Post-tensioning Strands Research Articles

Experimental investigation of tri-axial self-centering reinforced concrete frame structures through shaking table tests

This paper presents an experimental study on the seismic performance of a tri-axial self-centering reinforced concrete (RC) frame structure. In such a structure, the column-base joints were free to uplift, and the beam-column joints were relaxed to open. Post-tensioned (PT) strands passed through the RC beams and columns to provide re-centering capability. In order to accommodate the gap openings in beam-column joints, rubber pads were placed underneath the floor slabs to provide a smooth sliding motion between the RC beams and the slab. A 1/2.5-scale model of a tri-axial three-story self-centering RC frame structure was designed and tested on a shaking table under a series of earthquake excitations with increasing amplitudes. The results showed that the self-centering RC frame structure had desirable seismic performance with little damage even under extreme earthquakes. The reduction in natural frequencies were quite small after the extreme earthquakes, indicating that the structure had remained almost elastic. The test model showed a desirable tri-axial re-centering capability with negligible residual deformation. The sliding of the slabs was also measured throughout the tests, and the data indicated that the slabs re-centered to their original positions after the tests.

Multi-criteria optimization of lateral load-drift response of posttensioned steel beam-column connections

Posttensioned (PT) elements in steel buildings can substantially mitigate permanent seismic damages and the associated post-earthquake repair costs during earthquakes. In this paper, a response surface methodology (RSM) is used to predict and optimize the lateral response characteristics of PT steel beam-column connections with top-and-seat angles. The monotonic lateral response characteristics considered in the study include: initial stiffness, load capacity, and ultimate drift of PT connections, as well as load and drift levels corresponding to the gap-opening (decompression) in PT connections. Based on the results of finite element simulations and extensive sensitivity studies, six influential parameters are considered as input variables in this study. These parameters are posttensioning strand force, beam depth, beam flange thickness and width, span length, and column height. By using a desirability approach, the lateral response of PT steel beam-column connections is optimized. The optimization studies aim at maximizing the initial stiffness, load capacity, and ultimate drift of PT connections and/or minimizing the amount of steel in the beam section, which contributes to the final cost of frame structures. The multi-criteria optimization studies reveal the regions of factor space where optimal conditions are achieved. The optimized solutions are then confirmed by performing simulation runs with the optimal factor combinations. Among the results, it is shown that damage occurs earlier in PT connections with deeper beams and greater posttensioning strand forces. The dominant limit state for the PT connections was beam local buckling starting at early drifts of 1.2%, whereas the first occurrence of angle fracture was at about 4% drifts, and two limit states of strand yielding and bolt extensive yielding were not observed in the analyzed PT connections.

Finite element modeling and capacity analysis of post-tensioned steel frames against progressive collapse

Post-tensioned (PT) steel frames, in which beams are connected to columns through high-strength PT strands, have been successfully developed in the past decade as a novel earthquake-resistant structural system. Compared to conventional steel moment frames, a PT steel frame demonstrates superior seismic performance, notably the minimum damages in main structural components and the self-centering capability under a design basis earthquake. Despite the abundant research on the seismic behavior of PT steel frames, there is an apparent lack of study on their load-redistribution behavior upon the notional removal of critical load-bearing columns, a commonly used threat-independent local structural damage scenario that potentially triggers the progressive collapse of the column-removed frame. This paper presents a first-of-its-kind numerical investigation on the unique structural behavior of PT steel connections and frames in redistributing the unbalanced gravity loads due to column removal. High-fidelity finite element structural models are constructed and validated using the available experimental data in the literature. The capacity of PT steel frames subjected to a gradually increasing vertical displacement along the removed column line is systematically studied. It is found that, besides the resistance of energy-dissipating elements, beam arching action and strand catenary action are the major sources of structural capacity of a PT steel frame against progressive collapse. The corresponding failure modes are identified and the design implications are suggested.

