Relaxation Of Prestress Research Articles

Research on the control limits of cable length errors of cable-strut tensile structures for prestressed relaxation under load effect

Cable-strut tensile structures (CSTS) are mainly composed of cables, and the cable length errors are bound to exist in the manufacturing of cables. The cable length errors have significant impacts on the mechanical properties of CSTS under load effects. Meanwhile, CSTS generally uses flexible membrane material as roofs, therefore cable length errors will result in the wrinkle of roofs and then cause the secondary accumulation effects of rain and snow load. Therefore, it is necessary to reasonably control the manufacturing errors of cable length. In order to solve the problem, the method of solving the control limits of cable length errors of CSTS is proposed due to the prestressed relaxation under load effects. The reliability index of CSTS is firstly given based on the reliability theory. Then, based on the reliability index and the criterion of prestress relaxation under load effect, the critical state equation and constraint inequality group are constructed, and the coefficient matrix of constraint inequality group is derived. Meanwhile, the nonlinear finite element method for solving the coefficient matrix of constraint inequality group is proposed. The procedure of solving the control limits of cable length errors of CSTS is given based on the proposed method. The crossed spoke cable-truss structure is taken as an example to verify the feasibility of the new method. The research contents provide an effective solution for controlling cable length errors of CSTS for prestressed relaxation under load effect.

Evaluation of prestress relaxation loss and harping characteristics of prestressing CFRP systems

This paper presents partial results of an investigation about the use of prestressing carbon fiber reinforced polymer (CFRP) systems in prestressed bridge girders. First, stress relaxation tests were conducted on prestressing CFRP cables and bars and continued over a period of one year from the initial prestressing process. Stress relaxation loss found to be linearly related to logarithm of the time and invariant of the length of the specimen. The effect of CFRP anchor loss was also evaluated and used to develop equations for predicting prestress relaxation in both pre- and post-tensioning applications. Then, harping applications of CFRP systems under various harping angles and deviator sizes were experimentally investigated. Higher harping capacity for CFRP cable compared to bar was observed. Premature failure of the CFRP due to localized shear/contact stresses at deviaor location redulted in a significat harping capacity reduction. To maximize the contact surface and reduce the localized shear stress, new deviators were developed, which improved the results. Finally, a finite element (FE) model was developed and calibrated to better understand the harping behavior of CFRP. By performing parametric study on different harping profiles, jacking stress limitations were obtained for the use of CFRP in practical range of harping angles.

Evaluation of Prestress Relaxation Loss and Harping Characteristics of Prestressing Cfrp Systems

Evaluation of Prestress Relaxation Loss and Harping Characteristics of Prestressing Cfrp Systems

Effects of heat treatment on mechanical properties of 3D Si3N4f/BN/Si3N4 composites by PIP

The fabrication of three-dimensional silicon nitride (Si3N4) fiber-reinforced silicon nitride matrix (3D Si3N4f/BN/Si3N4) composites with a boron nitride (BN) interphase through precursor infiltration and pyrolysis (PIP) process was reported. Heat treatment at 1000–1200 °C was used to analyze the thermal stability of the Si3N4f/BN/Si3N4 composites. It was found after heat treatment the flexural strength and fracture toughness change with a pattern that decrease first and then increase, which are 191 ± 13 MPa and 5.8 ± 0.5 MPa·m1/2 respectively for as-fabricated composites, and reach the minimum values of 138 ± 6 MPa and 3.9 ± 0.4 MPa·m1/2 respectively for composites annealed at 1100 °C. The influence mechanisms of the heat treatment on the Si3N4f/BN/Si3N4 composites include: (Ⅰ) matrix shrinkage by further ceramization that causes defects such as pores and cracks in composites, and (Ⅱ) prestress relaxation, thermal residual stress (TRS) redistribution and a better wetting at the fiber/matrix (F/M) surface that increase the interfacial bonding strength (IBS). Thus, heat treatment affects the mechanical properties of composites by changing the properties of the matrix and IBS, where the load transfer efficiency onto the fibers is fluctuating by the microstructural evolution of matrix and gradually increasing IBS.

