Post-tensioning Bars Research Articles

Shear Strengthening of RC Beams Incorporating Post-Tensioned Bars and Engineered Cementitious Composite Reinforced with Palm Fronds

This paper investigates, experimentally and numerically, the shear strengthening of Normal Concrete (NC) beams using post-tensioning steel bars and Engineered Cementitious Composite (ECC) reinforced with chemically cured Palm Fronds (PFs). The benefits of strain-hardening ECC and the tensile strength of PFs cured with 6% wt Alkali NaOH solution beside post-tensioned bars have been employed herein. Seven full-scale Reinforced Concrete (RC) beams were fabricated and experimented with under three-point loading until failure. The test parameters include the strengthening technique, type, and configuration of the material used for strengthening. The strengthening process has been implemented through two techniques: Externally Bonded Reinforcement (EBR) and Near-Surface Mounted (NSM) Reinforcement. The strengthening materials have been configured and placed in horizontal, vertical, and inclined positions. The effectiveness of the strengthening methods has been evaluated by examining their cracking propagations, load-deflection responses, collapse modes, elastic stiffness, and absorbed energy. It was found that the proposed strengthening systems could significantly control the crack pattern and failure mode, and they could enhance the ultimate load amplitude up to 37% and 50% for NSM ECC with PFs and EBR post-tensioning steel bars, respectively. Nonlinear three-dimensional finite element models of the tested beams were developed and validated with the test data, where it was found that finite element models predict the structural performance of tested beams with a maximum error of only 2%.

Optimization and experimental study on seismic performance of precast dry-wet hybrid beam-column joint with local prestressing

In order to improve the seismic performance, a novel dry-wet hybrid beam-column joint was optimized based on the findings of the preliminary experiment. Four full-scale prefabricated specimens were tested under cyclic loading to further investigate their seismic behavior and explore the various factors, including the connection methods for beam top longitudinal reinforcement in the joint core, magnitude of prestressing force, forms of hoop reinforcement at beam ends, and types of grouting material used in corrugated steel ducts. The results indicate that the new joint failed in the flexural mode, and its experimental bearing capacity value was consistent with the theoretical value, indicating a good safety margin. Additionally, it exhibited remarkable ultimate deformation capacity and displacement ductility coefficient exceeding 3, comparable to cast-in-place structures. The performance indicators of post-tensioned (PT) bar specimens were similar to those of cast-in-place structures and superior to PT rod specimens. The energy dissipation of PT rod specimens accounted for approximately 80 % to 90 % of that of the PT bar specimen. The viscous damping ratios of specimens with PT bars and PT rods were approximately 20 % and 15 %, respectively. Two methods of connecting beam top longitudinal bars by grouting or couplers, accompanied by a local unbonded treatment, could both fulfill anchorage requirements. However, the later one resulted in loosening of steel bars within the connector during late loading stages. The anchoring performance of prestressed grouting material could satisfy the anchoring requirements of PT bars, but still failed to meet the slippage resistance requirements of PT rods. Increasing the prestressing force can enhance the initial stiffness, significantly reduce yield displacement, and improve ductility coefficient, while having minimal impact on strength and energy dissipation. The open composite stirrups at beam ends do not significantly impact overall performance of the joint. The optimal details for the novel joint involves a combination of high-strength PT bars, grouting in corrugated ducts with local debonding treatment, and open composite stirrups at beam ends.

