Self-Centering Behavior of Unbonded,Post-Tensioned Precast Concrete Shear Walls

Self-centering ability of unbonded post-tensioned precast concrete shear walls has been attributed to the presence of post-tensioning force. However, the experimental results presented in this paper indicate that the post-tensioning force may completely die out during cyclic loading while the walls are able to retain their superior self-centering characteristic. Moreover, the analytical study presented in this article indicates that with proper configuration of end-anchorages for post-tensioned tendons, self-centering of post-tensioned walls can be achieved even when the post-tensioning force vanishes. This study also investigates the effects of tendon layout, tendon end-anchorage configuration, and external vertical load on the self-centering ability of unbonded precast concrete shear walls subjected to earthquake loading.     

Experimental Evaluation of Seismic Performance of Squat RC Structural Walls with Limited Ductility Reinforcing Details

This article describes an experimental study carried out on of reinforced concrete (RC) walls of less confining reinforcement than that recommended by ACI 318. A total of eight RC walls with boundary elements comprising of five walls with aspect ratio of 1.125 and three walls with aspect ratio of 1.625 were tested by subjecting them to low levels of axial compression loading and simulated seismic loading, to examine the structural performance of the walls with limited transverse reinforcement. Conclusions are reached concerning the failure mode, drift capacity, strength capacity, components of top deformation, and energy dissipation characteristics of walls on the seismic behavior with limited transverse reinforcement. The influences of axial loading, transverse reinforcement in the wall boundary elements, and the presence of construction joints at the wall base on the seismic behavior of walls are also studied in this paper. Lastly, reasonable strut-and-tie models are developed to help in understanding the force transfer mechanism in the walls tested.     

Cyclic Response of Non-ductile RC Frame with Steel Fibers at Beam-Column Joints and Plastic Hinge Regions

An experimental study has been conducted on a reduced-scale gravity-load designed test frame to investigate its overall performance due to the addition of steel fiber-reinforced concrete (SFRC) at the critical regions. Two geometrically similar specimens, namely, reinforced concrete (RC) and SFRC, are tested under slow-cyclic lateral loading. End-hooked steel fibers (aspect ratio = 80) of 1.0% volume fraction were used in the SFRC mix for a distance of one-and-half times the member size near the joint regions. The addition of steel fibers improved the damage tolerance, lateral load resisting capacity, lateral stiffness, ductility, and energy dissipation of the frame.     

Computational Modeling of the Seismic Performance of Beam-Column Subassemblies

Analytical studies are carried out to investigate the effectiveness of finite element modeling procedures in accurately capturing the nonlinear cyclic response of beam-column subassemblies. The analyses are performed using program VecTor2, employing only default or typical material constitutive models and behavior mechanisms in order to assess analysis capabilities without the need for special modeling techniques or program modifications. The specimens considered cover a wide range of conditions, and include interior and exterior seismically and non seismically designed beam-column subassemblies. It is shown that finite element analyses can achieve good accuracy in determining the strength, deformation response, energy dissipation, and failure mode of reinforced concrete beam-column subassemblies under seismic loading conditions.     

Shaking Table Tests of a Structure Equipped with Superelastic Dampers

The energy dissipation capacity of the NiTi alloy was evaluated as part of a series of shake table tests. A superelastic damper was developed to take advantage of the hysteretic energy dissipation associated with this type of shape memory alloy. Each device was tested at different intensity levels. A vertical steel cantilever with 600 kg mass on top was subjected to a series of ground motions with different spectral characteristics. The dampers were placed as part of a tie system, restraining the horizontal movement of the top mass. The devices showed stable hysteretic behavior allowing for energy dissipation.     

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.     

CONCEPT AND DEVELOPMENT OF HYBRID SOLUTIONS FOR SEISMIC RESISTANT BRIDGE SYSTEMS

The development of alternative solutions for precast concrete buildings based on jointed ductile connections has introduced innovative concepts in the design of lateral-load resisting frame and wall systems. Particularly efficient is the hybrid system, where precast elements are connected via post-tensioning techniques and self-centring and energy dissipating properties are adequately combined to achieve the target maximum displacement with negligible residual displacements. In this contribution, the concept of hybrid system is extended to bridges as a viable and efficient solution for an improved seismic performance when compared with monolithic counterparts. Critical discussion on the cyclic behaviour of hybrid systems, highlighting the most significant parameters governing the response, is carried out.

