility of UPC beams with compression steel.

Experimental observation of stick-slip instability fronts

SUMMARY Stick-slip dynamic instability is a key mechanism governing frictional processes from microscale physics to earthquake faults and landslides; yet challenging questions are still open about its nucleation and propagation dynamics. Here we present novel observations on laboratory experimental faults where spontaneously nucleating fractures are produced, describing (1) an initial quasistatic, stable rupture front (propagating at about 5 per cent the shear wave velocity VS), accelerating to subshear and then to intersonic velocity; (2) the arisal of a higher degree of complexity when the friction to prestress ratio is increased on the sliding surface. The complex behaviour includes stop and go sequences, irregular propagation and rerupturing episodes within short-time intervals, implying rapid restrengthening of the surface and the formation of self-healing pulses, reproducing experimentally for the first time a behaviour observed on seismic faults.

Seismic behaviour of steel-concrete composite beams

Five steel-concrete composite beams are tested under low reversed cyclic loading. The failure patterns, hysteretic characteristics, hysteretic model, deformation restoring capacity, ductility, energy dissipation capacity, stiffness degradation and slips of the prestressed composite beams are discussed. Attention is paid to the effects of partial prestress ratio and degree of shear connection on the seismic performance of these beams. Studies show that the prestressed composite beams behave in a relatively ductile manner. Ductility of the composite beams decreases as the partial prestress ratio and the degree of shear connection increase. It was also observed that the applied prestressing is favourable for energy dissipation during downwards loading. Slips between concrete slabs and steel beams have adverse effects on energy dissipation in positive moment regions of composite beams. Tests also show that the effects of partial prestress ratio and the degree of shear connection are negligible to degradation of stiffness in composite beams.

Nonlinear behavior of concrete beams with hybrid FRP and stainless steel reinforcements

The full-range behavior of partially bonded, together with partially prestressed concrete beams containing fiber reinforced polymer (FRP) tendons and stainless steel reinforcing bars was simulated using a simplified theoretical model. The model assumes that a section in the beam has a trilinear moment—curvature relationship characterized by three particular points, initial cracking of concrete, yielding of non-prestressed steel, and crushing of concrete or rupturing of prestressing tendons. Predictions from the model were compared with the limited available test data, and a reasonable agreement was obtained. A detailed parametric study of the behavior of the prestressed concrete beams with hybrid FRP and stainless steel reinforcements was conducted. It can be concluded that the deformability of the beam can be enhanced by increasing the ultimate compressive strain of concrete, unbonded length of tendon, percentage of compressive reinforcement and partial prestress ratio, and decreasing the effective prestress in tendons, and increasing in ultimate compressive strain of concrete is the most efficient one. The deformability of the beam is almost directly proportional to the concrete ultimate strain provided the failure mode is concrete crushing, even though the concrete ultimate strain has less influence on the load-carrying capacity.

Behavior of Prestressed Ultra High Performance Concrete Beams

An experimental program was formulated to investigate the characteristics of complete stress-strain curve of UHPC in uniaxial compression and flexural behaviors of prestressed UHPC beams. The particular focus was the influence of the partial prestress ratio and jacking stress on the flexural response of UHPC beams. The tests of beams demonstrated that the UHPC beams have an excellent behavior in load carrying capacity, crack distribution and deformability; their ultimate deflection can reach 1/34～1/42 of the span. Based on this investigation, theoretical correlations for the prediction structure response of UHPC beams are proposed.

Behaviour of partially prestressed beams with external tendons

The behaviour of prestressed concrete structures with unbonded tendons differs significantly from that of those with bonded tendons after the working stage. To assess the safety of such structures, it is necessary to carry out full-range analysis with particular attention to the post-peak behaviour. This paper reports the initial findings of experimental investigations. A number of simply supported externally prestressed concrete beams with either steel or aramid fibre-reinforced polymer tendons are tested to failure, so as to study the effect of external prestressing on the ductility. The specimens tested all have T-sections prestressed by two external tendons with one on each side. In the experiments, different factors are taken into consideration. They include the level of prestressing force, area of non-prestressed steel reinforcement and partial prestressing ratio and so on. It is found that friction may vary significantly when a beam is subjected to large deformations. Comprehensive analysis of internal forces in selected specimens has been carried out. While the tendons play a key role in supporting loads during the initial stage of loading, their contribution decreases with further loading. At the same time, the role of bottom non-prestressed reinforcement increases and remains predominant after the apparent yielding of the specimen.

