Unbonded Post-tensioned Tendons Research Articles

Fragility-based methodology for evaluating the time-dependent seismic performance of post-tensioned timber frames

Since 2010, the construction of post-tensioned wooden buildings (Pres-Lam) has been growing rapidly worldwide. Pres-Lam technology combines unbonded post-tensioning tendons and supplemental damping devices to provide moment capacity to beam–column, wall–foundation, or column–foundation connections. In low seismic areas, designers may choose not to provide additional damping, relying only on the post-tensioning contribution. However, post-tensioning decreases over time due to creep phenomena arising in compressed timber members. As a consequence, there is a reduction of the clamping forces between the elements. This reduction affects the seismic response of Pres-Lam buildings in the case of low- and high-intensity earthquakes. Therefore, understanding and accounting for the post-tensioning losses and their uncertainty are paramount for a robust assessment of the safety of Pres-Lam constructions. So far, however, there have been no comprehensive studies which tackle the overall seismic performance of such systems in the presence of time-varying post-tension losses and the associated uncertainty. This study tackles this research gap by introducing a comprehensive seismic evaluation of Pres-Lam systems based on time-dependent fragility curves. The proposed fragility analysis is specifically designed to account systematically for time-varying post-tension losses and the related uncertainty. The method is applied to two case studies, designed, respectively, with and without supplemental damping devices. In terms of structural performance, results show that the use of additional dissipaters mitigates the effect of post-tensioning loss for earthquakes of high intensity. Conversely, performance under low-intensity earthquakes is strongly dependent on the post-tensioning value, as the reduction of stiffness due to the anticipated rocking motion activation would lead to damage to non-structural elements.

Experimental study on damage-controllable rocking walls with resilient corners

Polyurethane elastomer was employed in the concrete rocking element in an innovative way to limit the corner damage during rocking. A damage-controllable rocking wall consisting of rocking elements, resilient corners, connecting beams, unbonded post-tensioned tendons and replaceable dampers was developed. Cyclic tests on damage-controllable rocking walls were conducted by changing the conditions of dampers, corner elastomers and tendons to investigate their seismic performance. Two sets of reusable reinforced concrete rocking elements were repeatedly used seven times. The horizontal displacement of the specimen was mainly caused by rigid body rotation. The deformations of the rocking elements were quite limited, and no flexural cracks were observed in any tests. All the specimens performed well with a good capacity for low damage. Energy dissipation was generated by the attached dampers, and the equivalent viscous damping coefficient of the proposed wall was proportional to the yield strength of dampers. The damaged damper can be replaced conveniently after each test, whereas the corner elastomers remained elastic. Specimens without damping exhibited a bilinear elastic response and were completely self-centring after testing. Specimens with a suitable initial post-tensioned force and damping exhibited flag-shaped hysteresis with minor residual drifts.

Monitoring Dynamic Properties of a Pres-Lam Structure: Trimble Navigation Office

AbstractPres-Lam technology relies on unbonded post-tensioning tendons to provide moment capacity to timber beam-column, wall-foundation, or column-foundation connections. When designing buildings ...

Design criterion for the self-centering capacity of precast segmental UHPC bridge columns with unbonded post-tensioning tendons

This paper aims to provide a novel quantitative design criterion for the self-centering (SC) capacity of precast segmental ultra-high performance concrete (UHPC) bridge columns based on the concept of the SC factor, which is deemed capable of depicting the formation mechanism of residual drift. A refined numerical model was developed to simulate the seismic performance and was verified by the results of the cyclic loading test. Parametric analysis was conducted to research the influence of seven common design parameters on the SC factor based on the proposed numerical model. According to the results of the parametric analysis, a simplified formula was established to assess the residual drift when the SC factor and the maximum drift were known. Nonlinear dynamic analysis was conducted to validate the simplified formula for precast segmental UHPC bridge columns subjected to far-field, no-pulse near-field, and pulse near-field earthquake motions. A design criterion for the SC capacity was proposed based on the SC factor limits determined by the simplified formula. Research results show that the SC factor corresponding to the ultimate drift is generally conservative for assessing the SC capacity of precast segmental UHPC bridge columns. The residual drift is dependent on the SC factor and the maximum drift. The residual drift decreases with the increasing SC factor or decreasing maximum drift. The proposed formula is verified by the results of nonlinear dynamic analysis to be conservative to calculate the residual drift. According to the proposed design criterion, the SC factor limits are increased with the drift demand up at the same earthquake design level. The adverse effects of pulse near-field earthquake motions on the earthquake resilience of precast segmental UHPC bridge columns should be taken seriously.

