Unbonded Post-tensioned Tendons Research Articles

Seismic behavior of resilient composite steel plate shear wall using experimental investigations

Using composite encased steel plate shear wall (C-SPSW) has been attractively raised for enhancing the lateral seismic performance and increasing the useful floor area. Although, the experimental investigations have proved excellent dynamic behavior of C-SPSWs, structural damage as well as residual displacements are the main disadvantages of C-SPSWs, which are not consistent with low damage design (LDD) approaches. In the current study, the C-SPSWs performance is improved using the self-centering (SC) technique to control the both structural damage and reduce residual displacements as well. Five tests on C-SPSWs including three SC composite encased steel plate shear walls (SC-C-SPSWs) were conducted under gravity and lateral cyclic loads. The SC-C-SPSW consists of C-SPSW equipped with unbonded posttensioned tendons (UPT) to have resilience ability. For this aim, two symmetrical gaps were provided at the ends of the wall, and the UPTs were mounted at the wall ends and crossed into the foundation through the middle of the gaps. Under lateral load, the gap opening allows the elongation of UPTs that induce a high restoring forces, forcing the wall to have rocking movement above the foundation. In comparison with C-SPSWs, the SC-C-SPSWs exhibited low damages, higher shear strength capacity between 28% and 35%, larger ultimate drift capacity and more displacement ductility as 45–136% in average. Furthermore, the SC ability and the UPTs restoring forces effectively reduced the residual drift and the SC moment causes the recovery of the residual drift between 90% and 105%. Moreover, compared to C-SPSWs, the residual lateral strength of SC-C-SPSWs was improved by 30% in average.

Experimental and numerical evaluation of seismic behavior of coupled walls connected by post-tensioned, unbonded, precast concrete coupling beams

To investigate the seismic performance of coupled shear walls, which are connected by post-tensioned, unbonded, precast concrete coupling beams (PCCB), two 1/7-scale experiments with 8-story coupled shear walls under cyclic lateral loadings were conducted. Coupling of concrete shear walls is achieved by post-tensioning concrete beams to the wall piers using unbonded post-tensioning tendons (UPT) at the floor and roof levels. Experimental results show that little damage occurs in the coupling beams and the wall regions when the specimen undergoes large nonlinear displacements. The residual displacements of the specimen are small because of the restoring effect of the post-tensioning force. The seismic performance of three systems including CW-RC (coupled wall systems that use traditional RC coupling beams), CW-UP (coupled wall systems which use only post-tensioning tendons without steel reinforcement through the beam-wall joints), and CW-HUP (shear walls and PCCB that use combinations of steel reinforcement and post-tensioning tendons) are evaluated by means of nonlinear dynamic analysis. The calculation results indicate that the inter-story drift and peak lateral displacements of CW-HUP and CW-UP are greater than that of CW-RC, whereas the residual deformations are reduced because of the self-centering effect of the post-tensioning tendons.

Seismic behavior of self-centering precast segmental bridge piers with external auxetic steel shear panel dampers

Precast self-centering segmental bridge piers have gained significant attention in bridge construction. Among them, development of the energy dissipation devices that are applicable for the self-centering bridge piers has been a major focus over the past three decades. This study develops a novel post-tensioned precast segmental bridge piers (SC-PSBPs), incorporating replaceable external auxetic steel shear panel dampers (ASSPDs) at the bottom and connecting steel plates at the intermediate segments. The seismic behavior of the SC-PSBPs is investigated through experiments and numerical simulations. Auxetic metamaterials are utilized to develop two kinds of perforated steel plates, including elliptical and peanut-shaped perforations, for the ASSPDs. Eight SC-PSBP specimens are tested to study failure patterns, hysteresis response, residual displacement, segment gap opening behavior, and local response of steel components. Test results show that the proposed SC-PSBP, equipped with unbonded post-tensioning (PT) tendons and ASSPDs, exhibits stable hysteresis response and self-centering capacity with minimal concrete damage. Plastic deformation is concentrated in the ASSPDs, while the intermediate connecting steel plates behave elastically throughout the loading history. A numerical model is developed to investigate the influence of design parameters on the hysteresis behavior of SC-PSBPs. Parametric analyses using the validated FE models explore the seismic behavior of SC-PSBPs with different geometries of ASSPDs, connecting steel plates, and initial PT forces. Numerical results show that both the ASSPDs and the connecting steel plates are imperative to achieve the desired rocking and damage concentration mechanisms of SC-PSBPs. The numerical findings also indicate that decreasing the porosity of ASSPDs effectively enhances the bearing and energy dissipation capacity of SC-PSBPs, as well as increases their residual displacement.

