Post-tensioned Frame Research Articles

The seismic responses of the Post-Tensioned frame with Sliding Keys on Inclined Deflecting-cantilevers device (the PT-SKID frame)

The seismic responses of the Post-Tensioned frame with Sliding Keys on Inclined Deflecting-cantilevers device (the PT-SKID frame)

Seismic vulnerability assessment of post-tensioned timber building fitted with dissipative bracing systems

A post-tensioned (PT) timber building coupled with dissipative bracing systems is a sustainable seismic-resistant structural system. This system combines desirable qualities of self-centering, energy dissipation, and lightweight timber, which attracts lesser seismic force and contributes to a better resilient system. Despite significant advancement in the provision of these sustainable seismic-resistant timber-based structural archetypes, more study is required to derive a fuller knowledge of the behaviour of these innovative structural systems under a range of ground motion intensities and limited state capacities. Direct displacement-based design, a performance-based design approach, is used for the design of this system, and performance assessment is carried out using nonlinear dynamic analysis. This study investigates the performance of the braced PT frame under different limit state considerations using ground motion records consistent with the NBC 2020 seismic hazard.

Collapse performance of post-tensioned steel frames considering influence of infill walls

The aim of this paper is to study the effect of infill walls on load resisting behaviors of post-tensioned (PT) steel frames against collapse. Three planar PT steel frames are simulated using finite element (FE) method, one of which has no infill wall while the other two are installed with infill walls. The applied loads, lateral displacements, failure patterns, stress developments in infill walls, resisting mechanisms, as well as re-centering capabilities of three PT steel frames are studied and the role that infill walls play is discussed. Parametric analyses are conducted to investigate influences of magnitude of PT forces and critical material properties. Results indicate that infill walls work as compressive struts, significantly increasing initial stiffness, resistance and story drifts of PT steel frames, and play as energy dissipation devices during the collapse process. The existence of opening makes the stiffness distributed discontinuously throughout infill walls, greatly changing the failure pattern and increasing story drifts of PT frames. The resistances of infilled PT steel frames increase with increasing PT forces, bed joints strength and masonry unit strength in the initial stage. A design scheme is also developed to precisely predict the collapse resistance of infilled PT steel frame structures.

Experimental testing of post-tensioned steel-timber hybrid frames equipped with energy-dissipating braces

The weak properties of wood in perpendicular-to-grain direction have imposed significant design challenges for post-tensioned (PT) timber frame structures. To resolve the challenges, the authors of this study have introduced a novel solution, the steel-timber hybrid PT frame. To further enhance its lateral performance, additional damping and stiffness (ADAS) braces were integrated into the hybrid frame. In this study, the feasibility of the combination was experimentally examined at the system level. Cyclic tests were conducted to evaluate the performance of two 2/3 scaled frame specimens: a conventional PT timber frame and a PT steel-timber hybrid frame with ADAS braces. The combination of a hybrid frame with ADAS braces showed substantial enhancements in lateral strength, initial stiffness, system ductility, and energy dissipation compared to the conventional PT timber frame. The experimental results highlighted the significant role of ADAS braces in enhancing energy dissipation, while their contribution to stiffness was less dominant. Analytical analysis further revealed an inverse relationship between the increase in ADAS contribution to lateral stiffness and the self-centering competence of the frame. A design equation was proposed to assist practitioners in achieving an optimal balance between the self-centering capacity of the PT hybrid frame and the stiffness enhancement provided by ADAS braces.

Collapse performances of 3D post-tensioned RC frame structures considering combined influences of infill walls and floor slabs

This paper investigates the combined influences of infill walls and floor slabs on mechanical behaviors of post-tensioned (PT) reinforced concrete (RC) frame structures against collapse. Four PT frame structures modified from experimental specimens are studied. Applied loads, horizontal displacements, failure modes and resisting mechanisms are analyzed, and the roles that infill walls and floor slabs play are discussed. Parametric analyses are performed to study influences of opening in infill walls, diameter of wires in floor slabs and PT force in beams on collapse resisting capacity of PT frame structures. Results indicate that infill walls behave as equivalent compressive struts, increasing the horizontal deformation, changing the failure mode of PT frame structures; Floor slabs provide restraints on all columns and reducing the horizontal deformations of structures. Out-of-plane deformation develops in infill walls due to the two-way bending effect of floor slabs. The opening decreases the influence of infill walls, and the resisting capacity of PT frame structures enhances with increasing diameter of wires in floor slabs and PT forces in beams. A design method is developed to estimate collapse resistance of PT frame structures with good accuracy and efficiency.

