Prestress Level Research Articles

Cracking Analysis of Pre-stressed Steel–concrete Composite Girder at Negative Moment Zone

In order to analyze the influence position of prestressed reinforcement on the cracking load of composite girders, scale test and finite element analysis is used to study the cracking behavior of prestressed composite beam at the negative moment zone. The load–deflection curve and the cracking law of the scaled beam are obtained from the experimental results. The influence law of the resultant point and the spacing change through the symmetrical axis of the prestressed beam on the cracking load is obtained through the finite element parametric analysis. Furthermore, the prototype beam's mechanical performance in terms of the scaled test data is verified by both similarity theory and the prototype beam's numerical model. The results show that: Under different prestress levels, the cracking load is larger when the resultant force point of prestressed reinforcement on one side of the concrete slab's symmetric axis is within 0.15 ~ 0.50 times of slab half-width. In addition, the cracking moment of the specimen can be increased up to 11% by optimizing the prestress arrangement.

Optimization for energy absorption of 3-dimensional tensegrity lattice with truncated octahedral units

Tensegrity structures have recently found new applications as energy absorbers in the field of aerospace engineering. In this study, we introduce an optimization method to maximize the energy absorption capability of a 3D tensegrity lattice composed of truncated octahedral units. The stored strain energy of the tensegrity lattice subjected to forced vertical displacement is maximized under constraints on the stress, structural material volume, and volume occupied by the lattice. The force density of the bar that defines the shape of each unit is regarded as a parameter, and the design variables are the cross-sectional areas of cables and bars as well as the level of prestresses on the lattice. Buckling of the bars is extensively utilized to significantly increase energy absorption. The nonlinear behavior of bars allowing buckling is modeled as a smoothed function of bi-linear elastic material, in which imperfection in the bar shape is also taken into account. Numerical examples show that tensegrity lattices, with flexibility in the vertical direction and adequate (shear) stiffness in the horizontal direction, can be obtained by solving the optimization problem. It is also shown that energy absorption by the structure scales cubically when its sizes and prestress level are uniformly scaled.

Stress Intensity Factor Assessment for the Reinforcement of Cracked Steel Plates Using Prestressed or Non-Prestressed Adhesively Bonded CFRP.

The use of adhesively bonded carbon fiber reinforced polymer (CFRP) materials to reinforce cracked steel elements has gained widespread acceptance in order to extend the lifespan of metallic structures. This allows an important reduction of the stress intensity factor (SIF) at the crack tip and thus a significant increase of the fatigue life. This paper deals with the assessment of the SIF for repaired cracked steel plates, using semi-empirical analysis and finite element analysis. Metallic plates with only one crack originating from a center hole were investigated. Virtual crack closure technique (VCCT) was used to define and evaluate the stress intensity factor at crack tip. The obtained modeling results are compared with experimental investigations led by the authors for different reinforcement configurations including symmetrical and non-symmetrical reinforcement, normal modulus and ultra-high-modulus CFRP plates, and pre-stressed CFRP plates. Results show that finite element model (FEM) analysis can obviously simulate the fatigue performance of the CFRP bonded steel plates with different reinforcement configurations. Moreover, a parametric analysis of the influence of the pre-stressing level was also conducted. The results show that an increase of the pre-stressing level results in an increase of the fatigue life of the element.

Dynamic response analysis of bridge precast segment piers under vehicle collision

In recent years, the safety of the bridge structure has been challenge due to the vehicle collision on the pier, and the accident of vehicle collision has caused a lot of economic losses. With the development of precast bridge structure, the dynamic performance of precast segment pier has attracted extensive attention. The dynamic responses and failure modes of bridge precast segment piers under vehicle collision were investigated in this work. Firstly, the numerical model of precast segment column under impact loads was established, and its reliability was verified. Secondly, the numerical models of bridge precast segment pier and cast-in-place pier were established to analyze differences of dynamic performance under vehicle collision. The failure modes, impact force and displacement of bridge precast segment pier and cast-in-place pier were studied in detail. Finally, the factors of impact velocity, concrete strength, pre-stressing level and number of segments were considered respectively. The numerical results show that the damage of precast segment pier is smaller than that of cast-in-place pier under vehicle collision, and the peak impact force of precast segment pier is slightly lower than that of cast-in-place pier. The pre-stressing level and number of segments have no obvious effect on impact force of precast segment pier under vehicle collision. The relative displacements between precast segments under vehicle collision need to be taken into account in design.

