Post-tensioned Self-centering Research Articles

Mechanics and validation tests of a post-tensioned self-centering brace with adjusted stiffness and deformation capacities using disc springs

Self-centering braces have been developed to reduce the residual deformation of steel frames in earthquakes. The brace has a higher elastic axial stiffness before the activation, which may result in a higher base shear for frame design. This work was aimed to develop a new post-tensioned self-centering brace (PT-SCB) with adjusted stiffness and deformation capabilities such that the brace could be designed based on a specified yield or ultimate frame drift. Combining disc springs and one of the compression members together in the original SCB could achieve new structural characteristics, providing an alternative to tune the axial stiffness and deformation. This work first introduces the mechanics and deformation mechanism of the proposed SCB and performs a parametric study to investigate the effect of disc spring stacks on the hysteretic response of SCBs. A test program is then conducted on cyclic tests of disc spring stacks in different serial and parallel combinations and two PT-SCBs. Each 3800 mm-long brace is composed of high-strength steel tendons, compression steel members, disc spring stacks, and bolted friction device. The hysteretic responses of the new SCB with different design parameters are used to validate the mechanics and ability of the axial stiffness and deformation adjustment.

Seismic performance of an assembled self-centering buckling-restrained brace with controllable initial stiffness

An innovative assembled self-centering buckling-restrained brace (ASC-BRB) with controllable initial stiffness is introduced. The brace consists of a self-centering system and an energy dissipation system. Unlike existing post-tensioned self-centering energy dissipation braces, whose initial stiffness has high uncertainty and is sensitive to machining errors, the proposed ASC-BRB can obtain highly controllable initial stiffness because of its unique working mechanism. Therefore, structural seismic response uncertainties can be reduced significantly. The configuration, working mechanism, and theoretical restoring force model of the ASC-BRB are first presented. Quasi-static tests are then conducted to examine the feasibility of the brace, and typical flag-shaped hysteretic behavior and stable energy dissipation capability are observed. The main components of the ASC-BRB keep intact after repeated loadings, except for the core plates, which are damaged; however, the core plates can be easily replaced. This observation highlights the superiority of the ASC-BRB in terms of damage concentration and high recoverability. The proposed theoretical restoring force model can accurately predict the hysteretic response of the brace, especially for the initial stiffness, with the predicted value being within 5 % of the experimental value. A simplified numerical model is developed and used in a parametric study conducted to examine the effects of two key design parameters on the brace behavior. Furthermore, the effect of machining errors on the initial stiffness of the ASC-BRB is studied. An analysis of steel frames with ASC-BRBs indicates that the ASC-BRB can effectively reduce the residual inter-story drift and help achieve a peak floor acceleration response comparable to that of the frame with BRBs. However, the peak inter-story drift is amplified owing to the lower initial stiffness of the proposed brace. On the basis of a system-level parametric analysis, recommended optimized design parameters of the ASC-BRB are presented to facilitate efficient control of structural seismic responses.

Evaluating the effect of infill walls on seismic responses of post-tensioned self-centering concrete frames based on a new analytical model

This paper proposes a numerical model with the lump plasticity rotational springs for the simulation of post-tensioned self-centering (PTSC) concrete frames. The analytical connection model that is able to properly capture the joint responses and connection damage is incorporated in the rotational springs. The static and dynamic numerical results are validated against those obtained using the fiber-based structural model. Furthermore, the proposed numerical model is adopted in the seismic response prediction of PTSC frames with infill walls (IWs) and various energy dissipation components. Incremental dynamic analysis is performed through the scaling schemes based on Sa(T1) and PGA, and the effects of IWs and energy dissipators on the structural responses under different ground motion inputs are compared. The numerical results indicate a remarkable reduction in the inter-story drifts of the PTSC frames with IWs as compared to the bare frames under the same Sa(T1). Nevertheless, when subjected to ground motions with large PGAs, the damage of IWs would induce rapidly increased inter-story drifts in the lower floors, which may exceed the drifts of bare frames under the same earthquake intensity. This study highlights the accuracy of the proposed numerical model and the importance of considering IWs in the seismic response predictions of PTSC concrete frames.

