Thermal Prestress Research Articles

Thermal Prestress Losses in Normal- and High-Early-Strength Concrete

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Bonding Time and Prestress Loss in Precast Pretensioned Concrete during Steam Curing

During steam curing of concrete, the axial restraint of pretensioned strands causes thermal prestress loss of the strands accumulated until sufficient bonding occurs. A theoretical model is presented in this paper to predict the progressive slippage of strands with time, bonding time, and the amount of thermal prestress loss. The model incorporating the equivalent age-dependent local bond-slip relationships considers the effects of material-related factors, the layout of the prestressing system, and changes in the temperature. Three pairs of two monostrand pretensioned concrete prisms were fabricated to validate the developed model. The model was able to trace the experimentally measured progressive slippage of the strand reasonably well, and it predicted thermal prestress losses by 5.2%–5.5% of the initial prestress of 1,395 MPa for the tested prisms.

Experimental investigation and Mohr‐Coulomb plasticity modeling of glass fragments from pre‐stressed glass panes

The numerical treatment of the residual load‐bearing behaviour of laminated glasses (LG) is currently a much‐noticed topic, although there are only few approaches for an accurate representation of the experimentally observed behaviour. If one tries to characterize the load bearing behaviour, several mechanisms have to be mapped. In addition to the hyper‐viscoelastic behaviour of the interlayer and the bond between glass fragment structure and interlayer (delamination), another important aspect is the friction between the glass fragments. This interlocking of the glass fragments leads to a significant stiffening effect of the polymeric interlayer and will be examined in more detail in this paper. Therefore frame shear tests, which are common in geotechnical engineering to determine the shear strength of soils, are performed on glass fragments of different thicknesses and levels of thermal prestress. The test results are then used to calibrate a Mohr‐Coulomb material model of plasticity to represent a failure criterion. The goal is to transfer one of the decisive load‐bearing mechanisms of laminated safety‐glass (LSG) made of fully tempered glass (FTG) into a numerical model and thus contribute to the numerical calculation of the residual load‐bearing capacity.

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.

Data‐driven models for temperature distribution effects on natural frequencies and thermal prestress modeling

In vibration-based damage detection, it is fundamentally important to distinguish between changes in the dynamics of a structure and those related to environmental effects. Modal properties of structures are widely used as damage-sensitive features and indicate changes in physical properties due to structural damage or degradation. However, civil structures such as bridges are influenced by various operational and environmental conditions (such as traffic, wind, humidity, and, most importantly, temperature), and these variables also alter the modal properties of structures. Changes in modal properties caused by environmental effects can mask the effect of structural damage, resulting in unreliable structural damage assessments. It is thus fundamentally important to understand the relationship between environmental effects and modal properties. In this paper, a full-scale finite element (FE) model is used in a simulation-based study to investigate the effects of temperature distribution on natural frequencies. Two approaches for modeling the temperature effects on natural frequencies (thermal prestress imposed at an element level and temperature-dependent Young's modulus) are implemented in the FE model, and results are compared. Then, temperature profiles of the model are hypothetically simulated. These simulation results are used to develop a linear model and the three most popular machine learning models to reproduce the effects of temperature distribution on natural frequencies. Finally, a comprehensive comparison is made between the performances of these data-driven models.

Assessment of Thermal Prestress Loss with Re-Tensioning Tests

Assessment of Thermal Prestress Loss with Re-Tensioning Tests

Thermal Prestress in Composite Compliant Shell Mechanisms

This paper explores the ability to tailor the mechanical properties of composite compliant shell mechanisms, by exploiting the thermal prestress introduced during the composite laminate cure. An extension of an analytical tape spring model with composite thermal analysis is presented, and the effect of the thermal prestress is studied by means of energy landscapes for the cylindrical composite shells. Tape springs that would otherwise be monostable structures become bistable and exhibit greater ranges of low-energy twisting with thermally induced prestress. Predicted shell geometries are compared with finite element (FE) results and manufactured samples, showing good agreement between all approaches. Wider challenges around the manufacture of prestressed composite compliant mechanisms are discussed.