The concept of a flexible seismic design (displacement-based) of hybrid bridge piers and systems is proposed and its reliability confirmed by quasi-static cyclic (push-pull) and nonlinear time-history analyses based on lumped plasticity numerical models.     

Development and Seismic Behavior of Precast Concrete Beam-to-Column Connections

A new precast concrete beam-to-column connection for moment-resisting frames was developed in this study. Both longitudinal bar anchoring and lap splicing were used to achieve beam reinforcement continuity. Three full-scale beam-to-column connections, including a reference monolithic specimen, were investigated under reversal cyclic loading. The difference between the two precast specimens was the consideration of additional lap-splicing bars in the calculation of moment-resisting strength. Seismic performance was evaluated based on hysteretic behavior, strength, ductility, stiffness, and energy dissipation. The plastic hinge length of the specimens is also discussed. The results show that the proposed precast system performs satisfactorily under reversal cyclic loading compared with the monolithic specimen, and the additional lap-splicing bars can be included in the strength calculation using the plane cross-section assumption. Furthermore, the plastic hinge length of the proposed precast beam-to-column connection can be estimated using the models for monolithic specimens.     

Experimental Investigation on the Seismic Performance of Beam-Column Joints Reinforced with GFRP Bars

Glass fiber-reinforced polymer (GFRP) reinforcing bars were used recently as main reinforcement for concrete structures. The noncorrodible GFRP material exhibits linear-elastic stress-strain characteristics up to failure with relatively low modulus of elasticity compared to steel. This raises concerns on GFRP performance in structures where energy dissipation, through plastic behavior, is required. The objective of this research project is to assess the seismic behavior of concrete beam-column joints reinforced with GFRP bars and stirrups. Two full-scale exterior T-shaped beam-column joint prototypes are constructed and tested under simulated seismic load conditions. One prototype is totally reinforced with GFRP bars and stirrups, while the other one is reinforced with steel. The experimental results showed that the GFRP reinforced joint can sustain a 4.0% drift ratio and can recover its deformation without any significant residual strains. This indicates the feasibility of using GFRP bars and stirrups as reinforcement in the beam-column joints subjected to seismic-type loading.     

EXPERIMENTAL ASSESSMENT OF THE GLOBAL CYCLIC DAMAGE OF FRAMED R/C STRUCTURES

It is accepted that failure criteria for earthquake-resistant structures should be based on energy dissipation as well as on maximum ductility. Even though rational procedures to include low-cycle fatigue in the definition of the design spectra have been derived, major difficulties arise in the definition of the cyclic damageability of the structure. The work conducted so far was mainly based on cyclic tests conducted on simple structural elements, therefore, the extension to the complete structure is not an easy task. A final cyclic test was conducted at the ELSA reaction wall facility on a four-storey full-scale reinforced concrete frame designed according to Eurocode 8, at the end of a series of pseudodynamic tests. The results allow the performance during the pseudodynamic tests to be assessed and the cyclic damageability of a complete structure to be investigated. The effects of low-cycle fatigue on a high-ductility structure turned out to be more important than expected.     

COMPRESSIVE BEHAVIOUR OF CARBON FIBRE COMPOSITE JACKETED CONCRETE WITH CIRCULAR AND NON-CIRCULAR CROSS-SECTIONS

This paper describes the concentric compression test results of carbon fibre reinforced polymer (CFRP) composite sheet jacketed concrete specimens with circular, square and rectangular cross-sections. In the experimental program, pre-damaged specimens and repeated compressive loads were considered as well as undamaged specimens and monotonic compressive loads. The contribution of CFRP composite jackets to the compressive behaviour of the specimens is evaluated quantitatively, in terms of strength, longitudinal and lateral deformability and energy dissipation. Simple analytical expressions are proposed for compressive strength and ultimate axial strain that are valid for CFRP composite jacketed concrete with circular, square and rectangular cross-sections. The analytical results obtained by the proposed expressions are in good agreement with the experimental data obtained in this study, as well as the experimental data available in literature.     