Optimum design for unbonded posttensioned precast split shear wall system using genetic algorithm

SummaryPrecast concrete structures are increasingly being adopted by building designers in regions of high seismicity. An unbonded posttensioned (PT) precast split shear wall system (UPPSSW) was proposed by the Precast Seismic Structural Systems (PRESSS). The UPPSSW system is composed of two or more single precast concrete wall panels that are connected together with energy‐dissipating shear connectors and anchored to the foundation with unbonded PT tendons located at the panel center. In this paper, an optimum design program has been developed for designing this system. The objective of the optimum process is to find the optimum combination between PT tendons and shear connectors while keeping the moment capacity of the wall equal to the applied design moment and achieving zero residual drift simultaneously. In addition, MATLAB was employed to explore an optimization program using genetic algorithm. Compared with the existing design methods for the system, the optimum design program proposed in this research is accurate, efficient, and direct. Moreover, it can yield the optimum design automatically and quickly. As a result, the existing lengthy and manual design process of trial and error for the system can be avoided.

Analytical Model for Multi-Hazard Resilient Prefabricated Concrete Frame Considering Earthquake and Column Removal Scenarios

Research on multi-hazard prevention and mitigation in building structures is the most recent developing trend in civil engineering. In this study, an analytical model is proposed to calculate the structural resistance of a type of multi-hazard resilient prefabricated concrete (MHRPC) frame under earthquake and column removal scenarios. The MHRPC frame is assembled using prefabricated RC beams and columns, unbonded post-tensioning (PT) tendons, energy-dissipating steel angles, and large rotational shear plates. According to the experimental results, the MHRPC frame exhibits the features of low damage and self-centering under seismic loading. Meanwhile, when subjected to column removal scenarios, the MHRPC frame is proven to demonstrate a high progressive collapse resistance. In order to calculate the seismic and progressive collapse resistance of the MHRPC frame, analytical models for the critical components in the MHRPC frame (PT tendons and steel angles) are compared and selected based on the experimental results and numerical simulations. Furthermore, calculation methods for the seismic and progressive collapse resistance of the MHRPC frame specimens are proposed. The calculation results are validated using the experimental results. This study could provide a reference for the design of MHRPC frame structures, considering both earthquake and progressive collapse.

Ultimate tendon stress of strengthened and non-strengthened unbonded post-tensioned I-beams

There is a lack of sufficient information on the assessment of stress in unbonded tendons at ultimate in continuous post-tensioned members. Although many investigations have been conducted on simply supported members, few studies have focused on continuous members, particularly strengthened ones. An investigation into the stress in unbonded post-tensioned tendons at ultimate in continuous I-beams externally strengthened with carbon-fibre-reinforced polymer sheets is described, with special emphasis on the effects of non-prestressed steel and the implementation of ultrahigh-strength self-compacting concrete. Six two-span 9 m I-beams were constructed, monitored and tested until failure. Finite-element analysis (FEA) was carried out to analyse the ultimate stress in the tendons (fps) and generate the responses of non-strengthened and strengthened beams. The test results were compared with prediction equations proposed in design standards and other previous studies. The tendon stress approached the point of yielding at the onset of failure for the two groups of beams. This is an important finding, resulting in higher moment redistribution and ductility demands for such members. The FEA accurately predicted fps and was applicable to both non-strengthened and strengthened beams. The code equations were found to provide fps values for practical design on the safe side.

