Prestressing System Research Articles

Investigation of the protective airtight shell’s operational reliability at the construction stage

The actual values of the sealed enclosure internal parameters differ from the design results, in addition, the existing calculation models do not take into account the complex distribution of prestressed reinforcement, as well as the ropes’ behavior of the prestressing system. The article discusses the issues of developing an instrumental survey program to eliminate uncertainty in assessing the technical condition of a power generating unit.

Effects of un-bonded deviators on the out-of-plane buckling of steel H-beams pre-stressed by a straight tendon cable

The out-of-plane buckling characteristics of a pre-stressed (PS) system that consists of a steel H-beam, rectilinear tendon cable, anchorages and deviators is newly reported, in which the girder is subjected to combined compressive force and bending moment introduced by the tendon. For this, new lateral-torsional buckling theories were analytically formulated and solved for the PS system having no deviator, one intermediate deviator, two deviators at regular intervals and multiple deviators. In order to verify the validity of the proposed theories, numerical examples for the PS system are given and compared with the analysis results by ABAQUS models, which are composed of thin-walled beam, rigid beam and truss cable elements. The results showed that un-bonded deviators installed externally restrain lateral and torsional deformations of the steel beam sufficiently strongly to greatly improve its lateral-torsional buckling strength.

Stress-laminated timber decks in bridges: Friction between lamellas, butt joints and pre-stressing system

Stress-laminated timber (SLT) decks in bridges are popular structural systems in bridge engineering. SLT decks are made from parallel timber beams placed side by side and pre-stressed together by means of steel rods. SLT decks can be in any length by just using displaced butt joints. The paper presents results from friction experiments performed in both grain and transverse direction with different levels of pre-stress. Numerical simulations of these experiments in addition to comparisons to full-scale experiments of SLT decks presented in literature verified the numerical model approach. Furthermore, several alternative SLT deck configurations with different amounts of butt joints and pre-stressing rod locations were modelled to study their influence on the structural properties of SLT decks. Finally, some recommendations on design of SLT bridge decks are given.

Replacement of the containment prestressing system (CPTS) when the NPP unit life extension

The paper deals with the replacement problem of the containment prestressing system (CPTS) at the extending stage of CCB life characteristics. The features of domestic CPTS used in the blocks construction in the 80s and the system developed by the French company Francine - CPTS-M used today as a replacement for the units modernization are studied. The paper describes their features and some problems associated with the monitoring organization of the unit life extension period.

Exploratory study on simple hybrid or pre-stressed steel-glass I-beams under short-term bending – Part 1: Experiments

Due to glass intrinsic brittleness and unpredictable failure behaviour, several research studies are focused on the development of safe design concepts for structural elements, component details or composite systems. In this regard, the paper aims at assessing the bending performance of a relatively novel composite solutions for glass beams, and namely presents major experimental outcomes from full-scale bending tests performed on I-shaped hybrid (HB) steel-glass beams. In accordance with earlier literature efforts, such a design concept embodies additional steel flanges to traditional glass web, and thus can provide relevant ductility to the so assembled composite systems. Even further redundancy and ductility capacities can then be obtained by introducing in the so-assembled HB beams an external pre-stressing system of cables (PSHB). Key aspects of the design concept are however represented by detailing choices, as well as to the assembly strategies, and multiple aspects should be properly taken into account. Major experimental outcomes and comparative behaviours are hence discussed in the paper, based on a series of four full-scale prototypes, with a focus on the observed global deformations, measured strains and qualitative distribution of crack patterns.

Development of a Prestressing CFRP Laminate Anchorage System and Bridge Strengthening Application

For prestressed carbon fiber reinforced polymer (CFRP) tendon anchorage systems to become well established and used on a large scale, practical requirements for structure strengthening may be met by performing a relatively easy anchorage technique using prestressing CFRP laminates. From testing performed on a clip-type CFRP laminate anchorage system developed in our research group, it was revealed that this system could achieve the anchorage efficiency and the relaxation met the requirement of specification. Furthermore, the relevant indices of the anchorage system met the prestressed system standards. A test on the load-carrying capacity of a full-scale model beam demonstrated that the load-carrying capacity of the beam increased by more than 60% after it was strengthened with the anchorage system. The prestressing CFRP laminates and the bridge structure deformed and bore stress as a composite and exhibited excellent operating performance when working together.

