Post-tensioning Force Research Articles

Performance of segmental self-centering rubberized concrete columns under different loading directions

Segmental self-centering columns are an advantageous construction choice in earthquake prone areas due to their minimal or even zero residual deformation and their low repair/downtime costs. This research investigated the behavior of segmental rectangular column members with a high cross-sectional length to width ratio under different loading directions, with the aim of potentially extending the concept of segmental self-centering columns to wall members, such as shear walls and retaining walls. The main parameters of this study were the direction of loading: in-plane (strong-axis bending) and out-of-plane (weak-axis bending), provision (or absence) of reinforcement, and the type of concrete material (conventional concrete or rubberized concrete). Eight concrete columns with a cross-sectional length to width ratio of 2.5 and consisting of three concrete segments with dry joints in between, were tested under reversed-cyclic lateral loading. A pre-stressing force of 100 kN, corresponding to a stress of 2.8 MPa on the column, was applied using unbonded post-tensioning (PT) bars. The results indicated that although in-plane loaded columns had a higher load capacity, they exhibited a less ductile response, higher level of damage and higher loss in the PT force, compared with the out-of-plane loaded columns. The total equivalent viscous damping and its variation was very small in all tested specimens, and it increased slightly as the drift ratio increased. Empirical equations were developed to express the damping as a function of drift ratio. It was also concluded that in the out-of-plane loaded specimens, the effect of reinforcement on the load-displacement response was insignificant. In rectangular columns with high length-to-width ratio, under out-of-plane loading, if a minimum level of axial pre-stressing is applied (to prevent shear or sliding failure), no structural reinforcement is required. In addition, the strength reduction due to using rubber in concrete at the structural level was much lower than that at the material level. The results of this study can potentially be applied to segmental concrete walls due to the high length-to-width ratios of the tested columns.

Parametric Study on Effect of Adding Stiffener to Post-tensioned Steel Connection

Beam-to-column connections with post-tensioned (PT) elements are used in steel moment-resisting frames to provide adequate stiffness, strength, and ductility and to eliminate residual displacements under seismic loads. A steel PT connection has recently been developed with top and seat angles. In this connection, angles are used to dissipate energy under cyclic loading. This paper aims to investigate the influence of adding stiffeners to the top and seat angles on the cyclic responses of PT connections. To this end, a parametric study is performed to compare the effectiveness of stiffeners and some design parameters in the cyclic behavior of the PT connections. The design parameters are the angle thickness, the distance between the bolt line passing through the column and the heel of the angle (g2), the PT force of the strands, and number of strands. First, the results of finite element analyses are compared to previously conducted experiments on PT connections with top and seat angles through a verification study. In this way, finite element models of 23 PT connections are developed and analyzed under cyclic loading. Results indicate that adding the stiffeners to the angles, enhances the energy dissipation capacity, moment capacity, stiffness and ductility of the PT connection much higher than improving other design parameters.

The implications of post-tensioning losses on the seismic response of Pres-Lam frames

Since 2010, twelve post-tensioned timber (Pres-Lam) buildings have been constructed throughout the world. In high seismic areas, Pres-Lam technology typically combines unbonded post-tensioning tendons and supplemental damping devices to provide moment capacity to beam-column, wall-foundation or column-foundation connections. Over time creep within the timber elements leads to losses in post-tensioning forces reducing the connection moment capacity. This paper analyses how different post-tensioning loss scenarios, depending on the beam-column joint detailing, impact the building’s seismic response. Two case study buildings were designed and investigated using the Acceleration Displacement Response Spectrum (ADRS) method and Non-Linear Time History Analysis (NLTHA) to predict seismic performance. These buildings were considered to be located in areas of high and low seismic risk, leading to designs with and without the use of damping devices, respectively. The results show that the building with additional damping responded with similar peak displacements, even under extreme loss scenarios. In comparison, when supplemental damping was not used, peak displacements increased significantly with post tensioning losses.

Influencing factors of residual drifts of precast segmental bridge columns with energy dissipation bars

The energy dissipation bar has been proven to be an effective way to improve the lateral strength and the seismic capacity of the precast segmental bridge column. To date, however, investigations on how the main design parameters affect the residual drift of such bridge columns are quite limited. Focusing on this research gap, a novel numerical simulation method was proposed in this article to investigate the influencing factors of residual drift of precast segmental bridge columns with energy dissipation bars. First, the refined fiber element model was developed based on the OpenSees software package, considering the bond slip phenomenon at the column end and the boundary condition at the interface between the two adjacent segments. Then, the proposed numerical model was validated by comparing the simulated hysteretic behavior of the precast segmental bridge columns with available experimental data. After that, cyclic analyses were performed on 288 precast segmental bridge column models with different design parameters. Four important influencing factors were investigated, including the post-tensioning force, gravity load, energy dissipation bar ratio, and shear span ratio. According to the analysis results, some suggestions were provided for the practical design procedure of the energy dissipation bar reinforced precast segmental bridge columns.