Quasi-static Cyclic Tests and Paramatric Analysis on the Seismic Performance of Partial Precast Bridge Columns

Partial precast bridge column (PPBC) is an innovative column composed of upper precast segments and cast-in-place (CIP) base. The CIP base and precast segments are connected by unbonded posttensioned tendons. It is expected to achieve self-centering capacity as well as energy dissipating capacity during earthquakes. However, the research about damage mechanism and seismic performance of PPBC are still scarce. To investigate the seismic performance, quasi-static cyclic tests of one PPBC and a referenced CIP column are conducted in this study. Unlike CIP column, the PPBC shows excessive joint opening and obvious rocking characteristics during cyclic loadings. Slight concrete cracking in CIP base and severe concrete spalling at upper joints implies limited ductility of PPBC. Moreover, PPBC indicates smaller horizontal force, minor residual drifts and lower energy dissipation when compare with CIP columns. Based on damage distribution and deformable modes, increased prestress force, elevated CIP base and enlarged prestress ratios are analyzed to improve the anti-seismic behavior of PPBC. With cautious simulations and comprehensive comparisons, elevated CIP base and enlarged prestress ratios are validated to enhance the energy-dissipating capacity of PPBC.

Experimental study on the seismic performance of precast segmental unbonded post-tensioned frame piers

Prefabricated assembly technology offers significant benefits for the construction of urban bridges, including shortened construction periods and reduction of traffic congestion. It can be used to construct precast concrete structures that are capable of withstanding seismic loads. However, studies on the seismic performance of precast segmental unbonded post-tensioned frame concrete piers are lacking. Therefore, there is an urgent need to evaluate the seismic performance of piers under different prestressing degrees. In the study, three prefabricated frame concrete piers were designed and subjected to quasi-static tests: unbonded post-tensioned tendons (UPT) with the prestressing degrees of 0.5 and 0.7 combined with grouted splice sleeves (UPT-0.5 specimen, UPT-0.7 specimen); and grouted splice sleeve connection (a reference specimen PC-S). The seismic performance of the UPT-0.5, UPT-0.7, and PC-S specimens was evaluated based on various evaluation indexes such as bearing capacity, residual displacement, displacement ductility, displacement of column segments, stiffness degradation, and energy dissipation (ED) capacity. The test results showed that the three piers exhibited flexural failure; however, the damage to the UPT-0.5 specimen was the most severe. With the increase of prestressing force, the hysteresis curve of the UPT-0.7 specimen was greater, the yield load and peak load of UPT specimens both increased by more than 18%, the initial stiffness and failure stiffness increased by more than 10%, but the impact on the ductility, residual displacement, and displacement of column segments was relatively small. At the end of the test, the cumulative energy dissipation (ED) values of specimens UPT-0.5 and UPT-0.7 were 76% and 53% lower than those of the PC-S specimen, respectively. The combination of grouted splice sleeves and unbonded post-tensioned tendons endows precast segmental frame piers with the characteristics of rapid construction and excellent self-centring ability. These characteristics are particularly advantageous for the efficient post-earthquake rehabilitation of urban bridges in low-to-medium seismic areas.

Cyclic loading test of self-centering precast segmental concrete high-speed railway bridge columns with UHPC-filled duct connections

Precast concrete high-speed railway (HSR) bridge columns are attractive for standardized HSR bridges, but their applications are rare partly due to a research gap in their lateral behavior. Traditional under-reinforced HSR bridge columns have huge lateral stiffness and poor drift capacity, which means that their prefabrication is not a simple separation-assembly process and requires special research. This study aims to develop a novel self-centering precast segmental concrete (SPSC) HSR bridge column, which is designed to have five major improvements including adopting large-diameter deformed bars as energy dissipation (ED) bars, inter-locking multi-hoops, unbonded post-tensioning (PT) tendons, a cast-in-situ ultra-high performance concrete (UHPC) layer at the bottom joint, and UHPC-filled duct connections. A 1/6 scaled cyclic loading test was conducted on three specimens to validate prefabrication feasibility and assess seismic behavior. The research results show that inter-locking multi-hoops can enable the developed bridge columns to have ductile flexural failure and a drift capacity of up to 5 %. The bridge columns have increased lateral stiffness and strength as well as decreased ductility. UHPC-filled duct connections can cause the strain amplification of ED bars at the bottom joint. The bottom joint opening dominates lateral deformation while the influence of joint sliding can be neglected. The bridge columns have good ED capacity and the equivalent hysteretic damping ratio of about 20 % at the drift ratio of 5 %. The bridge columns have the excellent self-centering capacity to control residual drift ratio in an allowable range before flexural failure. The acceptable drift ratios are suggested to be 3.5 % and 4.0 %, respectively, for the SPSC HSR bridge columns without and with PT force.

