Unbonded Posttensioning Research Articles

Seismic Behavior of Monolithic Exterior Beam-Column Connections with Unbonded Post-Tensioning

Seismic Behavior of Monolithic Exterior Beam-Column Connections with Unbonded Post-Tensioning

Analysis and design of RC unbonded PT shear walls in isolation and integrated into buildings: A state-of-the-art review

Shear walls play crucial roles in buildings located in the seismically active regions by efficiently mitigating the impact of seismic forces. However, they have drawbacks regarding their non-recoverable deformation and, thus, lack of reusability once damaged during an earthquake. One potential solution to this problem is the use of post-tensioned (PT) shear walls. These walls rock over the base during seismic events and regain their original position after the event, showing self-centering behaviour. The self-centering behaviour makes these walls reusable and reduces the downtime of the structure, thus making them a sustainable solution over conventional shear walls. Often, they are fitted with either internal or external energy dissipators to enhance the energy dissipation capacity. The present study aims at reviewing past analytical and numerical works, design methods, and international codes on reinforced concrete (RC) PT shear walls. The study also focuses on the models used in the past to investigate the behaviour of these walls. The parameters that affect the performance of RC PT shear walls are presented. Also, the global behaviour and the system interaction of RC PT shear walls integrated into buildings have been discussed. Finally, this paper presents the summary and critical observations, along with future recommendations on RC PT shear walls.

Experimental and theoretical investigations of post-tensioned precast reinforced concrete bridge pier with external replaceable energy dissipaters

This study presents a post-tensioned (PT) precast reinforced-concrete bridge pier with external replaceable energy dissipaters (PRCB-EREDs). The EREDs provide both bending resistance and energy dissipation (ED) capacity, and the unbonded PT strands enable prefabricated construction and self-centering capabilities. This study introduces the construction, assembly process, and post-earthquake repair technology of the PRCB-EREDs. Subsequently, parametric quasi-static tests were performed by altering parameters, such as the initial PT force and the size of core energy dissipation steel plate in the ERED (ERED-CEDP). The test results indicated that the precast pier column remained essentially elastic, with damage localized within the ERED-CEDP, exhibiting good deformation and ED capabilities. The hysteresis curve of the specimen was full and stable, and the curves before and after repair were essentially identical, confirming the feasibility of the repair. With an increase in the initial PT force, both the stiffness and bearing capacity were improved, and better self-restoration effects were achieved. For test conditions with the same ERED-CEDP thicknesses but different initial PT forces, the ED capacities were almost the same. Finally, the load-resisting mechanism of the PRCB-ERED piers was analyzed and theoretical model of hysteretic behavior was proposed and verified.

Planar frame models considering system‐level interactions of a two‐story low‐damage concrete wall building

AbstractA system‐level shake‐table test of a two‐story low‐damage concrete wall building was conducted in 2019. The test building consisted of unbonded post‐tensioned (UPT) walls with perimeter frames and different wall‐to‐floor connections to provide deformation compatibility. The test results highlighted that the measured lateral strength of the building notably exceeded the design value. This over‐strength was largely attributed to the compatibility deformations imposed on the floor systems. Previous simulation results from planar models also underestimated the building global response and lateral strength, so a modified model was developed to improve the accuracy by accounting for the system‐level interactions. The floor slab out‐of‐plane strength was assessed by the floor slab models using shell elements to quantify their response when subjected to deformations induced by the UPT wall uplift. In addition, the unintended effects of the beam‐to‐floor connection strength were also calculated. The strength contributions from the floor and connections were included in the planar frame models using lumped rotational springs located at the slotted‐beam joints. Lastly, the models were also updated to include flexibility in the connections of the energy dissipations at the wall bases based on test observations. Nonlinear time history analysis of the updated models for unidirectional loading cases demonstrated an improved calculation of the test results for both the global and local responses. When considering the floor interaction in the updated models, the absolute relative errors of peak overturning moments reduced from 35.4% to 15.6% averagely for the longitudinal direction, and from 23.1% to 14.1% averagely for the transverse direction.