Experiment on crack resistance of prestressed CFRP tendons-steelreinforced concrete members under eccentric tension

In our country, there are many eccentric tension members, such as steel reinforced concrete truss transfer layer straining beam, low-rise building corner column. For these members, large-scale cracks easily appear during normal use stage. Aiming at these problems, this paper took the light, high-strength and anti-corrosion carbon fiber reinforced polymer composite (CFRP) tendons as the prestressed reinforcement, the prestressed CFRP-steel reinforced concrete structure system was designed by combining CFRP tendons with prestressed steel reinforced concrete, which can effectively restrain the crack. And on this basis, a systematic study on the crack resistance under eccentric tension was carried out. A total of eleven members were designed and fabricated with the prestress level, eccentricity, longitudinal reinforcement diameter and profile flange thickness as the main parameters, and eccentric loading was achieved through the self-developed tension-compression conversion truss system. The results indicate that the crack resistance of the prestressed CFRP tendons-steel/concrete members is greatly strengthened. Compared with the ordinary eccentric members, the cracking loads of the large tension members with prestressed CFRP tendons are increased by 64.8%–102.3%, the cracking loads of the small tension members with prestressed CFRP tendons are increased by 61.7%–117%. The crack resistance of the member is positively related to the prestress level, longitudinal reinforcement diameter and steel flange thickness, and negatively related to the eccentricity. With reference to the combined structural design specification, three positions of the neutral axis of the prestressed CFRP tendons-steel reinforced concrete members during the cracking stage are proposed, and the calculation formula of cracking load is derived. Compared with the test value, the agreement is perfect, which can provide a reference for the application of other composites in the prestressed eccentric tension system.

Numerical study of fatigue life of SMA/CFRP patches retrofitted to central-cracked steel plates

This study aims to evaluate the repair performance of central-notched steel plates reinforced by the self-prestressing shape memory alloy carbon fiber reinforced polymer patching system (SMA/CFRP patch) through the numerical and theoretical study of the fatigue life after experimental fatigue life results were validated with the numerical method. These methods were used to determine the effectiveness of the reinforcing technique. Different widths of SMA/CFRP patches, elastic modulus of CFRP, prestress level, initial damage degree and crack type were considered as parameters that influenced the repair effectiveness. The outcomes showed that the increase in the Young’s modulus, prestress level and width could significantly improve the fatigue life of central-notched steel plates. The effect of the prestress level was more prominent with wider width. Good agreement with the test data revealed that numerical and theoretical methods could predict the fatigue life with reasonable accuracy, and the strengthening was more effective for edge-cracked specimens than central-cracked specimens.

Experimental study on shear performance of RC beams strengthened with NSM CFRP prestressed concrete prisms

This paper presents an experimental investigation of the shear performance of RC beams strengthened with near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) prestressed concrete prisms (PCPs). The shear behaviour of strengthened beams can be affected by several design variables. In this research, the effect of the following parameters were considered: the prestress level, inclination and spacing of the CFRP-PCPs, and material type of the prism. The control beam had conventional shear steel reinforcement only while the other seven beams were shear strengthened with CFRP-PCPs by varying design parameters mentioned above. All the beams were tested under monotonic loading until they reached the failure load. The experimental results showed that the NSM CFRP-PCPs strengthening technique improves the shear performance of the beams effectively. The strengthened beams that applied the CFRP-PCPs at an inclination of 45° were more efficient in improving the shear capacity compared to vertical CFRP-PCPs. The shear capacity and deformation were enhanced with the increase of prestressing levels of CFRP rods and the decrease of CFRP-PCPs spacing. The failure modes of the strengthened beams were influenced mainly by the spacing and the inclination of the CFRP-PCPs. Moreover, the material type of the prism had little influence on the effectiveness of shear strengthening. The analytical model presented was developed to estimate the shear contribution of NSM CFRP-PCPs and the model was found to predict the shear capacity of the tested beams well.