Hybrid bridge bent using stretch length anchors with post-tensioning and shear key alternatives

Precast concrete bridge bents are good candidates for accelerating bridge construction. In seismic regions, in addition to construction speed, there is a need to improve seismic performance of bridges. One promising solution is self-centering bridge bents that use post-tensioning and external energy-dissipation devices as a hybrid system. This paper presents cyclic load experiments of hybrid two-column bridge bents to assess their performance regarding self-centering and hysteretic energy dissipation. The cyclic experiments were conducted on 42% scaled bridge bents that used unbonded post-tensioning (PT) bars for self-centering and stretch length anchors (SLAs) as external energy dissipators. A total of six experiments are investigated: three with PT bars and SLAs and three with PT bars only. The variables included the number of PT bars in each column (two or four), the initial post-tensioning force in the columns, and the shear key details in the footings and cap beam. The PT bars remained elastic until 4.0% drift ratio but some variation was observed in the post-tensioning forces especially for columns with two PT bars. For post-tensioned bridge bents with or without SLAs, increasing the number of PT bars in each column from two to four enabled efficient self-centering by limiting the residual drift ratio to 0.5%. There was no damage to the concrete for bents with four PT bars and there was repairable concrete damage for bents with two PT bars. The hysteretic energy dissipated by bridge bents with SLAs in addition to PT bars was 1.8 to 3.2 times the hysteretic energy of bridge bents with only PT bars.

Review and analysis of RC bridge half-joints strengthening techniques

In the past, the extensive use of the Gerber scheme in the construction of existing bridges has been justified by the advantages in terms of reduced stresses of the statically determined scheme and rapid execution. On the other hand, the presence of junction elements at the ends of the beams has over time led to a series of issues, especially related to their durability. In fact, the half-joints’ geometric configuration leads to the accumulation of degradation due to platform water seepage, resulting in the initiation of reinforcement corrosion, especially in the presence of chlorides derived from de-icing salts used in the winter season. Furthermore, these elements have been designed for lower loads than those specified in current regulations, and therefore, they may be in critical conditions where the strength reduction, due to existing degradation, can lead to failure. For these reasons, local repairs and/or strengthening interventions are necessary to ensure adequate levels of safety. Within this study, the topic of reinforcing half-joints is addressed, first by describing the currently available intervention techniques in the literature, and then by deepening an intervention technique based on the use of external post-tensioned bars, applied to a case study bridge through nonlinear numerical simulations. These analyses allow quantifying the performance improvements of this technique under both service conditions and ultimate loads, considering various parameters, including the pre-stress value applied to the external tendons and different scenarios of chloride-induced corrosion.

Enhanced Strategies for Seismic Resilient Posttensioned Reinforced Concrete Bridge Piers: Experimental Tests and Numerical Simulations

Recent studies have proposed and investigated the use of unbonded posttensioned (PT) bars in bridge substructure systems to improve their self-centering behavior. However, the lateral loading resistance and self-centering capacities of reinforced concrete (RC) piers with PT bars (i.e., the posttensioned RC bridge piers: PRC piers) could be easily compromised by early crushing of the base compression toe during a seismic event. Hence, in order to enhance the pier base integrity, the present study proposes and experimentally investigates three simple strategies for enhancing the seismic-damage resistance of PRC piers based on the use of (1) a steel tube to encase the PRC pier’s end segment; (2) ultrahigh performance concrete at the PRC pier’s end segment; and (3) engineered cementitious composite mortar bed underneath the pier bottom. The performance of these three PRC piers was assessed by comparing them with a conventional PRC pier under cyclic loading and considering the damage evolution and the cyclic force-displacement response. The comparison shows that the proposed enhanced PRC solutions allow improving the seismic performance of the system sustaining large lateral drifts with good self-centering behavior. Moreover, based on the test results, finite element models accounting for PT force loss and the rocking characteristics were validated to reproduce the piers’ cyclic response, thus highlighting the importance of considering PT force loss during numerical simulations.

