Post-tensioning System Research Articles

Performance of double skin FRP-concrete-steel self-centered segmental bridge piers subjected to forward-directivity near-fault ground motion

This paper investigates the damping energy and behavior of a proposed damage-resistant self-centering unbonded post-tensioned (PT) bridge pier system for accelerated bridge construction. The system is investigated with and without the addition of external energy dissipaters using two piers. The cross-section of each pier was a double-skin consisting of two tubes: outside glass fiber reinforced polymer (GFRP) tube and an inside steel tube. Self-consolidating concrete (SCC) was poured in between. The piers system has the benefits of accelerating construction using segments, self-centering capability due to rocking, and high energy dissipation from the steel bars. The piers were tested under ascending intensities of scaled ground motions. A near-fault pulse-like motion was chosen to examine the velocity impact effect on the piers. After being subjected to a series of ground motions of up to 250% of the design earthquake (DE), the piers had self-centering capability with almost no residual drift and no noticeable damage. The peak drift was 8.85%. The rocking motion reveals a complex induced dynamic behavior, in which a small change in the system can cause chaotic and butterfly effects. Hence, the energy of the system was investigated to better comprehend the dynamic behavior of rocking under forced vibrations. Two analytical models were developed to simulate the static and dynamic behaviors of the self-centering unbonded post-tensioned bridge piers.

A Case Study on Two-Span Post-Tensioned Concrete Bridge Decks with Different Span Lengths and Investigation on Prestressing Tendons with Comparisons

Post-Tensioned (PT) concrete system is one of the widely used bridge superstructure types because of its better load capacity performance. By increasing load carrying capacities on conventional reinforced concrete structures, PT is considered as an advanced technology in engineering. The aim of this paper is to study on finite element analysis of two-span PT concrete bridge decks with different span lengths. For three different span lengths, two-span bridge decks were investigated. To reduce the self-weight of the deck, at mid spans, deck section was used as voided section and at diaphragm regions, section was used as filled. Two-dimensional Finite Element (FE) deck models were created by frame elements. The FE software package Midas Civil was used. Permanent, transient and time-dependent loading types were considered. Boundary conditions were defined with their real mechanical properties. To balance the deck section stresses, prestressing tendons were used. By iterations, PT tendons layout and types were studied. Interaction between span lengths and PT tendons areas was investigated. Also, different effects on PT tendons were studied and relevant comparisons were submitted. The “best” tendon using was examined for the investigated deck sections. Concluded that optimization on PT tendon areas at bridge deck sections is possible by investigating the main effects on it. Results show that a good optimization enables us to have the optimal PT tendon using with lower cost.

Rapid heat-activated post-tensioning of damaged reinforced concrete girders with unbonded near-surface mounted (NSM) NiTiNb shape-memory alloy wires

Concrete girders can suffer from serviceability issues due to excessive cracking and deflection. In response to this problem, a novel heat-induced post-tensioning technique utilizing unbonded near-surface mounted nickel–titanium–niobium (NiTiNb) shape-memory alloy (SMA) wires was proposed and evaluated. SMAs are a class of smart materials that can recover seemingly permanent plastic deformation when heated. The post-tensioning forces, thus, can be generated by restrained heat-induced shape recovery of SMA wires. Material characterization tests showed that 3.92-mm diameter NiTiNb wires with 2.5% prestrain can generate recovery stress of approximately 500 MPa when actuated via Ohmic heating in a restrained condition. The proposed post-tensioning system was experimentally evaluated in reinforced concrete girders measuring 2.3 m in length and 23 × 41 cm in cross-sectional dimensions. The girders were initially cracked to simulate typical girder damage. NiTiNb wires were then installed in the bottom cover of the girders and anchored at both ends. Subsequently, the wires were actuated via Ohmic heating, which triggered shape-recovery and generated post-tensioning stresses in the girder. The post-tensioning technique reduced the crack widths by up to 74% (370 μm) and recovered the residual midspan deflection by up to 49% (1.52 mm) in the cracked girders. Following post-tensioning, flexural loading up to failure showed that the cracked stiffness and ultimate moment capacity of the girders had increased by up to 31% and 45%, respectively, with a relatively small NiTiNb reinforcement ratio of up to 0.17%.

