Unbonded Post-tensioning Strands Research Articles

Experimental study on the collapse-resistant performance of unbonded prestressed PC beam-column sub-assemblages with pinned connections

In precast concrete frame structures, it is imperative that beam-to-column connection has appropriate robustness and reliable axial resistance to defense against progressive collapse. Hence, a method of using pinned connection and unbonded posttensioning strands (UPSs) to mutually improve the collapse-resistant performance of precast concrete structures was proposed in this study. The pinned connections are used for connecting precast beams with columns at precast beam ends, and UPSs with straight profile are placed at the bottom of beam cross-section. In order to investigate whether the proposed method can enhance collapse-resistant capacity of precast concrete structures, static progressive collapse tests were conducted for the newly proposed precast concrete frame substructures with UPSs (UPPC frame substructures) and without UPSs (PC frame substructure). Moreover, influences of effective prestress level upon the collapse-resistant capacity of UPPC substructure were also investigated. Test results demonstrated that, similar to conventional RC frame structure, the compressive arch action (CAA) and catenary action (CTA) could also develop in the proposed UPPC substructures to avert progressive collapse. The UPPC substructure was able to develop 32% higher CAA capacity and 162% larger CTA capacity as compared to the PC substructure, indicating that the UPSs play a positive role in enhancing both its CAA and CTA. The CTA capacity of the PC substructure was 53.8% higher than that of CAA, which signifies that the pinned connection can be employed to improve structural robustness to mitigate progressive collapse. In addition, the increase of effective prestress level of UPSs is beneficial to the enhancement of CAA capacity of the UPPC substructure. However, higher effective prestress level is limited to improve its CTA capacity due to lower deformation capacity of UPSs. Subsequently, three-dimensional finite element models (FEMs) of the UPPC and PC substructures were developed by using ABAQUS software. By comparing experimental data with FEM results, it was concluded that the FEM could accurately predict the load–displacement curves, crack patterns and reinforcing ruptures of all the test specimens.

Experimental study on seismic behavior of the self-centering RCS joint with replaceable buckling restrained dampers

Reinforced concrete column and steel beam (RCS) joints are typically designed to ensure that plastic hinges develop at the end of the steel beams. The seismic energy is dissipated through the inelastic deformations of the steel beam. This design will result in difficult-to-repair damage and residual drifts. In this paper, a self-centering RCS joint with replaceable buckling restrained dampers and unbonded post-tensioning strands is proposed which can significantly eliminate residual drift under a strong earthquake. It dissipates seismic energy through replaceable buckling restrained dampers. It is easy to replace damaged dampers to ensure quick restoration of the capacity of the joint. Nine self-centering RCS joints were designed, constructed, and tested under cyclic loading. The experiments investigated the mechanical behavior, hysteretic curve, energy dissipation capability, self-centering capability, and replaceability of the self-centering RCS joints. The experimental results showed that the proposed joint experienced minimal residual drift and performed satisfactorily at the moment carrying capacity and energy dissipating capacity. Additionally, the specimens performed almost identically after the damaged dampers were replaced; thus, the effectiveness of damper replacement was verified.

Experimental study on seismic behavior of SCRC column base joints with replaceable dampers

Self-centering reinforced concrete (SCRC) column base joints can effectively reduce residual drift under horizontal seismic action as unbonded post-tensioning strands provide self-centering capacity. The connection also utilizes a steel tube collar, and a new type of replaceable damper to dissipate seismic energy. Six specimens of SCRC column base joints were designed, constructed and subjected to cyclic horizontal loading in conjunction with axial compression. The experiments investigated the mechanical behavior, hysteretic curve, energy dissipation capability, self-centering capacity, and replaceability of the SCRC column base joints. The experimental results showed that the SCRC column base joint performed satisfactorily in moment capacity, residual drift, and energy dissipation. The behavior of specimens with replaced dampers was almost the same as their behavior with the original dampers, indicating the efficacy of replacement. Moreover, the steel tube protected the concrete at the column base from local compression failure.

Progressive collapse of RC flat slab substructures with unbonded posttensioning strands after the loss of an exterior column

Reinforced Concrete (RC) flat slab structures are mainly composed of load-bearing columns and RC slabs, which are vulnerable to collapse once one or several local load-bearing columns fail due to accidental loads such as terrorist bomb attack or vehicle collision. To investigate the progressive collapse performance of flat slab structures after the loss of an exterior column, static collapse tests were conducted on three 1 × 2 bay RC flat slab substructures. Effect of prestress and layout of the unbonded posttensioning strands (UPSs) on the progressive collapse of flat slab structures was discussed. Experimental results showed that punching shear failure (PSF) foremost occurred at the slab-column connection (SCC) of the failed exterior column during collapse, and then successively occurred at the SCCs close to the corner column and the exterior column adjacent to the failed column. Primary cracks on the top slab surfaces around the failed exterior column generally presented arc-shaped distribution; while primary cracks on the bottom slab surfaces radially propagated from the failed exterior column. Comparing with the flat slab substructure without UPSs in the slabs, the load-bearing capacity of the substructures with UPSs in the slabs significantly improved. Flexural action was the fundamental collapse resistance mechanism in the initial collapse stage, and tensile membrane action gradually formed during collapse. The flexural action, the tensile membrane action, as well as the post-punching behaviors of the slabs resisted the collapse load together before the final collapse occurred. Most of the load applied on the failed exterior column mainly transmitted along the shorter slab bay under the exterior-column-removal scenario. Arranging posttensioning strands in the slabs contributed to reducing vertical deformation of the slabs. The UPSs in the column strip passing through the failed column played a more important role in resisting collapse load than those arranged in the middle strip. The residual flat slab substructures still possessed satisfactory load-carrying capacity even if PSF occurred in one or several SCCs.