Seismic performance of hybrid self-centring steel-timber rocking core walls with slip friction connections

While structures with conventional lateral force resisting systems are designed to meet the life safety criteria for the residents during and after a seismic event, they are allowed to tolerate the expected structural damage. This damage might be because of the residual displacements after the earthquake or the lack of ductility in the system. Despite the fact that the allowable damages are intended to be repairable, however, in most cases the repairs are highly uneconomical. A self-centring hybrid steel-timber rocking core wall system (SC-RW) is developed to provide sufficient ductility in addition to a significant amount of energy dissipation while it limits the residual drift and the associated damage. This system is comprised of one or more rocking cross laminated timber (CLT) walls with slip friction connections as the main lateral resisting system and steel beams and columns to resist against gravity loads. Horizontally oriented post-tensioned strands through the beams provide additional moment resistance at the beam-column interface to re-centre the structure after the earthquake. The efficiency of the proposed system is investigated under cyclic and seismic loading regimes. Furthermore, a preliminary displacement based design approach for a SC-RW system is introduced. Dynamic time-history simulations confirm an excellent behaviour in terms of drift capacity, residual displacement and peak roof accelerations.

Structural vibration control with the implementation of a pendulum-type nontraditional tuned mass damper system

Self-centering systems, such as base-rocking-wall systems, have been studied and demonstrated to be capable of achieving enhanced seismic-resisting performance. For these rocking-wall systems, unbonded post-tensioning strands and damage-permitted energy-dissipating fuses are usually implemented along the full height of the walls. Structural damage and residual deformation of main structures can be mitigated through deformation of the fuses which are replaceable after strong earthquakes. An innovative self-centering system referred to as a pendulum-type nontraditional tuned mass damper system is newly proposed in this paper, which can be deemed to be an inverted rocking system. Pendulum components can automatically return to their original position due to the effect of gravity, without any post-tensioning strands being required. Energy dissipators are only implemented between the bottom end of the pendulum component and the ground, and they can be easily fabricated and replaced. Both experimental and numerical studies have been carried out to examine seismic performance of the pendulum-type nontraditional tuned mass damper system, and it is found that satisfactory control of interstory drift and floor absolute acceleration can be achieved, while small movement space is required.

Experiments on Hybrid Precast Concrete Shear Walls Emulating Monolithic Construction with Different Amounts of Posttensioned Strands and Different Debond Lengths of Grouted Reinforcements

This paper proposed a hybrid precast concrete shear wall emulating monolithic construction (HPWEM) that utilized grouted vertical connecting reinforcements and unbonded posttensioned high-strength strands across the horizontal joint for the lateral resistance. The grouted reinforcements with predetermined debond length were used to provide strength by tension and energy dissipation by yielding. The posttensioned strands were mainly employed to offer the restoring force to reduce the residual displacement by elastic extension. The overlapping welded closed stirrups improved the confinement property of the restrained concrete, avoiding the brittle failure. Six HPWEM specimens, considering variables including the amounts of strands and the debond lengths of grouted reinforcements, as well as one referenced cast-in-place monolithic wall specimen, were tested under the low-cycle reversed lateral load. The HPWEM specimens were capable of providing strength, stiffness, ductility, and energy dissipation equivalent to that of the monolithic wall specimen under certain variable condition.

Long Term Monitoring of Prestressing Tension Force in Post-Tension UHPC Bridge using Fiber Optical FBG Sensor

This paper presents results of one-year monitoring on prestressing force of a 7-wire steel post-tensioning strand which is installed in a UHPC(ultra high performance concrete) bridge with 11.0 m long, 5.0 m wide, and 0.6 m high by using a FBG-encapsulated 7-wire steel strand. The initial prestressing forces and the prestress changes during a vehicle load test were measured using the FBG-encapsulated strand. The results show that the FBG-encapsulated 7-wire strand is very effective for monitoring the prestress forces even the change in the tension force is very small. Additionally, it was indicated that selection of the thermal expansion coefficient which is used for the temperature correction shall be carefully carried out.

