Theoretical analysis of natural frequency of externally prestressed concrete beam based on rigidity correction

A dynamic test on externally prestressed simply supported concrete beams separately with three typical types of tendon distributions was conducted. The results show that the natural frequencies of the beams increase with the increase in the prestressing force at the tensioning stage, and the natural frequencies decrease after the cracks occur in the beams. Following the calculation formula of natural frequency of externally prestressed beam, which was reported in a literature, the natural frequencies of the experimental beams are calculated, and big errors are found between the test results and the calculated ones of natural frequency values. As a result, this paper has tried to adopt two methods to correct the rigidity parameter of the concrete beam in the formula for natural frequency calculation, and to use the corrected formula to calculate the frequencies of the experimental beams. The calculation results indicate a good consistency with the experimental ones, which verifies the feasibility of the corrected formula.

Fatigue Damage Modeling for Partially Prestressed Concrete Beams under Repeated Loadings with Variable Amplitude

A full-range nonlinear analysis method for fatigue damage in prestressed concrete beams is presented. New damage accumulation models are proposed to describe the fatigue damage evolution in concrete and reinforcement respectively. Based on the stress analysis for cross section, the stress redistrbution in the fatigue damage process, due to the different damage mechanisms of concrete and reinforcement, is considered. The nonlinear damage analysis is achieved by means of piecewise linearity, and it is applicable on the condition of repeated loadings with variable amplitude. Fatigue damage modeling of a beam is implemented to illustrate that the proposed method can preferably fit the experimental results.

Experimental research on mechanical properties of prestressed truss concrete composite beam encased with circular steel tube

Tests of 4 simply supported unbonded prestressed truss concrete composite beams encased with circular steel tube were carried out. It is found that the ratio of the stress increment of the unbonded tendon to that of the tensile steel tube is 0.252 during the using stage,and the average crack space of beams depends on the ratio of the sum of the bottom chord steel tube's outside diameter and the secondary bottom chord steel tube's section area to the effective tensile concrete area. The coefficient of uneven crack distribution is 1.68 and the formula for the calculation of crack width is established. Test results indicate that the ultimate stress increment of unbonded tendon in the beams decreases in linearity with the increase of the composite reinforcement index β0. The pure bending region of beams accords with the plane section assumption from loading to failure. The calculation formula of ultimate stress increment of the unbonded tendon and the method to calculate the bearing capacity of normal section of beams have been presented. Besides,the method to calculate the stiffness of this sort of beams is brought forward as well.

Structural Behavior of Continuous Prestressed Steel Fiber Reinforced High Strength Concrete Beam

The flexural behaviors of continuous fully and partially prestressed steel fiber reinforced high strength concrete beams are studied by experiment and nonlinear finite element analysis. Three levels of partial prestress ratio （PPR） are considered, and three pairs of two-span continuous beams with box sections varying in size are designed. The major parameters involved in the study include the PPR and the fiber location. It is concluded that the prestressed high strength concrete beam exhibits satisfactory ductility; the influences of steel fiber on the crack behaviors for partially prestressed beams are not as obvious as those for fully prestressed ones; steel fibers can improve the structural stiffness after cracking for fully prestressed high strength concrete beams; the moment redistribution from mid-span to intermediate support in the first stage should be mainly considered in practical design.

Experimental Behavior of Partially Prestressed High Strength Concrete Beams

In the last few decades, prestressed concrete has been rapidly used in bridge engineering due to the enormous development in the construction techniques and the increasing need for long span bridges. High strength concrete has been also more widely spread than the past. It currently becomes more desirable as it has better mechanical properties and durability performance. Major defect of fully prestressed concrete is its low ductility;it may produce less alarming signs than ordinary reinforced concrete via smaller deflection and limited cracking. Therefore, partially prestressing is considered an intermediate design between the two extremes. So, combining high strength concrete with partial prestressing will result in a considerable development in the use of prestressed concrete structures regarding the economical and durability view points. This study presents the results of seven partially prestressed high strength concrete beams in flexure. The tested beams are used to investigate the influence of concrete compressive strength, prestressing steel ratio and flange width on the behavior of partially prestressed beams. The experimentally observed behaviors of all beams were presented in terms of the cracking load, ultimate load, deflection, cracking behavior and failure modes.

Analysis and calculation of factors on curvature ductility of unbonded prestressed concrete beams

In consideration that behavior of curvature ductility of interior support directly influences the degree of moment modification of unbonded prestressed concrete (UPC) continuous structures, constitutive relationships of concrete, non-prestressed reinforcement and prestressed reinforcement used for nonlinear analysis are given. Through simulation analysis on simple beams subjected to single loading at the middle of the span, the law of factors influencing curvature ductility, such as global reinforcing index, prestressing degree, effective prestress, strength of concrete and grade of non-prestressed reinforcement are explored. Based on these researches, calculating formula of curvature ductility coefficient of UPC beams is established, which provides basic data for further research on plastic design of UPC indeterminate structures.