Application of various creep analysis methods for estimating the time-dependent behavior of cracked concrete beams

A structural model is developed to predict the time-dependent behavior of cracked reinforced and partially prestressed concrete beams. The model considers creep, shrinkage, cracking, prestress relaxation and temperature. Three different methods to model the effect of creep are presented and compared, which include the effective modulus method, integral-type and rate-type formulations. The effect of temperature on both the creep and shrinkage is considered. A smeared cracking model is adopted and an iterative procedure is implemented at each time step to determine the cracked rigidities which vary through the length of the beam. The governing equations are solved using the multiple-shooting method at each time step. The validity of the proposed model is demonstrated through comparison to experimental data available in the literature. It was found that all three analysis methods provide equally reasonable prediction of the long-term behavior. A numerical study is also presented to demonstrate the capability of the model in simulating the time variation of stresses and strains in cracked concrete beams.

Fractional model for simulating the viscoelastic behavior of artificial fracture in shale gas

With the development of geothermal energy and unconventional reservoirs, shales have become the popular target formation. This kind of organic-rich or clay-rich geomaterial has significant viscoelastic behavior which may lead to severe accidents, due to the creep rupture and the relaxation of pre-stress. In order to understand the viscoelastic behavior and simulate it quickly and accurately, we have developed a novel procedure for solving viscoelastic problems based on fractional calculus. Fractional viscoelastic constitutive equations were built by introducing a springpot into the classical component models. Fractional and classical constitutive equations were compared by the fitting results of triaxial experimental data of shale samples. Fitting efficiency of fractional constitutive equation is enhanced if compared to classical equations. Besides, we also found that fractional order is directly correlated to the amount of total volume content of clay and kerogen. This implies that the physical meaning of fractional model is clearer and its extrapolation is more accurate. Fracture width creep model for unpropped fractures and closure stress relaxation model for well propped fractures were developed and analyzed. Their simulation results in terms of long-term and short-term scales differ tremendously and hence should be studied individually. The influence of lateral compliances of real shale samples on fracture width creep or closure stress relaxation is not significant and may be ignored in long-term simulations. Simulation results of closure stress relaxation could explain the abnormal proppant production, which occurs several days later than the start of natural flow-back.

Health monitoring and comparative analysis of time-dependent effect using different prediction models for self-anchored suspension bridge with extra-wide concrete girder

The structural health status of Hunan Road Bridge during its two-year service period from April 2015 to April 2017 was studied based on monitored data. The Hunan Road Bridge is the widest concrete self-anchored suspension bridge in China at present. Its structural changes and safety were evaluated using the health monitoring data, which included deformations, detailed stresses, and vibration characteristics. The influences of the single and dual effects comprising the ambient temperature changes and concrete shrinkage and creep (S&C) were analyzed based on the measured data. The ANSYS beam finite element model was established and validated by the measured bridge completion state. The comparative analyses of the prediction results of long-term concrete S&C effects were conducted using CEB-FIP 90 and B3 prediction models. The age-adjusted effective modulus method was adopted to simulate the aging behavior of concrete. Prestress relaxation was considered in the stepwise calculation. The results show that the transverse deviations of the towers are noteworthy. The spatial effect of the extra-wide girder is significant, as the compressive stress variations at the girder were uneven along the transverse direction. General increase and decrease in the girder compressive stresses were caused by seasonal ambient warming and cooling, respectively. The temperature gradient effects in the main girder were significant. Comparisons with the measured data showed that more accurate prediction results were obtained with the B3 prediction model, which can consider the concrete material parameters, than with the CEB-FIP 90 model. Significant deflection of the midspan girder in the middle region will be caused by the deviations of the cable anchoring positions at the girder ends and tower tops toward the midspan due to concrete S&C. The increase in the compressive stresses at the top plate and decrease in the stresses at the bottom plate at the middle midspan will be significant. The pre-deviations of the towers toward the sidespan and pre-lift of the midspan girder can reduce the adverse influences of concrete S&C on the structural health of the self-anchored suspension bridge with extra-wide concrete girder.