Ductility of a Structural Wall with Spread Rebars Tested in Full Scale

The experimental work focuses on the ductility of the reinforced concrete (RC) seismic structural walls in buildings of mid-rise height. A full-scale five-story structural wall was tested to obtain results, still scarce in literature, without the influence of size effect. An unusual detailing with large diameter longitudinal rebars uniformly distributed in the wall length was adopted to prevent premature web rebar fracture and shear sliding. The plastic hinge length and deformations were evaluated in detail. The results show the high ductility of the wall that reached a total drift of 2.5%, larger than those usually required in design.     

A numerical model for gravity wave dissipation in the thermosphere

Two simplified models of internal gravity wave dissipation due to viscosity, thermal conduction and ion-drag, in a multilayered, isothermal thermosphere are developed. Each of these models uses the WKB approximation, ray theory and the time-averaged equation of energy conservation, but whereas one of the models (A) employs all of the gravity wave equations appropriate to a dissipative atmosphere, the other (B) does not. Results derived from these models for one particular wave are compared to each other and also to some previously published results of Klostermeyer, which employed a full-wave, model. A breakdown of the WKB approximation in the lower, non-isothermal thermosphere leads to models A and B underestimating the total dissipation there. In the middle thermosphere model A estimates the dissipation reasonably well, while model B grossly overestimates the dissipation. In the upper thermosphere model A underestimates the total upward energy flux, probably as a result of the neglect of coupling into the dissipative waves at these levels, while no energy remains in model B. Results from model A show that when dissipation due to viscosity and thermal conduction are included correctly and simultaneously, the dissipation due to viscosity can exceed that due to thermal conduction by a factor of three. It is argued that ray theory may either overestimate or underestimate the energy flux reaching the upper boundary of a thermospheric model depending on both its height and the particular thermospheric model used.     

Experimental Study and Numerical Model Calibration for Earthquake-Induced Collapse of RC Frames with Emphasis on Key Columns,Joints, and the Overall Structure

A thorough investigation of earthquake-induced collapse of reinforced concrete frames is presented. The inherent correlation between the nonlinear behavior of key components and the collapse mechanism of overall frame is examined through concurrent collapse tests of both frame and key components. Important issues in the component models are investigated through calibration against experiments, leading to a comprehensive structural system model. Both test and simulation indicate that the seismic performance are predominately governed by the key columns, whereas the energy dissipation capacity is somewhat affected by the joints. This study offers systematic experimental data and numerical models for future collapse assessments.     

Experimental Investigation of Exterior Unreinforced Beam-Column Joints with Plain and Deformed Bars

Experimental tests on four full-scale exterior unreinforced reinforced concrete (RC) beam-column joints, representative of the existing non-conforming RC frame buildings, are carried out. The specimens have different longitudinal reinforcements (plain or deformed) and they are designed in order to be representative of two typical design practices (for gravity loads only or according to an obsolete seismic code). Different failure modes are observed, namely joint failure with or without beam yielding. The local response of the joint panel is analyzed. The different joint deformation mechanisms and their contribution to the deformability and to the energy dissipation capacity of the sub-assemblages are evaluated.     

Contributions of gravity waves to the momentum,heat and turbulent energy budget of the upper mesosphere and lower thermosphere

The role of gravity waves for the momentum and heat budget of the atmosphere between approximately 70 and 110 km height is considered. Parameterization schemes for vertical gravity wave diffusivity, generalized Rayleigh friction, viscous force, heat conduction and kinetic energy dissipation are reviewed. Eddy diffusion parameterization and its relation to the gravity wave approach is also discussed and it is shown that principal similarities exist in both concepts, especially when irregular (stochastic) contributions to the perturbations are modeled. Special attention is paid to the dissipation of perturbation kinetic energy and its contribution to the heat budget of the mesopause region. It is concluded that the amount of energy which can be attributed to the part of the gravity wave spectrum contributing to generalized Rayleigh friction above the mesopause is of the order of 10% of the total perturbation energy.     

