Post-tensioning System Research Articles

Risk-Based Selection of Inspection Method for External Post-Tensioning System of Bridges

The increasing complexity associated with the maintenance of bridges with post-tensioning tendons, along with growing public awareness to ensure higher levels of safety in bridges, has put additional pressure on the designers and the owners to find innovative solutions to ensure safe as well as economically viable solutions. Risk-based inspection and maintenance helps in finding such solutions and, thus, it is gaining more importance in the field of infrastructure management. Within the framework of current risk-based inspection methodologies, it is normally assumed that the method by which the inspection is performed is known beforehand. However, the selection of the inspection method by itself should be given importance and viewed as the first key step for any inspection. The lack of quantitative data in the initiation step makes this selection uncertain and the decision making rather subjective. Despite recent release of comprehensive reports and other publications on condition assessment of bridges with post-tensioning systems, a quantitative approach and a decision-making framework for the selection of the inspection method and associated protocol are still missing, and the inspection strategy and methods are determined purely by the experience of the inspector or the owner. In this paper, a simple and structured risk-based selection methodology is presented that can bridge the existing knowledge gap. The proposed methodology uses a statistical approach to quantify the likelihood of the inspection error utilizing a variety of applicable NDE (Non-destructive Evaluation) methods. To give the methodology both accuracy and practicality, the specifications for the national bridge inventory (SNBI) condition rating was incorporated in this methodology and the accuracy of the inspection methods are measured against determining the correct SNBI condition. Application and effectiveness of the proposed methodology are demonstrated using a case study inspection conducted earlier by the authors. The results, in this case, converged to the selection of one of the NDE methods, which consequently was accepted by the bridge stakeholders.

Effect of partition walls on the seismic response of mass-timber buildings with a post-tensioned rocking wall system

The structural-nonstructural interaction effects of nonstructural partition walls and post-tensioned CLT rocking walls in mass-timber buildings were evaluated in a parametric study. Representative 2D rocking wall units in 5-story and 12-story mass-timber archetype buildings were modeled in OpenSees. Moreover, concentrated spring models were developed to represent effective force-deformation for four different variations of partition wall detailing, and applied to the building models to represent wall densities associated with apartment and hospitality occupancies. Eigenvalue, pushover, and response-history analyses were performed on 2D models of the bare structure and those with partition wall variations. Including partition walls with fixed connection details was found to decrease story drifts and rocking wall story shears compared to the bare frame model by up to 50% in a service earthquake and up to 30% in a maximum considered earthquake. However, partition walls with deformation-compatible details were found to have negligible influence (less than 5%) on the response.

State-of-the-Art of Posttensioned Rocking Bridge Substructure Systems

Posttensioned bridge rocking systems have emerged as an alternative to traditional ductility design where substructure elements, such as columns and piles, are designed to dissipate energy through the formation of localized plastic hinges. Posttensioned rocking bridge systems have evolved from pure rocking structures to hybrid solutions that combine self-centering and dissipating capabilities, often in the form of central posttensioning and dissipation devices, respectively. Posttensioned rocking bridges provided a resilient solution with little to no damage or disruptions after a seismic event. For this reason, there has been increased interest amongst bridge asset managers and researchers in the development and application of these “low-damage” systems. This paper presents a state-of-the-art review of the evolution of posttensioned rocking bridge substructure systems, with an emphasis on column and joint details to reduce or eliminate local damage in bridge piers. This review also includes energy dissipation solutions and examples of operational bridges that have adopted posttensioned rocking column systems. This paper concludes with a comprehensive discussion that covers knowledge gaps and suggestions for future research.

Improved performance-based plastic design method for post-tensioned connection systems

Performance-based plastic design (PBPD) was initially proposed as a method for retrofitting existing structures. The PBPD method gradually became a general approach for designing new structures. This method is a direct method to obtain design base shear and leads to design structures being more economical compared to traditional methods. PT connections were developed after the Northridge earthquake. The self-centering could restore the connection to its original position without damage to the beams and columns. High-strength post-tensioned cables are used to restore the structure to its initial position in PT connections. Also, energy dissipaters (for example, top and bottom angles) are used to increase ductility. In this study, a proposed relationship between the ductility (µ), period (T), and response reduction factor (R) is presented for structures with pinching or flag-shaped behavior, and the effect of the parameters on R has been investigated. Also, a new design approach for structures with PT-steel connections with top-and-bottom angles is presented. The results show that R primarily depends on the post-yield stiffness ratio (α), the yield stress ratio (or energy dissipation) (β), ductility (µ), soil type, etc. Nonlinear time-history analysis indicates that designed models based on the proposed approach satisfy drift limits. The results show that the proposed relationship can be quite suitable for designing PT connections using the PBPD approach.