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.

Improving the Seismic Performance of Steel Frames under Mainshock–Aftershock Using Post-Tensioned Connections

In this study, the seismic responses of moment-resisting steel frames (MRSFs) with welded and post-tensioned connections under 28 artificial seismic sequences (mainshock–aftershock) are compared. For this aim, the mainshock are scaled at different ground motion intensity levels as a function of the spectral pseudo-acceleration corresponding to the fundamental period of vibration of the structure Sa(T1), whereas different intensity levels of the aftershocks are used for a percentage of the peak maximum acceleration of the mainshock. The seismic performance comparison of both structural systems is computed for the maximum, residual inter-story drift and hysteretic energy demands. The results show that post-tensioned frames significantly reduce the structural demands, especially in the case of residual inter-story drifts and hysteretic energy in comparison with moment-resisting steel frames with welded connections. The reductions in the structural response tend to be larger as the intensity of the aftershock tends to increase. Therefore, it is concluded that the use of post-tensioned connections is a great alternative to mitigate the seismic response of buildings subjected to seismic sequences.

The Sliding Keys on Inclined Deflecting‐cantilevers (SKID) device: Empirical and analytical sensitivity analysis with application in post‐tensioned frames

AbstractPreviously, the authors presented a new theoretical energy dissipating technology termed a Frictional Sliding on a Sprung Slope (FSSS) device. Its purpose was to provide significant energy dissipation to post‐tensioned (PT) frames structures. The FSSS device has a triangular‐shaped hysteretic curve, which has a zero‐force activation threshold. This enables activation for any earthquake magnitude and provides progressively greater energy dissipation with an increase in structural oscillations. In addition, it is designed to allow full self‐centring of the PT framed structure. In this paper, a physical prototype FSSS is designed, constructed and experimentally tested. The prototype replaces the theoretical springs, bearings and the constraints with pairs of cantilever bars. Consequently, it is renamed a Sliding Keys on Inclined Deflecting‐cantilever (SKID) device. Six scaled SKID configurations having different manufacturing parameters were built and tested. Also, five materials were used to investigate the sensitivity to the friction coefficient of the sliding pads. Both cyclic and fatigue tests were conducted to assess SKID performance empirically. A finite element model of SKID was then implemented in OpenSees and benchmarked against the experimental results. More than 700 one‐story one‐bay PT frames with different SKID devices (PT‐SKID frames) were numerically tested to investigate the dynamic characteristics of the structural system affecting the seismic response. Based on the extensive experimental and numerical analyses, design suggestions are provided, and the PT‐SKID self‐centring features are discussed.

Performance-based design of steel frames with self-centering modular panel

The self-centering modular panel (SCMP) is an emerging seismic resilient lateral force-resisting system. This paper intends to develop a performance-based design (PBD) method for steel buildings equipped with the SCMPs based on the concept of direct displacement-based design methodology. The theoretical hysteretic model of the SCMP consisting of the post-tensioned frame (PTF) and four replaceable hysteretic dampers (RHD) is proposed and validated in this paper. Based on the proposed theoretical hysteretic model, the equation for the equivalent damping ratio of the steel buildings with SCMP is developed and the PBD method is proposed. The 3-story and 6-story steel frames equipped with the emerging SCMP are established through the developed PBD procedure. Static analyses were performed to investigate the nonlinear responses of the established buildings. Twenty ground sequences were chosen and scaled to evaluate the seismic responses of the designed steel frames subjected to different seismic intensity levels through nonlinear dynamic analyses. The results of the research show that steel structures equipped with SCMPs established through the developed PBD procedure can display the expected hysteretic behavior and performance under earthquakes. The designed buildings can achieve excellent post-earthquake recoverability by keeping the residual displacement in each story less than 0.5% even under earthquakes with MCE intensity.