Research on non-linear structural behaviors of prestressed concrete beams made of high strength and steel fiber reinforced concretes

The flexural behaviors of fully and partially prestressed concrete beams made of high strength and steel fiber reinforced concretes are studied by experiment and non-linear finite element method. Three levels of partial prestress ratio (PPR) are considered, and for each PPR, a pair of two-span continuous beams with box-section are designed. In each pair of the test beams, one is constructed by the high strength concrete completely, and for the other one the steel fiber reinforced concrete is used in the negative moment zone (the mid-support zone). Such structural behaviors as the deflection of beam, the formation and the development of crack, the strain of concrete and reinforcement bar, and the moment redistribution are investigated. A non-linear finite element analysis program is developed and applied. The constitutive behavior of concrete is modeled by the microplane theory. A four-nodal six-DOF (degree of freedom) degenerated shell element is adopted to simulate the structural behaviors of the box-section beam. To obtain the ultimate limit state of the structure, the arc-length method is introduced in the non-linear solution. The analytical results agree well with test ones. The influences of the steel fiber reinforced concrete used in the negative moment zone on the structural behaviors are mainly concerned.

Effect of preloading on the cyclic strength of reconstituted sand

The effect of preloading on the cyclic soil strength under level ground was investigated by cyclic direct-shear tests where horizontal shear stress oscillated about a zero mean value. Tests on reconstituted samples were performed on two different sands and at varying prestress ratios, densities and vertical stresses. Test results showed a marked increase of the cyclic soil strength with the prestress ratio. The effect is more pronounced for the looser specimens. Arelationship giving the increase of the cyclic strength of sands with the prestress ratio is proposed. This relationship predicted reasonably well the increase in cyclic soil strength measured in previous laboratory studies. The domain of validity of this equation and the manner in which it can be used for ground modification projects under earthquakes are discussed.On étudie ici l'effet du préchargement sur la résistance cyclique du sol en dessous de la surface à niveau au moyen d'essais cycliques de cisaillement direct où la contrainte de cisaillement oscillait autour d'une valeur moyenne nulle. Des tests, réalisés sur des échantillons reconstitués, ont étéeffectués sur deux types de sables et pour des rapports de précontrainte, densités et contraintes verticales variables. Les résultats ainsi obtenus montrent une augmentation significative de la résistance cyclique du sol avec le rapportde précontrainte. Ce phénoméne est encore plus marqué pour les échantillons les plus meubles. Il est proposé uneéquation donnant l'augmentation de la résistance cyclique des sables en fonction du rapport de prélcontrainte. Cetterelation a permis de prédire de maniére relativement satisfaisante l'augmentation de résistance cyclique du solmesurée dans des é tudes de laboratoire précédentes. Ondiscute ici également du domaine de validité de cette équation et de la façon dont elle peut être appliquée auxprojets d'amélioration des sols soumis à des tremblementsde terre.

Ductility analysis of prestressed concrete beams with unbonded tendons

This paper describes a numerical method for the full-range analysis of prestressed concrete flexural members with unbonded tendons, taking into account the stress-path dependence of materials. The numerical results compare favourably with experimental results. Parametric studies are carried out to evaluate the influence of loading type, span–depth ratio, combined reinforcement index (CRI), partial prestressing ratio, concrete compressive strength, and ratio of compressive reinforcement, etc. on the ductility behaviour. The results indicate that the curvature ductility factor of prestressed concrete members with unbonded tendons decreases with the increase of CRI. The curvature ductility factors for members with bonded and unbonded tendons for given values of CRI are also analyzed and compared. It is generally observed that when the CRI is between 0.15 and 0.20, the ductility factor of an unbonded member is close to that of the bonded one. Above this range of CRI the ductility factor of an unbonded member is higher than that of a bonded one, while below this range the ductility factor of an unbonded member is lower than that of a bonded one.