Modeling of Bond Stress\u2013Slip Relationships of a Strand in Concrete during Steam Curing

The restrained thermal expansion of a pretensioned strand causes thermal prestress loss during steam curing until sufficient bond strength develops. The amount of thermal prestress loss is directly related to the characteristics of the interfacial bond stress–slip relationship at different maturity phases of concrete. For a rational assessment, the bond stress–slip relationship needs to be investigated experimentally at different maturity phases. In this study, a total of 12 pull-out tests were performed using seven-wire strand of 12.7 mm diameter, at different concrete equivalent ages of 7.8, 23.5, 53.8 and 85.2 h. Based on the test results, an empirical model of the bond stress–slip relationship was developed. The model comprised four segments: a curvilinear ascending region, a constant maximum region, a linearly descending region, and a region of constant frictional bond stress. The characteristic values in the model were expressed as functions of equivalent age. The model was able to predict the test results with reasonable accuracy.

Structural Behavior of a Long-Span Partially Earth-Anchored Cable-Stayed Bridge during Installation of a Key Segment by Thermal Prestressing

This study investigated structural behavior of long-span partially earth-anchored cable-stayed bridges with a main span length of 810 m that use a new key segment closing method based on a thermal prestressing technique. A detailed construction sequence analysis matched with the free cantilever method (FCM) was performed using a three-dimensional finite element (FE) model of a partially earth-anchored cable-stayed bridge. The new method offers an effective way of connecting key segments by avoiding large movements resulting from the removal of the longitudinal restraint owing to the asymmetry of axial forces in the girders near the pylons. The new method develops new member forces through the process of heating the cantilever system before installing the key segment and cooling the system continuously after installing key segments. The resulting forces developed by the thermal process enhance the structural behavior of partially earth-anchored cable-stayed bridges owing to decreased axial forces in the girders.

Dynamic isolation systems using tunable nonlinear stiffness beams

Vibration isolation devices are required to reduce the forcing into the supporting structure or to protect sensitive equipment from base excitation. A suspension system with a low natural frequency is required to improve isolation, but with linear supports the minimum stiffness is bounded by the static stiffness required to support the equipment. However, nonlinear high-static-low-dynamic-stiffness (HSLDS) mounts may be designed, for example by combining elastic springs in particular geometries, to give the required nonlinear force-displacement characteristics. Current approaches to realise the required nonlinear characteristics are often inconvenient. Furthermore, the weight of the supported equipment, the environment, or the structural stiffness may change. This paper investigates the design of HSLDS isolation mounts using beams of tunable geometric nonlinear stiffness. In order to obtain the nonlinear response required, we first study the case of generic beams subject to static loads that are able to tune their nonlinear force-displacement characteristics to ensure that the isolators have very low dynamic stiffness. Tuning is achieved by actuators at the ends of the beams that prescribe the axial displacement and rotation. Secondly, we study a composite beam with an initial thermal pre-stress, resulting in internal stresses that give the required nonlinear response.

Experimental load-carrying capacity evaluation of thermal prestressed H beam with steel brackets

To improve the load-carrying capacity of steel structures there are mainly two methods of using cover-plate which are the preflex method and multi-stepwise thermal prestressing method. However a prestress applied to the top flange of H beam is less than the bottom flange because these two methods have small eccentricity. It is necessary to develop an advanced feature of prestressing method which efficiently increases load-carrying capacity and prestressing efficiency. Therefore this study proposes a cover-plate prestressing method with increased sectional stiffness and prestressing efficiency, called multi-stepwise thermal prestressing with steel brackets. To evaluate the prestressing efficiency and load-carrying capacity of the proposed method, prestressing application tests and static loading tests were conducted on three H beams. The multi-stepwise thermal prestressing method with the steel brackets had higher displacement induced to the H beam and higher tensile stress in the top flange than in a case without the steel brackets. Also, when prestress is applied, the tensile stress in the cover-plate significantly decreased with the installation of the steel brackets. The static loading tests showed that the installation of the brackets increased the stiffness, the yield load, and ultimate load of the structure, compared with an unreinforced girder, which meant higher load-carrying capacity than a conventional thermal prestressed structure without brackets.