Seismic Analysis of Posttensioned and Hybrid Bridge Bents with Buckling Restrained Braces

A numerical model is presented for a rocking bridge bent specimen consisting of two precast concrete columns with posttensioning (PT) bars, two footings, and a cap beam, with or without a buckling restrained brace (BRB). First, a numerical model of the posttensioned bridge bent specimen without a BRB is developed; emphasis is placed on modeling the rocking mechanism at the joint between precast concrete members joined only through PT bars. The numerical model is extended to a hybrid specimen consisting of a two-column posttensioned bridge bent with a BRB as an external energy dissipation device. Experimental results obtained from quasi-static cyclic tests of the posttensioned and hybrid specimens are compared with the numerical models. Satisfactory agreement of the numerical models with the experimental results is observed in terms of structural response, hysteretic energy, and PT-bar forces. The numerical models are subsequently extended to a previously tested three-column bridge bent for two configurations: (1) a posttensioned bridge bent with PT bars, and (2) a hybrid bridge bent with PT bars and two BRBs. A fiber-based bridge bent model is built, which includes soil–structure interaction (SSI) using simplified springs. A comparison of the posttensioned and hybrid bridge bents using far-field and near-field ground motions shows that the hybrid bridge bent displays superior performance compared with the posttensioned-only bridge bent.

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.

Bridge deck analysis of transversely post-tensioned concrete box girder bridges

For rural bridges in Australia, a common design practice is pouring in-situ concrete on top of beams in order to tie all the beams together and distribute load. However, pouring concrete on-site creates more risk and contractors prefer to avoid it. Another method is using transverse post tensioning to tie beams. This article investigated the behaviour of transverse post-tensioning bars in providing load distribution between beams and ultimately comment on their effectiveness compared to in-situ poured decks. Currently, the industry has not completely investigated this matter in order to design post-tensioning accurately. Conservative estimates are currently used in industry today. Current practice is 50% of the design load on the beam where the load is applied in their design assumptions which is quite high. The team modelled concrete box girder bridges with transverse post-tensioning using grillage method. Several factors were investigated including bridge length and width, bridge skew and beam type. From the models, the team concluded that increasing the bridge span increases the load distribution, the load distribution difference is negligible for skew between 0 and 20 degrees and larger shear actions are observed with increased skew and width. It was determined that the worst-case total load on the beam where the load as applied was found to be 40.5%, 9.5% less than current practice. It is recommended that a similar investigation is conducted using a finite element method to gain a deeper understanding.

Post-tensioned rocking dissipative timber wall systems: Numerical prediction

Low-damage post-tensioned technologies were born with the aim to obtain structural solutions able to guarantee, in case of seismic events, not only the safeguarding of human life, but also the rapid re-use of the buildings without permanent damage. In this context, structural systems based on rocking dissipative timber walls have been introduced, in which rocking behavior is entrusted to post-tensioned bars, and the dissipative contribution is provided by steel dampers, easily replaceable after an intense seismic event. The paper presents a nonlinear numerical model to predict the response of different configurations of Post-Tensioned (PT)-timber wall systems, particularly useful to support the design phase. More in detail, the geometrical non-linearity due to the rocking behavior of the system and material non-linearities that characterize the dampers are taken explicitly into account. The proposed numerical modelling strategy has been validated on different experimental literature tests performed by non-linear static analyses on both single and double wall systems equipped with different damper configurations. The comparison between numerical and experimental results is performed to demonstrate the effectiveness of the proposed model in predicting the behavior of post-tensioned walls subjected to cyclic loads. Further, parametric analyses have been developed in order to investigate the effect of different parameters, such as wall dimensions, initial post-tensioned force in the bars, number of axial and shear dampers, on the global behaviour of post-tensioned timber systems.