Large axial actuation of pre-stretched tubular dielectric elastomer and use of oil encapsulation to enhance dielectric breakdown strength

Rolled dielectric elastomer actuators (DEAs) are subjected to necking and non-uniform deformation upon pre-stress relaxation. Though rolled up from flat DEAs, they performed much poorer than the flat ones. Their electrically induced axial strains were previously reported as not more than 37.3%, while the flat ones produced greater than 100% strain. Often, the rolled DEAs succumb to premature breakdown before they can realize the full actuation potential like the flat ones do. This study shows that oil encapsulation, together with large hoop pre-stretch, helps single-wound rolled DEAs, which are also known as tubular DEAs, suppress premature breakdown. Consequently, the oil-encapsulated tubular DEAs can sustain higher electric fields, and thus produce larger isotonic strain and higher isometric stress change. Under isotonic testing, they sustained very high electric fields of up to 712.7 MV m−1, which is approximately 50% higher than those of the dry tubular DEAs. They produced up to 55.4% axial isotonic strain despite axially stiffening by the passive oil capsules. In addition, due to the use of large hoop pre-stretch, even the dry tubular DEAs without oil encapsulation achieved a very large axial strain of up to 84.2% compared to previous works. Under isometric testing, the oil-encapsulated tubular DEA with enhanced breakdown strength produced an axial stress change of up to nearly 0.6 MPa, which is 114% higher than that produced by the dry ones. In conclusion, the oil encapsulation and large pre-stretch help realize fuller actuation potential of tubular dielectric elastomer, which is subjected to initially non-uniform deformation.

Influence of Reinforcement Placement on the Creep of Concrete

The creep of the concrete might cause comprehensive effect on the structure, such as stress redistribution and pre-stress relaxation. To better understanding the significant influence of the creep on the structure, the restriction of the reinforcement should be taken into account. In this paper, the creep of reinforced concrete was studied based on designed experiment with different concrete mixtures and reinforcement configurations. And Dischinger model was used to analysis the experiment results. It was found that, with the increasing of reinforcement ratio, the creep of all concretes could be reduced with a trend that the difference of the creep property between concrete mixtures would be delaminated by the reinforcement. And suggestion was proposed to modify the Dischinger model to better consider the influence of firm structure formed by rebar.

Experimental study on growth of time-dependent strain in SMA reinforced PSC beams

Gradual changes occur in the internal forces and deflections of reinforced pre-stressed concrete (PSC) flexural members owing to the combined effects of concrete creep and shrinkage, and relaxation of pre-stress. For wide-scale use of PSC members in civil engineering applications (e.g. long-span bridge girders), this long-term behaviour must be minimised. The ability of shape memory alloy (SMA) bars to restrict the growth of time-dependent and thermal strains in reinforced PSC beams was investigated up to concrete age of 350 days using four laboratory-scale models. It was observed that the growth of time-dependent strain was less in the treated PSC beams reinforced with a combination of SMA and conventional steel bars than in controlled PSC beams with only conventional steel bars. Analysis of the results of the experimental beams showed that the presence of SMA bars increases the cracking moment and effective moment of inertia of the treated specimens. This makes the treated beams stiffer and restricts the growth of time-dependent strain and deflections. The experimentally obtained strain values were compared with predicted strain values of similar size and grade of plain concrete (PC) beam using model code ACI 318, and it was found that the strain in treated PSC beams was less than that in PC beams.