Cyclic loading test of self-centering precast segmental unbonded posttensioned UHPFRC bridge columns

Three 1/3-scale precast segmental bridge columns, manufactured with ultrahigh-performance fiber-reinforced concrete (UHPFRC) incorporating river sand and coarse aggregate, were tested under cyclic loading. Energy dissipation (ED) bars, embedded in ultrahigh-performance concrete (UHPC) grout, maintained continuous across segment joints and unbonded at the bottom joint. Self-centering prestressing force was provided by unbonded posttensioning (PT) tendons. The research parameters included PT force level and the amount of ED bars. Test results showed that all the specimens exhibited no less than 8% drift capacities, which were remarked with the first fracture of ED bars. No obvious cracking and limited UHPFRC spalling were observed. Both PT force level and the amount of ED bars have notable effects on stiffness, lateral strength, and ductility. Increased PT force may improve ductility with the total axial loading ratio less than 0.08. All PT tendons were elastic and no yield or rupturing was found, but the stress loss was significant. The equivalent unbonded length can be evaluated with 0.007dbfy for ED bars embedded in UHPC grout. The rotation of the bottom joint dominated lateral deformation and the contribution of joint sliding can be neglected. The contribution λED of ED bars to lateral strength should be no more than 25% to maintain self-centering capacity.

Self-tensioning long-span T-shaped spruce and oak web floors with a CLT upper flange. An experimental approach

• A self-tensioned system was developed to design small depth and long-span timber floors. • The device automatically apply a post-tensioned force in reponse to gravitational loads. • The T-shaped floors showed a noticeable reduction in their average relative deflection.

Response surface analysis and optimization of controlled rocking steel braced frames

As an alternative to conventional seismic force resisting systems, controlled rocking steel braced frames (CRSBFs) can effectively eliminate permanent structural damage after earthquakes. Together with the rocking action in the braced frame, post-tensioning (PT) elements and fuse members are used to provide self-centering and energy dissipation, respectively. This study firstly aims to assess the influence of design parameters related to the fuse and PT materials on the seismic response of CRSBFs. These factors include the yield strength, initial stiffness, and strain hardening ratio of the fuse, the initial force and modulus of elasticity of the PT strands, and the gravity load on the rocking column. Additionally, different analysis cases are considered to include the effects of frame aspect ratio and earthquake intensity level. Nonlinear response history analyses are performed with factor combinations generated using a design of experiment methodology. The second goal of the study is focused on optimizing the seismic response of CRSBFs with respect to the influential factors identified in the sensitivity analyses. Using a response surface methodology and desirability approach, multiple-response optimization is applied to determine the design variable values needed to simultaneously minimize the maximum transient and residual roof drift ratio and peak floor acceleration. Among other results, it is found that the fuse strain hardening ratio and PT strand modulus of elasticity do not significantly influence the seismic response demands in CRSBFs. The results of the multi-response optimization demonstrate that the initial PT force is most useful for minimizing all three seismic response demand parameters.

Seismic testing of post-tensioned Pres-Lam core walls using cross laminated timber

With the increasing demand for multi-storey timber buildings in areas with high wind loads and high seismic activity, stiff lateral load resisting systems are becoming a crucial design component. Post-tensioned Pres-Lam mass timber lift shafts and stairwell core walls not only provide a strong and very stiff lateral load resisting system, but also damage limiting response in the case of a large seismic event.This paper describes the results of experimental tests on Cross-Laminated Timber (CLT) Pres-Lam core walls tested under bi-directional quasi-static seismic loading. In the first configuration the CLT wall panels were connected in the corners with screws, while in the second configuration, steel pivotal columns were introduced at the corners and the CLT wall panels were connected to the steel columns with dissipative U-shaped Flexural Plates (UFPs).Overall the testing showed that the Pres-Lam system, when used for structural timber core walls subjected to bidirectional loading regimes, sustains nominal damage after large drift demands. By adding ductile screw-connections or steel columns with UFPs at the corners additional strength and dissipation capacity is obtained.Friction between the CLT panels improved the seismic performance of the structure, which in Serviceability Limit State (SLS) conditions led to rigid behaviour of the splices between the panels. Displacement incompatibilities between the floor diaphragm and the core walls were accommodated by locating the connections at the centre of the walls, or by pinned connections in the corner pivotal columns. Relative displacements between orthogonally running connector beams were accommodated by using flexible connections out-of-plane. Under low axial forces there was horizontal sliding of the walls at the foundation level, but this was not observed when larger post-tensioning forces were applied.