Cyclic model of post-tensioned low damage timber walls with dissipative devices

Post-tensioned rocking dissipative timber wall systems born from the idea to obtain structural solutions able to guarantee, after a seismic event, a rapid re-use of the buildings without permanent damage, as well as to guarantee the safeguarding of human life. In this framework, the rocking of timber walls is entrusted to post-tensioned bars, placed in ad hoc cavities of the panel, while the dissipative contribution is provided by steel dampers, which are sacrificial elements subjected to damage easily replaceable after an intense seismic event. In this context, it can be helpful to have a numerical model in order to support the design phase; indeed, the paper presents a non-linear numerical model of post-tensioned rocking dissipative timber walls able to predict the response of this innovative system. In particular, geometrical and material non-linearities, due to the rocking behavior of the system and to the material that characterize the dampers, are taken explicitly into account. Further, the proposed numerical modelling strategy has been validated on literature experimental campaigns performed on both single and double wall systems, with different damper configurations. The comparison between experimental and numerical cyclic responses are shown in term of global results, i.e. force/displacement curves and post-tension variation of the bars. The comparison demonstrates the effectiveness of the proposed model in predicting the behavior of post-tensioned walls subjected to cyclic loads.

Quantifying Effect of Post-Tensioned Bars for Precast Concrete Shear Walls

A hybrid concrete wall is made up of a traditional reinforced concrete wall with post-tensioned technology. Recent research has shown that post-tensioned (PT) reinforced concrete shear walls have a wide range of advantages when it comes to resisting lateral forces and lateral moments caused by earthquake loading. To explore the PT reinforced concrete wall behavior subjected to seismic load, the concrete models were augmented with various material models, including the KCC, CDP, and Winfrith models for a PT 2D wall under pushover analysis. To ensure that the overall behavior forecast was qualitatively acceptable, the models’ performance was compared to experimental findings. Then, the post-tensioned modeling approach was implemented with the 3D wall in order to predict structural responses of the PT 3D wall. The well-validated finite element models were ultilized to estimate the effects of the post-tensioned bars on lateral resisting capacities of the precast concrete shear wall.

Experimental evaluation of post-tensioned bridge bent under cyclic loads and comparison to hybrid bridge bents

Experimental results for a re-centering bridge bent whose concrete columns were post-tensioned with unbonded high-strength steel bars are presented. The initial post-tensioning forces to achieve column re-centering were obtained using rigid body analogy. Steel collars were provided at the top and bottom ends of the columns for confinement. The column-to-footing interface contained a polyurethane layer inside the shear key to reduce column damage. For the initial post-tensioning forces implemented in the experiment, concrete damage was observed at the bottom of the columns. The post-tensioned (PT) bars remained elastic up to the design drift ratio of 2.0% and yielded at a 5.0% drift ratio; the column longitudinal mild steel bars yielded at a drift ratio of 6.0%. The performance of the post-tensioned bridge bent without any external energy dissipators (PT-Only bent) was compared with that of two similar hybrid bridge bents reinforced with external energy dissipators: the first incorporating tension-only stretch length anchors, and the second incorporating a buckling restrained brace. The two hybrid bents displayed excellent performance compared to the PT-Only bent in terms of strength, stiffness, and hysteretic energy dissipation; however, the PT-Only bent had a superior re-centering efficiency compared to the two hybrid bents.

Experimental and numerical investigations of the seismic behavior of socket and hybrid connections for PCFT bridge columns

The present paper investigates an innovative seismic-resilient bridge column-footing socket connection (SC) for post-tensioned concrete-filled steel tube (PCFT) columns defined according to the Accelerated Bridge Construction (ABC) requirements. The PCFT-SC column uses an embedded steel tube to dissipate the seismic energy and unbonded post-tensioned (PT) bars to improve the self-centering capacity. Experimental tests were performed on a PCFT-SC column, and the seismic response was compared with the PCFT column with hybrid connection (HC) previously investigated by the authors. The experimental tests confirmed that the PCFT-SC column could develop large strength, stiffness, and ductility capacity and that, compared with the PCFT-HC column, its seismic performance is competitive for drifts up to 2.8%. For larger drifts, the performances are slightly reduced due to the steel tube local buckling and the resulting PT force loss. However, even in these cases, the PCFT-SC column could be preferred due to its constructional advantages. Simplified models of PCFT columns with the SC and HC were developed in OpenSees, including modeling details accounting for the effects of the PT force loss. The numerical results, calibrated against the experimental data, showed the importance of considering PT force loss in the cyclic load analysis for PCFT columns.