Experimental and numerical investigations of self-centering post-tensioned precast beam-to-column connections with steel top and seat angles

There is growing interest in the development of earthquake resilient structures. This paper proposes a new kind of self-centering post-tensioned precast beam-to-column connection, assembled using unbonded posttensioning strands, and steel top and seat angles (PTSA). A series of cyclic loading tests along with numerical simulations were performed to investigate the seismic behavior of the PTSA connection. In the experimental tests, eight subassembly specimens were prepared with varying specimen parameters including the initial posttensioned force, beam depth, and type of steel angles. Test results show that the properly designed specimens had large initial stiffness, good recentering capacity, and high ductility. Permanent deformation was concentrated in the steel angles, which were relatively easy to replace. Precast beams, columns, and PT strands in each subassembly behaved almost elastically up to 3.5% drift. Additional tests on a previously tested PTSA specimen with newly replaced steel angles demonstrated that the cyclic responses were almost identical to the original counterpart. Numerical analyses were carried out using the platform OpenSees to examine the effects of the concrete initial compressive stress, the initial prestress, the concrete compressive strength, and the beam cross-sectional width on the mechanical response of PTSA. Results indicate that appropriate areas and initial prestress of the PT strands, and concrete compressive strength were beneficial to the load-carrying and deformation capacities of PTSA. Increasing the beam cross-sectional width may improve the ductility of PTSA.

Experimental evaluation of self-centering hybrid coupled wall subassemblies with friction dampers

Post-tensioned hybrid coupled wall system has desirable seismic characteristics including self-centering capability and tolerance for large nonlinear displacements with limited damage. In this study, the behavior of concrete coupled wall subassemblies with post-tensioned steel coupling beam was experimentally evaluated. In this system, the coupling beam is not embedded into the walls, and the coupling of the concrete walls is accomplished by post-tensioning the steel coupling beam to the walls using unbonded post-tensioning strands. Steel angles and friction dampers were used at the beam-to-wall connection region for energy dissipation. In the first series of tests, a friction damper with different values of the clamping force was tested under two different loading frequencies. In the second series, seven quasi-static cyclic tests were conducted on 2/3-scale subassemblies of the post-tensioned hybrid coupled wall system. The test specimens included a control specimen without energy dissipation device, a specimen with steel angles and four specimens with friction dampers. In addition, one of the tested specimens with dampers was re-tested to investigate the structural behavior and residual capacity of the system during a strong aftershock. The test parameters included the type of energy dissipation device, the initial stress of post-tensioning strands and the amount of damper normal force. The test results show that the subassemblies equipped with friction dampers exhibit excellent lateral stiffness, strength, ductility and energy dissipation and are capable of withstanding large nonlinear cyclic deformations up to the drift of 8%, without residual displacements and without significant damage to the system. Whereas the subassembly equipped with steel angles provide lower energy dissipation capacity and because of the low-cycle fatigue fracture of angles, energy dissipation and load carrying capacities of the specimen decrease significantly.

Nonlinear dynamics of self-centring rocking steel frames using finite element models

Rocking post-tensioned steel frames capitalise on the use of rocking joints, and unbonded post-tensioning strands to provide self-centring action. Investigations on the complex and unconventional nonlinear dynamics of tied rocking steel frames, exclusive of supplemental damping methods, are presently limited. Increasing levels of energy-dissipation reduce the probability of observing nonlinear dynamic phenomena such as co-existing (high/low) amplitude responses at and around the system's nonlinear resonance. To this end, a finite element (FE) modelling framework is presented, validated and extended to multi-storey steel buildings. It is shown that the simulation strategies proposed enable an accurate representation of the complex nonlinear dynamics of self-centring structures, over a wide range of excitation frequencies and amplitudes. The methodology, applied to multi-storey steel frames, captures the presence of sub-harmonic resonances and higher-modes. It is also demonstrated that the additional demands observed in the rocking columns are the consequence of the asymmetry of the member boundary conditions.

Progressive Collapse Resistance of Posttensioned Concrete Beam-Column Subassemblages with Unbonded Posttensioning Strands

In this study, the behavior of posttensioned reinforced concrete (PC) beam-column subassemblages subjected to the loss of a middle column is investigated experimentally. The influence of unbonded posttensioning strands (UPS) with a parabolic curve on the behavior of reinforced concrete (RC) frames to resist progressive collapse is also quantified. Test results indicated that UPS have little effect on the yield load and first peak load of frames to resist progressive collapse. However, UPS could significantly increase the ultimate load capacity of the frames because stretching of strands could provide considerable additional vertical load resistance. UPS will aggregate the damage in the beam ends near to the middle column, although they may relieve the damage in the beam ends near to the side column. Moreover, UPS may change the load-resisting mechanism of the RC frame. No reliable compressive arch action developed in PC beams to resist progressive collapse because the UPS changed the distribution of the compressive stress along the beams. In addition, the effects of span/depth ratio and effective prestress in UPS on the progressive collapse resistance of PC frames are investigated. It is found that the span-depth ratio has a significant effect on the performance of RC frames to resist progressive collapse, but not the PC frames. The effective prestress in UPS has little effects on the yield load and initial stiffness of the PC frame, but it may significantly affect the ultimate deformation capacity and ultimate load capacity of the PC frame.

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.

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.