International Concrete Abstracts Portal

This paper investigates the seismic performance of exterior beam column joints in special moment frames (SMFs) with varying axial load ratios. Cyclic testing of four additional specimens with an axial load ratio of 0.45 is compared with four companion specimens at 0.10. Each specimen was designed and constructed with Grades 60, 80, or 100 (No. 420, 550, or 690) reinforcement in accordance with ACI 318-19 provisions for special moment frame joints, except for the provisions of joint shear and confinement. While ACI 318-19 tightens confinement requirements for SMF columns and joints, especially under high axial loads, this study reveals that increasing the axial load ratio benefits joint behavior. The study also demonstrates the feasibility of using high-strength reinforcement in exterior beam-column joints of SMFs, provided that appropriate modifications are made. The findings in this study have influenced modifications from ACI 318-19 to the Building Code Requirements for Concrete Structures in Taiwan.

Concrete confinement using fiber-reinforced polymers (FRPs) has been vastly used for strengthening of reinforced concrete (RC) columns. The strengthening of RC columns belongs to the realm of existing structures, which has been recognized as distinct from the design of new structures. Code development efforts for the strengthening of RC columns should follow a reliability-based framework similar to the one used in the code development for new structures. In this process, a number of additional issues arise: the mechanical model of FRP confinement representing existing RC columns, the statistical description of the design variables, and the target reliability to be attained. In this study, the reliability levels of 288 axially loaded, FRP-RC short columns of circular cross sections, strengthened according to ACI 440 guidelines, are assessed. Monte Carlo simulation is used in the probabilistic description of column strength and computation of the probability of failure. An FRP confinement model that explicitly accounts for the presence of transversal steel and attendant model errors associated to the estimation of ultimate stress and ultimate strain are used in the computation of the FRP-RC column capacity. The values of the reliability index are in the range of 3.92 to 4.61, satisfying the target reliability suggested for both new and existing structures. The research findings presented herein provide further support for the efforts of ACI Committee 440 in the development of standards related to the FRP strengthening of RC columns.

Previous studies on reinforced concrete (RC) beam-column subassemblies under a column removal scenario are helpful to understand the load-resisting mechanisms of RC structures against progressive collapse, but most of these studies failed to simulate actual boundary conditions, which were simplified as fixed boundaries to allow sufficient development of the load-resisting mechanisms. These studies were unable to reflect the response of joints and side columns under progressive collapse. To fill this gap, an experimental program on six half-scale beam-column subassemblies with joints and side columns was designed and tested to fully understand the effects of boundary conditions on the structural behavior of RC planar frames against progressive collapse. Three subassemblies were specially designed, while the other three were ordinarily designed to quantify the benefits of special detailing. The test results show that the effects of boundary conditions on the development of load-resisting mechanisms are marginal, whereas the effects of special detailing are significant. Specifically, specimens under a middle-column removal scenario and a penultimatecolumn removal scenario develop similar compressive arch action (CAA) capacities and catenary action (CA) capacities. The CAA capacity dominates the load resistance of specimens with ordinary detailing. In contrast, the CA capacity governs the load resistance of specimens with special detailing mainly due to the larger areas of longitudinal reinforcing bars and the greater rotation capacities of beam ends. However, boundary conditions can greatly affect the failure mode of specimens with ordinary detailing. Finally, an analytical study was performed to demonstrate the contributions of axial force and shear force to load resistance. According to test results and analytical analyses, RC frames with special detailing have sufficient rotational capacity to develop adequate tie forces to resist progressive collapse.

Corrosion of steel reinforcing bars in reinforced concrete (RC) structures is a matter of concern among practicing engineers and researchers are perpetually working over it. The development length of reinforcing bars at joints of RC structural frames are more prone to severe corrosion. Due to this, the design stress that needs to be developed in reinforcing bars is largely reduced. In addition, the development lengths of reinforcing bars create congestion at frame joints. This paper is an attempt to overcome these issues. In this paper, an epoxy-grouted nut coupler system is proposed that generates the required design stress in reinforcing bars with a very short development length at end anchorages, due to which congestion of the reinforcing bar at the joints can be avoided. The experimental investigation on the effect of corrosion on bond strength and development length of reinforcing bar in this epoxy-grouted nut coupler is also carried out by performing pullout tests. Statistical models are developed to predict the bond strength between the coupler and reinforcing bar corroded to different levels. This epoxy-grouted nut coupler is an effective tool for developing required stress in reinforcing bars by reducing the actual development length of reinforcing bars in the case of new structures. It is also useful and convenient in regeneration of stress in reinforcing bars at end anchorages that has been lost in corrosion-damaged structures.

This research aims to investigate the cyclic behavior of prestressed assembled precast frame structures that contain various amounts of recycled aggregates. A series of half-scale model precast frame structures with different recycled aggregate substitutions, varying from 0 to 100%, were constructed and tested to failure to evaluate their structural performance indexes in terms of strength, stiffness, cumulative energy dissipation, and damping ratio. The tests showed that all tested prestressed assembled precast frame structures maintained the features of a low-damage and selfcentering seismic-resistant system before the drifts attained 4%. The occurrence of adverse effects in seismic behavior is more likely if more natural aggregates are substituted by recycled aggregates. However, those adverse effects observed in the experiments are not severe. Therefore, this type of recycled aggregate concrete precast frame has the potential to provide adequate seismic capacity. It is definitely possible to be used in regions of high seismicity with a rational design. Empirical equations for estimating the equivalent damping and self-centering efficiency are proposed.