Shaking table test of a 10‐story self‐centering tube building

AbstractThe severe damage of conventionally designed buildings and consequent socioeconomic impacts after major seismic events have spurred the development of self‐centering systems with high seismic resilience. Post‐tensioned (PT) walls with rocking interfaces allowed to open and close and unbonded PT bars that provide most of the re‐centering ability have been developed extensively as a promising self‐centering technology. Aiming at providing essential insights into the seismic performance of self‐centering tube systems, a 1/6‐scale ten‐story self‐centering tube building was designed, constructed, and tested on a shaking table subjected to a series of unidirectional and bidirectional ground motions with increasing intensities. The test building consisted of a self‐centering tube that included four PT walls and provided the primary lateral force resistance in both directions, a perimeter rocking frame that was designed mainly to carry gravity loads, and particular connections with slots to accommodate the potential displacement incompatibility among the PT walls and surrounding components. The experimental results indicated the excellent seismic performance of the test building that had reliable structural integrity and experienced only confined damage even when subjected to extremely high intensities. The building also exhibited satisfactory re‐centering ability with minimal residual deformations at the conclusion of testing. It was demonstrated that the self‐centering tube system is a desirable low‐damage alternative to traditional tube systems in earthquake‐prone regions. The benchmark data from this test provided unique suggestions and support for further research on the design methods, numerical modeling, and experimental testing of self‐centering tube systems.

Nonlinear Behavior of Bonded and Unbonded Two-Way Post-Tensioned Slabs Pre-Strengthened with CFRP Laminates

In this study, hybrid nonlinear finite element models (FEM) were developed to examine the flexural performance and the ultimate load capacity of bonded and unbonded two-way reinforced concrete post-tensioned (PT) slabs that were pre-strengthened with external carbon-fiber reinforcement polymer (CFRP) laminates. Full 3D simulations, using ANSYS models, have been created for five different slab samples that were selected from a previously available experimental study. The model results were assessed to enable further numerical analysis. The result calibration included measurements of first crack loads, ultimate loads, deflections, strains in the extreme fiber of concrete, strains in CFRP laminates, and failure modes. The results proved a good correlation between FEM output and experimental ones. Based on this, the influencing parameters that affect plate stiffness, as well as the bending capacity of PT slabs, were examined by performing a detailed parametric study. The parameters included real-life load simulation, cable-to-CFRP strength contribution, and CFRP laminate location selection. The results demonstrated that strengthening using CFRP laminates have significantly increased the ductility index of both bonded and unbonded PT concrete slabs by 62.18% and 59.87%, respectively. In addition, strip strengthening locations near supports are much more effective than in the middle of slabs. Additionally, the CFRP strengthening contribution is very considerable in slabs with low PT ratios.

Experimental study on a dry-assembled precast concrete frame with buckling-restrained braces

This paper proposes a new dry-assembled precast concrete frame with buckling-restrained braces (BRB-DPCF) to reach the full potential of building industrialization and promote the popularization of precast structures in earthquake-prone zones. A new mechanical coupler and an unbonded posttensioning (PT) technology were adopted for the column-to-column joint and beam-to-column joint, respectively, of the BRB-DPCF. In this paper, the authors designed and tested a monolithic concrete frame with buckling-restrained braces (BRB-MCF) and a BRB-DPCF to investigate the seismic performance of the BRB-DPCF. The two specimens were two-story, single-bay concrete frames with a story height of 1750 mm and a bay of 2500 mm. The test results demonstrated that the BRB-DPCF exhibited stable and fat hysteretic loops and a lateral-resistance capacity comparable to the BRB-MCF. The BRB-DPCF showed a greater initial stiffness and smaller energy dissipation capacity than the BRB-MCF. The column damage of the BRB-DPCF was similar to that of the BRB-MCF, while the girder damage of the BRB-DPCF was negligible compared with that of the BRB-MCF. The BRB-DPCF featured a greater repairable ability after earthquake events than the BRB-MCF.

Quasi-Static Cyclic Testing of Hybrid Posttensioned Bridge Column Supported on a Monopile Foundation

In this research, a low-damage posttensioned rocking solution referred to as dissipative controlled rocking (DCR) connection is proposed for bridge columns supported on monopile foundations. The DCR connection is a low-damage system that replaces the plastic hinge mechanism in a column and aims to minimize and potentially eliminate the repair time and costs after an earthquake. A DCR bridge system combines unbonded posttensioning and replaceable dissipaters to provide self-centering and energy absorption capabilities, respectively. Current research on DCR column systems has focused on the performance of bridge bents founded on a fixed foundation, neglecting soil damping effects. In this research, a one-third scaled bent supported on a monopile with passive soil resistance was tested under quasi-static cyclic loading to validate the lateral seismic response of a DCR bridge bent with the contribution of the soil–foundation–structure interaction. When developing the details for the DCR connection, particular emphasis was made on utilizing conventional construction materials and forms for a low-damage bent that yields a similar construction cost to a monolithic joint. Nevertheless, the proposed DCR connection can be easily and quickly repaired or replaced after a significant seismic event. The performance of the bent was compared against a benchmark structure with an emulative connection type that resembles a monolithic cast-in-place joint. Results from testing suggest a greater performance of the low-damage DCR column. The dynamic response of the DCR column built on a flexible foundation differed from that of a fixed-base structure, which is primarily due to the energy dissipation capability of the flexibly supported structure. Flexibility in the soil foundation increased the structural period of the structure and reduced the seismic response of the DCR column. Damage to the DCR bent was limited to concrete crushing at the rocking interface, which proved to be easily repairable. In contrast, the bent with an emulative connection developed plastic hinging at the bottom of the column, which was nonrepairable.