Full-scale test and simulation of a PBL anchorage system for suspension bridges

In suspension bridges, cable anchorage system is the critical structure for cable tension transmission. For traditional rear anchor girder anchorage system and pre-stressed anchorage system, problems such as local compression and inconvenient construction are inevitable. This paper presents the development of a novel PBL anchorage system used in the 4th Nanjing Yangtze River Bridge. This PBL anchorage system adopted grouped perfobond rib shear connectors (known as PBL shear connectors) in the independent perforated steel plate. Two full-scale specimens were extracted from the practical design. The load–slip curves and the strain developments were obtained. The results revealed the reliable load-carrying capacity and very slight residual slip after the cyclic unloading from the design load. The load carried by each row of PBL shear connectors proved the distributed load-carrying capacity. Finite-element analyses were conducted in ANSYS, which agreed well with the test results. Further parametric analyses focusing on the spacing and rows of PBL shear connectors were conducted. Based on the analysis results, the design methods of the PBL anchorage system were suggested.

A shape memory alloy torsion actuator for static blade twist

Active blade twist is an option to increase helicopter performance, for instance moving its condition from hovering to cruise. Shape memory alloys give the possibility of realizing compact devices, with high energy density. Several devices have been proposed in literature, showing limitations in terms of effectiveness and necessary room. In this article, the capability of a shape memory alloy torque tube to induce a certain twist law along the blade, while preserving its integrability within the structure, has been exploited. The study refers to a complex theoretical model, made of different specialized modules. In detail, transmitted twist action by the shape memory alloy actuators, aerodynamic effects caused by the induced geometrical change, inertial impact following the motor system integration, and system layout influence on the blade response have been taken into account. Through this model, a parametric investigation has been organized to highlight the importance of selected design variables. Tube thickness, mass, and length have been considered. Two different configurations have been initially taken into account, distinguished for the twist transmission mode and their outline. In the first hypothesis, a pre-stressed wire system converts tensile stress into a rotary action. In the second sketch, a pre-twisted solid tube connects two different stations of the blade, transmitting relative rotation. After the first trade-off, the second architecture has been selected for further analysis, focusing on its performance in terms of net transmitted twist, aerodynamic effects, while paying attention to a proper mass balance. In the chosen approach, the actuator has been installed at the torsion center. A finite element model has been used to validate the assessed analytical representation and has permitted establishing the applicability domain. Apart elastic forces, acting both in the shape memory alloys and the blade components, centrifugal forces have been taken into account by considering an increased stiffness of the reference structural element. Aerodynamic forces have been evaluated after the target configuration has been reached; helicopter trim has been considered to this purpose. The researchers aim at developing this concept by integrating the reverse action of the aerodynamic field and evaluating the importance of the actuator position along the chord. The research herein presented has been carried out within the SABRE project, project ID 723491, gratefully funded by the European Union within the Horizon 2020 program.

Innovative prestressing method for externally bonded CFRP laminates without mechanical anchorage

Strengthening of reinforced concrete (RC) structures by externally bonded carbon fiber reinforced polymer (CFRP) laminates has been widely accepted as an effective and cost-efficient method. It is well known that advantages offered by the bonded CFRP laminates can be further increased by prestressing the laminates prior to bonding. Mechanical anchors are essential, in this case, to prevent debonding since interfacial stress at areas close to the ends of the strengthening laminate is several times higher than the strength of the concrete substrate. Common anchorage solutions often consist of bolted metallic plates to clamp the prestressed CFRP laminate. Besides labor-intensive installation operation, the anchor plates are vulnerable to galvanic corrosion, which further complicates the inspection and increases the maintenance costs. There are also doubts about the long-term performance of such anchorage systems as it highly depends on the quality of the adhesive layer between the plate and laminate, and the level of pre-tension in clamping bolts.This paper presents the work conducted at Chalmers University of Technology on the development of an innovative prestressing method and a tool which allow for the application of prestressed CFRP laminates without mechanical anchors. The principles of the novel method and the prestressing system are explained. Experimental and numerical work carried out on an RC beam strengthened with this method is presented. Results indicate that CFRP laminates with high prestressing forces (approximately 30% of CFRP tensile strength) can be safely anchored without the need for mechanical anchors. Numerical results based on finite element analyses show that the proposed prestressing method can reduce the interfacial shear stresses in the CFRP-concrete adhesive joint below the bond strength with a reasonable safety margin.

Investigation of parameters of pre-stressed metal combined cable-stayed systems with hanging transport equipment

The previous tension in building structures allows the distribution of force between the elements of the structure so that in each of them, at least at one combination of loads, the actual stress is equal to or close to the calculated resistance of the material. Pre-stressed metallic combined cable-stayed systems with hanging transport equipment have been investigated. They have been calculated with optimization of distribution of forces and material according to the method and algorithm proposed by the author. Optimization has been done executed using linear programming. With the help of the computer program created by the author, calculations of metal constructions of combined cable-stayed coverings of industrial buildings with 60, 85 and 105 meters spans were performed on the permissible combinations of permanent, snow, wind and crane loads. Comparison of calculations results was carried out on maximum deformations and the actual weight of the structure, using the graphs made by the author, concerning dependence of construction coefficients of weight of the cables on their length. Recommendations are given for the calculation of such structures in accordance with the undeformed scheme, the dismemberment of the beam of rigidity on the panel, the method of coupling the beam of stiffness, pylons and risers, the number of suspension guy, the height of the pylons, the slope angle of the guy-wire, the choice of the material of the cables, The table and examples of graphs are presented, which illustrate some results for individual runs of the investigated constitutions.