A verification of simplified analysis method for thermal prestressing in continuous composite bridge

The use of thermal-prestressing method (TPSM) prevents the occurrence of tensile transverse crack in concrete decks located in negative bending moment regions of continuous composite bridges. An accurate representation of the structural response can be obtained by means of refined analyses using three-dimensional finite element models to carry out both heat transfer and thermo-elastic calculations. So, a simplified approach was proposed for easily using the TPSM effect in design to reduce the amount of computational work of the refined analysis, but its application to real structure was not confirmed. Therefore, in this study, the verification of a simplified analysis approach has been performed to explain TPSM effects in the analysis of continuous composite bridges. For this purpose, experimental results on 2-span thermal prestressed continuous composite bridge specimens were compared with simplified analysis and refined analysis results as well as analytical approach on 2-span and 3-span continuous composite bridges. From the verification results, the proposed simplified TPSM analysis methodology shows the good predictions of the thermal prestressing effect to apply design of TPSM applied continuous composite bridges.

Plate Girder Bridge Strengthened with Multi-Stepwise Thermal Prestressing Method

The strengthening of a plate girder bridge with external prestressing tendons is a commonly used method of upgrading existing bridges. Indeed, it has been known to offer advantages such behaviors as increased elastic behaviors under higher loading, increased ultimate resistance, and reduced deflection under service loads. However, this method has notable disadvantages such as stress concentration at anchorages and inefficient live load-carrying capacity. The thermal prestressing method (TPSM) for steel bridges proposed in this study uses thermal expansion and contraction of a cover-plate to provide a combination with the prestress effects of an external prestressing tendon and the section enlargement benefits of the external bonding method. In this study, the basic concepts of the proposed strengthening protocol using multi-stepwise TPSM are presented. The existence of the strengthening effect is well substantiated and the proposed analytical approach is also rigorously verified. The significant strengthening effects on a simply supported plate girder bridge are analyzed.

The Effect of Thermal Prestress on the Deformation of Micromirror Chip Embedded with Through-Silicon Vias

The thermal expansion mismatch problem for a chip due to temperature decrease from processing temperature to room temperature may cause residual stress inside the chip structure. The thermal prestress accumulated and may affect the chip reliability when the chip was subjected to the thermal loading again. In this paper, the effect of thermal prestress on the micromirror chip embedded with copper through-silicon vias (TSVs) was investigated by the finite element method. In analysis, the micromirror chip embedded with TSVs was analyzed first under thermal loading which resulted from temperature decrease between the stress free processing temperature and room temperature. This process produced a thermal prestress in the micromirror chip. The chip was then subjected to a heat source at the bottom while in operation and the heat transfer analysis was used to simulate this situation. Finally, the thermal stress analysis was carried out to obtain the deformation and the stress distribution in the chip. The results show that the thermal prestress had strong effect on the chip reliability and should be reduced as much as possible. This paper proposed a three steps analysis method to obtain the deformation and the stress distribution in the chip, in which the effect of thermal prestress on the chip reliability was evaluated effectively.