Seismic performance of controlled spine frames with energy-dissipating members

Recently, various controlled rocking systems have been proposed in seismic design to prevent damage concentration and to achieve self-centering against a wide range of input ground motion intensities. However, several obstacles must be overcome before these systems can be applied to actual buildings; for example, the requirement for large, self-centering post-tensioned strands and special treatment at uplift column bases must be addressed. This paper proposes a non-uplifting spine frame system with energy-dissipating members without post-tensioned strands, its self-centering function is achieved by envelope elastic-moment frames. The system is applied to an actual building constructed in Japan. Conventional shear damper and uplifting rocking systems with post-tensioned strands developed in prior studies are also applied to the same building structures, and the performances of the three systems, including damage distribution, energy dissipation, self-centering, robustness against severe earthquakes, and irregular stiffness, are compared and discussed through numerical simulations based on practical design criteria.

Cyclic Loading Test of Unbonded and Bonded Posttensioned Precast Segmental Bridge Columns with Circular Section

Four 1/4-scale precast segmental bridge columns (PSBC) with different reinforcement types and arrangements and one monolithic reference column (MRC) were designed and tested under cyclic quasi-static loading. Both UPC (PSBC with unbonded tendons only) and UPCE (PSBC with unbonded tendons and bonded mild steel bars) use unbonded posttensioned (PT) strands to connect the segments. Bonded energy dissipation (ED) bars were added in UPCE to increase the ED ability of unbonded PSBC. Specimens BPC (PSBC with fewer bonded PT bars) and BPCII (PSBC with more bonded PT bars) use bonded PT bars to combine the segments together. The unbonded length was set in ED bars to delay the low-cycle fatigue damage. Test results showed PSBC only has minor concrete cracks or crushing at lower joints. The unbonded PT PSBC with ED bars had higher lateral strength, lower residual drift, and comparable ED with MRC. The bonded PT bar PSBC with appropriate bar arrangement also showed high lateral strength and medium energy-dissipation ability. However, bonded PT PSBC experienced large PT stress loss, which leads to a residual drift of 3% when unloading from 6% drift.

Finite-Element Simulation of Posttensioned Steel Connections with Bolted Angles under Cyclic Loading

AbstractSteel beam-column connections with posttensioned (PT) elements are proven systems that can provide adequate stiffness, strength, and ductility, while eliminating permanent deformations in a moment-resisting frame subjected to seismic loading. In this study, detailed three-dimensional finite-element (FE) models of steel beam-column connections with PT strands are developed and analyzed under cyclic loading. Efforts are made to overcome challenges in performing the nonlinear FE analysis of large-scale PT connections, which involves gap opening and closing behavior as well as contact and sliding phenomena. Geometric and material nonlinearities, preloaded bolts and strands are also considered in the modeling. Through a verification study, the results from the FE models are validated against prior experiments on interior PT connections with top-and-seat angles. Parametric studies are also conducted to investigate the effects of three factors on the cyclic performance of PT connections. The factors inve...

A numerical study on seismic response of self-centring precast segmental columns at different post-tensioning forces

Precast bridge columns have shown increasing demand over the past few years due to the advantages of such columns when compared against conventional bridge columns, particularly due to the fact that precast bridge columns can be constructed off site and erected in a short period of time. The present study analytically investigates the behaviour of self-centring precast segmental bridge columns under nonlinear-static and pseudo-dynamic loading at different prestressing strand levels. Self-centring segmental columns are composed of prefabricated reinforced concrete segments which are connected by central post-tensioning (PT) strands. The present study develops a three dimensional (3D) nonlinear finite element model for hybrid post-tensioned precast segmental bridge columns. The model is subjected to constant axial loading and lateral reverse cyclic loading. The lateral force displacement results of the analysed columns show good agreement with the experimental response of the columns. Bonded post-tensioned segmental columns at 25%, 40% and 70% prestressing strand stress levels are analysed and compared with an emulative monolithic conventional column. The columns with a higher initial prestressing strand levels show greater initial stiffness and strength but show higher stiffness reduction at large drifts. In the time-history analysis, the column samples are subjected to different earthquake records to investigate the effect post-tensioning force levels on their lateral seismic response in low and higher seismicity zones. The results indicate that, for low seismicity zones, post-tensioned segmental columns with a higher initial stress level deflect lower lateral peak displacement. However, in higher seismicity zones, applying a high initial stress level should be avoided for precast segmental self-centring columns with low energy dissipation capacity.