Hybrid simulation of a post-tensioned timber frame and validation of numerical models for seismic design

The post-tensioned frame is one of the recently emerged structural systems for multi-story timber buildings. It is characterized by a high level of prefabrication and quick erection on the construction site. The post-tensioned frame developed at ETH Zurich is based on post-tensioned beam–column connections with hardwood reinforcement of the column in the connection region and column base connections with glued-in steel rods. Such a construction system is suitable for low- and mid-rise buildings that are located in regions characterized by low to moderate seismicity.This paper presents a series of hybrid simulations of the response of a two-story two-bay post-tensioned timber frame subjected to ground motion excitation. Nonlinear numerical models of both beam–column and column base connections to be used for design purposes are validated based on the experiments.

Dynamic experiments on post-tensioned beam-column substructures following the sudden removal of a single interior column

Most experimental investigations about post-tensioned (PT) substructures have been conducted statically, neglecting the dynamic characteristic of progressive collapse. Dynamic performances of PT frame structures against progressive collapse are imperative to study under the sudden column removal scenario, which can reflect the actual progressive collapse more convincingly. This paper presents experimental programs on six half-scaled beam-column substructures, which were tested to evaluate dynamic responses after the sudden removal of a single interior column. During the tests, extensive instruments were installed in prescribed locations to record various dynamic responses such as displacements, loads, strains, and crack patterns. By comparison, the influences of post-tensioned strands (PTS) on the structural mechanisms of reinforced concrete (RC) frames against collapse were demonstrated. The findings cover the span–depth ratio of beam, bonded form of bonded post-tensioned strands (BPTS), and unbonded form of unbonded post-tensioned strands (UPTS). Experimental results highlight that the initial stiffness of RC substructures can be significantly enhanced by the PTS. Resistant mechanisms are mobilized to resist progressive collapse, such as compressive arch action and catenary action. The catenary action develops to resist progressive collapse if a larger span–depth ratio is applied in the design. Moreover, both UPTS and BPTS can increase the damping ratio of the corresponding RC, and the damping ratio of the PT specimen can reach up to 217.23% of that of the corresponding RC. And also, the proposed analytical model and theoretical equations can be well verified by the experimental results.

Progressive collapse mechanisms of post-tensioned reinforced concrete frames considering effect of infill walls

This study numerically investigates progressive collapse mechanisms of post-tensioned RC frames under column removal scenarios by using the finite element method. The primary objective of this study is to investigate the influences of infill walls on mechanical behavior of post-tensioned (PT) frames against progressive collapse. Three experimental conventional reinforced concrete (RC) frames are modified into PT reinforced concrete frames: PT bare frame, PT full infilled frame and PT partial infilled frame. The numerical study was performed and the applied vertical load, horizontal displacement, failure process, load redistribution mechanisms, and re-centering capacity of PT frames are investigated under middle column pushdown scenario. Parametric analyses are conducted to study influences of material properties, arrangement of PT tendons, and magnitude of PT forces. The results show that masonry infill walls enhance the resisting capacity; however, change the failure modes and reduce the re-centering capacity of the PT frames against progressive collapse. The presence of the opening reduces the contribution of the infill walls. The PT bare frame have better resilience performance compared with the conventional bare frame. The bed joints, strength of masonry bricks, arrangement of PT tendons and magnitude of PT force greatly influence the progressive collapse resisting capacity of PT frames. Finally, a preliminary design method is proposed to predict the progressive collapse resisting capacity of PT frames with good accuracy and efficiency.

Seismic behavior of dual buckling-restrained steel braced frame with eccentric configuration and post-tensioned frame system

The buckling-restrained steel braced frames with eccentric configurations (BRBFEs) have high energy dissipation capacities and an architectural advantage to create openings. However, recent investigations have shown that after the earthquake at the MCE hazard level, the residual drift in buildings with buckling-restrained steel braced frames (BRBF) is remarkable. In the present study, to reduce the residual drift in a BRBFE system, it has been suggested to add a post-tensioned frame (PTF) in parallel and create a BRBFE-PTF combined system. Moreover, a method has been proposed to design this combined system. In this method, the values of the two parameters of the self-centering ratio and cable cross-section area are selected arbitrarily. Then, after verifying the modeling, the pushover, cyclic, and time-history analyses of the prepared numerical models were performed. The results showed that if appropriate values were selected for the self-centering ratio and cable cross-section area, the proposed system had the desirable seismic performance in maximum drift demand and residual drift.