Research on seismic behavior of unbonded post-tensioned concrete wall with vertical energy-dissipating connections

An unbonded post-tensioned (PT) concrete wall system with vertical energy-dissipating connections (UPTW-VEC) is proposed that consists of precast wall panels, several X-shaped metal dampers (XMD), mild steel energy-dissipating bars (ED bars) and PT tendons. In this wall system, the precast wall panels are assembled through XMDs along the vertical joints and ED bars along the horizonal joints to provide the energy-dissipating capacity. The PT tendons installed at the wall toes are used to provide the self-centering capacity. In addition, each wall toe is protected by a steel jacket from the concrete corners spalling or crushing. Firstly, the hysteretic behavior of XMD was studied experimentally. Then, the seismic behavior of the unbonded post-tensioning concrete wall with different parameters were investigated experimentally based on four specimens. It was shown that the wall panels were resilient after tests and the cracks were only observed near the embedded plate in wall panels. In addition, a simplified analysis method was proposed, and the theoretical results matched well with the experimental results. Finally, the finite element analysis (FEA) model of UPTW-VEC was established. Following validating through the experimental results, a parametric study was conducted to investigate the influence of critical parameters on the seismic behavior of UPTW-VEC.

Seismic behavior of dual buckling-restrained steel braced frame with eccentric configuration and post-tensioned frame system

The buckling-restrained steel braced frames with eccentric configurations (BRBFEs) have high energy dissipation capacities and an architectural advantage to create openings. However, recent investigations have shown that after the earthquake at the MCE hazard level, the residual drift in buildings with buckling-restrained steel braced frames (BRBF) is remarkable. In the present study, to reduce the residual drift in a BRBFE system, it has been suggested to add a post-tensioned frame (PTF) in parallel and create a BRBFE-PTF combined system. Moreover, a method has been proposed to design this combined system. In this method, the values of the two parameters of the self-centering ratio and cable cross-section area are selected arbitrarily. Then, after verifying the modeling, the pushover, cyclic, and time-history analyses of the prepared numerical models were performed. The results showed that if appropriate values were selected for the self-centering ratio and cable cross-section area, the proposed system had the desirable seismic performance in maximum drift demand and residual drift.

Cyclic response of CLT Post-Tensioned Walls: Experimental and numerical investigation

This paper presents the main outcomes of full-scale cyclic tests on CLT post-tensioned walls. Investigated systems were constituted by rocking dissipative timber walls, whose main features were the re-centering of walls, low damage in case of seismic actions, additional hysteretic dissipation, and easy replacement of dampers. Referring to previous experimental campaign on LVL and CLT structures available in literature, the proposed post-tensioned CLT wall system was conceived employing an innovative and replaceable damping system and realizing all the technological details in order to minimize local and global damages due to horizontal cyclic loading. Two kinds of post-tensioned systems were tested: single wall and double walls with different set-up configurations and patterns of the dampers. Specifically, two different levels of initial post-tensioning force were applied and different patterns of damper system were experimentally investigated in order to define the energy dissipation contribution provided by the individual damper types installed. These different configurations were investigated by using the same CLT wall system which, despite having been subjected to repeated cyclic tests, showed limited residual damage and therefore fulfilled the low-damage requirement.In order to properly design the experimental set-up, an advanced Finite Element nonlinear numerical model was developed in the OpenSEES framework and then calibrated on the local response of the individual components of the system. Model calibration is presented and the comparison with the experimental results is critically discussed both in terms of local and global response of the PT-wall systems. The proposed model well captured the hysteretic behaviour of the wall system highlighting the reliability of the modelling strategy.