Dynamic response of post-tensioned rocking structures with inerters

Post-tensioned rocking systems have proved to be highly effective in controlling structural damage during strong ground motions. However, recent events have highlighted the importance of looking at both the structural and non-structural components within a holistic framework. In this context, the high rotations and accelerations associated with the rocking motion can cause significant non-structural damage and affect the performance and functionality of the entire system. In this paper, we analytically examine the fundamental dynamics of post-tensioned rocking structures and investigate the benefits of using supplemental rotational inertia to reduce their seismic demands and improve their overall performance. The newly proposed strategy employs inerters, a mechanical device that develops a resisting force proportional to the relative acceleration between its terminals. Analyses conducted for a wide range of acceleration pulses and real pulse-like ground motions show that post-tensioned structures equipped with inerters consistently experience lower demands and have reduced probabilities of exceeding limit states typically associated with damage. Importantly, the new vibration control strategy advanced in this paper opens the door for an expedient modification of the fundamental dynamic response of rocking systems without altering their geometry.

Material-based models for post-tensioned shear wall system with energy dissipators

The Prestressed-Laminated timber system, Pres-Lam, developed in New Zealand, is a low damage innovative wood-hybrid system that utilizes post-tensioned (PT) mass timber components along with various types of energy dissipators. In addition to testing, numerical modelling is another important approach used to investigate the structural performance of Pres-Lam system. In this study, a material-based modelling method was proposed to investigate the seismic performance of Pres-Lam cross-laminated timber (CLT) wall system. Correspondingly, specific material-based numerical models were developed for PT-only and Pres-Lam CLT walls in general purpose finite element software package, ABAQUS. Only the material and geometrical properties of panels, connections and energy dissipators are required as inputs for the material-based models. These models are different from other current simplified numerical models by the fact that they can eliminate the time- and resource-consuming wall tests for the models’ calibration, and also can predict the structural performance of the Pres-Lam CLT wall system to complete failure which is challenging to obtain by testing. The response of the developed models agreed well with the experimental results of all tested wall configurations. The influence of PT force level, aspect ratio of the wall panel, and spacing and number of energy dissipating devices on the response of the system was investigated using the developed models. The material-based models provide structural engineers and researchers with a cost-effective method for the analysis and design of the Pres-Lam CLT shear walls. The analysis results will form the basis for developing future design guidelines for PT-only and Pres-Lam mass timber systems under in-plane lateral loads.

Structural strengthening of square spread footings using circular external prestressing

Footing enlargement method is widely used to increase the load carrying capacity of obsolete spread footings. The traditional method involves addition of new concrete and dowel bars, along with splicing of steel bars, which is labor intensive and impractical. This paper presents a new strengthening system that utilizes an unbonded post-tensioning system. The original square footing was turned into a circular footing, using ordinary reinforced concrete. The new system is more applicable, as the connections are achieved by the confinement actions provided by the prestressing strands. To investigate the proposed system, finite element models were built and analyzed in ABAQUS. The original as well as all strengthened footings failed in punching shear, and the proposed system proved to be efficient in improving the punching shear capacity. The investigated parameters include amount of prestressing force, shear-span to depth ratio, eccentricity of strands, flexural reinforcement ratio, and notch length. The finite element results suggest that the amount of prestressing force had the most significant influence on the punching shear behavior of strengthened footing. For original spread footings, the empirical model for punching shear presented in Eurocode 2 (acri = d/2) was found to serve as a good analytical model, as the predictions from this model yielded the best agreements with the FEA results. For the strengthened footings, an analytical model was proposed by modifying the empirical model adopted by Eurocode 2. A new factor η was introduced, which reflects the effects of all investigated parameters.

Anchor Gallery Designs for Tainter Gate Active Anchorage Systems

This paper reviews the design of accessible anchor galleries that allow the inspection, testing, and replacement of post-tensioned trunnion girder anchorage systems. Various means of egress were reviewed along with features to remove drainage, prevent inundation, and provide air circulation. An elastic analysis of an example gallery was performed and discussion provided regarding reinforcement detailing, safety features, and constructability.