Cyclic experimental response of self-centering concrete frames with slotted columns

A slot, which is a partial separation between a column and a base, provides the rotation of columns in a moment-resistant frame under strong ground motion, and prevents severe concrete crushing due to concentrated plastic deformation at the bottom end of the column. In a slotted column, unbonded post-tensioned tendons are arranged at the centerline of the cross-section to reduce residual drift, that the moment-resistant concrete frame performed the capacity of self-centering. The aim of this study was to determine the cyclic lateral response of a self-centering concrete frame with slotted columns. Three 1/2.5 scale frames, with different graded longitudinal reinforcements as energy dissipators in the slots, were tested under cyclic lateral loads combined with different axial loads. The failure of specimens was observed without the presence of plastic hinges near the column end, and the energy dissipation capacity was quantitatively analyzed. The fracture of several exposed longitudinal reinforcements in the slots during the test had a lesser influence on the energy dissipation capacity. The residual drifts of the three specimens were less than 1.00% for the elastoplastic seismic design at 2.00% lateral drift, and less than 1.75% at failure. By investigating the rules of the loading and unloading stiffnesses at different loading stages, a hysteretic model was developed to provide a satisfactory prediction of the structural residual drift, in which the residual drift was considered as constant after the fracture of several longitudinal reinforcements in the slots.

The implications of post-tensioning losses on the seismic response of Pres-Lam frames

Since 2010, twelve post-tensioned timber (Pres-Lam) buildings have been constructed throughout the world. In high seismic areas, Pres-Lam technology typically combines unbonded post-tensioning tendons and supplemental damping devices to provide moment capacity to beam-column, wall-foundation or column-foundation connections. Over time creep within the timber elements leads to losses in post-tensioning forces reducing the connection moment capacity. This paper analyses how different post-tensioning loss scenarios, depending on the beam-column joint detailing, impact the building’s seismic response. Two case study buildings were designed and investigated using the Acceleration Displacement Response Spectrum (ADRS) method and Non-Linear Time History Analysis (NLTHA) to predict seismic performance. These buildings were considered to be located in areas of high and low seismic risk, leading to designs with and without the use of damping devices, respectively. The results show that the building with additional damping responded with similar peak displacements, even under extreme loss scenarios. In comparison, when supplemental damping was not used, peak displacements increased significantly with post tensioning losses.

Design Approach to Predict Post-Tensioning Losses in Post-Tensioned Timber Frames

AbstractSince 2010, twelve post-tensioned timber (Pres-Lam) buildings have been constructed throughout the world. The technology relies on unbonded post-tensioning tendons to provide moment capacit...

Cyclic loading test of self-centering precast segmental unbonded posttensioned UHPFRC bridge columns

Three 1/3-scale precast segmental bridge columns, manufactured with ultrahigh-performance fiber-reinforced concrete (UHPFRC) incorporating river sand and coarse aggregate, were tested under cyclic loading. Energy dissipation (ED) bars, embedded in ultrahigh-performance concrete (UHPC) grout, maintained continuous across segment joints and unbonded at the bottom joint. Self-centering prestressing force was provided by unbonded posttensioning (PT) tendons. The research parameters included PT force level and the amount of ED bars. Test results showed that all the specimens exhibited no less than 8% drift capacities, which were remarked with the first fracture of ED bars. No obvious cracking and limited UHPFRC spalling were observed. Both PT force level and the amount of ED bars have notable effects on stiffness, lateral strength, and ductility. Increased PT force may improve ductility with the total axial loading ratio less than 0.08. All PT tendons were elastic and no yield or rupturing was found, but the stress loss was significant. The equivalent unbonded length can be evaluated with 0.007dbfy for ED bars embedded in UHPC grout. The rotation of the bottom joint dominated lateral deformation and the contribution of joint sliding can be neglected. The contribution λED of ED bars to lateral strength should be no more than 25% to maintain self-centering capacity.

Shake-table testing of a self-centering precast reinforced concrete frame with shear walls

The seismic performance of a self-centering precast reinforced concrete (RC) frame with shear walls was investigated in this paper. The lateral force resistance was provided by self-centering precast RC shear walls (SPCW), which utilize a combination of unbonded prestressed post-tensioned (PT) tendons and mild steel reinforcing bars for flexural resistance across base joints. The structures concentrated deformations at the bottom joints and the unbonded PT tendons provided the self-centering restoring force. A 1/3-scale model of a five-story self-centering RC frame with shear walls was designed and tested on a shake-table under a series of bi-directional earthquake excitations with increasing intensity. The acceleration response, roof displacement, inter-story drifts, residual drifts, shear force ratios, hysteresis curves, and local behaviour of the test specimen were analysed and evaluated. The results demonstrated that seismic performance of the test specimen was satisfactory in the plane of the shear wall; however, the structure sustained inter-story drift levels up to 2.45%. Negligible residual drifts were recorded after all applied earthquake excitations. Based on the shake-table test results, it is feasible to apply and popularize a self-centering precast RC frame with shear walls as a structural system in seismic regions.