Experimental and numerical investigation of jointed self-centering concrete walls with friction connectors

A jointed self-centering (SC) concrete wall system with friction connectors is proposed, in which the precast concrete wall panels are assembled through unbonded posttensioning (PT) tendons to provide self-centering capacity, and friction connectors at the vertical joint provide supplementary energy dissipation as the walls move relatively to each other during a major earthquake. The wall toes are protected by the steel jackets from crushing as the walls rock. The behavior of the jointed wall system was experimentally investigated through nine cyclic load tests, based on which the influence of different initial PT forces and friction forces on structural performance of the walls was discussed. Besides, strains on the steel jackets during the tests were measured to validate the effectiveness of steel jackets, and three-dimensional laser scanning technique was applied to measure the in-plane and out-of-plane deformations of the walls. It is observed that the main structure remained resilient after the nine tests with the maximum drift ratio of about 2.98%, though cracking in concrete cover at the eastern connectors was found after a number of reversal loading, which calls for improved structural details. A numerical simulation method of the jointed SC wall system is proposed, and the simulated hysteretic behaviors match well with the test results.

Effect of post-tensioned spandrel beam on wide beam–column connections

Reinforced concrete wide beam flooring systems are used increasingly in seismic regions, despite little information being available on their performance under seismic actions. This paper presents the results of a set of experiments performed on two full-scale exterior wide beam–column connections, which were tested under lateral quasi-static reversed cyclic loading. The control specimen had both longitudinal and transverse reinforcement in its spandrel beam. The second specimen was built in the same way but provided with post-tensioning force. The specimens were tested under a combination of axial force and quasi-static reversed cyclic lateral displacements. Results show that the control specimen reached its expected beam flexural capacity at a drift ratio of 3%, and then the joint shear failure and spandrel beam torsional failure controlled the behaviour. The seismic performance of the specimen with a post-tensioned spandrel beam was considerably improved compared to the control specimens. The specimen sustained a 5% drift without any significant drop in strength. No signs of joint shear failure or spandrel beam torsional failure were observed in this specimen. The results indicate that adopting a post-tensioned spandrel beam can not only prevent torsional failure, but can also improve the joint shear capacity and displacement ductility.

A simplified numerical model for post-tensioned steel connections with bolted angles

Post-tensioned (PT) self-centering beam–column connections has been developed for its good seismic performance. Many researchers have investigated its mechanical behavior by numerical or experimental method. Prior researches have indicated that the analysis by elaborate FE models is very time consuming. To overcome this disadvantage, a simplified numerical model was established in this paper. The accuracy of results derived by this model was validated against prior investigations on interior PT connections with top-and-seat angles. Influence of initial PT force on mechanical behavior of PT connection was investigated. The Geometric and material nonlinearities, and strands can be considered in the modeling. A planar steel frame structure was established and hysteretic analysis was conducted. Results indicated that the computational cost can be greatly reduced by this model.

Optimum Reduction of Flexural Effect of Axially Symmetric Cylindrical Walls with Post-tensioning Forces

Post-tensioning of axially symmetric cylindrical walls has been used to prevent cracking and serviceability failures. In the case of axially symmetric cylindrical walls used for storing water, post-tensioning improves impermeability by reducing the tension loads generated by fluid loads. However; achieving these improvements in axially symmetric cylindrical walls requires determination of a suitable design for post-tensioning loads. By optimizing the locations and intensities of the post-tensioning loads, the effects on axially symmetric cylindrical walls can be minimized. In this study, four metaheuristic algorithms such as harmony search (HS), flower pollination algorithm, bat algorithm and teaching-learning based optimization are investigated for optimum design of posttension forces on axially symmetric cylindrical walls. The design involves the location and intensity of the post-tensioning cables. In the optimization process, analyses of cylindrical walls are conducted by a developed module using the superposition method (SPM). According to the analyses results, the optimization methods investigated are effective in reducing maximum longitudinal moments on cylindrical walls, but major difference in performance of the methods are seen.