Experimental Study on Application of Thermal Prestressing Method to Continuous Composite Girder Bridges

Experimental Study on Application of Thermal Prestressing Method to Continuous Composite Girder Bridges

Analytical Study on Behavior of Creep and Drying-Shrinkage for Continuous Composite Bridges Applied by Thermal Prestressing Methods

Analytical Study on Behavior of Creep and Drying-Shrinkage for Continuous Composite Bridges Applied by Thermal Prestressing Methods

A Microresonator Design Based on Nonlinear 1 : 2 Internal Resonance in Flexural Structural Modes

A unique T-beam microresonator designed to operate on the principle of nonlinear modal interactions due to 1 : 2 internal resonance is introduced. Specifically, the T-structure is designed to have two flexural modes with natural frequencies in a 1 : 2 ratio, and the higher frequency mode autoparametrically excites the lower frequency mode through inertial quadratic nonlinearities. A Lagrangian formulation is used to model the electrostatically actuated T-beam resonator, and it includes inertial quadratic nonlinearities, cubic nonlinearities due to midplane stretching and curvature of the beam, electrostatic potential, and effects of thermal prestress. A nonlinear two-mode reduced-order model is derived using linear structural modes in desired internal resonance. The model is used to estimate static pull-in bias voltages and dynamic responses using asymptotic averaging. Nonlinear frequency responses are developed for the case of resonant actuation of a higher frequency mode. It is shown that the lower frequency flexural mode is excited for actuation levels above a certain threshold and generates response component at half the frequency of resonant actuation. The effects of damping, thermal prestress, and mass and geometric perturbations from nominal design are thoroughly discussed. Finally, experimental results for a macroscale T-beam structure are briefly described and qualitatively confirm the basic analytical predictions. The T-beam resonator shows a high sensitivity to mass perturbations and, thus, holds great potential as a radio frequency filter-mixer and mass sensor.

Basic Problems of Coupled Thermoelasticity with Thermal Relaxation and Pre-Stress: Aspects Observed in Exact and Asymptotic Solutions

Some 1- and 2-D dynamic problems of coupled thermoelasticity in an unbounded solid, half-space and slab are treated. The problems are basic, and have been addressed in the literature. This treatment considers (possibly finite) pre-stress and both Fourier and thermal relaxation models, and generates analytical results from exact solutions or inversions of asymptotic expressions for exact transform solutions. The results illustrate the role of wave speeds in solution behavior, how pre-stress affects these speeds, and how the models themselves define solution behavior, especially near wave fronts.

Multi-stepwise thermal prestressing using a cover-plate in steel structures

This paper introduces an innovative prestressing method for steel structures utilizing a multi-stepwise thermal prestressing method (M-TPSM). The proposed prestressing method is founded on a simple concept of thermal expansion and contraction of steel. A cover-plate, a high strength steel cover-plate used as a post-tensioning member in this method, is heated by electrical heaters before it is attached to the main member. After the cover-plate is attached to the main member by using high tension bolts at several points or by welding, the electromagnetic induction heaters are removed, and the resulting contraction force of the cover-plate induces eccentric axial forces to the main member. In this study, a fundamental investigation involving analytical and experimental study is carried out to evaluate the feasibility of the M-TPSM for steel structures. Also, static loading tests were performed on the rolled beam specimens for assessment of the behavior of the steel beam prestressed with M-TPSM.

Innovative Key-Segment Closing Method Using Thermal Prestressing Technique for Partially Earth-Anchored Cable-Stayed Bridges

This paper proposes an innovative key-segment closing method for partially earth-anchored cable-stayed bridges erected using free cantilever construction method, and examines its feasibility by performing a construction sequence analysis. The proposed method is based on the thermal prestressing technique, which gives an artificial heat to the girder in a cantilever state, in order to introduce the closing force required for connecting the key-segment to the adjacent segment by means of thermal expansion. The new method not only provides a feasible closing method for partially earth-anchored cable-stayed bridges, but also has a significant effect in reducing the axial force in the girders. A construction sequence analysis of an example bridge shows that the axial forces in the girders and the uplift forces at the bearings are reduced by a considerable amount due to application of the thermal prestressing method, whereas the member forces of cables and pylons are slightly changed. Therefore, it is verified from this study that the structural capacities of the girders and the bearings of the partially earth-anchored cable-stayed bridges can be effectively enhanced by application of the proposed method.