Experimental investigation of damaged RC joints retrofitted by stiffened angles and bars

Experimental studies reveal that weakness in joint core leads to the formation of a shear hinge in this area, loss of load carrying capacity in the both regions of cyclic curve and decrease of ductility. Generally, the recent mentioned results are related to the previous studies, in which the effect of slabs and lateral beams (2D joints) have been neglected. However, these effects can lead to different responses in real joints compared to ideal 2D joints, in the term of changes in capacity and demand. Consequently, in this study the effect of slab and lateral beam has been considered (3D joints). Four 3D external joints with a scale of 1/2 are made in the laboratory. Specimens include two control joints and two damaged joints retrofitted by stiffened angles and post-tensioned bars. The results show that the use of retrofit method for damaged joints results in the formation of a flexural plastic hinge in the beam and shift of the shear hinge out of the joint core. Furthermore, the effect of slab and lateral beam in the bare non-seismic joint may lead to the increase of the ductility, compared to the similar 2D joints in previous studies.

Seismic performance of precast shear walls prestressed via post-tensioned high strength bars placed inside grouted corrugated pipes

The seismic performance of precast concrete shear walls depends on the mechanical behavior of the connections to the foundation and especially on the energy-dissipation capacity that is strongly related to connection deformability. With the focus on wall-to-foundation connections, the seismic performance of four vertically-elongated shear walls (height-to-width ratio close to two) is investigated by testing as many full-scale specimens, either cast-in-place (one “reference” specimen) or precast (three specimens). The three precast specimens are equipped with front and back metallic corrugated pipes into which three types of reinforcements are placed in concrete close to the vertical sides, i.e., ordinary bars (PW1), high-strength bars (PW2) and post-tensioned bars (PW3), whereas ordinary spliced bars are placed in the central part of the precast specimens. The cast-in-place specimen (CW) is vertically reinforced only with ordinary continuous bars. After bar placement, the pipes are filled with grout. In this way, the connection between the bars protruding from the footing and the bars reinforcing the shear wall is very effective face to seismic action. Coupling post-tensioned bars with ordinary spliced bars improves the stiffness of the shear walls. The specimens are tested under low-cycle loading, to investigate the lateral resistance, the energy dissipation capacity and ductility, to monitor crack evolution and to study hysteretic behaviour. The local deformation of the concrete at the wall-footing interface (into which epoxy-coated concrete keys are incorporated) is also examined. The test results show that (a) the four shear walls have a similar bearing capacity (PW2 and PW3 have slightly stronger bearing capacities than CW and PW1); (b) the shear walls PW2 and PW3 exhibit similar ductility to the shear wall CW under cyclic loading; and (c) the shear wall PW1 is the least ductile of the investigated specimens.

Structural testing of ungrouted post-tensioned cantilever masonry walls under unidirectional out-of-plane monotonic and cyclic loading

Post-tensioning techniques improve the tensile strength of masonry substantially, endowing posttensioned masonry with performance comparable or superior to that of reinforced masonry cantilever walls. A useful structural application is for retaining walls, and although investigations have been made in posttensioned masonry cantilever walls, most of them focused on grouted elements and in-plane loads. This study investigated the influence of initial prestress and masonry strength in the behavior of ungrouted cantilever prestressed masonry walls with laterally unrestrained post-tensioned bars and which are intended as earth-retaining structures. The study also addresses the accuracy of different prestressed masonry code expressions to calculate the ultimate flexural capacity. The results show the ability of this structural system to withstand large lateral displacements with a limited and concentrated damage zone. The impact of masonry strength when the walls are loaded to ultimate conditions is less evident than that of initial prestress, and the USA and Canadian design code procedures show better correlation between numerical predictions and experimental data than do other worldwide codes.