Numerical study on damage mechanism of post-tensioned concrete box bridges under close-in deck explosion

Abstract To understand the behavior of post-tensioned concrete box bridges under blast loads, a typical concrete cantilever bridge is exposed to close-in deck explosion. The objectives of the current study are investigating the blast wave resonance entered into the box (confined condition) and the bridge's main components damage mechanism. For this purpose, 5, 150, 355, 550, 700 and 1200 kg TNT weights are chosen as load varying cases and parametric study analysis are done in two main explosion scenarios. Averagely, after the explosion, high impulsive blast loads hit the bridge deck and penetrate into the deck. Less than 5% of the incident blast overpressure succeeds to propagate into the box-shaped deck. However, according to the confined conditions within the box, blast waves can be amplified up to about 2.7 times. In most load cases, the concrete is damaged in the anchorage zone of some pre-stressing tendons, leading to the loss of effective post-tensioned forces. On the other hand, those tendons directly exposed to the blast impact are observed to be failed and the crushing of the concrete reduces the effective post-tensioned forces and stability of the entire bridge remains uncontrolled. Therefore, it is necessary to apply some energy absorbing or properly fused systems to reduce the close-in deck explosion effects. The obtained results can help bridge designers recognize weak points along post-tensioned box bridges under explosion loads and contribute into easier identification of retrofit and strengthening methods.

Investigation on seismic performance and functionality of self-centring post-tensioned segmental columns

The precast post-tensioned segmental columns react against lateral seismic loading with low residual displacement, predominantly because of the self-centring capability of post-tensioning strands. However, their application is still limited in high seismic zones due to their unknown behaviour during and following severe earthquake loading. In this work, experimental study is conducted to inspect the functionality of post-tensioned segmental columns after suffering severe damages. In earlier work by the authors, partial yield of central strands had been occurred during the unloading stage in the last cycle at 4% drift level. In current study, the experimental work is extended and the specimen retested under loading and unloading up to 4.0% drift. The results indicate that the first segment shows approximately 10 mm horizontal movement on the surface of foundation. However, the specimen showed identical strength at 4.0% drift level. In addition, using finite element method (FEM), a numerical study was conducted in order to investigate the influence of various parameters such as mild steel ratio, concrete strength, number of segments, post-tensioning force and axial loading. Finally, design recommendations were proposed for the yield displacement, ductility and correlation of residual displacement and displacement ductility of post-tensioned segmental columns with various aspect ratios.

Damage Detection in Post-Tensioned Slab Using 2D Wavelet Transforms

Earthquake force, loading more than structural capacity, cracking, material fatigue, and other unpredicted events are undeniable in the structure life cycle in order that environmental conditions of the structure would be changed and treats health. Damage of structures such as crack, corrosion of the post-tension cables from inappropriate grouting of the post-tension structures, etc., can lead to the collapse of the structure. Accordingly, damage detection precedes the damage is essential. In this article, damage detection in the post-tensioned slab through 2D wavelet transform investigated. Hence, the structure has been modeled with finite element method. Damaged cables have been modeled by a reduction of the post-tension force. The vertical displacement has been derived through static analysis of the intact and damaged structure. Damage location will be detected with wavelet transform on the difference of the vertical displacements. To demonstrate the ability of 2D wavelet transform single and multiple scenarios of the post-tension cables, are tested. The influence of noise in the vertical displacement data for these scenarios is also considered. Using 2D wavelet transform and damage index W, an empirical equation has been proposed to calculate damage severity of the post-tensioned slab cables.

Improved estimation of the reverse-cyclic behavior of fully-grouted masonry shear walls with unbonded post-tensioning

The use of unbonded post-tensioning to develop fully-grouted masonry shear wall systems that can respond by rocking has recently gained significant momentum. The behavior of these walls has been routinely characterized using monotonic analysis with primary consideration to the rocking mechanism, concentrating the wall deformations at the base. However, experiments have indicated that these walls experience noticeable flexure and shear responses, in addition to rocking, which has been inadequately addressed by previous analysis methods. Moreover, when these walls are subjected to reverse-cyclic loads, their initial post-tensioning forces may reduce significantly due, in part, to masonry degradation at the wall base, producing responses that cannot be captured accurately using monotonic analysis. This paper presents an analysis method that can address these issues by accounting for the reverse-cyclic loading and the three mechanisms of rocking, flexure, and shear. Rocking is estimated using inelastic fiber-element sectional analysis at the wall base, while flexure and shear are modelled as elastic mechanisms, assuming they are small components of the total wall responses. The proposed method is evaluated using experiments and finite element analyses of fully-grouted masonry shear walls with unbonded post-tensioning. The method adequately captures the experimental force-displacement responses, hysteretic energy dissipation, and displacement component responses of the walls. Estimation of the post-tensioning forces experienced by the walls is significantly improved compared to monotonic analyses.