ABAQUS modeling for post-tensioned reinforced concrete beams

Finite element analysis (FEA) using ABAQUS software was performed to investigate the behavior of post-tensioned concrete beams. This study is an attempt to examine the concrete damage behavior using a concrete damaged plasticity (CDP) model in ABAQUS, as well as the effect of an external post-tensioning (EPT) steel rod system. Concrete is a well-used material in many architectural and civil structures, with behavior exhibiting different characteristics in compression and tension; it also shows an inelastic-nonlinear behavior. These properties of concrete make modeling or simulation of the material difficult. In reinforced concrete, there is particular difficulty with respect to the bond-slip relationship between concrete and steel. However, in this paper the finite element analysis for concrete beams through ABAQUS simulation has been carried out with some assumptions, including perfect bond of steel and concrete and the CDP model for concrete property. In comparing analysis and experimental results, the simulated tensile deformations are similar to actual crack patterns in tests and the analytical responses such as strength, deflection, and stress of external rods are in good agreement with the measured responses.

A CASE STUDY ON PRE-TENSIONING & POST TENSIONING SYSTEMS OF A PRESTRESSED CONCRETE

In Present days the pretensioning and post tensioning systems are very popularly using in the major constructions of a structure. The Pre-tensioning and Post-tensioning both methods are used under pre-stressing process. In which has few edges over the orthodox non-stressed structures like greater span to depth ratio, higher moment and shear capacity. These methods are generally adopted in the making of PSC girders, sleepers, Bridges, Slabs in buildings, Concrete Pile, Repair and Rehabilitations, Nuclear Power Plant etc.

Development and implementation of CFRP post-tensioning system for bridge strengthening

The paper presents development, testing and on-site implementation of a new CFRP (Carbon Fibre Reinforced Polymer) post-tensioning system for flexural strengthening. Main elements of the system include the tensioning device and steel anchorages mounted on both ends of a single CFRP strip. Development of the system itself and the results of a system efficiency examination on full-scale beams in the lab have been briefly described in the paper. After laboratory testing, the system has been implemented on site to strengthen two beam bridges, built of reinforced concrete and steel. Bridge load tests before and after strengthening have been carried out to assess system efficiency.

Rigid-block analysis of archeological elements retrofitted with external post-tensioning system

In Europe, many archeological sites require continuous maintenance interventions capable of preserving the cultural value of this type of heritage. To this end, particular attention must be paid to ruins, the remains of larger portions and archeological artifacts. These elements, generally made of ancient stone masonry materials are vulnerable to external action such as are earthquakes, differential settlements, degradation, etc.. In order to reduce their structural vulnerability, repair interventions are usually required using repair techniques that preserve the artistic and aesthetical values of the archeological element. One of these techniques is the use of external post-tensioning system through pre-stressed cables.In this contribution, some examples of the application of post-tensioning to an archeological element are presented. The application of rigid-block analysis to archeological elements (retrofitted with external post-tensioning system) is developed and discussed.

Magnetic flux leakage technique to detect loss in metallic area in external post-tensioning systems

Post-tensioned girders with internal and external tendons are widely used for the construction of bridges because of their eminent advantages. However, owing to poor grout quality, damage to protective ducts, exposure to environmental conditions, overstressing of strands, and overloading of the girder, the external tendons are susceptible to corrosion and breakage of strands. It is important to identify loss in cross-sectional area of the strands, in order to take preventive actions to avoid loss in capacity, and in some cases even failure of the tendon. Magnetic Flux Leakage (MFL), a non-destructive evaluation technique, is used to detect and quantify metallic defects in external post-tensioning systems. In order to evaluate this technique in in-service bridges under field conditions, a full-scale post-tensioned girder specimen was constructed as a control specimen. Metal defects that include section loss, corrosion, and breakage in wire and single or multiple strands with known loss in cross-sectional area, were fabricated and placed in the external tendons. The MFL device is then used to evaluate loss in metallic area (LMA) in the control girder specimen. The MFL device consistently detects LMA greater than 5% of the total strand cross-sectional area for the group of strands in a tendon. The measurements are found to be repeatable and reproducible. Even though there are errors associated with the estimated LMA percentages, especially for defects with greater than 50% loss in cross-sectional area, the estimated magnitude of the loss in total strand cross-sectional area gives a good indication about the severity of the defect.