Rection as a Synthesis of Reciprocal and Tensegrity Structure

There are several statements mentioning the possibilities of combining reciprocal and tensegrity structure. One of those is called reciprocal-tension (rection). This research was aimed to propose rection as a new structural system that also differs from both reciprocal and tensegrity. It defined rection structure and its characteristic, basic configuration and structural behaviour. Three simulations were conducted to test the hypothesis using dynamic relaxation method. The result showed that rection structure had the characteristics of (1) bar under bending stress, that do not touch each other, that hangs to each other reciprocally, and (2) self-equilibrated and pre-stressed system for enough rigidity. The research also concluded that the basic configurations of rection structure consist of upward fan reciprocal and positive double curvature configuration. Structural analysis also had been done on a 1:1 scale experiment and dynamic relaxation simulation.

Design of prestressed cold-formed steel beams

Structural design rules for prestressed cold-formed steel beams, considering both the prestressing and imposed vertical loading stages, are presented herein. In the proposed approach, the cold-formed steel member is designed as a beam-column using linear interaction equations in conjunction with the Direct Strength Method (DSM), while the prestressed cable is designed by ensuring that its tensile capacity is not violated during the two loading stages. In the present paper, the design approach and the failure criteria, which define the permissible design zone for the prestressed system, are first introduced. The suitability of the design recommendations is then assessed by comparing a set of parametric finite element (FE) results for several combinations of prestress levels, beam geometries and cable sizes, with the corresponding design predictions. Finally, following reliability analysis, the implementation of the design recommendations is illustrated through a practical worked example.

Steel columns strengthened/reinforced by prestressed CFRP strips: Concepts and behaviors under axial compressive loads

A novel strengthening method using prestressed (PS) carbon fiber reinforced polymer (CFRP) strips was proposed. This method can be used for repairing existing steel columns or constructing new columns for high-rise buildings, power transmission towers and other buildings. Specifically, a prestressing system (PS system) can be used to prestress CFRP strips without using a hydraulic jack. A steel column, PS CFRP strips and the PS system comprise a self-balanced system that offers much greater stiffness against overall buckling than a pure steel column. To verify this strengthening method, axial compression tests are conducted. The results show that the buckling load of the PS-strengthened specimen is 2.5 times greater than that of an unstrengthened specimen, and the failure mode ranges between the first order symmetric and the second order asymmetric overall buckling mode. In addition, finite element analysis (FEA) was adopted to obtain the optimal prestress value, which can also be used as a guide in design. Finally, a simple calculation method of axial bearing capacity was proposed.

Sposoby wzmocnienia dźwigarów żelbetowych

In the article, strengthening of precast reinforced concrete girders with span of 15 m by using prestressed CFRP stripes is described. These girders (together with columns) represent the main structural elements of the canopy by the industrial hall. The necessity to perform the strengthening was caused by excessive deflections of these elements, being a result of rheological processes in the concrete – shrinkage and creep. Possible ways of executing the strengthening by using CFRP strips were analyzed and compared, results of calculations of deflections of the girders before and after their external prestressing were described and the technology of the chosen prestressing system was presented.

Frost heaving of foundation pit for seasonal permafrost areas

Frost heaving can cause support structures to crack and even instability of the foundation pit. This paper describes the frost heaving features of the steel pile pre-stressed tendon composite foundation pit support system (SPPTCFPSS). As a combined rigid–flexible support system during the overwintering stage in Northeast China, these systems were used to investigate the transient heat conduction and fixed boundary one-dimensional frost heaving stress equations. The axial force sensors of the tendons used for the in-situ test accurately recorded the changing values of the axial forces of the pre-stressed tendons during the integrated working period for the foundation pit frost-heaving effect. Practical support data for the frost heaving stress analysis of the system were thus provided. The thermo-physical properties were obtained from the soil experiments, including the coefficient of thermal conductivity, specific heat of the foundation soil, and thermal expansion factor, among others. Base on this, the fluid effective velocity, saturation, and temperature fields were received from the heat flow coupling analysis of finite element methods (FEM).The results show that the actual axial force applied to the SPPTCFPSS is approximately equal to the theoretical value of the pit frost-heaving force calculated for the one-dimensional fixed boundary conditions corrected by saturation index from FEM. The SPPTCFPSS can adapt to a large-scale frost-heaving deformation to enable a reasonable increase in adaptive capacity in a region that has seasonal periods of frozen soil.