Numerical modelling and analysis of post-tensioned timber beam-column joint

As a part of sustainable development initiatives, post-tensioned timber structural systems have been developed in recent years with the intention of implementation in seismically active regions. Ductility is achieved through rocking connections between members and optional steel yielding elements. Numerical investigations on such systems have been mostly limited to macro-modeling to avoid complexities arising from the wood anisotropy. This paper examines the lateral cyclic behavior of post-tensioned Laminated Veneer Lumber (LVL) beam-column subassemblies using three-dimensional continuum finite element (FE) modeling. The results of a testing program on full-scale interior joints with unreinforced and steel reinforced configurations at the connection interface are presented. A description of the pre-processing and solution of the FE models, including material, geometric and contact nonlinearities, element mesh, and analysis is provided. The FE results are then validated against the experiments through a verification study. Focus is placed upon the anisotropy of the material as well as the difficulties emerging from performing the nonlinear analysis for such systems.

Seismic Behaviour of Spring Anchored Unbonded Post-Tensioned Rocking Systems

ABSTRACT The construction of low damage, self-centering, unbonded post-tensioned seismic systems requires special techniques and equipment such as hydraulic jacks to stress tendons that may not be readily available in developing communities. The recently proposed spring anchored rocking system (SARS) uses a spring at the anchorage of the post-tensioning tendon to facilitate applying force to the tendons using simple techniques such as torque wrenches. The seismic behaviour of the SARS is investigated using finite element models developed in OpenSees. The self-centering lateral force resisting system of a 3-story timber structure was anchored with springs having four different configurations, and subjected to a set of 22-pair ground motions. The finite element models consisted of elastic beam column elements to model the frame members, nonlinear truss element to model the post-tensioning tendon, spring element to model the anchorage of the tendon, and gap element to model the spring closure. It is found that the spring at the post-tensioning anchorage introduces flexibility, reduces the tendon force, and generally increases the rigid body deformations of the SARS compared to conventionally anchored systems using nuts set on bearing plates.

Design of prestressed, jointed columns for enhanced seismic performance

Jointed, prestressed columns have been shown to enable accelerated construction and to offer superior seismic performance, as compared with cast-in-place columns without prestressing. During an earthquake, such columns deform primarily through concentrated rotations at the joints, and special detailing at the ends of the columns prevents crushing and spalling. Unbonded prestressing steel, running vertically through the columns, provides a restoring moment that returns the columns to plumb after the ground motion stops. The use of prefabricated substructure elements speeds up the bridge assembly by eliminating building formwork, fixing steel, casting and curing concrete on-site. A variety of detailing strategies have been developed for jointed, prestressed columns, but their fundamental behavior is similar. More importantly, this behavior differs from that of conventional reinforced concrete columns for which current seismic code provisions were developed. This paper develops a displacement-based procedure for designing jointed, prestressed columns that is based on the framework provided by the AASHTO Guide Specifications for LRFD Seismic Bridge Design. To account for the unique characteristics of these systems, the design procedure introduces new performance criteria; it also provides recommendations for proportioning the prestressed and non-prestressed steel, calculating the effective column stiffness, and estimating the column displacement capacity. The focus of the paper is on columns with pretensioned strands and internal energy dissipaters, although the procedure would work for post-tensioned systems and those with external dissipaters as well. This design approach is critically evaluated through comparison to the results of both cyclic tests of cantilever columns and shaking table tests of a two-span bridge system.

Evaluation of Chloride Intrusion along Concrete–Grout Interfaces for Post-Tensioned Concrete Durability

A pour-back is a critical component of a post-tensioned concrete system, affecting its durability by protecting anchorages and vents from corrosion. However, the material interface between the grout pour-back and the concrete member may serve as an accelerated pathway for chloride ion intrusion and may lead to premature corrosion of post-tensioning components. This study evaluates the vulnerability of concrete–grout interfaces through two experimental protocols: a novel electrical migration test and the standard electrical resistivity test. The results are used to predict the service life of pour-back systems using a concrete life-cycle assessment model. This research serves as a first step toward defining the chloride permeability of concrete–grout interfaces.

Post-tensioning of glass beams: Analytical determination of the allowable pre-load

The effectiveness of post-tensioning in enhancing the fracture resistance of glass beams depends on the level of compressive pre-stress introduced at the glass edge surface that will in service be exposed to tensile stresses induced by bending. Maximum pre-load that can be applied in a post-tensioned glass beam system, yielding maximum compressive pre-stress, is limited by various failure mechanisms which might occur during post-tensioning. In this paper, failure mechanisms are identified for a post-tensioned glass beam system with a flat stainless steel tendon adhesively bonded at the bottom glass edge, including the rupture of the tendon, glass failure in tension and adhesive/glass failure in the load introduction zone. Special attention is given to the load introduction failure given that the transparent nature of glass limits the use of vertical confinement usually applied in concrete. An analytical model for determination of the allowable pre-load in post-tensioned glass beams is proposed, based on the model applied for externally post-tensioned concrete beams. The model is verified with the results of a numerical model, showing good correlation, and applied in a parametric study to determine the influence of various beam parameters on the effectiveness of post-tensioning glass beams.