An Experimental Study on Interrelation of Influential Parameters on Unbonded Tendon Stress

The purpose of this study is to investigate the relations between unbonded tendon stress and its influential parameters, i.e. bonded reinforcement ratio, span/depth ratio, and loading type. To this end, the influence of such parameters was examined with twenty eight test results of previous studies. Afterwards, an experimental study was carried out with twenty one test specimens. The investigation of previous and current experiments revealed the followings; (1) The bonded reinforcement ratio and prestressing ratio were proved to be important variables on the unbonded tendon stress. (2) The ratio of span to depth and the type of loading affected the unbonded tendon stress partially although their effects varied with bonded reinforcement ratio. (3) AASHTO LRFD Code and Moon/Lim's design equations predicted the experimental results well with the safety margin.

緊張材の付着特性を考慮したプレストレストコンクリート圧着骨組の曲げ挙動に関する解析研究

An analytical work was conducted on a prestressed concrete sub-frame assembled by post-tensioning. Layered element analysis considering bond-slip characteristics of prestressing tendon was used. The larger ratio of average effective prestress to compressive strength of concrete in the section and the smaller ratio of average effective prestress to 0.2% offset vield stress of the tendon were assigned, the larger difference of flexural strength between the cases with and without consideration of bond-slip was observed. Furthermore, the flexural behavior of the member was significantly influenced by the bond yield stress, while the effect of the bond stiffness was hardly found.

Serviceability of Concrete Beams Prestressed by Carbon-Fiber-Reinforced-Plastic Bars

The use of carbon-fiber-reinforced-plastic (CFRP) as prestressing reinforcement for concrete structures, has increased rapidly in the last ten years. The non-corroding characteristics of CFRP reinforcement could significantly increase the service life of concrete structures. In addition, CFRP prestressing reinforcement has the advantages of high strength-to-weight ratio, good fatigue properties, and low relaxation. However, the linear elastic behavior of the material up to failure requires special design consideration to ensure sufficient deformability of the members. In this paper, partial prestressing using low-jacking stresses, is proposed for the design of concrete members prestressed by CFRP reinforcement to reduce the cost and improve their deformability. The paper summarizes a study undertaken to examine the flexural behavior of concrete beams partially prestressed by CFRP reinforcement. The experimental program consists of testing eight concrete beams prestressed by CFRP bars and two beams prestressed by conventional steel strands. The parameters considered in this experimental program are the prestressing ratio, degree ofprestressing, and distribution of the CFRP bars in the tension zone. The various limit states behavior of concrete beams prestressed by CFRP bars in terms of cracking and deflection prior and after cracking are examined. The different modes offailure and the deformability of such beams are discussed.

Finite element analysis of partially prestressed steel fiber concrete beams in shear

Abstract This paper presents a finite element formulation for the modeling of the behavior of partially prestressed steel fiber concrete beams in shear. Based on a secant modulus approach, the formulation treats steel fiber concrete as an orthotropic material, characterized by appropriate constitutive relations in the principal compressive and tensile directions. An experimental program with the partial prestressing ratio, the shear span:effective depth ratio, and the volume fraction of steel fibers as test variables was carried out and the deflections of the beam, concrete, and steel strains were monitored and compared with the results of the finite element analysis. The finite element formulation was found to predict the deformational characteristics and the ultimate load of the test beams well. Steel fibres were observed to improve the beam stiffness after the occurrence of first shear crack and to enhance the shear strength of partially prestressed concrete beams significantly.

Inelastic Buckling of Prestressed Sandwich or Homogeneous Arches

Inelastic buckling of prestressed homogeneous and sandwich arches is studied using a nonlinear finite element model. Prestressed arches are made by intentional buckling of thin struts into elastica shapes. In the case of sandwich construction, each layer is first buckled into shape, then the prebuckled layers are connected to form a composite section. The fabrication process is modeled by a special algorithm that accounts for the change in element type from three single-layer homogeneous elements into a single three-layer sandwich beam element. A corotational formulation is used for the large displacement, large rotation analysis of an elastic, linear strain-hardening material. Shear deformations and membrane and flexural rigidities of all layers are included. A parametric study is carried out to include the effects of plastic tangent modulus, relative thickness of the core layer, and the ratio of prestress to the yield stress of the material. Symmetrical and asymmetrical buckling modes are both considered. The results indicate that plastification of the facings could reduce the buckling loads by as much as 55% compared to the linear elastic buckling loads.