A frictional sliding on a sprung slope (FSSS) device that axiomatically confers energy dissipation with re-centring to post-tensioned (PT) frames: A conceptual study

A conceptual novel frictional sliding on a sprung slope (FSSS) device is proposed for application with post-tensioned (PT) frames (which have a nonlinear elastic characteristic). This FSSS device has an amplitude-dependent hysteretic resistance nature. The mechanical schematization and design implementation are presented, followed by its analytical formulations. Theoretical analysis indicates that the estimated equivalent viscous damping ratio of the PT frame with the FSSS device must increase with the amplitude of drift ratio oscillations. Three heuristic PT frames with and without the FSSS device are analysed numerically using OpenSees as a numerical proof-of-concept. Both quasi-static (cyclic push-over) and nonlinear dynamic structural analyses are performed. The numerical results of the PT part are benchmarked against the experimental test, and those of the FSSS part are verified by the theoretical formulations. Quasi-static cyclic tests indicate that the PT frame with the FSSS device has a triangular flag-shaped hysteretic curve rather than a parallelogram flag-shape, as in traditional self-centring systems. This triangular flag-shape is an advantage as it does not inhibit re-centring of the PT frame as its activation force threshold is effectively zero. Incremental dynamic analyses (IDA) are also carried out for a set of ground motions. The IDA results show that the device significantly reduces the earthquake response of the PT frame in terms of drift ratios. Additionally, no residual deformations are observed after the earthquake excitations, validating the axiomatic self-centring feature of the PT frames with FSSS devices.

Applications of simplified 3D FE model in seismic analysis for friction-damped posttensioned steel frame structures

The post-tensioned (PT), energy-dissipating connection for steel frames has drawn the attention of many researchers due to its role in ensuring a favorable seismic performance. The energy-dissipating capacity of these frames is often improved by various friction mechanisms, such as friction-damped PT (FDPT) steel connections. Numerous researchers have investigated the seismic behavior of FDPT by using either the numerical or experimental method. However, previous studies have mostly relied on elaborate, time-consuming FE models, and analyzing a 3D frame structure is nearly impossible. To overcome these limitations, this paper proposed and analyzed a mechanical model of 3D FDPT as well as a 3D FE model of FDPT frame structures. The seismic responses of traditional and FDPT frame structures were also analyzed and compared. The results provide a foundation for the future analysis and design of PT frame structures.

Experimental testing of a self-centring concentrically braced steel frame

Conventional concentrically braced frames (CBFs) undergo many cycles of inelastic deformation during seismic excitation. This inelastic deformation leads to the possibility that a structure will remain in an out-of-plumb position, even if it has performed as required by current design codes. This paper presents an improved steel braced framing system that eliminates such residual deformations in the structure by using a post-tensioning arrangement to ensure the structure self-centres following an earthquake. This is achieved by combining the bilinear elastic rocking response of a post-tensioned frame with the inelastic behaviour of tubular steel bracing members to give a system that both dissipates hysteretic energy and ensures self-centring behaviour, termed the self-centring concentrically braced frame (SC-CBF). This SC-CBF system distinguishes itself from previous self-centring CBFs by employing post-tensioned rocking beam-column connections as opposed to a globally uplifting frame. The mechanics behind the behaviour of the SC-CBF are first described, followed by a discussion of an experimental test setup to validate the concept under quasi-static cyclic testing. Results from a total of nine tests are presented to demonstrate the self-centring behaviour of the SC-CBF. Comparisons with analytical expressions developed for the system demonstrate the SC-CBF performs as anticipated and presents a novel system for the seismic design of steel structures.