Unbonded Post Tensioned Concrete Slabs in Fire - Part II - Modelling Tendon Response and the Consequences of Localized Heating

This is Part II of a two part paper dealing with the current state of knowledge of the fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures. Part I provided detailed results of nineteen transient high temperature stress relaxation tests on restrained UPT tendons of realistic length and parabolic longitudinal profiles. Experimentation identified several credible concerns for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating, which may result from a number of possible causes in a real structure. The real world response of continuous UPT tendons both during and after heating is largely unknown, and is dependent on factors which are not currently accounted for either in standard fire tests or by available prescriptive design guidance. This second part of the paper presents and applies a numerical model to predict the time-temperaturestress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real concrete building in a real fire. The model is used, along with previously developed and validated computational models for heat transfer and prestress relaxation in UPT tendons, to assess existing prescriptive concrete cover requirements for UPT slabs. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, and that current prescriptive code procedures based on concrete cover alone are, in general, insufficient to prevent this. Based on the data presented it appears that minimum code prescribed concrete covers for UPT structures require revision if premature tendon rupture during fire is to be avoided.

Submerged floating tunnels (SFTs) for Norwegian fjords

Submerged floating tunnels (SFTs) weigh roughly the same as the surrounding water. The loads on the tunnel depend on the variation of the forces on the tunnel. The forces come from variation in traffic, current, temperature, waves, weight of water, weight of concrete, growth on the tunnel, wear of asphalt, dust and debris, relaxation of prestress and shrinkage and creep in the concrete. The last six variations are slow and can be counteracted by altering weights in the tunnel.All structures above sea level are subject to gravity, which tends to limit their spans. In SFTs buoyancy counteracts gravity. This speaks for longer spans, but the slope of roads limit the depth to raise ratio of downward arched SFTs. This tends to limit the free spans. In this article the design of SFTs is discussed. Finally there is a comparison between materials needed for two SFTs and a suspension bridge between Vallavik and Bu in Eidfjord in western Norway. The fjord is ∼500 m deep.It is many years since the author did serious research on submerged tunnels. He has written this contribution in the vain hope that some of his ideas on submerged floating tunnels might be of value to somebody. The author got the idea of SFTs at the technical university in Trondheim from his teacher Erik Ødegård [1]. However the idea is much older. E.J Reed, MP. applied for a patent in 1884. Up to 1968 the following Norwegian names are connected to the idea: Olsen 1923, Sam Lorgen 1968, Sverre Mo 1968.The author apologizes for not mentioning many other Norwegians who have done valuable work on SFTs. His best publication on submerged tunnels is [2]. It has a list of 32 references.

Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.

Tensile properties of the human glenoid labrum

Human fresh-frozen cadaveric glenoid labrae from 16 donors were harvested and ten of these had no gross degeneration. These ten were divided into eight equal circumferential sections. Each section was cut to produce test-samples from the core layer with a cross-section of 1 x 1 mm. Tensile testing was performed within a controlled environment unit at 37 +/- 1 degrees C and 100% relative humidity. Each test-sample was precycled to a quasi-static state to alleviate the effects of deep-freezing, prior to final testing. The tangent modulus was calculated for each test-sample before and after a 5-min period of stress relaxation and at yield. The mean elastic modulus and yield stress of the glenoid labrum were 22.8 +/- 11.4 and 2.5 +/- 2.1 MPa, respectively. The anterosuperior portion had a lower elastic modulus and lower yield stress than the inferior portion (both P < 0.02). The pre-stress relaxation tangent modulus was significantly lower than the post-stress relaxation tangent modulus for all portions of the labrum. The glenoid labrum has similar tensile material properties to articular cartilage. Its elastic modulus varies around its circumference. This suggests that the labrum may encounter different forces at different positions.

