International Concrete Abstracts Portal

Design recommendations are presented for calculating the immediate deflection of cracked prestressed concrete members under service load. Inconsistency and sometimes confusion regarding the calculation of immediate deflection for the different approaches presently available highlight the need for a rational approach to computing deflection. The ACI 318-19 approach for reinforced (nonprestressed) concrete is broadened to include prestressed concrete. This involves the implementation of an effective moment of inertia taken together with an effective eccentricity of the prestressing steel used to define the effective curvature and/or camber from the prestressing force. Proposed revisions to ACI 318 are presented for prestressed Class T and Class C flexural members and clear steps are provided for calculating immediate deflection. The effectiveness of the new approach is validated against an extensive database of test results, showing reasonable accuracy and reliability in predicting deflections. The paper concludes with practical recommendations for implementation and a worked-out example to illustrate the proposed methodology. These findings aim to enhance the accuracy and consistency of deflection predictions in prestressed concrete design, contributing to better serviceability and performance of concrete structures.

Lap splicing of longitudinal reinforcing bars in shear walls is often encountered in practice, and the transfer of forces in lap-spliced reinforcing bars to the surrounding concrete depends on the bond strength. Buildings with shear walls during an earthquake develop plastic hinges in the shear walls, particularly where the reinforcing bars are lap-spliced. Brittle failure is commonly observed in reinforcing bar lap-spliced shear walls, which needs to be minimized by choosing the appropriate percentage of lap-spliced reinforcing bars. Therefore, it is essential to address the detailing of the lap-spliced regions of reinforced concrete (RC) shear walls. Several seismic design codes provide guidelines on lap-spliced detailing in shear walls related to its location, length of lap-splice, confinement reinforcement, and percentage of reinforcing bars to be lap-spliced. In this study, the percentage of reinforcing bars to be lap-spliced at a section is examined with staggered lap-splicing of 100, 50, and 33% of longitudinal reinforcing bars, in addition to a control RC shear wall without lap-splicing. This study tested four half-scale RC shear walls with boundary element (BE), designed as per IS 13920 and ACI 318, under quasi-static reversed cyclic loading. From the experimental study, it is observed that the staggered lap splicing of reinforcing bars nominally reduces the performance of shear walls under cyclic load in terms of the reduced flexural strength, deformation capacity, energy dissipation, and ductility of the shear walls compared to the control shear wall without lap splicing. It is also observed that the unspliced reinforcing bars do not sustain the cyclic loading in staggered lap-splice after the post-peak. Current provisions of ACI 318, EC2, and IS 13920 recommend staggered lap-splice detailing in shear walls. However, from the current study, shear walls with different percentages of staggered lap splice show that the staggered lap-splice detailing in shear walls does not improve its seismic performance.

This research proposes a standardized arrangement of longitudinal reinforcement using Grade 690 MPa (100 ksi) high-strength steel and D32 (#10) or D36 (#11) large-diameter threaded bars to alleviate reinforcement congestion and construction difficulties. Four full-scale column specimens with the proposed standardized arrangement were tested using double-curvature lateral cyclic loading to examine their seismic performance. Test results showed that all the columns exhibited a combined axial and flexural failure mode, with ultimate drift ratios ranging from 4.07 to 5.98%, ratios of measured to nominal moment strength based on actual material strengths ranging from 1.18-1.51, and relative energy dissipation ratios satisfying the requirement of ACI 374.1-05. No shear or bond-splitting failures were observed. Based on the test data from this research and the literature, two modifications were proposed in the calculation of l_d to relax the requirement of 1.25l_d≤l_u/2 as required by ACI 318-19.

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 Gr.60 (420), Gr.80 (550), or Gr. 100 (690) reinforcement in accordance with ACI CODE-318 provisions for special moment frame joints, except for the provisions of joint shear and confinement. While ACI CODE-318 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 CODE-318 to the Building Code Requirements for Concrete Structures in Taiwan.

To secure emulative seismic performances of precast concrete (PC) special moment frame buildings, two capacity-based connection design options (that is, strong and ductile precast connections) are provided in the current version of ACI 318. However, the evolving performance-based seismic design and response evaluation requires a reasonable estimation of the energy dissipation and corresponding hysteresis damping characteristics so that their potential performance level can be properly predicted. Therefore, this study focuses on the seismic performances, especially the energy dissipation and damping performances of the Code- compliant PC wide beam-column connections. Three PC wide beam-column connection specimens under the ductile connection design principle with different joint details and a reinforced concrete (RC) control specimen were fabricated and tested under reversed cyclic loadings. In addition, an energy-based macro-modeling method was developed to characterize the cyclic responses, including the damping response of PC wide beam-column connections. The test results revealed that the Code-required overstrength of shear-friction strength between PC beam members and cast-in-place (CIP) concrete is crucial to achieving the ductile performance of precast connections. It also appeared that the energy-based macro-modeling method could capture the hysteresis features through the relationship between the equivalent viscous damping (EVD) ratio and the ductility capacity of PC wide beam-column connections.