Component-based model for posttensioned steel connections against progressive collapse

A component-based method for the estimation of the posttensioned (PT) steel frame against progressive collapse is proposed and presented in the current paper. A mechanical model of PT steel connections is developed and benchmarked with experimental data of a PT beam-column substructure from literature. The developd mechanical models of four PT connections are able to capture the initial elastic stiffness, decompression load, and residual stiffness under lateral loading. Then, analysis of a reduced-scale three-storey two-bay PT steel frame is carried out with sufficient accuracy by incorporating the proposed joint model into the frame analysis. The proposed method is then applied to assessing progressive collapse of a one-storey two-bay PT frame under middle column removal scenario, and is verified against existing experimental and ANSYS finite element results. Three resistance mechanism for progressive collapse of the PT frame are evaluated, which consists of angle flexural mechanism, beam compression arching action and strand tensile catenary action. Finally, parameter analyses of the PT frames are conducted to investigate the effects of the connection details on the behavior and resistance of progressive collapse. The proposed model can be used to predict the quasi-static behavior of PT frames under monotonic vertical loading conditions with satisfactory accuracy.

Seismic Performance of Post-tensioned Precast Concrete Joints Improved with Curved Steel Braces

ABSTRACT The moment frames that consisted of prefabricated concrete beams and columns, which were linked by the unbonded post-tensioning, have been proposed for multi-story buildings. However, their lower structural strength and stiffness limited the applicability of their systems in the earthquake-prone areas. Curved steel braces were proposed in this study to improve the structural performance of the post-tensioned precast reinforced concrete (RC) beam-column joints, along with four joint specimens consisting of braces with different cross section height and eccentricity were tested under the reversed cyclic loads while the parametric analysis was also carried out using the finite element analysis (FEA) model. Also, the failure mode, strength, stiffness, and energy dissipation were analyzed. Based on the results, the whole joints have a stable mechanical behavior with an inter-story displacement angle of 5%, concrete crushing at the end of the beam, and a bending yield of curved braces form the failure mode. The curved steel braces could provide good strength, stiffness, and energy dissipation for the precast joints and these features of the joints could gradually increase with the expansion of the brace section height and the reduction of the brace eccentricity. The equivalent viscous damping coefficient at 4% inter-story displacement angle ranged from 0.12 to 0.2, which was two to three times higher than the general post-tensioned precast RC joints. When the eccentricity of the curved steel brace was less than 0.3, the joint strength was greatly impacted by the eccentricity and changed a bit when it was greater than 0.3. The results showed the practical feasibility of the post-tensioned frames with curved steel braces.

Buckling-enabled composite bracing for damage-avoidance rocking structures

Post-tensioned rocking frames have been proposed as damage-free seismic-resistant structures. However, currently available load resisting systems for rocking frames rely on sacrificial yielding components that accumulate damage during strong dynamic action. To address this shortcoming, this study proposes a novel thoroughly damage-avoidance solution by means of bracing elements with carefully controlled buckling behaviour. To this end, a proof-of-concept study is presented, whereby the elastic buckling response of buckling-enabled composite bracing (BECB) elements with circular-arc shaped cross-section is numerically investigated. Varying geometric properties are considered and validated against analytical approximations. Besides, a finite element study of a single-storey steel post-tensioned frame under static and dynamic actions is performed. The case study incorporates BECB elements made from glass-fibre reinforced polymer (GFRP). It is demonstrated that BECB enhances the non-linear static and dynamic response of rocking frames by providing stability and significantly reducing storey drifts and accelerations without accumulating damage.

Seismic Analysis of Friction-Damped Self-Centering Coupled-Beams for Moment-Resisting-Frames without Floor Elongation

The common characteristic of self-centering (SC) post-tensioned (PT) connections is the gap opening and closing at the beam-column interface, which often face the problem of deformation incompatibility with floor system and cause severe damage of floor slab. To address this drawback of PT frame expansion, an innovative friction-damped SC coupled-beams (CBs) that incorporate PT strands to provide a SC capacity along with friction devices (FDs) to dissipate energy is presented. The global mechanics of this system are briefly explained. An individual FD is tested to investigate the wear resistant properties and friction coefficients of friction materials for the energy dissipation system of SC-CBs. Detailed three-dimensional finite element models (FEMs) of a single-story single-bay frame using SC-CBs are developed with consideration of different FDs installation positions. Analytical results indicate that SC-CBs can sustain large lateral deformations without structural damage under cyclic loading. Then, simplified computational models are validated based on the solid models and applied in nonlinear dynamic analysis of a five-story MRFs using SC-CBs. The frame using SC-CBs minimizes the residual drift and displays good energy dissipation capacity under both the design basis and maximum considerable levels of seismic loading.