Artificial Neural Network Model for Predicting the Tendon Stress in Unbonded Posttensioned Concrete Members at the Ultimate Limit State

Artificial Neural Network Model for Predicting the Tendon Stress in Unbonded Posttensioned Concrete Members at the Ultimate Limit State

Novel panel system for concrete masonry walls

Masonry is one of the oldest materials that humans use for construction. It is durable, as seen on ancient structures built thousands of years ago, yet still in excellent shape. Recently, masonry walls have struggled to be competitive among other construction materials such as precast concrete, tilt-up walls, timber, etc. This is mainly due to construction practices and the lack of a standardized modular masonry system that prevents offsite construction. In this paper, new construction methods for concrete masonry walls are proposed. These methods are based on building partially grouted masonry panels that can be built onsite or offsite, then transported and assembled on the site. The methods commonly use a restrained unbonded post-tensioned (UPT) threaded bar to join panels. A full-scale experimental program campaign at the University of Alberta (UOA) is currently undergoing to test the proposed methods. Numerical models were developed before the experimental investigation to assess the behaviour of such techniques under out-of-plane loading. Bare joints, connecting panels reinforcement, and grouting joints are the three types of panel connections proposed here. The influences of the magnitude of prestressing and lateral restraints stiffness are also studied. The results show that the proposed approaches are comparable to traditionally constructed walls with improved serviceability. Also, because of the relative ease of construction and improved post-cracking behaviour, connecting panel reinforcement is the preferred method. Whereas the increasing level of prestressing enhances service conditions but leads to premature failure. Also, lateral restraints with high stiffness, such as grout or steel, are essential to enhance post-cracking behaviour.

Fragility-informed seismic design of multi-column bridge bents with post-tensioned concrete columns for accelerated bridge construction

Accelerated Bridge Construction (ABC) is playing an increasingly large role in bridge construction within urban centers due to faster installation, improved quality and lower cost. Concrete columns are prefabricated and connected together with footings and cap beams using either grouted splice sleeves, socket connections, or through post-tensioning. Precast unbonded post-tensioned (PT) concrete columns are protected from damage due to their ability to mitigate seismic demand through rocking. A numerical model is developed for rocking columns which is validated using published data. Seismic assessment of a two-column bridge bent constructed with precast PT concrete columns is carried out; column height-to-width ratio and ratio of initial posttensioning force to axial column capacity are selected as the assessment parameters. Performance limits are defined for the bridge model using non-linear pushover analysis. Probabilistic seismic hazard analysis is carried out using nonlinear time-history analysis for a set of eighty ground motions. The results show that PT concrete columns provide sufficient re-centering to the bridge bent so that it remains serviceable after a large earthquake. An ABC bridge bent with unbonded PT concrete columns is designed using the outcome of seismic fragility analysis. The seismic performance of the fragility-designed PT bridge bent is compared to that of a bent with conventional cast-in-place (CIP) concrete columns using nonlinear time-history analysis. The seismic performance of the ABC PT bridge bent using fragility analysis satisfied performance objectives similar to a monolithic CIP bent designed according to the latest seismic design codes; in addition, the residual drift of the PT bent was practically zero. The fragility-informed approach for designing ABC bridge bents constructed with unbonded PT concrete columns presented is expected to promote post-earthquake serviceability and functional recovery after a large earthquake.