Cyclic testing of unbonded post-tensioned concrete wall systems with and without supplemental damping

A series of cyclic lateral-load tests were conducted on four different unbonded post-tensioned precast concrete wall systems, including two Single Rocking Walls (SRW) and two PREcast Wall with End Columns (PreWEC). The main purpose of these tests was to systematically investigate the cyclic response of post-tensioned concrete walls with varying amounts of supplemental damping while keeping the initial post-tensioning force, wall dimensions, and confinement details constant. A secondary objective was to validate the wall panel design including the appropriate selection of axial force ratio and design of confinement and armouring details. All of the test walls exhibited excellent performance with uplift and rocking at the wall base with only minor damage observed, consisting of small amounts of spalling in the wall toes. There were consistent observations and measurements of the wall damage, concrete compressive strains, and wall neutral axis depths for both the SRW and PreWEC systems with the same wall panel dimensions. Based on these observations it is concluded that the behaviour of the wall panel in a PreWEC system is independent of the number of energy dissipating O-connectors. The O-connectors increased the hysteretic energy dissipation in the wall system and provided between 1.1% and 1.4% of additional equivalent viscous damping per connector for the PreWEC walls tested. Overall, the behaviour of the four walls tested confirmed the design procedures used, with both the global force-displacement response and local response parameters predicted with sufficient accuracy using an existing simplified analysis method.

Moment and energy dissipation capacities of post-tensioned precast concrete connections employing a friction device

This paper describes an analytical investigation into the design of unbonded post-tensioned (PT) precast concrete beam-column connections that employ a friction device. Design formulas related to the moment capacities, hysteretic energy dissipation capacities, and self-centering capabilities of the PT connections are developed in normalized form. The validation of the design formulas and the effect of normalized parameters such as friction force, initial post-tensioning force and the location of the friction device on the moment and energy dissipation capacities are investigated. A design procedure to achieve desired design moment and energy dissipation capacities for the PT connections is also developed using the design formulas. Contour plots for the linear limit moment and hysteretic energy dissipation ratio are provided for determining the required values of the normalized parameters. The investigation is based on the analytical responses from a nonlinear section analysis method for a number of PT connections with various sets of normalized parameters. The results confirm the adequacy of the design formulas and design procedure.

Modelling of steel-timber composite connections: Validation of finite element model and parametric study

This paper investigates the mechanical behaviour of lap steel-timber composite connections using a 3-D continuum-based finite element (FE) model. In the FE models developed, the non-linear behaviour and failure of the timber is captured by a stress-based failure criterion formulated in the framework of continuum-damage mechanics. Yielding of the steel plates and fasteners (i.e. coach screws and bolts) is captured by an elastic-hardening plastic constitutive law. These FE models are validated by experimental (push-out) tests conducted on laminated veneer lumber (LVL)-steel and cross-laminated timber (CLT)-steel composite lap connections. It is shown that the FE models developed in the paper can replicate adequately the load-slip response and failure mode of the hybrid steel-timber composite connections tested. The validated FE model is used to undertake a parametric study that elucidates the influence of the yield strength and the length of the fasteners and the post-tensioning force in the bolts on the stiffness, load carrying capacity and load-slip behaviour of hybrid steel-timber composite connections.

Finite Element analysis of post-tensioned SG-laminated glass beams with adhesively bonded steel tendons

Taking advantage of past full-scale experimental test results, the bending performance of laminated glass beams with post-tensioned, adhesively bonded steel tendons is explored via refined Finite-Element (FE) models. As far as the primary advantage of the post-tensioned glass beam concept is to provide an initial state of compressive stresses in glass, a marked enhancement of the expected structural performance is expected (i.e. increase of the initial fracture load and redundancy), compared to typically brittle, unreinforced laminated glass beams. Several key aspects can affect the overall performance of such beam typology, first of all the adhesive joint providing the structural interaction between the glass beam and the steel tendon, as well as the geometrical and mechanical properties of each beam component, in relation to the amount of initial post-tensioning force. Based on a first validation of a reference full 3D FE model towards the available past full-scale experimental test results, an extended parametric study is presented in this paper, giving evidence to the effects of several mechanical and geometrical parameters (i.e. steel tendon section, level of the applied post-tensioning force, adhesive joint type and size, etc.) in the bending performance of post-tensioned laminated glass beams at room temperature under quasi-static load.