Experimental and numerical investigations on glass fragments: shear-frame testing and calibration of Mohr\u2013Coulomb plasticity model

The numerical treatment of the residual load-bearing behavior of laminated glasses (LG) in the post-fractured state is highly topical. Nevertheless, currently only few numerical approaches for an accurate representation of the experimentally observed behavior are existent. In order to model the characteristics of the load-bearing behavior of glass laminates in the post-fractured state, the behavior of the interlayer, the behavior of the glass fragments as well as the bonding between glass and interlayer need to be characterized correctly. This paper focuses on the modeling of the frictional contacts between the glass fragments itself. In order to allow for the calibration of failure criteria for the fractured glass particles, framed shear tests which are a common experimental technique in geomechanical testing to determine the shear strength of soils, are performed on glass fragments of different thicknesses and levels of thermal pre-stress. The test results are subsequently used to calibrate non-associated Mohr–Coulomb criteria, which are widely applied to the description of failure and frictional sliding of soils, to the experimental data of four distinct kinds of glass fragments. The obtained parameters of the Mohr–Coulomb models are in magnitude similar to the parameters of standard soils such as sand or gravel. The experimental data further show, that the Mohr–Coulomb model in general can be used to approximate the stress failure plane of the glass fragments but lacks for capturing correctly the plastic volumetric strains (dilation) in Finite Element modelling. Numerical investigations by the Finite Element method showed, that it is possible to reproduce experimental data by using Mohr–Coulomb plasticity models and hence the numerical models are validated for further investigations.

Minimal mass design of active tensegrity structures

Tensegrity structures have been widely utilized as lightweight structures due to their high stiffness-to-mass and strength-to-mass ratios. Minimal mass design of tensegrity structures subject to external loads and specific constraints (e.g., member yielding and buckling) has been intensively studied. However, all the existing studies focus on passive tensegrity structures, i.e., the structural members cannot change their lengths actively and the structure has to passively resist external loads. An active tensegrity structure equipped with actuators can actively adapt its internal forces and nodal positions and thus can actively resist external loads. Therefore, it is expected that active tensegrity structures use less material compared to passive tensegrity structures thus leading to a smaller mass. Due to the integration of the active control system, the design of active tensegrity structures is different from passive tensegrity structures. This study proposes a general approach for the design of minimal mass active tensegrity structures based on a mixed integer programming scheme, in which the member cross-sectional areas, prestress, actuator layout and control strategies (i.e., actuator length changes) are designed simultaneously. The member cross-sectional areas, prestress level, and actuator control strategies are treated as continuous variables and the actuator layout is treated as a binary variable. The equilibrium condition, member yielding, cable slackness, strut buckling, and the limitations on the nodal displacements as well as other practical requirements are formulated as constraints. Three typical active tensegrity structures are designed through the proposed approach and the results are benchmarked with the equivalent minimal mass passive designs. It is illustrated that the active designs can significantly decrease the material consumption compared with the equivalent passive designs thus leading to more lightweight tensegrity structures.

Self-centering mechanism and seismic response of steel tension-only concentrically braced beam-through frames

Tension-only concentrically braced beam-through frames (TCBBFs) are a newly proposed structural system that has great potential in damage mitigation for seismic design of low-rise steel buildings. In TCBBFs, columns usually have cold-formed hollow structural section (HSS) and are connected to the flanges of H-shaped beams at each floor level as an easy main frame assembly, and tension-only braces are installed to the main frame to provide additional lateral stiffness. The braces also serve as the replaceable fuse elements that contain the self-centering and energy dissipation properties of the system. This paper investigates the self-centering mechanism, structural behavior, and seismic performance of TCBBFs through integrated analytical and experimental studies. Particularly, shaking table tests were conducted to investigate the seismic response of TCBBFs with tension-only braces designed with four different methods. The effects of yielding, prestress level, and design methods of the braces on structural response are evaluated. The experimental results show that TCBBFs are capable of archiving uniform story drift response with self-centering ability upon unloading. Due to the self-centering mechanism of the system, the tension-only braces can be easily replaced and the main frame can be reused after the shaking of major earthquakes.