Quasi-Static Testing of RC Infilled Frames and Confined Stone-Concrete Bearing Walls

This article presents an experimental investigation of the seismic performance of gravity load-designed RC infilled frames and confined bearing walls of limestone masonry backed with plain concrete. Five infilled frames and two bearing walls were constructed at one-third scale and tested using reversed cyclic lateral loading and constant axial loads. Effects of openings, axial loading, and infill interface conditions were examined using quasi-static experimentation. The two structural systems exhibited similar lateral resistance and energy dissipation capacities with higher global displacement ductility for the infilled frames. Hysteretic behavior of the infilled frame models exhibited pinching of the hysteretic loops accompanied by extensive degradation of stiffness whereas loops of the bearing walls were free of pinching. Test results confirmed the beneficial effect of axial loading on lateral resistance, energy dissipation, and ductility of the bearing walls. Higher axial loading resulted in a substantial decrease in ductility with no significant effect on lateral resistance of the infilled frames. Openings within the infill panel reduced significantly the lateral resistance of infilled frames. Using dowels at the infill panel interfaces with the base block and bounding columns enhanced the maximum load-carrying capacity of infilled frames without impairing their ductility.     

Numerical Simulation and Seismic Fragility Analysis of a Self-Centering Steel MRF with Web Friction Devices

This article presents the seismic fragility analysis of a self-centering steel moment-resisting frame (SC-MRF) with web friction devices. A detailed numerical model of the SC frame was developed using the Open System for Earthquake Engineering Simulation (OpenSees) and the elastoplastic responses of the SC-MRF were studied, including the strength degradation under cyclic loading, tendon rupture, beam buckling, bolt bearing and friction loss, etc. The proposed simulation approach is validated by comparing the simulated results with those in existing hybrid-simulation tests, quasi-static pushover test and low cyclic tests, where good agreement is observed. In addition to the well-established performance limit states (i.e., immediate occupancy, collapse prevention and global dynamic instability), two unique performance limit states (i.e., the recentering and repairable limit states) are defined for the SC-MRF. Finally, incremental dynamic analyses are conducted to evaluate the seismic fragilities regarding the five performance limit states.     

BEHAVIOUR OF REHABILITATED STRUCTURAL WALLS

An experimental program for identifying the causes of failures in structural walls under earthquake loading and investigating potential rehabilitation schemes was undertaken. Large-scale models of the plastic hinge region of the walls were tested. An innovative test setup that provides the possibility of controlling the ratio of the shear force to both bending moment and axial load was constructed. A control wall was tested and failed prematurely in shear reproducing the failure observed in the field. Two different rehabilitation schemes to improve the behaviour of the wall using biaxial fibre reinforced polymer (FRP) sheets were designed to prevent the shear failure. To improve the ductility, the end column elements of the walls were confined using anchored FRP. The two schemes were tested and proved to be effective in increasing shear strength, ductility, and energy dissipation capacity of the walls.     

CAPACITY OF RC JOINTS SUBJECTED TO EARLY-AGE CYCLIC LOADING

The influence of cyclic loading applied on T-shape beam-column subassemblies at an early stage of the concrete curing period on the capacity of the joints after the maturing period is experimentally investigated. A series of 20 joints constructed for this purpose were subjected first to an early-age cyclic loading and then retested at the age of 28 days. From the results it is deduced that the early-age loading has significantly influenced the capacity and the overall behaviour of the specimens after the concrete curing period. Specimens subjected to early-age loading 12 or 24 hours after casting exhibited at the age of 28 days peak load values and energy dissipation higher than the ones of the specimens subjected to early-age loading two or seven days after casting, due to the high recoverability of the material at the very early stages of concrete curing period. Emphasis has been placed on the study of the influence of the early-age loading sequence and the slippage of the bars anchorage on the final capacity of the examined specimens. Finally, an attempt is made to assess the damage caused by early-age loading based on the observed stiffness degradation.