High-performance cementitious grout with fly ash for corrosion protection of post-tensioned concrete structures

In order to provide corrosion protection of tendons in post-tensioned (PT) concrete systems, the congested interstitial spaces between the strands and duct are ideally to be filled with cementitious grouts. However, many currently used grouts do not have the desired fluidity and resistance to segregation – leading to formation of voids, frothy grout materials, and premature corrosion of strands. This paper presents a 3-phase study on the development of high-performance, pre-blended grouts. Several grouts were formulated with various proportions of ordinary portland cement (OPC), fly ash and chemical admixtures. The fluidity, bleed (i.e., standard, wick-induced, pressure-induced, and -inclined tube), softgrout, rheological, and shrinkage parameters of the formulated grouts were assessed; based on which a systematic selection of grouts was performed in three phases. Finally, a grout with 52 and 48% of OPC and fly ash, respectively (by volume) and optimal dosages of chemical admixtures was formulated and pre-blended in an industrial facility. This grout satisfies a set of performance specifications on fluidity, resistance against segregation (say, bleedwater and softgrout formations), and shrinkage. Therefore, the grout is capable of filling the ducts and providing long-term corrosion protection of PT strands.

Cost Evaluation of Post-tensioned Slabs in Multi-storey Buildings Considering Seismic Effect

Currently, many owners and consultants prefer to use post-tensioned PT slab systems in multi-storey buildings due to their considerable savings in materials, labor, and time. On the other hand, many contractors still use the traditional RC flat slab systems due to their simple construction process. This research focuses on the comparison between the two systems taking into consideration: cost, time, and structural behavior. Detailed cost and time analysis is performed for each system considering all construction resources. The analysis and design of the buildings are conducted using many structural and project management computer programs. These include ETABS, SAFE, RAM, REVIT, and Primavera. The results indicated that, PT systems provide significant savings for spans larger than 6 m. The structural analysis of both systems also showed that PT slabs are more effective in resisting seismic straining actions.

Anchorage System in Old Post-Tensioned Precast Bridges

Abstract The first precast post-tensioned bridges in Slovakia are approaching 60 years of their service life. Facing an aging infrastructure, it is necessary to assess their present structural condition and residual life expectancy. Different types and methods of anchorage system were developed and available for first post-tensioned bridges around the world. The reliability and functionality of anchorages contribute to the proper behaviour of post-tensioning system. The considerable effect of aggressive environment (frost, chloride salts, humidity) and environmental load (CO2) leads to corrosion of anchorages and prestressing steel with the risk of a sudden unexpected damage to bridge. The research work deals into finding more details of this structural part. During the demolition of damaged post-tensioned bridges built in 1960s several anchorages were removed and analysed. The material properties, size parameters and performed tensile tests of a wedge anchorage system are summarised in this paper. The findings from this study can be used as input parameters for numerical simulation and durability assessment of similar aged existing post-tensioned bridges.

Cobiax and post-tension slabs: A comparative study based on the construction duration time and cost

Slabs are one of the most basic components concerning the cost of the structure, because of the number of building materials utilized. It is of incentive to think about the current technologies available, meaning to make these slabs cost less or even improve its behaviour. Cobiax and post-tension slabs systems show up as an appropriate alternative. In light of this reason, this paper focuses on demonstrating the advantages and disadvantages of both Cobiax and post-tension systems. Nowadays, these systems are very popular all around the world. It is very necessary to have a wide understanding of the differences in cost and the construction duration between these two systems and have good knowledge about the advantages and disadvantages of both types. Firstly, the properties of these types of slabs have been reported and the advantages and disadvantages for both systems and then the cost and time has been compared, which are the most important factors for the owners. Finally, it has been made a comparison between the reinforcement quantities, concrete quantities, final cost and duration to finalize the construction stage. As a result, it has been found that the post-tension system is more economical than the Cobiax system according to reinforcement quantities, on the other hand, Cobiax system is more economical in concrete quantities which leads to lighter buildings and fewer reinforcement quantities in foundations. When the construction duration has been considered, it has been found that the post-tension is the quickest system in the construction field because it is increasing the strength and durability of the structure which the Cobiax system cannot provide.