Prestress Losses in Partially Prestressed High Strength Concrete Beams

The results of a comprehensive parametric study on prestress losses in prestressed and partially prestressed high strength concrete beams are reported. Attention is focused on the influence of the partial prestressing ratio (from no prestressing to full prestressing) and the compressive strength of concrete, from 6 ksi (41 MPa) up to 10 ksi (69 MPa). Different beam cross sections, representing building and bridge girders with various spans and spacings, were studied. Additional parameters investigated include the strength and type of prestressing steel (stress-relieved of low-relaxation), the yield strength of reinforcing steel, the relative humidity of the environment and curing conditions (steam- or moist-curing). Time-dependent prestress losses were computed through an accurate time-step analysis procedure, which was implemented in a computer program. Conclusions and recommendations are drawn for use in design practice. The results are applicable to pretensioned as well as post-tensioned members. Proposed design recommendations are suggested to replace current lump sum design tables suggested in the AASHTO Specifications and the ACI 343R-88 report on analysis and design of reinforced concrete bridge structures.

Service Load Behavior of Concrete Members Prestressed with Unbonded Tendons

The results of experimental and analytical investigation of the behavior of concrete members prestressed with unbonded tendons under static loading are described. In the experimental section, a total of 26 beam specimens are tested, and several parameters, including reinforcing index, partial prestressing ratio, member span‐to‐depth ratio, and type of load application, are investigated. In the following analysis a general method of section analysis of concrete members prestressed with unbonded tendons is developed and used to reproduce the experimentally observed service load stresses in the tensile reinforcement. Also, available methods proposed in the technical literature to predict the service load deflection of bonded prestressed and partially prestressed concrete members are generalized to account for the slip of prestressing steel in unbonded posttensioned members and then compared with the current experimental results.

Calculation of stress in prestressing steel at ultimate conditions

This paper reviews different methods available for the calculation of stress in prestressing steel at ultimate, ƒps, and the methods are evaluated using a nonlinear analysis computer program. The development of a nonlinear model for the analysis of partially prestressed concrete flexural members is described. The model takes the material nonlinearities into account by the use of appropriate stress–strain relationships for steel and concrete. A parametric study of partially prestressed beams was conducted to study the effect of various parameters on stress in prestressing steel at ultimate conditions. Two types of sections, rectangular and T sections, were studied. The prediction equation, recommended in the Canadian Code Can3-A23.3-M84, for calculating the stress in prestressing steel at ultimate is found to be very conservative for all the cases studied, which covered all possible ranges of partial prestressing ratio. The present restriction in the Canadian Code, for the use of their prediction equation only when the ratio of the neutral axis depth, Cu, to the depth of the centroid of prestressing steel, dps, is less than or equal to 0.3, is found to be too restrictive. An alternative equation for ƒps, suggested by Loov, is found to be more suitable than the present equation in the Canadian Code; however, a more practical upper limit for cu/dps ratio is suggested in the paper. It is shown that the proposed equation can be used conservatively over the complete range of partially prestressed concrete. Key words: analysis, concrete (prestress), flexure, partial prestressing, prestressing, strain, stress, steel.

Losses in partially prestressed concrete

The calculation of prestress losses by the age-adjusted effective modulus method is analyzed and compared with the Ontario highway bridge design code predictions for partially prestressed concrete. Specifically, the effect of nonprestressed reinforcement on prestress losses is studied. The age-adjusted effective modulus method for calculating prestress losses is outlined, and plots of prestress losses versus partial prestressing ratio are presented and analyzed. It is shown that prestress losses decrease with increasing amounts of nonprestressed reinforcement. Also, the Ontario highway bridge design code expressions, which are intended for use with fully prestressed sections, are not suitable for use in the design of partially prestressed concrete members. Key words: concrete (prestress), design, partial prestressing, prestress losses.