Experimental study on the flexural behavior of RC beams strengthened with prestressed BFRP laminates

In this study, the flexural behavior of RC beams strengthened with prestressed basalt FRP (BFRP) laminates was investigated. Compared with the conventional carbon FRP (CFRP) laminate, BFRP exhibits a larger failure strain but lower modulus, in addition to its high strength and prominent creep rupture behavior; this makes BFRP a suitable option for cost-effective strengthening of existing beams. Nine specimens were prepared to investigate the effects of the FRP type, laminate thickness, strengthening method, reinforcement ratio, prestressing level, and adhesive on the flexural behavior of RC beams. The differences in the failure mode, L-D curve, cracking behavior, and strain development of concrete, steel, and FRP laminate were analyzed. The results demonstrated that specimen strengthened with BFRP laminate could achieve 85% of the loading capacity of the specimen strengthened with CFRP laminate under the same laminate section and but ensured a superior crack control effect and ductility. Meanwhile, a higher prestressing level led to a higher material utilization of BFRP, and the prestressing method lowered the reinforcement ratio effectively. Moreover, the bonded system had a better crack control effect and higher laminate strength utilization ratio than the unbonded system did.

Pre-stressed steel strips for the strengthening of axially loaded masonry walls

The current paper presents an experimental investigation on the working mechanism of pre-stressed steel strips, used for the strengthening of brick masonry walls when subjected to axial loads. Three strengthening schemes were used, involving vertical and in some cases horizontal strips, with or without pre-stress, installed onto the walls through binding bolts. A total of nine unreinforced masonry walls were prepared and tested up to failure, by using the level of pre-stress on the steel strips as well as the axial preload level on the walls, as parameters. The test results showed that pre-compression on the vertical strips increases the crack initiation and the crack distribution loads by percentages as high as 60%, and 50% respectively, whereas the level of preloading, not resulting in the initiation of cracking, do not play a significant role in the overall behavior of the walls. Pre-tension on the horizontal steel strips on the other hand, controls the behavior of masonry at the ultimate limit state and increases the axial load bearing capacity of the walls by almost 43%. Moreover, the pre-tension on the horizontal strips, enhances the lateral support conditions of the vertical counterparts and changes the post-buckling behavior from unstable to stable. In general, the intervention scheme which involved pre-stressing in the horizontal strips performed better, in terms of the ultimate and the serviceability limit states. Based on the observed behavior of the strengthened walls, a design procedure is proposed with the use of a failure criterion for bi-axially compressed anisotropic masonry.

Effect of the chloride environmental exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates

Carbon fibre-reinforced polymer (CFRP) plates are widely used to strengthen reinforced-concrete (RC) structures. At present, most studies focused on the short-term performance of strengthened RC beams with a prestressed CFRP plate, and very limited information is available on their durability, which is of importance in certain cases, such as high-piled wharfs and bridges in marine environments. This study investigates the effects of chloride environment exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates. In total, seven RC beams, four of which were strengthened by a self-anchored prestressed CFRP plate, were fabricated and tested under three-point bending. The testing parameters included 25% and 40% prestress level of the ultimate tensile strength of the CFRP plate, 5% and 10% tensile steel rebar corrosion ratio of mass loss and 10% and 20% anchor corrosion ratio in the chloride environment. The evolution of time-dependent prestress losses in the strengthened beam was determined, and the flexural behaviour of the strengthened beams was analysed. A theoretical model is proposed to predict the flexural strength of prestressed CFRP strengthened RC beams in a chloride environment. The experimental results indicated that the coupling of the corrosion expansion of the metal anchor and rebar in the chloride environment had a large negative effect on the flexural performance of the beams strengthened by the self-anchored prestressed CFRP plates. After exposure to the chloride ion solution, the failure mode of the RC beams strengthened by the prestressed CFRP changed from concrete crushing in the compression section to anchor pullout. The proposed model, which considered the effect of a chloride environment to predict the flexural strength of the strengthened beams, agreed well with the test results.