Design and analysis of a seismic resilient steel moment resisting frame equipped with damage-free self-centering column bases

Many recent research studies focused on the development of innovative seismic resilient structures by chasing the objectives of minimising both seismic damage and repair time, hence allowing the definition of structures able to go back to the undamaged, fully functional condition, in a short time. In this context, the present study investigates an innovative type of self-centring damage-free steel column base (CB) connection and its beneficial effects when used within steel moment-resisting frames (MRFs). The proposed connection consists of a rocking column equipped with a combination of friction devices, providing energy dissipation capacity, and post-tensioned bars with disk springs, introducing restoring forces in the joint. Contrary to conventional steel CBs, the proposed connection exhibits moment–rotation behaviours that can be described by simple analytical equations, allowing the definition of an easy-to-apply design procedure. Numerical models of the connection, developed in OpenSees, are validated against experimental results and successively implemented within a four-storey case study steel MRF. Incremental Dynamic Analyses are performed to derive the samples of the demand for the engineering demand parameters of interest while accounting for the record-to-record variability. Fragility Curves show the effectiveness of the proposed solution in reducing the residual storey drifts and in protecting the first-storey columns from damage, hence providing significant advantages in terms of repairability, and hence resilience of the structure with a negligible increase on the overall cost. The results show that the damage-free behaviour of the CBs is a key requirement when self-centring of MRFs is a design objective.

Evaluation of Seismic Performance of Improved Rocking Concentrically Braced-frames with Zipper Columns

Concentrically braced frames (CBFs) as one of well-known stiff and common lateral force resisting systems often show limited ductility capacity under severe earthquakes. This study proposes rocking zipper braced frame (RZBF) to improve the drift capacity of CBFs which is based on combination of rocking behavior and zipper columns. In the RZBF system, rocking behavior permit the braced frame to uplift during the earthquake and then restoring force induced through post-tensioned bars self-center the frame to its initial state. Also, zipper columns can decrease the concentration of damage by distributing the unbalance force at the mid bay over the frame’s height. To assess the performance of RZBF, a comparison study is carried out considering CBF, rocking concentrically braced frame, zipper braced frame and RZBF. For this purpose, some frames structures are designed and nonlinear time history analysis conduct under a set of earthquake records. Seismic responses such as roof drift ratio, gap opening at the column-base interface, forces of top story braces and post-tensioned bars are taken into consideration. The results show that the proposed RZBF has better performance among the others and zipper columns can improve the behavior of rocking systems.

Seismic Collapse Risk Assessment of Posttensioned Controlled Rocking Masonry Walls

AbstractThe use of vertical unbonded posttensioned (PT) bars in masonry walls results in a controlled rocking behavior that can provide a high drift capacity and recenter the wall to its vertical a...

Seismic response evaluation of concentrically rocking zipper braced frames

In this study an innovative rocking zipper braced frame (RZBF) is proposed to overcome the deficiencies of common concentrically braced frames. RZBF is an improved rocking concentrically braced frame which is based on combination of rocking behavior and zipper columns. The base rocking joints and post-tensioned bars provide rocking response and restoring force, respectively. Also, zipper columns distribute the unbalance force over the frame height and reduce the damage concentration. To evaluate seismic performance of RZBF, a comparison study is carried out considering concentrically braced frame, zipper braced frame, rocking concentrically braced frame and RZBF. Thereby, a suite of non-linear time history analyses had been performed on four different types of archetypes with four, six, eight, ten and twelve stories. Frames were designed and non-linear time history analyses were conducted in OpenSees. To compare the seismic behavior of the archetypes, roof drifts, residual roof drifts, story drifts, the forces of first and top story braces, PT bars forces, column uplift and base shears were taken in to consideration. Results illustrate that using RZBF, can reduce the damage due to reduced residual drifts. Zipper columns enhance the seismic performance of rocking systems. As the number of stories increase in the RZBF systems, larger top story braces were needed. So the RZBF system is applicable on low and midrise buildings.