Seismic design guidelines for solid and perforated hybrid precast concrete shear walls

This paper presents recommended seismic design and detailing guidelines for special unbonded postten- sioned “hybrid” precast concrete shear walls, including a full-scale worked design example and supporting experimental evidence from six 0.4-scale test speci- mens. Hybrid precast concrete walls use a combina- tion of mild steel bars (Grade 400 [Grade 60]) and high-strength unbonded posttensioning steel strands for lateral resistance across horizontal joints. The mild steel bars are designed to yield in tension and com- pression, providing energy dissipation. The unbonded posttensioning strands provide self-centering capabil- ity to reduce the residual lateral displacements of the structure after a large earthquake. The proposed design guidelines are aimed to allow practicing engineers and precast concrete producers to design American Concrete Institute–compliant hybrid shear walls with predictable and reliable seismic behavior. Ultimately, the results from this project support the U.S. code approval of the hybrid precast concrete wall system as a special reinforced concrete shear wall for moderate and high seismic regions.

Time-Variant Flexural Reliability of Posttensioned, Segmental Concrete Bridges Exposed to Corrosive Environments

Posttensioned (PT) bridges are used on major thoroughfares because they are economical structures for traversing long spans. Special inspections of these bridges have revealed corrosion of the strands at void locations in the tendons. The integrity of these strands has significant influence on the safety of these bridges. Inspections indicate that many ducts (parts of the tendon that is supposed to protect strands from exposure to aggressive environments) are cracked and many grout holes and vents are opened, allowing direct ingress of moisture and chlorides. This paper presents a framework for assessing the flexural reliability of PT bridges exposed to various environmental conditions. The moment capacity of a PT girder is formulated using probabilistic models for the tension capacity of corroding PT strands exposed to various void and environmental conditions. Using Monte Carlo simulations, the flexural reliability of an example PT bridge is assessed. The research indicates that the flexural reliability index reaches a value below the recommended value within a relatively short period of time when moisture and chlorides infiltrate the tendons. These findings emphasize the critical need for new inspection, assessment, and repair methods for these bridge types.

Behavior of Post-Tensioning Strand Systems Subjected to Inelastic Cyclic Loading

AbstractPost-tensioning (PT) strands have been employed in a number of self-centering seismic force-resisting systems as part of the restoring force mechanism that eliminates residual building drif...

Corrosion failure of post-tensioning tendons in alkaline and chloride-free segregated grout: a case study

This paper describes a case study of corrosion of post-tensioning strands in segregated grout that led to failure of one of the external tendons of a bridge after less than 2 years from the construction. Unexpectedly, severe corrosion attacks took place in areas where the ducts were filled by the injection grout, but segregation of the grout led to the presence of an unhardened whitish paste. A detailed inspection of the failed tendon, as well as other tendons removed afterwards, showed that corrosion occurred mainly in the inclined parts of the external post-tensioning tendons, in the presence of whitish grout. Analysis on samples of segregated grout showed that it was highly alkaline and had a high content of sulphate ions, but was free of chloride ions, excluding thus the normal causes of corrosion of steel in cement-based materials. Microstructural analysis of failed wires also excluded the failure for hydrogen embrittlement.

Test and simulation of full-scale self-centering beam-to-column connection

A new type of beam-to-column connection for steel moment frames, designated as a “self-centering connection,” is studied. In this connection, bolted top-and-seat angles, and post-tensioned (PT) high-strength steel strands running along the beam are used. The PT strands tie the beam flanges on the column flange to resist moment and provide self-centering force. After an earthquake, the connections have zero deformation, and can be restored to their original status by simply replacing the angles. Four full-scale connections were tested under cyclic loading. The strength, energy-dissipation capacity, hysteresis curve, as well as angles and PT strands behavior of the connections are investigated. A general FEM analysis program called ABAQUS 6.9 is adopted to model the four test specimens. The numerical and test results match very well. Both the test and analysis results suggest that: (1) the columns and beams remain elastic while the angles sustain plastic deformations for energy dissipation when the rotation of the beam related to the column equals 0.05 rad, (2) the energy dissipation capacity is enhanced when the thickness of the angle is increased, and (3) the number of PT strands has a significant influence on the behavior of the connections, whereas the distance between the strands is not as important to the performance of the connection.