Unbonded Post-Tensioned Structural Masonry Wall with Rubber Interface for Limited-Damage Systems

Unbonded post-tensioned (UPT) tendons have been used in structural masonry walls to enhance the self-centering capability of the walls. However, the lateral displacement capacity of masonry walls with UPT tendons can be compromised by early crushing of the compression toes. To prevent toe crushing, this research study employed rubber pads underneath the bottom corners of a full-scale one-story wall. This concept was investigated by subjecting the wall sequentially to free vibration and quasistatic tests, which minimized damage to the masonry and reduced the strength degradation of the wall with increasing lateral drift. Two major damping components were identified: one is due to the instantaneous impact of the wall on the foundation base, and the other is due to the inelastic action occurring within the rubber pads during the continuous phase of rocking motion. Using the test results, a procedure is presented for designing masonry walls with rubber pads and UPT tendons.

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.

Experimental testing of hybrid sliding‐rocking bridge columns under torsional and biaxial lateral loading

AbstractHybrid sliding‐rocking (HSR) bridge columns are precast concrete segmental columns with unbonded posttensioning, end rocking joints, and intermediate sliding joints. Having been developed for construction in seismic regions, HSR columns offer self‐centering, energy dissipation, and low damageability. Recent advances in the computational modeling of HSR columns have resulted in the proposal of a number of refinements to their original design, leading to the introduction of Second‐Generation HSR columns. Compared to First‐Generation HSR columns, Second‐Generation HSR columns use a reduced number of sliding joints made of high‐performance polytetrafluoroethylene (PTFE) materials to provide desirable levels of sliding and frictional energy dissipation. The seismic performance of Second‐Generation HSR columns was recently investigated through an extensive experimental study. The present paper discusses the major results of the tests performed on two half‐scale HSR column specimens under combined lateral‐torsional loading and biaxial lateral loading to demonstrate their performance under such loading scenarios. A variety of loading protocols were used to test each column specimen, including both cyclic and arbitrary paths. Both columns sustained minimal damage under displacement demands representing 975‐ and 2475‐year seismic hazards, meeting their design objectives. Because of the torsional sliding at sliding joints, the column subjected to lateral‐torsional loading did not exhibit any distinct torsional damage. The biaxial lateral loading was, however, found more damaging to HSR columns than torsional and uniaxial lateral loading.

Experiments on the Hysteretic Response of Self-Centering RC Columns Prestressed by Unbonded Posttensioned Tendons

ABSTRACT The prestressed column, where unbonded posttensioned tendons are arranged along the axis of the RC column, showed self-centering behavior after lateral loading. To investigate the detailed hysteretic response, nine prestressed RC columns with different axial loads and transverse reinforcement arrangements were tested and subjected to the combined action of cyclic horizontal loading and constant axial loading. The experimental results revealed the failure modes, and the drift capacity was analyzed using the ductility and plastic hinge zone length. The controlling effects on residual displacements varied over time during testing, and their mechanism was explained from the point of view of energy dissipation capacity.

Experimental investigation on the seismic behaviour of innovative self-centring precast steel-concrete hybrid frames

An innovative self-centring precast steel-concrete hybrid (SPH) frame was proposed. The proposed SPH frame was assembled with reinforced concrete (RC) columns and prefabricated steel-concrete hybrid (PH) beams using steel end plates and high-strength bolts. The RC columns consisted of a traditional RC section and an unbonded post-tensioned (PT) screw-thread steel (STS) bar, and the PH beam was composed of two steel beam ends, a prefabricated RC beam unit and four unbonded PT strands. The self-centring ability of the structure system was mainly provided by the PT steel reinforcements; the energy dissipation capacity was mainly provided by the bolted web friction devices (WFDs) applied to the beam and the plastic deformation of the steel beam ends. To examine the seismic behaviour of this innovative structural system, cyclic tests were conducted on two SPH frames and one non-prestressed precast steel-concrete hybrid (NPH) frame. By analysing failure modes, hysteretic loops, skeleton curves, energy dissipation, displacement ductility ratios, stiffness degradation and the relative self-centring efficiency (RSE), the effects of the initial prestress, the pretorque of the high-strength bolts at the WFDs and the construction measure at the column base were thoroughly assessed. The test results indicated that the proposed noncontact oil jack prestressing technique used in the PH beams was reliable; during the testing process, gap opening and closing could be clearly observed at the interfaces between the RC beam unit and the steel beam end; the SPH frames exhibited better self-centring and energy consumption capabilities than the NPH frame; and the SPH frame exhibited a typical two-stage cyclic behaviour, namely, a self-centring stage and an energy consumption stage. In the self-centring stage (before the drift ratio of 2.0%), the SPH frame could effectively control residual deformation, and the RSE remained at approximately 85%; in the energy consumption stage (after the drift ratio of 2.0%), the SPH frame could effectively consume the absorbed energy. Based on the test results, a moment calculation method, which could distinguish the gap opening in the PH beam, was proposed, and the calculated results agreed well with the experimental results.