Digital Twin for Monitoring In-Service Performance of Post-Tensioned Self-Centering Cross-Laminated Timber Shear Walls

A digital twin (DT) can be defined as a multiphysics, multiscale model in which a digital model, such as a building information model (BIM), is updated based on data obtained from a physical system, such as sensor data, results from probabilistic simulations, and material/structural models. This study describes sensor data integration within a BIM as the first critical step toward the implementation of DTs to support structural health monitoring (SHM). In particular, the study defines a methodological approach used to integrate the as-built geometry of existing buildings, as well as their material properties and sensor data into a digital model to assist in accessing sensor data to assess a building’s structural performance. A mass-timber structural system consisting of post-tensioned cross-laminated timber (CLT) self-centering shear walls at the George W. Peavy Forest Science Center (“Peavy Hall”) at Oregon State University was used as a case study to test the proposed approach. The BIM of the shear walls was developed using a Scan-to-BIM approach by converting light detection and ranging point clouds into a BIM. Sensors in the building recorded environmental and structural parameters influencing the long-term performance of the shear walls. Measurands included relative humidity, air and wood temperature, wood moisture content, displacements, and deformations of shear walls. The precise placement of these sensors and the possibility to associate the measured parameters of these entities within a BIM is hypothesized to assist with data management by adding a spatial element to data and analysis results. In addition, the integration into the IFC-BIM platform of a material- and phenomena-specific warning tool allows to promptly identify areas of concern in the monitored building. This can support facility managers in planning inspection and maintenance activities and eventually could lead to the prolonged service life of a building.

Seismic performance of post-tensioned self-centering concrete frames under near-fault pulse-like ground motions

Post-tensioned self-centering (PTSC) frames have been identified as a class of high-performance structures with capabilities of mitigating damage and minimizing residual deformations. However, the seismic behavior of PTSC frames subjected to near-fault pulse-like ground motions (NPGMs) is not fully understood yet. This study provides a comprehensive assessment on the seismic performance of PTSC frames under NPGMs. 4-story and 12-story PTSC frames with different energy dissipation capacities are numerically modeled in the OpenSees platform. Various degrees of structural damage of the PTSC frames are properly described by particular limit states. Incremental dynamic analysis (IDA) is performed using a set of 91 near-fault ground motions with a wide range of pulse periods. Fragility surfaces for different damage states are developed for the exemplar PTSC frames with respect to both seismic intensity and pulse period. The numerical results indicate that the impacts of pulse period on the PTSC frames of different stories are quite different. Besides, the most unfavorable pulse period changes with the severity of structural damage. The effects of energy dissipation ratio and pulse period on the exceedance probabilities of different damage states are compared, and the limitation of the commonly adopted friction dampers is discussed. Based on the results and observations of structural performance with respect to the pulse characteristics, guides and recommendations for the design of PTSC frames are provided.

The influence pulse-like near-field earthquakes on repairability index of reversible in mid-and short-rise buildings

Abstract Conventional earthquake-resistant systems, often experience inelastic behavior in a part of the structure during a large earthquake and eventually causing residual deformation and damage to the structure. Repairing these damages are unaffordable and often leads to structure destruction. Therefore, the use of structures with the ability to focus damage on interchangeable elements, which leads to reduced earthquake damage, is very important. Due to the importance of the performance of self-centering structures to reduce their damage against various earthquakes, in this study, the Repairability Index of Post-tensioned Self-Centering Frame for Near-Field earthquake (RIPSCF-N) has been developed. According to the 12 models of the studied building, a building that can be repaired, that the maximum rotation in its connection after the earthquake does not exceed the rotation of the immediate occupancy performance. Based on this, the output data of Incremental Dynamic Analysis (IDA) in OpenSees were drawn according to the connection of relative rotation and spectral acceleration. According to the predicted performance levels of Garlock for each acceleration level, the value of the connection opening is divided by the opening of the Design Basis Earthquake (DBE) level. The resulting curve shows the repairability index according to spectral acceleration, which if less than one, the repairability target is achieved. To evaluate the damage of angles, the Angle Failure Probability of Post-tensioned Self-Centering Frame for Near-Field earthquake (AFPPSCF-N) has been developed. This index equation is determined according to the fragility curve and the intensity of damage in each building.