Static pushover response of spring anchored unbonded post-tensioned rocking systems

This paper describes the static pushover response of a proposed lateral force resisting system that uses spring anchored post tensioning (PT) tendon to create a rocking system. The spring at the PT tendon anchorage is aimed at facilitating manual post tensioning of the PT tendon which helps to implement rocking systems in developing communities subjected to high seismic hazards without the need for trained workers or specialized equipment. The reduction in the lateral stiffness of this system at high intensity loads causes rocking behavior that is initiated by a gap opening at the base of the system. The PT tendon is unbonded over the length of the system to prevent yielding and the loss of prestressing force. Because the prestress force in the unbonded PT tendon is maintained, the lateral force resisting system exhibits self centering behavior. This paper derives closed form expressions that describe the lateral force deformation behavior of the proposed system. A simple finite element analytical model is presented to verify the closed form expressions and to study the effects of the system parameters on the static pushover response. Results of this study show that the closed form expressions display an excellent agreement with the analytical model including the predictions of the gap opening, clamping of the spring element, yielding and fracture of the PT tendon.

Seismic demand assessment of self-centering steel plate shear walls

In previous studies, a post-tensioned rocking system has been introduced to steel plate shear wall (SPSW) system as a means for reducing its residual drifts. The success of this method and the performance of self-centering SPSWs (SC-SPSWs) is evaluated, in this study, by nonlinear dynamic analysis of previously-designed structures. For this purpose, a numerical model is developed and validated against experimental results. This model is, then, used for representing five 3- and 9-story SC-SPSWs that reflect various design assumptions. The median and dispersion of various response parameters are then extracted using a set of 20 ground motion records scaled to three different intensity levels. A comparative assessment is next performed between response of the studied structures in presence and absence of the rocking system. This assessment reveals that the rocking system is not effective in reducing the residual deformations of SPSW system. Nevertheless, the rocking behavior is shown to effectively mitigate the ductility demand at the beams' end portion and provide a more uniform distribution of plasticity along beams and within web plates of different stories.

Analytical and numerical study of curved-base rocking walls

In light of the socio-economic impact of earthquakes, several novel lateral force resisting systems have recently been developed to improve the seismic performance and resilience of structures. One promising class of systems, and the focus of this work, are post-tensioned rocking systems. Despite the many benefits of rocking systems (e.g. small residual drift and simplified repairs after extreme events), several additional opportunities for improved performance remain. This paper introduces the concept of curved-base rocking walls, a redesign over traditional rectangular profile walls, and explores the potential benefits of this simple geometric modification. Analytical equations are developed to study the static and dynamic behavior of curved-base rocking walls in concert with gravity frames. These equations also provide insight into the parameter sensitivities and identify key components that drive the behavior. A finite element modeling approach is also proposed for modeling curved-base rocking structures. Finally, a preliminary investigation of how curved-base rocking walls may improve rocking wall performance and how the modifications effect the nonlinear dynamic response is presented.

포스트텐션-프리캐스트 압착구조 시스템 접합부의 횡방향 구조거동

포스트텐션공법은 프리캐스트 콘크리트 접합부의 일체성을 획기적으로 향상시킬 수 있다. 이 연구에서는 슬래브 유무, 다발 또는 단일 강연선 적용, 그라우팅 여부, 포스트텐션 도입길이를 변수로 한 실대형 크기의 포스트텐션-프리캐스트 압착구조 접합부 실험체를 제작하여 반복가력실험을 수행하였다. 모든 실험체들의 최대강도는 구조기준(KCI 2012)에서 제시하는 설계강도를 만족하는 것으로 나타났다. 또한 ACI 374.1-05에서 제시하는 허용기준(acceptance criteria)을 이용하여 각 실험체별 성능 검증을 수행하였으며, 1개를 제외한 모든 실험체에서 허용기준을 만족하는 것을 확인하였다.