Tendon Model for Nonlinear Analysis of Prestressed Concrete Structures

A tendon model based on the finite-element method, that can represent the interaction between tendon and concrete of the prestressed concrete member, is proposed. Especially for multitendon cases, this model is capable of simulating the slip of tendon in concrete duct so it can deal with the prestressing transfer of posttensioned structures and calculate unbonded prestressed structures more efficiently. Another relevant model is also proposed to represent friction and bond at the interface of tendon and concrete. By using these models, a numerical procedure is established for material and geometric nonlinear analysis of prestressed concrete structures, including time-dependent effects due to load history, relaxation of prestress, and creep and shrinkage of concrete. The procedure can predict the responses of pretensioned and bonded or unbonded posttensioned plane concrete structures such as elastic and plastic deformation, cracking, and damage patterns throughout their prestressing transfer and service-load history. A numerical example is presented to demonstrate the validity and efficiency of the present method. Comparison of computed results with experimental ones shows good agreement.

Northumberland Strait bridge: analysis techniques and results

The Northumberland Strait at the Atlantic coast of Canada will be crossed by a 13 km bridge. The major part of the superstructure will consist of 44 spans, each of length 250 m and a box cross section of variable depth 4.5–14 m. The superstructure, divided into units of maximum length 192 m, will be produced in a yard by segmental casting and multistage prestressing. These units will be assembled at their final position on top of the piers. Each pier is composed of two pieces, also produced in the yard and connected on site by cast in situ concrete. This paper describes selected analysis problems and their solutions employed in the structural design. The analysis problems are concerned with (i) dynamic response to ice forces; (ii) movements of pier footings and stiffness of the subgrade; (iii) variation of stresses and deformations during construction and during the life of the structure, considering the effects of creep and shrinkage of concrete and relaxation of the prestressed steel; and (iv) thermal stresses. Key words: bridges, concrete, creep, prestress relaxation, segmental construction, shrinkage, strait crossing.

Effect of Creep Relaxation of Prestress on the Integrity of Axisymmetric Wire-Wound Cylindrical Pressure Vessel

Abstract The creep relaxation of prestress in a wire-wound cylindrical vessel based on power function creep law is obtained. In this analysis, it is assumed that the wire is wrapped perpendicular to the axis of the cylinder. The relaxed stress at each layer of wire at any time t is then employed to determine the capability of the cylinder to safely contain an internal pressure. The maximum allowable pressure is obtained as a function of time.

Nonlinear Analysis of Prestressed Concrete Slabs

An efficient numerical procedure for the material and geometric nonlinear analysis of reinforced and prestressed concrete slabs and panels including the time‐dependent effects due to load history, temperature history, creep, shrinkage and aging of the concrete and relaxation of prestress is developed. The procedure, based on the finite element method, is capable of predicting the response of these structures throughout their service load history as well as throughout elastic, cracking, inelastic and ultimate load ranges. An updated Lagrangian formulation is employed to take the effects of changing structural geometry on the response of planar structures into account. Slabs or panels having an arbitrary tendon layout with either pretensioned, or bonded or unbonded post‐tensioned tendons can be analyzed. Two numerical examples are presented to study the validity and applicability of the present method. The results obtained are compared with experimental and/or analytical results obtained by other investigators.

Nonlinear Analysis of Prestressed Concrete Frames

A numerical procedure based on the finite element method for the material and geometric nonlinear analysis of plane prestressed concrete frames including the time dependent effects due to load history, temperature history, creep, shrinkage and aging of concrete and relaxation of prestress is developed. The procedure is capable of predicting the response of these structures throughout their service load history as well as throughout elastic, inelastic and ultimate load ranges in one complete analysis. Time dependent variation of concrete properties is recognized. Concrete creep is evaluated by an efficient numerical procedure based on the age and temperature dependent integral formulation. Varied material properties within a frame element is accounted for by a composite concrete and reinforcing steel layer system. The contribution of prestressing steel is incorporated directly. A series of numerical examples is presented.

Some results concerning the viscoelastic relaxation of prestress in a surface rock anchor : Selvadurai, A P S Int J Rock Mech Min Sci, V16, N5, Oct 1979, P309–317

Some results concerning the viscoelastic relaxation of prestress in a surface rock anchor : Selvadurai, A P S Int J Rock Mech Min Sci, V16, N5, Oct 1979, P309–317