Preliminary Study on the Design of Structures with a Responsive External Post-tensioning System

External post-tensioning has proven to be an effective solution to introduce intermediate supports in structures in order to decrease their spans and to allow a more efficient distribution of bending moments. This leads to important savings in terms of material. However, this increase in effectiveness is achieved only for a specific loading, which generally corresponds to the permanent load distribution. This paper introduces the possibility of expanding the benefits of this technique to situations where relevant live loads act through the use of a responsive external post-tensioning system. The responsive behaviour is materialized through an actuator located between the beam and the post-tensioning system. After a brief overview of the existing responsive structures, this research studies the static structural behaviour of responsive externally post-tensioned structures. Furthermore, the main design variables are parametrically analysed and the applicability of the system is evaluated with the construction of a small-scale prototype. Finally, results are discussed and the effectiveness of this approach in terms of material savings is claimed.

Flexural behaviour of post-tensioned glass beams: Experimental and analytical study of three beam typologies

The concept of post-tensioned glass beams builds on the concrete analogy of the reinforced glass beams. By additionally prestressing the reinforcement in a post-tensioned system, a compressive pre-stress is applied on the glass, enhancing the initial fracture resistance in bending. This paper presents three post-tensioned beam systems tested in four-point bending. The effects of both adhesive bonding and mechanical anchoring of tendons are explored in order to define an optimised beam system which can provide a significant level of compressive pre-stress and maximise the efficient use of material. The tests demonstrate the feasibility of post-tensioning by providing a substantial level of additional load capacity and redundancy to commonly applied laminated glass beams. Several specimens have demonstrated premature failure governed by lateral-torsional buckling or by shear. Since glass beams are generally not strengthened through transversal reinforcement, as concrete elements, further investigation of the mechanism of shear failure is particularly important. Analytical models, based on the pre-stressed concrete theory, provide expressions for determination of the initial fracture resistance, initial stiffness and ultimate flexural capacity. Compared with the experimental results, the models provide close prediction values for the initial fracture loads, while the predictions of the initial stiffness and ultimate post-fracture load capacity show less consistency. Closer prediction requires a better understanding of the effect of specific anchoring, the level of composite action and the post-fracture behaviour of laminated glass (including long-term and temperature effects).

Conceptual design of externally posttensioned curved structures for live loads

AbstractBending moments due to permanent loads can be strongly reduced in a generally shaped structure by introducing a set of additional forces through an external posttensioning system. The response of these structures under live loads has not been studied yet; therefore, this work investigates the behavior of these structural systems under load distributions that differ from the permanent ones. First, a definition of a moment‐based efficiency is proposed. Next, a parametric analysis is carried out to evaluate the performance of these structural systems by varying the two main design variables and other relevant parameters (strut connection and orientation, discretization, efficiency definition, etc.). Finally, the methodology is applied to a case of study where several posttensioned alternatives are compared to the standard one (i.e., without posttensioning) to prove the benefits of adopting external posttensioning in terms of material‐saving.

Sensitivity analyses of the optimum design parameters of prestressed post-tensioned I-girder and deck-slab system

The main objective of this paper is to analyze the sensitivity of the optimum design parameters of a girder-deck system and hence obtain an effective solution. The optimization problem by which optimum values of design parameters are obtained is based on a global optimization algorithm named “Evolutionary Operation”. This problem contains a total of 14 design parameters and bounded by 14 explicit constraints along with 46 implicit constraints. For sensitivity analyses, deviations of these parameters are considered on both the upper and lower sides of the optimum solutions which reflect their realistic possible ranges of deviations. For small deviations, compliance with the explicit and implicit constraints are examined. Characteristics of cost variations are also identified. It is observed that deck slab thickness is the most sensitive parameter in terms of structural adequacy. The approach is applied to a real-life bridge project. The slab thickness parameter in the conventional design procedure is less sensitive compared to optimized slab thickness. Finally, an attempt is taken to obtain an effective solution for optimum design with reduced sensitivity.