Progressive collapse analysis of precast reinforced concrete beam-column assemblies with different dry connections

Beam-column joints using dry connections usually experience severe damage under large deformations, causing precast concrete (PC) frames with dry connections to become highly vulnerable to progressive collapse. Hence, this study conducted a static pushdown test adopting uniformly distributed loading on four PC beam-column assemblies using dry connections and one reinforced concrete (RC) substructure to investigate the collapse-resisting behavior under the middle-column removal scenario. The tested dry connections included top-and-seat angle connection (TA), strengthened top-seat angle connection (STA), and two refined connections with unbonded post-tensioning tendons (UPTA and UPSTA, respectively). Results showed that compared with specimen RC, the resistance of specimen TA at the beam flexural and compressive arch action (B-CAA) and catenary action (CA) stages dropped by 41% and 27%, respectively, due to the buckling of steel angles and endplates (B-CAA) and the fracture of steel angles and bolts (CA). Specimen STA had a 7% greater resistance of B-CAA because of the strengthened steel angles. However, the ductility decreased, with the displacement of rebar fracture decreasing by 37%, leading to a 30% resistance reduction of CA. The unbonded post-tensioning tendons dramatically improved the structural integrity, contributing to 52% and 97% increase in the resistance of the CA of UPTA and UPSTA, respectively. Further numerical analyses indicated that the restraint gaps reduced the initial stiffness of the collapse responses, and a good rebars welding anchorage condition could remarkably improve the resistance in the B-CAA and CA stages. An improved joint was proposed based on UPTA and rebar welding anchorage to improve deformation capacity while retaining reasonable collapse resistance. Finally, the resistance contributions from different mechanisms and components were identified through a developed calculation procedure.

Experimental Assessment of Resilient Controlled Rocking Masonry Walls with Replaceable Energy Dissipation

In recent years, several studies have been conducted to evaluate the seismic response of controlled rocking masonry walls (CRMWs) that depend on unbonded post-tensioning (PT) tendons. The current paper describes the quasi-static cyclic testing of proposed controlled rocking masonry walls that omit PT but incorporate externally attached energy dissipation (ED-CRMWs). The proposed external energy dissipation (ED) is a flexural yielding device, named a flexural arm, that is bolted to the wall through a special steel hollow block to allow simple and fast replacement after an earthquake. As such, this wall system overcomes the limitation of internal ED devices (e.g., unbonded axial yielding bars) of being unreachable and unreplaceable after damage due to yielding or fracturing. In addition, the paper reports the retesting of the proposed controlled rocking wall after being repaired and subsequently compares the performance to that of the original wall in order to evaluate the wall resilience following seismic events. The experimental results are discussed in terms of the failure modes and damage pattern, force–displacement response, wall lateral load capacity, residual drifts, displacement ductility, and ED capacity. The results show that using a special hollow steel block at strategic locations produced a high displacement capacity of 5.0% drift without strength degradation and preserved the intended self-centering with residual drifts of less than 0.2% until at least 3.7% lateral drift. The flexural arm also had significant ductility capacity, where the walls reached 5.0% drift without buckling or fracturing of the arms. Both the original and the repaired walls exhibited limited and localized damage at the wall toes, thus positioning the proposed ED-CRMWs as a resilient system for masonry construction. Finally, the paper presents design recommendations that could be considered for introducing ED-CRMWs with flexural arms in future relevant standards.

Analysis and experimental testing of a self-centering controlled rocking wall with buckling-restrained braces at base

A new self-centering controlled rocking wall (CRW) with buckling-restrained braces (BRBs) at the base (SC-CRWBB) is proposed to improve the energy dissipation and self-centering capacities of CRWs. The SC-CRWBB was established by adding unbonded posttensioned tendons (UPTs) at the center and BRBs at two corners of a CRW. The quasi-static test results demonstrated that the proposed SC-CRWBB achieved a satisfactory seismic performance. Structural damage was concentrated in the BRBs, while a few cracks occurred in the surface between the CRW and mortar layer and between the BRBs and CRW. Then, a finite element model (FEM) of the test specimen was created in ABAQUS software to study the influence of the initial stress of the UPTs and sectional area of the BRB core plate on the residual deformation and lateral carrying capacity of the SC-CRWBB. Finally, an analytical method for the investigation of SC-CRWBBs was validated against the test and FE analysis results.