Experimental and theoretical analysis on the natural frequency of external prestressed concrete beams

Analyses and computations of the natural frequencies of external prestressed concrete structures are the bases for studying the dynamic properties of such structures. We carded out dynamic tests on three types of external simply supported beams, pulling the beams gradually. Then vertical loads were applied to the beams while the frequencies and related coefficients were measured at every step. We calculated natural frequencies and the results indicate that natural frequencies increase as the prestressing force rises in the pre-crack stage, and decrease significantly during the post-crack stage. Substantial incoincidences exist between the calculated and experimental results for the frequency and its tendency to changel Based on the experimental results, we modified the stiffness and other parameters in the equations. The results calculated using the modified equations agree with experimental results well, so the modified eauations can be used nractically.

Influences of Installment Tensioning Technology on Bearing Capacity of Rectangular Prestressed Concrete Beam

To obtain the relationship between use of installment tensioning technology and the bearing capacity of prestressed concrete beams, tensile and cyclic loading tests are conducted on five rectangular prestressed concreted specimens. Two types of prestressed tensioning methods are adopted in the tests, one uses installment tensioning technology to tension beams on days 4, 12, and 8, 12, respectively, and the other adopts the conventional method to tension the beam on day 28 only. The age of concrete, number of times tensioned, and the prestress value of early tensioning are considered in tests. Results show that use of installment tensioning technology has no effect on the bending failure pattern of prestressed concrete beams, but it reduces prestress loss and increases crack and yield loads. © 2017, Tianjin University and Springer-Verlag Berlin Heidelberg.

Fatigue Behavior of Unbonded Partially Prestressed Concrete Beams with Flexure

A study on fatigue behavior of unbonded partially prestressed concrete beams is presented. Model tests have been carried out in static loading and cyclic compressive loading on 15 beams with flexure. The ratios of the lower limit to the upper limit of fatigue load are 0.5 and 0.3 respectively, and the frequencies of cyclic loading are 8 Hz and 4.5 Hz respectively. The experimental results of the strains of the concrete and steel bars, the deflection of test beams, and the crack width of normal section are analyzed. According to statistics and analysis of test results, the corresponding calculation models are developed and presented.

Simulation of Fatigue Stiffness Degradation in Prestressed Concrete Beams under Cyclic Loading

In order to investigate and research the fatigue cracking of prestressed concrete fatigue properties and loading and stiffness degeneration process,cyclic loading tests were carried out on six prestressed concrete beams and the stiffness degradation under fatigue was investigated. A simulation model of stiffness degradation is proposed based on the stiffness analysis of the fatigue-damaged section. The elastic modulus of damaged concrete and the effective residual area of steel were introduced as well as an adjusted three-stage concrete fatigue damage evolution model. The strip method was used to analyze concrete damage due to changing stress along the depth of the beam section. The simulation and test results were compared and a method of predicting fatigue deflection was presented based on the simulation model. The predicted results were compared with that of the neural network method. It is in good agreement for the simulation results with the test results. It is only less than5% error for the simulation model which can reveal the two-stage degradation of prestressed concrete beams under cyclic loading. It is more precise for the simulation prediction method under proper conditions.

Shear Behavior of Novel Prestressed Concrete Beam Subjected to Monotonic and Cyclic Loading

Prestressed steel ultrahigh-strength reinforced concrete(PSURC) beam is a new type of prestressed concrete beam, which not only has a considerable compressive strength attributed to the ultrahigh strength concrete, but also ensures a certain degree of ductility at failure due to the existence of structural steel. Five of these beams were monotonically tested until shear failure to investigate the static shear performance including the failure pattern, loaddeflection behavior, shear capacity, shear crack width and shear ductility. The experimental results show that these beams have superior shear capacity, crack control ability and shear ductility. To study the shear performance under repeated overloading, seven PSURC beams were loaded in cyclic test simultaneously. The overall shear performance of cycled beams is similar to that of uncycled beams at low load level but different at high load level. The shear capacity and crack control ability of cycled beams at high load level are reduced, whereas the shear ductility is improved. In addition, the influences of variables including the degree of prestress, stirrup ratio and load level on the shear performance of both uncycled and cycled beams were also discussed and compared, respectively.