Pushover Analysis of Precast Segmental UHPFRC Bridge Columns with Unbonded Posttensioned Tendons

Cyclic loading tests were carried out to research seismic behavior of precast segmental ultrahigh-performance fiber-reinforced concrete (UHPFRC) bridge column with unbonded posttensioned (PT) tendons. Energy dissipation (ED) bars embeded in ultrahigh-performance concrete (UHPC) grout maintained continuous across segment joints and unbonded at the bottom joint. The equivalent method was proposed for ED bars with unbonded length and unbonded PT tendons to obtain the equivalent fiber section conforming to the flat section assumption. A method based on the equivalent plastic hinge model was given to conduct pushover analysis for precast segmental UHPFRC bridge columns. Test results showed that all the specimens exhibited no less than 8% drift capacities, which were remarked with the first fracture of ED bars. Compared with test data, the proposed model was validated to have good accuracy to provide lateral skeleton curve.

Experimental and numerical investigation of jointed self-centering concrete walls with friction connectors

A jointed self-centering (SC) concrete wall system with friction connectors is proposed, in which the precast concrete wall panels are assembled through unbonded posttensioning (PT) tendons to provide self-centering capacity, and friction connectors at the vertical joint provide supplementary energy dissipation as the walls move relatively to each other during a major earthquake. The wall toes are protected by the steel jackets from crushing as the walls rock. The behavior of the jointed wall system was experimentally investigated through nine cyclic load tests, based on which the influence of different initial PT forces and friction forces on structural performance of the walls was discussed. Besides, strains on the steel jackets during the tests were measured to validate the effectiveness of steel jackets, and three-dimensional laser scanning technique was applied to measure the in-plane and out-of-plane deformations of the walls. It is observed that the main structure remained resilient after the nine tests with the maximum drift ratio of about 2.98%, though cracking in concrete cover at the eastern connectors was found after a number of reversal loading, which calls for improved structural details. A numerical simulation method of the jointed SC wall system is proposed, and the simulated hysteretic behaviors match well with the test results.

Axial compression ratio limit for self‐centering precast segmental hollow piers

Axial compression ratio limit was investigated for self‐centering precast segmental hollow piers based on post‐earthquake residual axial loading capacity. The analytical method of unbonded posttensioning tendons (PT) stress increment and the simplified method of compression zone height were developed and validated by the finite element model verified by experiments. An analytical formula was deduced to calculate the critical axial compression ratio, which was regarded as the limit and can be obtained when the capacity provided by the noncompression zone at the bottom is equal to the initial axial compression loading. Parameter analysis was conducted to study effects of eight common design parameters on the critical axial compression ratio. The conclusions are summarized that there is good accuracy between the proposed methods for both unbonded PT stress increment and compression zone height and the finite element analysis. The axial compression ratio limit is proposed to be 0.25 when the ratio of energy dissipation (ED) bars is <1.5%. It is an inadvisable solution that more ED bars are matched with higher initial tensioning force of unbonded PT to fulfill the precast segmental piers with good ED capacity and self‐centering capacity in practice.

Fragility analysis of self-centering prestressed concrete bridge pier with external aluminum dissipators

A novel self-centering prestressed concrete pier with external energy dissipators has been developed to realize seismic resilient performance and enhanced corrosion-resisting property. Self-centering capacity of the pier is provided by the unbonded post-tensioned tendons and damage is mostly concentrated on the replaceable dissipators. To investigate the seismic behavior of the proposed pier, a detailed analytical model considering interface opening and dissipator deformation was developed and verified through existing cyclic load tests. Based on the proposed model, a prototype reinforced concrete pier and a self-centering prestressed concrete pier with similar backbone curves are designed, and fragility analyses are conducted on the two piers through incremental dynamic analysis. One maximum drift-based performance limit state (i.e. collapse prevention) and two residual drift-based performance limit states (i.e. emergent usage and reconstruction) are defined for seismic capacity evaluation. Fragility curves indicate that the self-centering prestressed concrete pier has a slightly higher peak drift demand owing to its inferior dissipating capacity as compared with the reinforced concrete pier, while sustains a much lower residual drift demand due to its inherent self-centering characteristic.