RC beams strengthened by prestressed CFRP plate subjected to sustained loading and continuous wetting condition: Time-dependent prestress loss

Prestress loss of CFRP during prestressing and service stage is one of the critical technical issues in concrete structures rehabilitation. In this work, the prestress loss of reinforced concrete (RC) beams strengthened by a prestressed CFRP plate subjected to sustained loading and continuous wetting condition were experimentally studied. Eight RC beams strengthened by prestressed CFRP plate were prepared with different prestress levels of 20% and 40% respectively. The initial prestress loss was monitored up to 28 days and the loss ratios of all specimens were around 5%, which was dominated by slippage of the end anchorage system. Then, the specimens were sustainedly loaded at two levels under wet or dried environments for 170 days. The average prestress loss ratios during this period were all less than 2%, which indicates that the CFRP plate end anchorage system had good long-term performance. Moreover, the adhesive bonding was weakened by the moisture penetration, and the specimens with lower prestress levels were more susceptible to the wet condition. Nevertheless, regardless of the environmental exposure, the maximum crack widths of 40% prestressed specimens remained less than 0.2 mm even subjected to high sustained load level, which is within the limitation of the prestressed concrete structures.

Numerical investigation of the parameters influencing the behavior of dapped end prefabricated concrete purlins with and without CFRP strengthening

Dapped end prefabricated concrete purlins (PCPs) suffer from shear damages due to coating, snow, wind load and also their dead loads. In this present study, a series of numerical modelling is performed with the aid of finite element program ABAQUS in order to investigate this behavior of PCPs through parametric study. In addition to the mechanical properties of the PCPs, the strengthening of the PCPs with the help of carbon fiber reinforced polymers (CFRP) is considered as a parameter in the models. Pursuant to this aim, longitudinal steel reinforcement ratio, shear friction reinforcement ratio, bending reinforcement ratio, suspension reinforcement ratio, concrete and steel mechanical properties, pre-stressing level, CFRP ply orientation, the number of CFRP plies and material properties of CFRP composite were selected as the parameters. First of all, numerical models were verified using experimental test data of three PCP test specimens and five CFRP coupon tests. Later, a total of 50 different numerical models was created to investigate the behavior of PCPs thoroughly. Vertical load- displacements curves are compared and explained in detail. The result of the parametric study revealed that the effects of the parameters related to reinforced concrete except longitudinal reinforcement and material properties of concrete are very limited when compared to the effects of the parameters related to CFRP. The effect of FRP ply orientation is the most effective parameter that increases significantly the shear capacity of PCPs. More importantly is the general FRP layout is proposed to delay or prevent shear cracks for the beams and the proposed layout is proved through numerical analyses. [±45°] fiber orientation is recommended to use to prevent shear damage.

Parametric Analysis of Tensegrity Plate-Like Structures: Part 2—Quantitative Analysis

The study includes a parametric analysis of a group of tensegrity plate-like structures built with modified Quartex modules. The quantitative assessment, including the calculation of the structure’s response to constant loads, was carried out. A static parametric analysis was performed, with particular emphasis on the influence of the initial prestress level on the displacements, the effort, and the stiffness of the structure. A geometrical non-linear model was used in the analysis. A reliable assessment required introducing a parameter for determining the influence of the initial prestress level on the overall stiffness of the structure at a given load. The stiffness of the structure was found to depend not only on the geometry and material properties, but also on the initial prestress level and external load. The results show that the effect of the initial prestress on the overall stiffness of the structure is greater with less load and that the effect of load is most significant with low pre-stressing forces. The analysis demonstrates that the control of static parameters is possible only when infinitesimal mechanisms occur in the structure.

Corrosion Characteristics of Anchor Cables in Electrolytic Corrosion Test and the Applicability of the Test Method in Study of Anchor Cable Corrosion

The selection of corrosion test method in the corrosion study of the prestressed anchors is an important issue. In this paper, the corrosion test of anchors was conducted with electrolytic corrosion test method. The corrosion characteristics of the anchor cables were examined. The effects of sodium chloride solution concentration, current, test time, and prestress level on corrosion were studied. The applicability of electrolytic corrosion method in anchor cable corrosion study is discussed subsequently. The results show that the corrosion of the anchor appears to be uniform corrosion. With the corrosion of the anchor, the central wire of the cable was basically not corroded, and the cross-sectional shape of the outer wire changes from a round to fan shape. The sodium chloride concentration and prestress level have no obvious effects on the corrosion of the anchor. The variation of test time does not affect the difference between the measured and theoretical calculated results, while a proper current in the electrolysis test may help reduce the difference. The measured corrosion rate fluctuates from −4% to 10% and tends to be higher compared with calculated results based on Faraday’s law. The study indicates that the electrolytic corrosion test is applicable in the anchor corrosion study.