Systematic Assessment of Nondestructive Evaluation Techniques for Post-Tensioning and Stay Cable Systems

Nondestructive evaluation (NDE) can provide invaluable information about the condition of tendons in post-tensioned and cable-stayed bridges, which can facilitate preventative actions to mitigate further deterioration. To date, the use of nondestructive testing methods for routine maintenance of bridges has been relatively low, partly due to limitations of available technologies but mainly owing to a lack of know-how regarding existing NDE techniques and guidelines. This study aims to develop guidelines to assist bridge owners and engineers in selecting the most appropriate NDE method or combination of methods for assessing the condition of in-service post-tensioning and stay cable systems. An extensive experimental study is conducted on a full-scale post-tensioned girder specimen and four large-scale stay cable specimens to evaluate the performance of various NDE methods in detecting location and severity of numerous strand and grout defects. Decision metrics are developed considering multiple criteria such as precision, accuracy, ease of use, inspection requirements, and cost. The decision metrics are intended to facilitate selecting the most appropriate NDE method for assessing a particular defect condition. Overall scores for the tested NDE methods in identifying various defect conditions in different parts of the post-tensioning and stay cable systems are calculated using a weighted sum model. These scores are used to develop a list of ranked NDE methods for evaluating a particular defect condition within a specific location and for a specific duct material.

Capacity Design Procedure for Rocking Braced Frames Using Modified Modal Superposition Method

Rocking braced frames (RBF) are innovative structural systems that maintain near-zero residual drift through controlled rocking and a self-centering posttensioning system. The braced frame members are designed to remain elastic, with inelasticity concentrated in energy-dissipating elements. Recently developed capacity design procedures are reviewed and compared with a newly proposed method, called the modified modal superposition (MMS) method, for a set of seven RBFs using nonlinear time-history analyses. The proposed design method employs a structural model whose boundary conditions mimic the rocked configuration of the structure and reduction of the forces associated with the first-mode response determined by the base moment demand at maximum considered earthquake (MCE). The proposed MMS procedure is shown to be equally or more accurate than other capacity design procedures in estimating story shear forces and brace and column axial forces in the RBFs. A reliability study supports applying a load amplification factor of 1.3 to the MMS force demands to meet reliability targets for capacity designed force-controlled members under MCE hazard demands.

Hysteretic behavior investigation of self-centering double-column rocking piers for seismic resilience

Seismic resilient structures with low structural damage and self-centering behavior after an earthquake are focus issues of current earthquake engineering. Unbonded pretensioned rocking columns offer advantages for accelerated bridge construction in seismic regions, which utilize column-uplifting mechanisms, high strength post-tensioning, and replaceable energy dissipation devices to enhance seismic performance and resilience. The experiment of three 1:3 scale unbonded, post-tensioned rocking bridge bents with different types of external replaceable energy dissipation devices subjected to quasi-static loading protocols, were carried out according to practical engineering demand in high-intensity earthquake regions. An improved analytical model was proposed for predicting the force-displacement relationship of the post-tensioned rocking bridge bent systems considering the neutral axis depth. Minimal physical damage was observed for the post-tensioned rocking bridge bent systems, which exhibit good energy dissipation and self-centering behavior. Especially, the external replaceable buckling-restrained plates dissipaters can be easily replaced if severely damaged subjected to higher than expected ground motions. The satisfactory analytical and experimental comparisons are presented as a validation of the reliability for the high-performance seismic-resistant bridge bent systems and the modelling techniques in this study.

Novel Laser and Post-Tensioned Wire-Based System for Short-Term and Long-Term Monitoring Deflections in Bridges

The monitoring of static vertical deflections on bridges is still a first-level technological challenge. These data are very useful to understand and to assess the behavior of structural elements in civil engineering, especially for the case of long-term monitoring. The reason is that the material degradation processes and the changes of the mechanical properties of the structure due to aging (because of, for example, creep and shrinkage in concrete and composite bridges) have a direct impact on the exhibited static vertical deflections. This is even a greater challenge in case of high-span high-rise bridges. This paper introduces and evaluates an approach to measure displacements of structures using an original system specially designed to monitor the vertical deflection in high-span high-rise bridges and viaducts, called PTW system. This idea is based on the post-tension technology and its principles, in combination with non-contact laser displacement transducers. In this paper, the conceptual design of the device is described. Additionally, one short-term field testing, carried out to validate the system, is presented and discussed. The results demonstrate that the proposed system is robust, accurate, and reliable, and also inexpensive, which are essential requirements for field implementation.