Natural Frequency of Full-Prestressed Concrete Beam

Dynamic tests of three bonded and two unbonded full-prestressed concrete beams were carried out.The purpose was to seek the relation between prestressing force and natural frequency.Test results indicate that the frequency of prestressed concrete(PSC)beam increases with the increase in prestressing force approximately.The results are different from the dynamic characteristics of isotropic material beam subjected to compressive axial force which were put forward by Clough et al.The reason is that the beams were considered as isotropic,homogeneous,linear elastic material in the traditional analysis method.However,more accurate results are required in the analysis of frequency of PSC beam.The constitutive mode of PSC member is analyzed based on microstructure of concrete in this paper.The orthotropic linear elastic mode is used to analyze the relation between dynamic frequency and prestressing force of concrete beam,at the same time the equivalent stiffness of prestressed tendon relating to the prestressing force is added to the bending deformation stiffness of the beam.The analytical value agrees well with the test result,indicating that the current analysis method in this paper is feasible to full-prestressed concrete beam.

Rational concrete cover of prestressed concrete beams and slabs for fire resistance

As the influence of the ratio of the characteristic value of structural resistance to that of frequent combination effect, K, on the thickness of concrete cover of prestressed concrete （PC） beams and slabs for fire resistance is not considered sufficiently in design criteria for concrete structures in China and other countries at present, numerical analyses were carried out by ANSYS software on unbonded prestressed concrete （UPC） oneway simply-supported and continuous slabs and beams, and bonded PC simply-supported and continuous beams subjected to fire in 186 cases. In the analysis, K, section dimensions and the thickness of concrete cover are regarded as independent variables and some calculation parameters determined by trial and error based on test resuits. Calculation formula for fire endurance of UPC simply-supported and continuous slabs was proposed, and the recommended thicknesses of cover of PC beams and slabs was presented in consideration of the influence of K and section dimensions as well as fire endurance requirements. Comparison analysis of relevant design criterions in China and other countries was performed to verify the rationale of the proposed values.

The Design and Analysis of Long-Span and Low-Depth Prestressed Composite Steel-Concrete Beam Bridge

The design scheme of long span and low depth composite steel concrete beams is introduced, and the methods of avoiding the cracking of concrete deck in the negative moment regions are proposed. Moreover, significant exploration for problems of the composite beams has been made, such as optimizing construction steps to regulate the stress, applying jacking technique to exert prestress on the concrete deck, investigating the uplifting force principle of the shear connectors by means of model test and non linear finite element analysis, and pointing out the countermeasure to reduce tension force of the shear connectors.

Experimental study on retarding mortar and its application in prestressed concrete

Based on a large number of orthogonal tests and theoretical analyses, the retarding mortar which meets the requirements of retard-bonded prestressed concrete was prepared. Initial setting time of the retarding mortar may vary from several hours to 15 d at 5 ℃-35 ℃ due to quantities and average curing temperature. And its 28 d compressive strength is above 35 MPa. Thus the influence of quantities on setting time and 28 d compressive strength, and the relationship between setting time and average curing temperature were investigated. The optimum quantities were obtained by studying the interaction of admixtures, and the retarding mechanism was discussed. Based on 52 retard-bonded prestressed strands by manual work from 24 retard-bonded prestressed concrete T-beams, static friction drag, change factor κ and friction factor μ were obtained from the test when retard-bonded prestressed strands were tensioned. Application of the retarding mortar will be vast in practical concrete projects.

FLEXURAL CAPACITY OF RC BEAM STRENGTHENED WITH PRESTRESSED C/AFRP SHEETS

A method for strengthening concrete structures by prestressed intraply hybrid C/AFRP sheets(composed of aramid fiber and carbon fiber)is proposed,aiming at overcoming shortages of some current strengthening methods.The bending capacity of beams strengthened by prestressed intraply hybrid C/AFRP sheets is analysed.The result shows that the bending resistance,the extending performance and the economical efficiency of intraply hybrid fiber sheets are better than those of interlaminar hybrid fiber sheets and single-fiber sheets.Moreover,prestressed intraply hybrid C/AFRP sheets can improve the performance of the strengthened beam,effectively inhibit the expansion of the cracks,and reduce distortion and risk of early failure.