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.

Fatigue of Diabolo-Deviated Unbonded Post-Tensioning Tendons

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An analytical design approach for post-tensioned concrete slabs with unguided tendons

This article contains practical information on the design and dimensioning of prestressed concrete solid flat slabs using unbonded post-tensioned tendons, in accordance with the European standard EN 1992-1-1 and German standard DIN 1045-1. The calculation method and the mathematical equations necessary for the dimensioning of the non-adherent unguided tendons are given. The paper presents a case study where a slab floor is dimensioned for deflection using prestressing only in the column strips. The deflection of the slab is calculated using equivalent loads (unguided tendon), a mixt tendon profile with 4th degree curve segments (unguided tendon) and parabolic tendon profile with 2nd degree curve segments (guided tendon-classical solution).

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.

LOAD RESISTING MECHANISM OF PRECAST STRUCTURE UNDER EXTERIOR COLUMN FAILURE

In order to study the anti-continuous collapse mechanism of an unbonded post-tensioning precast structure with prestressed spliced connections under the condition of side column failure, four one-half scaled precast beam-column substructures were tested in the laboratory. The performance of three types of precast beam-column connections are investigated by using a pushdown loading approach. The test results demonstrated that different connection types exhibit different failure modes. The installation of unbonded post-tensioning connections is an effective way to resist the progressive collapse of precast concrete frames. The hybrid connection achieved greater load resistance than the summation of the load resistance of the unbonded post-tensioning connection and steel angle connection individually. Higher effective prestress in unbonded post-tensioning tendons could increase the load resistance in a relatively small deformation stage. However, the higher effective prestress may decrease the ultimate load capacity of the frame at a large deformation stage. Finally, high-fidelity finite element models, which were relied on commercial software ANSYS/LSDYNA, were developed. After validation, the results, which could not be measured during tests, were presented and discussed. Then, parametric studies were carried out based on the validated models. The numerical results showed that bonded post-tensioning tendons could increase the resistance of the specimen with unbounded post-tensioning tendons. Increasing the concrete strength could also increase the load resistance significantly. However, higher axial compressive forces at side columns will aggregate the P-Δ effects, which leads to lower deformation capacity and ultimate load resistance capacity.

Unbonded Post-tensioned Precast Concrete Walls With Rocking Connections: Modeling Approaches and Impact Damping

Over the past two decades, precast concrete members have been utilized in seismically resilient structures. In developing these structures, different techniques have been used for connecting the precast members to the foundation. In building construction, unbonded post-tensioning (PT) tendons can anchor a precast wall to the foundation, resulting in the so-called rocking wall system. The rocking wall system develops a dry connection with the foundation and provides moment resistance by means of the PT tendons. The PT tendons remain elastic when the wall is subjected to design-level ground motions to preserve the re-centering capability of the wall. Moreover, the structural damage is concentrated near the wall toes and can be minimized with proper detailing of the toes. Rocking wall systems can consist of a Single precast Rocking Wall (SRW), which uses no supplemental damping, or walls with supplemental damping in the form of viscous or hysteretic energy dissipating devices. In addition to the supplemental damping, rocking walls dissipate the seismic energy through their impacts on the foundation base, their inherent viscous damping, and the hysteresis of concrete near the wall base. While the investigation of rocking walls continues to gain interest, there is no widely accepted means of modeling their dynamic behavior. This paper investigates two popular approaches for modeling rocking walls with and without supplemental damping: the finite element method and analytical modeling. The ability of the two approaches to capture the local and global responses of the walls is evaluated against shake table tests of walls with multiple-level intensity base motions. Next, the behavior characteristics of the two modeling approaches and their ability to simulate impact damping are discussed.

Pulse-like near-fault ground motion effects on controlled rocking steel cores

Controlled rocking steel cores (CRSCs) with reliable self-centering capability have been known as a low-damage seismic force-resisting system. The flag-shaped hysteresis response of CRSCs can be provided using unbonded post-tensioning tendons and replaceable energy dissipation devices. This paper examines the seismic performance of CRSCs subjected to a suite of pulse-like near-field and far-field ground motions. Non-linear dynamic response analysis is performed for 3-, 6-, 9-, and 12-story archetypes with single and coupled configurations. Results are quantified for different seismic responses, and the influences of earthquake type and structural configuration are compared from various aspects. The findings indicate remarkable changes in the performance of CRSCs due to modeling and ground motion properties. The outcomes reveal that rocking-core archetypes are efficient for mitigating residual damage and restricting peak displacements under both far- and near-field motions.

