Post-tensioning System Research Articles

Ultimate behaviour and design of post-tensioned composite slabs

Post-tensioned composite slab systems consist of concrete slabs poured on thin-walled profiled steel sheeting and prestressed soon after casting. This paper presents six experimental tests that investigate the influence of different parameters on the ultimate behaviour of post-tensioned composite slabs. For this purpose, the samples were tested to failure in a simply-supported configuration. The parameters considered in this study included the slab thickness, the number of prestressing strands, the span length and the continuity of the profiled sheeting. A design model is also presented for the evaluation of the ultimate moment capacity by extending the applicability of procedures currently available for composite and prestressed floor slabs. The proposed theoretical approach was validated against the measurements presented in the paper and available in the literature. The comparisons carried out between the experimental measurement and the calculated capacities showed close agreement.

Post-tension near-surface-mounted strengthening systems of full-scale PSC girders

In this paper, a post-tension near-surface mounted (NSM) strengthening system is proposed to strengthening concrete structures with a relatively low amount of labor, without altering their appearance or dimensions. The main objective of this study was to investigate the flexural behavior of a prestressed concrete (PSC) girder using post-tension NSM strengthening systems. Two 21.8-m full-scale PSC girders were fabricated. The behavior of the girders was examined through a four-point loading test up to failure. The test results demonstrated that the post-tension NSM strengthening system was an effective method of increasing the flexural capacity. A finite element analysis model was also developed and verified against a full-scale test result. In addition, parametric studies were conducted to investigate the effects of the prestressing force and the number of grooves in the NSM strengthening system on the behavior of a test unit by nonlinear finite element analyses.

New opportunities for the conceptual design of material-efficient antifunicular structures

Non-structural design criteria (e.g., usability, architectural needs, esthetics) may prohibit the selection of purely funicular or anti-funicular shapes. In response to this issue, this paper illustrates the possibility of achieving an axial-only behavior, even if the geometry departs from the ideally bending-free shape. This is achieved by adding forces through an external post-tensioning system, with a layout defined through graphic statics. The paper briefly illustrates examples of this approach and its implementation within a design-driven software where structural performance and geometric variation are embedded within an interactive and parametric working environment.

Comparative Study of Prestress Losses

This paper compares the prestress losses as calculated by four different design codes; British standard CP110, Comite Europeen du Beton 70 and 78, American Concrete Institute 77 and the Prestressed Concrete Institute method (PCI). The comparison is done by determining the total losses which take place in a rectangular prestressed concrete beam for both pre-tensioning and post-tensioning systems. The results show that total losses calculated for the post-tensioning method are higher than those calculated for the pre-tensioning method, which is not the usual case. It seems that the PCI method may be required for special structures or for simply supported slender members which may be sensitive to small changes in deflections. However, for non-special structures, or where actual losses have little effect on the design, it is better to compute losses by the ACI method because it is simple and does take into considerations interactions between the various sources of losses. However, it is not possible to conclude which method gives the more accurate prediction of shrinkage and creep without direct co-relation to realistic insitu data.

Cyclic testing of unbonded post-tensioned concrete wall systems with and without supplemental damping

A series of cyclic lateral-load tests were conducted on four different unbonded post-tensioned precast concrete wall systems, including two Single Rocking Walls (SRW) and two PREcast Wall with End Columns (PreWEC). The main purpose of these tests was to systematically investigate the cyclic response of post-tensioned concrete walls with varying amounts of supplemental damping while keeping the initial post-tensioning force, wall dimensions, and confinement details constant. A secondary objective was to validate the wall panel design including the appropriate selection of axial force ratio and design of confinement and armouring details. All of the test walls exhibited excellent performance with uplift and rocking at the wall base with only minor damage observed, consisting of small amounts of spalling in the wall toes. There were consistent observations and measurements of the wall damage, concrete compressive strains, and wall neutral axis depths for both the SRW and PreWEC systems with the same wall panel dimensions. Based on these observations it is concluded that the behaviour of the wall panel in a PreWEC system is independent of the number of energy dissipating O-connectors. The O-connectors increased the hysteretic energy dissipation in the wall system and provided between 1.1% and 1.4% of additional equivalent viscous damping per connector for the PreWEC walls tested. Overall, the behaviour of the four walls tested confirmed the design procedures used, with both the global force-displacement response and local response parameters predicted with sufficient accuracy using an existing simplified analysis method.