Seismic Performance of Steel Self-Centering, Moment-Resisting Frame: Hybrid Simulations under Design Basis Earthquake

AbstractThe behavior of a self-centering moment-resisting frame (SC-MRF) is characterized by the connection gap opening and closing at beam-column interfaces under earthquake loading. A SC-MRF uses high-strength posttensioning strands to precompress the beams to the columns and to close the gaps that develop under earthquake loading, returning the frame to its initial position (i.e., the frame is self-centering). In this study, a beam web friction device is included in each beam-column connection to dissipate energy under seismic loading. Unlike a special steel moment-resisting frame with welded connections (W-SMRF), a SC-MRF can be designed to survive the design basis earthquake (DBE) without structural damage, leading to the potential for immediate occupancy performance, and to suffer only modest damage under the maximum considered earthquake, leading to collapse prevention performance. A 7-bay, 4-story SC-MRF prototype building was designed for a location in the Los Angeles area. A 0.6-scale model of t...

Llyn Alaw reservoir post-tensioned anchor installation

As a result of leakage along the downstream face of the dam, a section 10 inspection carried out in February 2006 contained a recommendation for a study to be undertaken into the uplift pressures and stability of the concrete gravity dam impounding Llyn Alaw reservoir. The study was subsequently carried out and showed the dam to have inadequate factors of safety for stability during an extreme seismic event. Dwr Cymru Welsh Water awarded a contract to undertake the design and supervision of works to ensure the stability of the structure. The selected method of stabilising the structure was to install 44 no. post-tensioned strand anchors along the length of the dam, vertically through the crest and into the bedrock foundation. This paper describes the stages of the project from feasibility studies and design through to construction of the scheme, which was completed in mid-2013.

An experimental study on the ultimate behaviour of simply-supported post-tensioned composite slabs

Post-tensioned composite steel-concrete slabs consist of thin-walled profiled steel sheeting, post-tensioned strands and concrete. This paper presents the preparation and results of two ultimate test series aimed at evaluating the contribution of the profiled sheeting on the load carrying capacity of this form of construction, and whether this is affected by time effects. For this purpose, six samples were prepared and tested to failure in a simply-supported static configuration by means of two transverse line loads applied near the mid-span. These consisted of two post-tensioned solid slab and four post-tensioned composite ones cast using Stramit Condeck HP® and Stramit PrimeForm® profiled sheeting. The flexural strengths observed for the composite specimens prepared with Condeck HP and PrimeForm profiles were in average 77% and 20% greater, respectively, than the moment capacity of the post-tensioned solid slab counterpart. These values did not appear to be affected by time effects for the material and geometric properties considered in this study. All samples exhibited a ductile response and all tests were terminated due to concrete crushing. A theoretical model was proposed to account for the partial shear connection behaviour of the composite slabs. The model was then applied to evaluate the influence of different geometric and material parameters on the moment capacity calculated for different post-tensioned composite slabs. It is expected that further comparisons with experimental results need to be carried out to properly validate its adequacy for use in routine design.

Identification of corrosion mechanisms by univariate and multivariate statistical analysis during long term acoustic emission monitoring on a pre-stressed concrete beam

Acoustic Emission (AE) technique was used to monitor Hydrogen Assisted Stress Corrosion Cracking (HASCC) activated by using ammonium thiocyanate solution during a long term laboratory corrosion test on a pre-stressed concrete beam using post-tensioned steel strands. By univariate analysis, three stages of damage mechanisms were identified: activation, propagation and rupture. We therefore proposed a combination of principal component analysis and self organising maps algorithms as a new procedure to identify the progression of different damage mechanisms during time until final failure. Initiation, propagation and critical rupture phases were clearly identified and associated to specific features of the AE events.