Cyclic tests of precast post-tensioned concrete filled steel tubular (PCFT) columns with internal energy-dissipating bars

In this paper, a promising construction option, which combines the concrete filled steel tubular (CFT) column and the self-centering rocking system, is introduced and its seismic behavior is experimentally investigated. In this hybrid system, named as post-tensioned concrete filled steel tubular (PCFT) column, the unbonded post-tensioned (PT) tendons and energy dissipation (ED) devices are employed to provide recentering and ED properties, respectively. The steel tube acts as a protective jacket, stay-in-place formwork, and transverse reinforcement. Six PCFT columns with different amounts of initial PT force and ED bars were tested. All columns failed due to the fracture of ED bars and the crushing of mortar bed which though was cast from ultra-high performance concrete (UHPC). The column shafts of specimens suffered little damage after testing. The amount of ED bars greatly affected the self-centering capacity and ED capacity of the PCFT columns when the ratio of ED bars was less than 1.2%. Beyond this ratio, its effect was negligible. Additionally, increasing initial PT force made only a little difference to the load-carrying capacity but significantly enhanced the self-centering capacity of the column.

Shaking table test for externally-hung self-centering rocking wall structure

This paper proposes an innovative externally-hung self-centering rocking wall (ESRW) structural system for retrofitting existing reinforced concrete structures. Reinforced corbels are installed on the columns of an existing structure to which rocking walls having unbonded post-tensioned tendons are connected using H-shaped connectors. The seismic performance and self-centering ability of the ESRW were investigated through a series of shaking table tests. For comparison, an unretrofit control structure (UCS) was also tested. The UCS was severely damaged under peak ground acceleration (PGA) = 1.2 g test conditions: some reinforcing bars were fractured and the UCS was close to collapse. For the ESRW, the damage was significantly reduced: there were only minor cracks appearing at PGA = 1.5 g test conditions and no damage appeared at the corners of the rocking walls. The maximum story drift and residual deformation of the ESRW were reduced 88% and 92%, respectively, relative to the UCS test. The ESRW structure showed desirable seismic performance and self-centering characteristics during shaking table tests.

Experimental and numerical study of low-damage self-centering precast concrete frame connections with replaceable dampers

A kind of low-damage self-centering precast concrete (LDSCPC) frame connection with replaceable dampers was proposed. Precast components with embedded steel connectors were assembled by unbonded post-tensioned (PT) tendons, and the modular energy dissipation devices made them have great upgrading and expanding capacity. Based on the cyclic load tests of three specimens of LDSCPC frame joint and one specimen of RC frame joint, the influence of initial PT force, location of PT tendons and contribution of dampers on the mechanical performance of LDSCPC joint and the significant difference with RC joint were studied. The test results showed that the load-displacement behavior of the LDSCPC joint presented two-stage characteristics, with the excellent self-centering ability and stable mechanical properties. The stress level of the precast beam of the LDSCPC joint at 3% chord rotation could reach the stress level that of the RC joint at 0.25% chord rotation, and only a few 0.08 mm wide cracks appeared on the concrete surface. The precast components were at lower stress levels and had excellent low damage characteristics. The elaborate solid-element FE model was established, and the results of numerical simulation proved the behavior and characteristics of the LDSCPC joint in the experiment. The parametric study related to the damper was carried out, and some design comments and suggestions were also made.

Overview of Connection Detailing for Post-tensioned Dissipative Timber Frames

The concept of post-tensioned rocking structures was successfully applied to timber in 2005, in prestressed laminated timber (Pres-Lam) technology. When applied to moment-resisting frames, the technology relies on unbonded post-tensioning tendons passing through hollow beams and the supporting column to provide beam–column connections with moment capacity. Supplemental dissipation devices can be included in the design, resulting in a recentring/dissipative system. Since seismic movements are accommodated through a control rocking mechanism between the beam and the column, the detailing of the beam–column joint can significantly affect the seismic performance of the structure. Among the main concerns, the shear transfer system should be able to transfer shear forces without interfering with the rocking motion, and ignoring the face friction and joint deformation should not compromise the drift activation of the gap opening. Attention should also be paid to minimizing post-tensioning losses while providing access for potential retensioning. As a result of the continuous and rapid development of Pres-Lam frame systems, a wide range of alternative solutions for connection detailing has been tested in the laboratory or implemented in existing buildings. This paper presents an overview of the beam–column detailing implemented in the five Pres-Lam frame buildings currently operative in New Zealand.