Post-Tensioned Self-Centering System Efficiency against Extreme Wind Loads

Post-Tensioned Self-Centering System Efficiency against Extreme Wind Loads

Lifetime seismic performance assessment on post-tensioned self-centering concrete frames considering long-term prestress loss

Prestressing force acts as the main provider of the moment resistance and self-centering capacity of post-tensioned self-centering (PTSC) concrete frames. As the constantly compressed members are subjected to long-term deformations induced by the shrinkage and creep of concrete, the prestress loss would inevitably arise in PTSC frames. This study presents a comprehensive approach to assess the long-term seismic performance of PTSC concrete frames, in which the prestress loss is estimated and considered as the major degradation effect. Time-dependent fragility functions are derived for each floor to detect the variations in the story performance and damage distribution of the structure. The proposed approach is applied to two PTSC concrete frames with and without appendant energy dissipators, respectively. Results indicate that the prestress loss imposes greater impact on the higher floors, and would change the position of the most vulnerable story. Moreover, the frame without energy dissipators suffers larger performance degradation as compared to the damped building.

Influence of connection properties on seismic performance of post-tensioned self-centering concrete frames

This paper investigates the impacts of connection characteristics on the seismic performance of post-tensioned self-centering (PTSC) concrete frames. The considered connection properties include the level of energy dissipation capacity and the extent of beam end confinement. In addition to the common range of energy dissipation ratio for maintaining the self-centering behavior of connections, the PTSC connections with large energy dissipation capacity that induces considerable residual deformations are also adopted. The behavior of the connections with various parameters, in particular the different levels of energy dissipation and beam end confinement is investigated first. Then, bivariant seismic fragility analyses with the maximum and residual deformations as the performance criteria are conducted for the structural performance assessment. Comparisons are made in respect of the seismic capacity and demand of exemplar frames, as well as the fragility curves. The results demonstrate the benefit of PTSC connections with large energy dissipation capacity in reducing the collapse probability while still being able to maintain the functionality of structures after earthquake. It is also found that the enhancement of beam end confinement is more effective for PTSC concrete frames with small energy dissipation capacities.

Probabilistic analysis on time-dependent mechanical behavior of post-tensioned self-centering concrete connections

This paper presents a probabilistic assessment on the time-dependent mechanical behavior of post-tensioned (PT) self-centering (SC) concrete connections. First, the prestress loss in PTSC frames induced by the shrinkage and creep effects of concrete is estimated according to the probabilistic analysis with the inclusion of possible sources of uncertainties. The finite element models of PTSC subassemblies with different structural parameters are developed and utilized to investigate the impacts of span number, reinforcement ratio and initial prestress level on the loss degree of prestress. Subsequently, a time-dependent analytical model is proposed for the PTSC concrete connections. The long-term performance of PTSC joints is analyzed with the incorporation of prestress loss and other time-dependent effects, such as the continuing hydration of cement. Finally, the key indicators for the mechanical properties of PTSC connections are extracted and quantitatively evaluated.

Assess the Effectiveness of SMA on Response of Steel Connection with PT Strands

Using the post-tensioned (PT) elements can significantly plummet the persistent damages and the relevant costs to post-earthquake repair and maintenance, facing earthquake hazards. Rooted in a PT self-centering steel connection, the deformation and residual drift of the main members are limited to the linear elastic branch derived from existence of steel strands by omitting the gap between beam-column connections. The shape memory alloys (SMA) material has super elastic property according to the conversion of austenite to martensite phases due to changes in the temperature and stress. In this research, a nonlinear behavior of PT steel connection with top-and-seat angles was analyzed due to vulnerability of the connection during the cyclic loading, so that the connection was equipped with stiffened angles and SMA material in order to improve seismic behavior of the steel connection. The results illustrated that the using the equipped angles with SMA property flourish a logical performance in the steel connection under cyclic loading, by which the connection witnesses an appropriate energy dissipation (ED) capacity and dramatically effect on initial stiffness.

Modeling of force-displacement behavior of post-tensioned self-centering concrete connections

This paper provides an analytical procedure for deriving the force-displacement relations of post-tensioned (PT) self-centering (SC) concrete connections. The restoring force of PT tendons is defined using the section analysis, in which direct estimation of maximum concrete compressive strain is incorporated instead of adopting the commonly used “monolithic beam analogy” method. Analytical models for the PTSC beam-column joints with and without energy dissipating devices are provided according to the force and deformation characteristics of each case. Using the validated finite element models, a parametric study including 100 PTSC connections is conducted with the ABAQUS platform to detect the variation pattern of maximum concrete strain and estimate its values for connections with different section dimensions, concrete strength and initial PT forces. The accuracy of proposed analytical models is verified based on comparisons with the finite element results and available experimental data.