Experimental investigation of multi-span post-tensioned glass beams

This paper presents a study on post-tensioned glass beams in a statically indeterminate system. In order to increase the safety of structural glass beams, ductile reinforcement can be added to glass beam sections providing secondary load carrying mechanism in case of glass breakage. In the here investigated post-tensioned system, the reinforcement tendons are additionally pre-tensioned, introducing compressive pre-stress in the beam in order to increase the apparent tensile strength of glass. The system is tested in five-point bending at 23 and 60,^{circ }hbox {C} in order to investigate the basic structural performance and the influence of temperature increase on the initial cracking load and the behaviour of the cracked beam. The benefit of the here investigated statically indeterminate system is a more economical design, i.e. lowering of the bending moment in the span of an equivalent simply supported system by continuing the beam over the central support. The efficiency of the applied system is compared to a reinforced beam system produced in the same batch with similar overall dimensions. The results show an increase of initial cracking load of the post-tensioned beams due to the applied pre-stress and a ductile post-cracking response, reaching high ultimate loads prior to failure. At 60 , ^{circ }hbox {C} both reinforced and post-tensioned beams show lower initial cracking loads and limited post-cracking ductility but still significant load reserve with ultimate loads well above the initial cracking loads, providing safe failure behaviour.

Effect of Partial Shear Connection on Strengthened Composite Beams with Externally Post-Tension Tendons

Composite steel-concrete beams are used widely in bridges and buildings construction as the main structural elements in flexure. These structures have a design life and this may be reduced if loads are increased or environmental degradation could happen. These changes may reduce the design life and strength of such members and thus replacement or retrofitting may need to be considered. The present study focuses on evaluating the effect of partial shear connection on strengthen composite beams with externally post-tension tendons. Using three dimensional F.E. modeling it’s able to simulate the overall flexural behavior of composite beams which are strengthen with many shapes of tendons profiles. A fundamental point for the structural behavior and design of composite beams is the level of connection and interaction between the steel section and the concrete slab. The use of partial connection provides the opportunity to achieve a better match of applied and resisting moment and some economy in the provision of connectors, taking into account the demonstrated advantages of externally post tension system like: Increase in ultimate moment capacity of structure, Enlarge the range of elastic behavior before yielding for the structure with the introduction of internal stresses.

Reduction of inelastic seismic demands in a mid-rise rocking wall structure designed using the displacement-based design procedure

Summary Precast post-tensioned rocking wall structural system has been developed in the recent past as a damage-avoidance structural system for seismic regions. For a widespread use of this structural system, suitable design procedures are required to ensure a reliable and well-predicted performance under different levels of seismic hazard. In the current study, a mid-rise 20-story rocking wall structure is selected and designed using the displacement-based design procedure. Furthermore, two different capacity design procedures are used to predict the increased force demands due to higher mode effects. The time history results against moderate and severe level of seismic hazards show the effectiveness of displacement-based design procedure in predicting and controlling the displacement and drift demands, while the simplified procedure and the modified modal superposition procedure for the capacity design are found to be unconservative and conservative, respectively. To further investigate the seismic demands, modal decomposition of inelastic seismic responses is carried out, and the contribution of different modes in the total responses is calculated. Based on this improved understanding, a mitigation technique of dual gap opening is employed. A detailed discussion about the location and design strength of the extra gap-opening is carried out by considering different performance parameters. Copyright © 2016 John Wiley & Sons, Ltd.

Computer modeling of dynamic behavior of rocking wall structures including the impact-related effects

Precast post-tensioned concrete rocking wall system is an innovative damage avoidance structural system for seismic regions. Past experimental works on the dynamic performance of rocking walls have identified the presence of high-frequency acceleration spikes in both lateral and vertical directions during the impact of wall base with the foundation. These acceleration spikes, acting together, can cause shear slip at the wall–foundation connection. This study is focused on the development of a computer model that can predict these acceleration spikes along with the identification of their effects on the dynamic performance of rocking walls. For this purpose, impact phenomenon at the wall–foundation joint has been discussed in detail and some general guidelines are set for the two important parameters of impact or contact modeling named as contact stiffness and contact damping. The finite element numerical models, based on the proposed guidelines, are found to predict the overall dynamic behavior of rocking walls along with the acceleration spikes quite efficiently. The acceleration spikes are found to be dependent on the lateral velocity at impact and the initial contact stiffness. So a velocity-dependent energy dissipation device along with a soft contact is found to be suitable for reducing these effects.

Fuse-Type External Replaceable Dissipaters: Experimental Program and Numerical Modeling

AbstractThe paper presents the experimental and numerical study of a mild steel replaceable dissipation device. The dissipater is made of a milled-down mild steel bar that is confined by a steel tube filled with either grout or epoxy. The use of a reduced area concentrates yielding and plastic straining over a defined length while the tube prevents buckling. The device has been widely adopted in posttensioned rocking systems, and extensive experimental research was carried out on wall and beam-column subassemblies using this external dissipater. As part of those experimental programs, some component testing was carried out; nevertheless, a more comprehensive study was necessary to cover a wider range of geometric and mechanical parameters. The main objective of this research was to calibrate and suggest design parameters to support the design of the dissipation device. The paper shows a comprehensive experimental program under quasi-static cyclic loading carried out with varying geometrical parameters. Th...