Investigating the behaviour of steel end-plate connections with shape memory alloy bolts

Conventional extended end-plate connections with normal high-strength steel (HSS) bolts show considerable residual deformation following an earthquake. The application of shape memory alloys (SMAs) has recently been considered in seismic regions because of their capabilities, such as the ability to tolerate cyclic deformations and dissipate energy. In this paper, extended end-plate connections equipped with SMA bolts are investigated. A hybrid extended end-plate connection utilising SMA bolts only for the outer rows and conventional HSS bolts for the remaining rows is a new and viable alternative to existing connections. Finite-element models of connections with different bolt lengths and pre-stressing levels have been developed using Ansys software. The cyclic performance of the models has been assessed and compared. Results indicated that conventional and hybrid connections have better capabilities in energy absorption compared to SMA connections. The hybrid connections and also connections with 69 mm long SMA bolts are classified as high-ductility connections according to the American Institute of Steel Construction (AISC). Increasing pre-stressing over 50% fy has no significant effect on the cyclic performance of connections.

Static and dynamic camber behavior of uniaxial and biaxial post-tensioned fully, limited, and partially unbonded pre-stressed concrete slab

Pre-stressed members in various structures are gaining popularity among engineers in many parts of the world because pre-stressed strands offer better stability, serviceability, economy, aesthetics, and structural efficiency. The profile of the strand greatly influences the tensile strength of concrete. The force exerted by the strand on the concrete counterbalances internal tensile forces. A construction engineer’s principal goal is to build an excellent strength structure without sacrificing agility and cost-effectiveness. This study’s main objective is to model numerically different pre-stressed concrete slabs to understand and predict the upward deflection (camber) behavior of uniaxial and biaxial pre-stressing of unbonded concrete strands in the static and dynamic behavior and the maximum moments of pre-stressed concrete members by considering previous experimental work as a benchmark for validation. Particular emphasis was placed on the unbonded post-tensioned pre-stressed slab parameters that influence the mid-span upward deflections and internal moments in linear and nonlinear crack analysis. This study also investigated the effect of strand profiles, strand areas, number of strands, strand eccentricities, loading types, and level. It looked into full and partial pre-stressing with uniaxial and biaxial pre-stressing directions. The numerical dynamic characteristics in terms of members’ natural frequency with such parameters were found. This study used a finite element numerical model for the analysis of linear and cracked sections and concluded that the upward deflection (camber) of uniaxial and biaxial one-way and two-way partially unbounded pre-stressed concrete slabs is affected by the strand’s profile, area, and number and eccentricity; the loading type and value; and the pre-stressing level in static and dynamic analyses.

Seismic details for exterior connections of post-tensioned flat slabs to steel columns using steel plates, vertical stiffeners and bolts

One of the commonly used structural systems in buildings worldwide includes flat slabs supported by steel columns. A key issue in these systems is ductility of the connection between slab and column. Common details for nonprestressed slabs to steel column joints are recommended by well-known references, but specific details for post-tensioned slabs are rarely reported. In this study, specific details have been proposed for connections of post-tensioned flat slabs to steel columns. The details include top and bottom steel plates, vertical stiffeners, and post-tensioned high-strength bolts. To examine the details, a half-scale specimen is subjected to reversed-cyclic loading with increasing amplitude. The test shows that the connection remains elastic up to a drift of 1%, exhibits stable nonlinear response up to a drift of 3.5%, and could tolerate relatively large lateral drifts up to 6% without losing gravity load capacity. The test also indicates that prestressing level, gravity shear force magnitude, and dimensions of horizontal steel plates are important factors that affect seismic behavior of the proposed connection.