Parametric study of hexagonal castellated beams in post-tensioned self-centering steel connections

The effects of important parameters (beam reinforcing plates, initial post-tensioning, and material properties of steel angles) on the behavior of hexagonal castellated beams in post-tensioned self-centering (PTSC) connections undergone cyclic loading up to 4% lateral drift have been investigated by finite element (FE) analysis using ABAQUS. The PTSC connection is comprised of bolted top and bottom angles as energy dissipaters and steel strands to provide self-centering capacity. The FE analysis has also been validated against the experimental test. The new formulations derived from analytical method has been proposed to predict bending moment of PTSC connections. The web-post buckling in hexagonal castellated beams has been identified as the dominant failure mode when excessive initial post-tensioning force is applied to reach greater bending moment resistance, so it is required to limit the highest initial post-tensioning force to prevent this failure. Furthermore, properties of steel material has been simulated using bilinear elastoplastic modeling with 1.5% strain-hardening which has perfectly matched with the real material of steel angles. It is recommended to avoid using steel angles with high yielding strength since they lead to the yielding of bolt shank. The necessity of reinforcing plates to prevent beam flange from local buckling has been reaffirmed.

Seismic design and performance evaluation of self-centering timber moment resisting frames

Motivated by the recent concept of seismic resilient structures and societies, the seismic resilient timber solutions were developed to provide the lateral drift capacity while reducing the structural damage through controlled rocking and self-centering motion. However, very limited studies have evaluated the seismic performance of the post-tensioned self-centering timber frame at the building level, restricting the real-world applications of such structure. This study provides a post-tensioned self-centering timber frame solution for a building hypothetically designed in a high seismic zone in China. Besides, the same architectural design is provided with another comparable structural solution, i.e., a typical post and beam frame system with timber braces, conforming to the design code requirements. Subsequently, the nonlinear numerical models are carefully developed and their crucial component behaviors are calibrated with the existing experimental results. Finally, the seismic performance of the two structural solutions are systematically evaluated. The peak inter-story drift, the residual inter-story drift, and the Park-Ang damage index at the system level are investigated. Moreover, the fragility curves are generated demonstrating the self-centering feature of the seismic resilient timber solution. The advantages as well as challenges of such post-tensioned self-centering timber frame structures are clearly captured in the study.

Development Behavior for Post-Tensioned Self-Centering Steel Connection under Cyclic Loading

One of the newest steel beam-column joints to replace conventional welded connections, post-tensioned connection steel is with the upper and lower angles. In this connection are high-strength steel strands that parallel beam web and angles between beams and column. Actually high resistance strands and upper and lower angles respectively are provider centralization properties and energy dissipation capacity of the connection. The benefits of post-tensioned steel can be used in connection with the centralization and lack of relative displacement (drift) persistent, stay elastic core components such as connecting beams, columns and fountains connection, appropriate initial stiffness and joint manufacture with materials and traditional skills. . In this study, numerical modelling in Abaqus software, the results of the analysis were compared with the results of laboratory samples and the results showed that the two together are a perfect match. After validation, parameters influential centrist connection then pulled the thick angles in three numerical models were evaluated. The results show that by increasing the thickness of the angles, increase energy dissipation capacity and ductility connection and the β₁ value does not experience tangible changes with changes in angle thickness.

Probabilistic seismic performance assessment of self-centering prestressed concrete frames with web friction devices

A novel post-tensioned self-centering (SC) concrete beam-column connection with web friction devices has been proposed for concrete moment-resisting frames. This paper presents a probabilistic performance evaluation procedure to evaluate the performance of the self-centering concrete frame with the proposed post-tensioned beam-column connections. Two performance limit states, i.e., immediate occupancy (IO) and repairable (RE) limit states, are defined based on peak and residual story drift ratios. Statistical analyses of seismic demands revealed that the dispersion of residual drifts is larger than that of peak drifts. Due to self-centering feature of post-tensioning connections, the SC frame was found to have high probabilities to be re-centered under the design basis earthquake (DBE) and maximum considered earthquake (MCE) ground motions. Seismic risk analysis was performed to determine the annual (50-year) probability of exceedance for IO and RE performance limit states, and the results revealed that the design objectives of the SC frame would be met under the proposed performance-based design approach.