Experimental calibration of parallel-to-grain strain penetration length for internal epoxied bars

Tensile strain penetration effects have been first investigated in the study of reinforced concrete components as experimental studies highlighted that theoretical values for the equivalent plastic hinge length did not reflect the observed experimental results. This is due to the elongation of the bars beyond the theoretical interface of the structural element with another component (e.g. beam-to-column or column/wall-to-foundation connections). The concept of an equivalent strain penetration length was first introduced for flexural analysis of concrete elements and the definition of the element’s plastic hinge length.Experimentally calibrated expressions for the equivalent strain penetration length have been developed in literature and extensively adopted in the design of traditional reinforced concrete connections.Internal partially unbonded bars have more recently been used is precast concrete dissipative post-tensioned rocking systems, developed during the PREcast Seismic Structural Systems (PRESSS) program, and more recently in timber rocking (Pres-Lam).As per bars embedded or post-grouted into concrete, internally glued bars in timber are affected by tensile strain penetration effects. An important parameter involved in the design of dissipative rocking section as well as more traditional epoxied rods connections is the additional slip and consequent loss of stiffness, strength and energy dissipation due to strain penetration.The paper shows the determination of an equivalent strain penetration length in internal epoxied bars through the experimental testing of several specimens accounting for different geometric parameters. The final part of the paper discussed the experimental results in terms of bar slip proposing simplified analytical formulas.

An electromechanical impedance-based method for tensile force estimation and damage diagnosis of post-tensioning systems

We propose an effective methodology using electromechanical impedance characteristics for estimating the remaining tensile force of tendons and simultaneously detecting damages of the anchorage blocks. Once one piezoelectric patch is attached on the anchor head and the other is bonded on the bearing plate, impedance responses are measured through these two patches under varying tensile force conditions. Then statistical indices are calculated from the impedances, and two types of relationship curves between the tensile force and the statistical index (TE Curve) and between statistical indices of two patches (SR Curve) are established. Those are considered as database for monitoring both the tendon and the anchorage system. If damage exists on the bearing plate, the statistical index of patch on the bearing plate would be out of bounds of the SR curve and damage can be detected. A change in the statistical index by damage is calibrated with the SR curve, and the tensile force can be estimated with the corrected index and the TE Curve. For validation of the developed methodology, experimental studies are performed on the scaled model of an anchorage system that is simplified only with 3 solid wedges, a 3-hole anchor head, and a bearing plate. Then, the methodology is applied to a real scale anchorage system that has 19 strands, wedges, an anchor head, a bearing plate, and a steel duct. It is observed that the proposed scheme gives quite accurate estimation of the remaining tensile forces. Therefore, this methodology has great potential for practical use to evaluate the remaining tensile forces and damage status in the post-tensioned structural members.

Combined half precast concrete slab and post-tensioned slab topping system for basement parking structures

It is known that the prestressed half PC (precast) slab system has many advantages such as a reduced construction period and use of long-span one-way slab systems over reinforced concrete and half PC slab systems without prestressing. For this reason, prestressed half slabs are commonly used for both residential and basement parking structures in Korea and other countries. The prestressed half PC slab systems currently being used only have the pre-tensioning system only for PC slabs. To ensure better performance of the half PC slab system with larger span-to-depth ratio, a new system using both pre-tensioning and post-tensioning methods is suggested for basement parking structures in this paper. The main purpose of this paper was to analyse the suggested model and verify its safety and serviceability for a given span and a given load. The second purpose is to suggest a preliminary calculation process for the suggested system that satisfies the ACI 318 requirements. A parametric study is conducted in order to find the best and most economic model using four variables: (1) depth of precast concrete; (2) depth of topping concrete; (3) number of pre-tensioned tendons; and (4) number of post-tensioned tendons. The obtained model satisfies the requirements of ACI 318 in terms of working stress design.

Behavior of unbonded post-tensioning monostrand anchorage systems under short duration, high amplitude cyclical loading

This study evaluates the behavior of monostrand post-tensioning anchorages under both monotonic and short duration high amplitude (post-yield) cyclical loading representative of use in self-centering structural applications designed to provide seismic resistance. Anchorages from two different manufacturers were tested in coupled and uncoupled configurations. In addition, a modified wedge geometry was evaluated to determine if the performance of the system could be improved. Strain within the strand was recorded using extensometers and strain gages for different samples. A detailed evaluation of the relationship between the strain within the strand and the elongation measured by the testing frame was conducted to ensure that the strain within the strand was calculated accurately throughout the tests. The results of this research indicate that the strain within the strand is not linearly related to the elongation recorded by the testing frame, and a more accurate method was developed to allow for this conversion. Additionally, it was determined that couplers did not significantly affect the capacity of the system, but the capacity was affected by the manufacturer of the anchorage system and the loading regime used to test the sample. Based upon the observed variations in capacities, this study recommends the design of monostrand post tensioned systems using cast anchorages should be limited to a strain of 0.01mm/mm.