Deformation and cyclic buckling capacity of external replaceable hysteretic dampers for unbonded post-tensioned precast concrete walls

This paper proposes and validates a type of external replaceable hysteretic dampers to be mainly applied to unbonded post-tensioned (UPT) precast walls. The main aim of the proposed damper was to provide the UPT walls with large energy dissipation without significant buckling. To evaluate the efficiency of these dampers, a cyclic test was conducted. The damper specimens consisted of a mild steel round bar with a reduced diameter over a fused length and an additional threaded length at each side, for installation purposes. Moreover, a round steel tube was installed over the fused length and filled with grout to delay the buckling. The proposed dampers, with different geometries and conditions, were subjected to cyclic loads to evaluate their deformation capacity and fracture mechanism. The specimens displayed a regular and stable hysteretic curve until strains of 9%, after which a flexural buckling was observed at the ends of the fused length. However, this flexural buckling did not affect the energy-dissipating capacity in any specimen. Consequently, this type of damper is considered suitable for applications in self-centering rocking walls. Furthermore, the proposed dampers fractured by a combination of low-cycle fatigue and excessive rotations at the transition length. In addition to the experimental validation, an extensive numerical investigation was conducted on the proposed damper to complement the experimental data and to provide analytical tools for design.

Flexural Performance of Unbonded Posttensioned Rectangular Concrete Filled FRP Tube Beams

Abstract This paper experimentally investigates the effect of using the posttensioning (PT) steel strands on the flexural performance of rectangular concrete filled fiber-reinforced polymer (FRP) t...

Effect of Major Design Parameters on the Seismic Performance of Bridges with Hybrid Sliding–Rocking Columns

This paper explores the effects of major design variables on the seismic performance of the so-called hybrid sliding–rocking (HSR) bridge columns. The HSR bridge columns are precast concrete segmental columns with internal unbonded posttensioning, end rocking joints and intermediate sliding joints, which offer significant self-centering and energy dissipation capabilities. HSR columns introduce additional design variables, the effects of which on the seismic response of such columns have remained unclear. Using a recently developed robust computational modeling approach, this study identifies four major design variables, investigates their effects on the dynamic response of HSR columns, and proposes component-level design recommendations. The four design variables include the number of the sliding joints and their distribution over the column height, the incipient sliding base shear, the incipient bearing sliding amplitude, and the peak achievable sliding capacity. This study further investigates the effect of the vertical component of the earthquake excitation on the response of HSR columns and compares the seismic performance of an HSR column with that of a rocking-only column of the same strength. The findings indicate that HSR columns can be more damage resistant than rocking-only columns under earthquake excitations. The response of HSR columns was found to be insensitive to the vertical component of the earthquake excitation.

Experimental comparisons of repairable precast concrete shear walls with a monolithic cast-in-place wall

Previous research has shown that nonemulative (jointed) precast walls with unbonded posttensioning can achieve superior seismic performance (e.g., self-centering) compared to conventional monolithic cast-in-place reinforced concrete walls. However, the progression of significant damage and failure in nonemulative precast walls is through the yielding of ductile steel reinforcing bars (referred to as energy dissipating bars) followed by the crushing of concrete at the wall toes, greatly limiting the ability to repair these walls after a major earthquake. In this paper, a nonemulative precast wall system with replaceable, external buckling restrained plates (BRPs) for energy dissipation, and steel jackets for confinement of the concrete at the wall base is investigated through reversed-cyclic lateral loading tests. Four precast wall specimens with varying cross-sectional area of the BRPs and number of horizontal joints were tested and compared against a monolithic cast-in-place reinforced concrete wall. The precast walls exhibited no significant unrepairable tensile or compressive damage with increasinglateral strength up to 4% drift, showing advantages over the cast-in-place wall and non-jacketed precast walls in previous research. With out-of-plane buckling restrained, the BRPs yielded in tension and compression, providing the precast walls with desirable energy dissipation. To highlight the reparability of the precast wall system, one of the specimens was repaired after being subjected to a complete cyclic loading history up to 4% drift. The repair was done by replacing the damaged BRPs, after which the wall was re-tested. The repaired wall restored most of its original energy dissipation, lateral stiffness, and strength, with limited cumulativedamage in the wall. In general, the proposed precast wall system with buckling restrained plates is desirable for seismic regions, offering excellent reparability and seismic resilience.