Analytical model validation and influence of column bases for seismic responses of steel post-tensioned self-centering MRF systems

The first generation for the post-tensioned (PT) self-centering (SC) system, which incorporates the PT technology to beam-to-column connections, exhibits good seismic performance with small residual deformations except for the first floor. Instead of using the fixed column base, the column PT to the base affects the seismic performance of frames, especially for residual deformations. Recently, the seismic response of a SC frame subassembly, which applied the PT technology to the connection and column base, was verified in shake table tests. In this paper, a three-dimensional analytical model with rotational springs in the PT connection and PT column base was introduced to capture shake table test results of the frame subassembly. The same modeling approach was adopted to one MRF and three SC frames to study the effects of column bases on the seismic responses of frames under the design-based and maximum-considered earthquakes. The monotonic, cyclic pushover, and time–history analyses were conducted for these frames. Analytical results showed that (1) the residual drift of the first floor could be significantly minimized by using the PT column base but the maximum interstory drift in the SC frame increased with decreasing fixity at the column base, (2) the largest maximum interstory drifts of the SC frames were larger than those of the MRF due to the low-to-medium structural period and high yield strength, and (3) the SC frame with the PT column base effectively decreased column restraining forces to the first floor compared to that with the fixed column base.

Seismic design and shake table tests of a steel post-tensioned self-centering moment frame with a slab accommodating frame expansion

Post-tensioned (PT) self-centering moment frames were developed as an alternative to welded moment-resisting frames (MRFs). Lateral deformation of a PT frame opens gaps between beams and columns. The use of a composite slab in welded MRFs limits the opening of gaps at the beam-to-column interfaces but cannot be adopted in PT self-centering frames. In this study, a sliding slab is used to minimize restraints to the expansion of the PT frame. A composite slab is rigidly connected to the beams in a single bay of the PT frame. A sliding device is installed between the floor beams and the beams in other bays, wherever the slab is allowed to slide. Many shaking table tests were conducted on a reduced-scale, two-by-two bay one-story specimen, which comprised one PT frame and two gravitational frames (GFs). The PT frame and GFs were self-centering throughout the tests, responding in phase with only minor differences in peak drifts that were caused by the expansion of the PT frame. When the specimen was excited by the 1999 Chi-Chi earthquake with a peak ground acceleration of 1.87g, the maximum interstory drift was 7.2% and the maximum lateral force was 270 kN, equal to 2.2 times the yield force of the specimen. Buckling of the beam bottom flange was observed near the column face, and the initial post-tensioning force in the columns and beams decreased by 50 and 22%, respectively. However, the specimen remained self-centering and its residual drift was 0.01%. Copyright © 2011 John Wiley & Sons, Ltd.

Friction Damped Posttensioned Self-Centering Steel Moment-Resisting Frames

A new connection for steel moment-resisting frames that incorporates posttensioning elements to provide a self-centering capacity along with friction mechanisms to dissipate energy is proposed and experimentally validated. The restoring force from the posttensioning elements in the connection makes the structure return to its undeformed state, even after experiencing large inelastic drifts. A bolt-prestressed friction mechanism with a frictional interface consisting of stainless steel and new nonasbestos organic break lining pads dissipates seismic input energy as the system undergoes lateral deformations. Cyclic tests were conducted to investigate the efficiency of the proposed friction interface and its performance under loading conditions that are expected during seismic loading. The test results showed that the frictional behavior is stable, repeatable, and predictable, although its friction coefficient is relatively low. Exterior and interior self-centering moment connections equipped with the proposed friction dampers were tested to study their structural behavior under cyclic loading. The results confirmed that friction damped posttensioned self-centering connections are capable of developing similar stiffness and strength characteristics to welded connections. They are also capable of undergoing large deformations with good energy dissipation characteristics, but without introducing inelastic deformations in the beams or the columns and without residual story drifts. Even at the ultimate stage, i.e., beyond the self-centering limit, the proposed connections can be detailed to exhibit a ductile response with the formation of flexural hinges in the beam sections, thus avoiding the sudden loss of strength and stiffness that occurs when the posttensioning elements are overloaded or when the beams buckle under excessive combined axial loads and bending moments.