Funicularity through External Posttensioning: Design Philosophy and Computational Tool

AbstractFunicular geometries, which follow the idealized shapes of hanging chains under a given loading, are recognized as materially efficient structural solutions because they exhibit no bending under design loading, usually self-weight. However, there are circumstances in which nonstructural conditions make a funicular geometry difficult or impossible. This paper presents a new design philosophy, based on graphic statics, that shows how bending moments in a nonfunicular two-dimensional curved geometry can be eliminated by adding forces through an external posttensioning system. An interactive parametric tool is introduced for finding the layout of a posttensioning tendon for any structural geometry. The effectiveness of this approach is shown with several new design proposals.

Estimation of Friction Coefficient Using Smart Strand

Friction in a post-tensioning system has a significant effect on the distribution of the prestressing force of tendons in prestressed concrete structures. However, attempts to derive friction coefficients using conventional electrical resistance strain gauges do not usually lead to reliable results, mainly due to the damage of sensors and lead wires during the insertion of strands into the sheath and during tensioning. In order to overcome these drawbacks of the existing measurement system, the Smart Strand was developed in this study to accurately measure the strain and prestressing force along the strand. In the Smart Strand, the core wire of a 7-wire strand is replaced with carbon fiber reinforced polymer in which the fiber Bragg grating sensors are embedded. As one of the applications of the Smart Strand, friction coefficients were evaluated using a full-scale test of a 20 m long beam. The test variables were the curvature, diameter, and filling ratio of the sheath. The analysis results showed the average wobble and curvature friction coefficients of 0.0038/m and 0.21/radian, respectively, which correspond to the middle of the range specified in ACI 318-08 in the U.S. and Structural Concrete Design Code in Korea. Also, the accuracy of the coefficients was improved by reducing the effective range specified in these codes by 27–34 %. This study shows the wide range of applicability of the developed Smart Strand system.

Experimental response and design of O-connectors for rocking wall systems

A unique series of nine cyclic tests were performed on energy dissipating O-connectors for use in seismic resisting unbonded post-tensioned rocking wall systems. The effect of the O-connector profile, steel material and welding process for attaching the O-connectors was investigated and two different failure mechanisms were identified. The welding process used to install the O-connector was found to significantly influence the type of failure mechanism and ductility as such a new O-connector profile was developed to overcome this sensitivity. Design equations to estimate the ultimate strength of the O-connector were proposed and used to develop a bilinear force–displacement approximation.

Comparative Performance Studies on Bonded Post-Tensioned and RC Flat Slab

In a developing country like India the benefits of Post-Tensioning system are yet to be recognized. The post-tensioning method is now a day’s being adopted widely due to its applications. By using post-tensioning method one can achieve the most economical and safe design for commercial as well as residential buildings. The present study focuses on Post-Tensioned flat slab systems with respect to strength, service and cost estimation. The designs were analysed using ADAPT Builder software with different panel sizes. The results indicate that PT slabs have better moment carrying capacity & Deflection; also the costs are better than the RC flat slab systems.

Design of the Digester Tanks of the Atotonilco Water Treatment Plant in Mexico

This paper discusses the design processes of the 15 000 m3 post-tensioned concrete digesters of the Atotonilco Water Treatment Plant in Mexico. It introduces the project with a brief description of the plant and focuses on the structural challenges faced by the engineers during the design phases, and the measures taken to resolve these issues pertaining to the tanks. This challenging design was solved by a double post-tensioned system, circumferential and vertical, fixed into the wall, which ensures the structural integrity of the tank under the design loads, compatible with the desired rapid methods of construction for the 30 digesters of the plant.

Performance and Design of LVL Walls Coupled with UFP Dissipaters

This article presents recent research on the seismic resistance of coupled post-tensioned timber walls for use in multi-story buildings. The walls are constructed from laminated veneer lumber (LVL), post-tensioned with unbonded vertical tendons, and coupled together with mild steel U-shaped flexural plates (UFPs) as energy dissipating elements. The timber wall design follows the same principles as used for post-tensioned precast concrete wall systems, using U-shaped plates to obtain a “hybrid” system, where energy is dissipated through yielding of the plates, while the vertical post-tensioning provides the restoring force. In this project, the same principles are applied to timber coupled walls. A series of quasi-static cyclic and pseudo-dynamic tests have been carried out to verify the applicability of the concept and the feasibility of the construction technology in timber buildings. The U-shaped plates showed stable energy dissipation characteristics and, in combination with the post-tensioning, desirable re-centering hysteretic behavior typically referred to as “flag-shape”. Because of the simplicity of these elements and the low cost of implementation, they have good prospects for practical application.