Experimental study on emulative hybrid precast concrete shear walls

The reversed cyclic lateral loading experiments were conducted on three, full scale, emulative hybrid precast concrete shear wall specimens with different post-tensioned forces and one reference monolithic RC shear wall specimen, and the measured results were then discussed in this paper. The emulative hybrid wall combined grouted vertical reinforcements with unbonded post-tensioned high-strength multi-strand tendons for lateral resistance, attempting to emulate the cast-in-place RC wall. Because of the presence of the unbonded post-tensioned tendon, by contrast to the monolithic wall specimen, the emulative hybrid wall specimens possessed higher strength, higher stiffness, and smaller residual displacement. The opening gap, resulted from the discontinuity of concrete and the locally debonding of vertical connecting reinforcements, was limited mainly due to the vertical connecting reinforcements across the joint distributed over total cross-section. Meanwhile, the emulative hybrid wall specimens were capable of providing energy dissipation and ductility capacity comparable to that of the monolithic specimen, and the negative effect of the post-tensioned force on energy dissipation was more obvious than on ductility. Moreover, the post-tensioned tendon remained elastic throughout the test and the yielding of the connecting reinforcements was delayed.

The effects of lubricant and tendon mass variances on the coefficient of friction in unbonded post-tensioning tendons

In unbonded post-tensioning tendons, the coefficient of friction varies from one design standard to another. This variation is caused by the large number of complex factors that must be considered in design. These factors include the thickness of lubricant or grease present in the system, clearance between the strand and the plastic sheath, and surface characteristics of the strand and plastic sheath. In order to determine the effect of these factors a series of friction tests were performed on two different diameters of strand, namely 12.7 mm and 15.24 mm diameters. Through a regression analysis, it was found that the frictional force decreases with an increase in the thickness of the grease, and that friction increases with an increase in the mass of the strand. The degree of friction was found to be dependent on the surface characteristics of the strand and plastic sheath, clearance between the plastic sheath and the strand, and the extrusion process of the plastic sheath.

Nonlinear quasi-static analysis of hybrid sliding–rocking bridge columns subjected to lateral loading

In this paper, a nonlinear three-dimensional finite element (FE) model for a hybrid sliding–rocking (HSR) column is developed. The HSR columns are segmental members incorporating internal unbonded post-tensioning, rocking joints at the member ends, and intermediate sliding joints along the member height. The HSR columns introduce new design parameters (e.g., frictional properties at the sliding joints, amplitude of joint sliding, number/distribution of sliding joints along the column height) and additional modeling challenges (e.g., contact interactions between adjacent segments and between segments and the internal unbonded post-tensioning tendons) as compared with conventional monolithic or rocking-only members. In the proposed modeling approach, contact interaction amongst adjacent segments and between segments and unbonded tendons are captured. Concrete segments are modeled using solid elements, mild steel reinforcement is modeled by beam elements embedded into the concrete segments, and the unbonded tendons are modeled using truss elements. The FE model was validated against available experimental data and was then used to conduct a parametric pushover study. Variations of the external vertical load were found to have minor effects on the lateral column response. Increase of the initial post-tensioning force resulted in early onset of inelastic response of the tendons, while the peak lateral column strength remained unchanged. Joint sliding increased the lateral deformation capacity of the column. Propagation of sliding from bottom to top and vice versa had small effects on the lateral response of the column. Propagation of sliding was controlled by the friction properties at the sliding joints.

Quasi-Static Cyclic Testing of Two-Thirds Scale Unbonded Posttensioned Rocking Dissipative Timber Walls

AbstractPosttensioning low-damage technologies were first developed in the late 1990s as the main outcome of the U.S. Precast Seismic Structural System (PRESSS) program coordinated by the University of California, San Diego, and culminated with the pseudo-dynamic test of a large-scale five-story test building. The extension of posttensioned techniques to timber elements led to the development of new structural systems, referred to as Pres-Lam (prestressed laminated timber). Pres-Lam systems consist of timber structural frames or walls made of laminated veneer lumber, glue laminated timber (Glulam), or cross-laminated timber (CLT). Pres-Lam walls consist of a rocking timber element with unbonded posttensioned tendons running through the length and attached to the foundation, which provides a centering force to the wall, while energy dissipation is supplied by either internal or external mild steel dissipaters. Previous tests carried out on posttensioned timber walls focused on small-scale (one-third) speci...