Nonlinear deformation analysis for precast pre-stressed concrete beam systems

The structural systems such as road bridge beam systems (span superstructure) or beam systems used in high-rise buildings with precast pre-stressed reinforced concrete might be subjected to the load higher than the calculated load of the design process. In the process of operation the internal forces of the beams reach to a certain value, the section stiffness in elements will be changed. The change in section stiffness will lead to redistribution of internal forces in the whole system. However, that has not been included in the design process. The paper presents nonlinear deformation analysis of precast pre-stressed beam system used in road bridge span superstructure or high-rise buildings using nonlinear model of materials according to Russian construction codes to build the relationship between internal forces and section stiffness. From the calculation results, some comments will be recommended about increased load capacity in comparison with the results obtained from analysis using linear model of material.

Forced Vibration of the Non-Homogeneously Pre-Stressed System Consisting of the Hollow Cylinder and Surrounding Medium

Forced Vibration of the Non-Homogeneously Pre-Stressed System Consisting of the Hollow Cylinder and Surrounding Medium

Human-induced vibration serviceability of arch pre-stressed concrete truss system

Human-induced vibration has become a serious serviceability problem due to the larger opening of girder, lighter floor systems and longer spans designed and used in practice. Vibration tests were undertaken in laboratory to research the vibrational characteristics of the arch pre-stressed concrete truss (APT) system spanning 16.0 m. Results from ambient vibration, impulse excitations (heel-drop and jumping) and steady-state incentives (walking and running) were presented. Dynamic characteristics such as natural frequencies, damping ratios, mode shapes and acceleration responses were studied and checked against the existing codes. Experimental results show that the investigated APT girder possesses high fundamental frequency and low damping ratio. Moreover, the perception factors based on the root-mean-square acceleration, vibration dose value (VDV) and psychological comfort data were obtained. Lastly, the threshold accelerations and VDVs were suggested for evaluating the human-induced vibration.

Flat prestressed unbonded retrofit system for strengthening of existing metallic I-Girders

In this study, a novel retrofitting system is developed to strengthen existing metallic I-girders using prestressed unbonded carbon fiber reinforced polymer (CFRP) plates. The system relies on a pair of mechanical clamps. Each clamp holds two CFRP plates (each having cross-sectional dimensions of 50 × 1.4 mm) and anchors their prestressing forces to a metallic I-girder via friction. A finite element (FE) model was established to optimize the design of the required mechanical components, and a set of pull-off tests was performed to evaluate the capacity of the optimized system. The proposed flat prestressed unbonded retrofit (FPUR) system was then applied on a 6.4-m-long steel I-beam, and the excellent performance of the system, in terms of stress reduction in the beam bottom flange, was confirmed based on the results of a set of static four-point bending tests. Moreover, a fatigue four-point bending test was conducted on the steel I-beam, strengthened using the proposed FPUR system with a prestressing level of 53% (i.e., approximately 1120 MPa prestress in the CFRPs). Experimental results of the high-cycle fatigue test confirmed the excellent fatigue performance of the proposed FPUR system, as no prestress loss was observed after 20 million fatigue cycles. An analytical model is proposed to accurately predict the stress state in an I-girder strengthened with the proposed FPUR system, while design recommendations are provided concerning the practical use of the system. Relying on the laboratory experimental test results, the proposed FPUR system was used to strengthen the metallic cross-girders of a 121-year-old bridge in Australia.

Experimental and numerical studies on the vibration serviceability of fanshaped prestressed concrete floor

An extensive experimental and numerical research was undertaken to study the vibration serviceability of a long-span fanshaped prestressed concrete floor system to be used in the lounge of a major airport. Specifically heel-drop and jumping impact tests were conducted to obtain the natural frequencies and modal damping ratios of the floor system, followed by the discussion on the distribution of peak accelerations. Running tests and simulation were also performed to capture the acceleration responses. Moreover, the finite element method was used to evaluate the dynamic characteristics. The floor system is found to have a low fundamental frequency (≈6.00 Hz) and the corresponding modal damping ratio (average ≈2.20%). The comparison of the experimental results with the published American institute of steel construction (AISC) design guide indicates that the prestressed concrete floor system exhibits satisfactory vibration perceptibility overall. The study results also show that the intensity and the location of impact excitation have a significant influence on the rate of acceleration decay. A crest factor βrp is proposed based on the test results to calculate the maximum root mean square acceleration for running for convenience.