重载铁路桥梁体外预应力筋加固性能研究

运营提载会导致重载铁路桥梁损伤累积和刚度退化加剧,对重载铁路预应力混凝土桥梁进行加固可以改善桥梁的受力状况,提高桥梁承载能力。为了研究重载铁路桥梁体外预应力筋加固的性能,分别制作重载铁路24 m超低高度预应力混凝土梁的未加固足尺试验模型与体外预应力筋加固足尺试验模型,并开展200万次疲劳试验与疲劳后静力试验。建立未加固预应力混凝土梁与体外预应力筋加固预应力混凝土梁的精细化有限元模型,通过试验与有限元分析,研究加固前后重载铁路预应力混凝土梁的静力与疲劳性能,揭示加固前后梁体挠度和应力变化规律,并分析不同轴重列车作用下加固预应力混凝土梁的力学性能。研究结果表明:重载铁路桥梁外加预应力筋的加固效果良好,疲劳试验中加固梁刚度退化幅度、跨中挠度、梁内预应力筋应力、梁底及下翼缘混凝土应力显著比未加固梁的小;在33 t轴重列车与1.2倍中等荷载(ZH)轴重作用下,外加预应力筋加固的24 m超低高度预应力混凝土梁的适应性良好,预应力混凝土梁挠度与应力响应均符合规范要求。研究结果可为重载铁路桥梁的设计、养护、维护与加固提供基础与参考。

部分预制预应力型钢混凝土梁受力性能试验与设计方法研究

为简化现场施工工序,创新将预应力技术与装配式技术集成应用于型钢混凝土结构中,提出部分预制预应力型钢混凝土(PPPSRC)梁。通过5个梁试件的受弯性能试验及5个梁试件的受剪性能试验,研究了预应力施加顺序、预应力度、预应力筋高度及预制混凝土种类对PPPSRC梁的破坏形态和截面应力发展的影响,并结合试验结果对各试件的裂缝发展规律、承载及变形能力进行了分析研究。研究结果表明:预应力施加于梁预制部分或施加于整个梁对PPPSRC梁受力性能影响较小,不同预应力施加顺序的试件开裂荷载与峰值荷载差值均在5%以内;预应力度和预制部分混凝土种类对PPPSRC梁的抗裂性能影响显著,其中PPPSRC梁的开裂荷载相较于普通型钢混凝土梁最高可提高166%,而预制部分采用超高性能混凝土相比预制部分采用普通混凝土的试件开裂荷载可提高33%。基于试验结果建立了适用于PPPSRC梁的受弯、受剪承载力以及开裂荷载的计算公式,计算结果与试验结果吻合较好。

基于缩尺模型的高铁桥梁预应力损失试验研究

【目的】为更好的评估在役预应力混凝土桥梁结构预应力损失状况。【方法】选用国内高铁中应用最广的32 m预应力混凝土箱梁作为研究对象,按照1:16的缩尺比例将箱梁转化为T形截面制作了26根C50混凝土试验模型梁。通过对试验梁预应力钢绞线应变、伸长量的瞬时和长期监测得到预应力损失的实际值,并将此实际值与目前国内外常用预应力损失规范计算所得值进行对比分析。【结果】预应力损失主要发生在前期即瞬时损失,约占总损失的70%;84.6%的试验梁锚固损失占瞬时损失总和的70%以上,最高达80.57%;试验梁的长期损失在锚固后的前10 d内变化较快,后期逐渐趋于稳定;将5种规范的计算结果与试验实测值进行对比,对于长期损失,数据显示实测数据与《铁路桥涵设计规范》(TB 10002—2017)计算结果最为吻合。【结论】研究结果可为实际工程中预应力梁的预应力损失计算提供参考。

局部受热锈蚀预应力混凝土梁力学性能试验研究

随着我国基础建设的发展,大量混凝土桥梁在恶劣环境影响下发生腐蚀和老化,腐蚀桥梁结构的灾害破坏机理研究对桥梁结构的运营安全具有重要意义。为研究已经锈蚀受损的桥梁在局部受热状态下的高温力学性能,设计10根预应力混凝土梁试件。其中对照组的2根试件在常温下加载,试验变量为预应力筋初始应力,试验组的8根试件在高温下加载,试验变量分别为钢筋锈蚀、预应力筋初始应力、初始荷载。首先,基于电化学原理对4根预应力混凝土梁进行钢筋加速锈蚀处理,然后利用复合高温试验炉对8根预应力混凝土梁(4根锈蚀、4根未锈蚀)在跨中进行三面受火试验。最后,采用数值方法对局部受热预应力混凝土梁进行了仿真分析。研究结果表明:局部受热条件下,预应力混凝土梁的承载能力会发生明显降低,下降幅度约为11.3%~14.9%;钢筋锈蚀导致的混凝土开裂会对预应力混凝土梁的抗火性能产生不利影响,主要表现为2个方面,1)相同试验条件下,锈蚀后的试件预应力增量更大;2)锈蚀试件在局部受热条件下承载能力下降幅度更大,约为19.4%~21.9%;增大预应力筋的初始应力可以减小预应力混凝土梁在局部受热时的高温蠕变变形;局部高温作用下预应力混凝土梁对初始荷载较为敏感,当试件初始荷载值提升20%时,三分点位移量约增大60%。研究成果可以为实际锈蚀桥梁的防火设计和灾后应急提供理论依据。