Hung-Jen Lee

Analytical Model for Predicting Shear Strengths of Interior Reinforced Concrete Beam-Column Joints for Seismic Resistance

A softened strut-and-tie model was previously developed for determining the shear strengths of exterior beam-column joints for seismic resistance. This existing model originates from the strut-and-tie concept and satisfies equilibrium, compatibility, and the constitutive laws of cracked reinforced concrete. This paper examines the applicability of the previously proposed model to interior beam-column joints. The calculated shear capacities of 56 interior joints were compared with the experimental results, and good agreement was obtained.

Role of hoops on shear strength of reinforced concrete beam-column joints

This study investigated the effect of joint hoops on the shear strength of exterior reinforced concrete beam-column joints subjected to earthquake-type loading. The study included 9 exterior reinforced concrete beam-column subassemblages that were tested under reverse cyclic loading. All test specimens were designed to have adequate shear strength of joints, determined by the softened strut-and-tie model. The parameters considered include the amount and detail of joint hoops. Test results showed that the major function of the joint hoop is to carry shear as a tension tie and to constrain the width of crack. The authors note that a lesser amount of hoop reinforcement with a wider spacing could be used without significantly affecting the performance of joints. For beam-column joints, where there is a need for sustained strength under deformation reversals, the authors recommend the design of joints with adequate shear strength and with a sufficient number of hoops to remain in elastic range under earthquake loading.

SHEAR STRENGTH PREDICTION FOR DEEP BEAMS

This paper proposes a softened strut-and-tie model for determining the shear strengths of deep beams. The proposed model originates from the strut-and-tie concept and satisfies equilibrium, compatibility, and constitutive laws of cracked reinforced concrete. The shear strength predictions of the proposed model and the empirical formulas of the American Concrete Institute (ACI) 318-95 Code are compared with the collected experimental data of 123 deep beams. The comparison shows that the performance of the softened strut-and-tie model is better than the ACI Code approach for all the parameters under comparison. The parameters reviewed include the ratios of horizontal and vertical reinforcement, concrete strength, and the shear span-depth ratio. The softened strut-and-tie model can be further developed to improve the current deep beam design procedures by incorporating the actual shear resisting mechanisms in predicting shear strength supply of deep beams.

SHEAR STRENGTH PREDICTION FOR REINFORCED CONCRETE CORBELS

Corbels are brackets that project from the faces of columns and are used extensively in precast concrete construction to support primary beams and girders. This paper proposes a softened strut-and-tie model for determining the shear strength of corbels. The proposed model originates from the strut-and-tie concept and satisfies equilibrium, compatibility, and the constitutive laws of cracked reinforced concrete. The shear strength predictions of the proposed model and the empirical formulas of American Concrete Institute (ACI) 318 are compared with the collected experimental data of 178 corbels. The comparison shows that the performance of the softened strut-and-tie model is better than the ACI 318 approach for all the parameters under comparison. The parameters reviewed include the concrete strength, the shear span-depth ratio, and the amount of horizontal hoop. It is therefore recommended that the current shear design procedures for corbels be reformed to incorporate the actual shear-resisting mechanisms as postulated by the softened strut-and-tie model.

Cyclic Response of Exterior Beam-Column Joints with Different Anchorage Methods

This paper presents the cyclic response of six exterior beam-column joints with or without eccentricity to evaluate the use of mechanical anchorages in place of hooked bar anchorages. In high seismic zones, the hooked beam bars often cause steel congestion in a joint at building comers. From previous tests of beam-column joints, the use of mechanical anchorages in place of hooked bar anchorages provides a promising solution for steel congestion, but it has not been verified in eccentric beam-column joints. The presented experimental program demonstrates that eccentric beam-column joints with mechanical anchorages can exhibit satisfactory performance and adequate anchorage capacity for a limiting drift ratio. Extending ACI design methods to cover the use of mechanical anchorages for eccentric beam-column joints is an appropriate code modification. Test results also indicate that the cyclic behavior of exterior beam-column joints can be significantly improved by attaching double mechanical devices on each beam bar within the joint.

Eccentric reinforced concrete beam-column connections subjected to cyclic loading in principal directions

This paper describes cyclic loading responses of five reinforced concrete corner beam-column connections with one concentric or eccentric beam framing into a rectangular column in the strong or weak direction. The specimen variables are the shear direction and the eccentricity between the beam and column centerlines. Results from the experiment showed that two joints connecting a beam in the strong direction were capable of supporting adjacent beam plastic mechanisms. The other three joints connecting a beam in the weak direction, however, exhibited significant damage and loss of strength after beam flexural yielding. Eccentricity between beam and column centerlines had detrimental effects on the strength, energy dissipation capacity, and displacement ductility of the specimens. Although the current American Concrete Institute design procedures appear to be acceptable for seismic design purposes, they could not prevent the failure of corner connections at a large drift level of 4-5%.

Cyclic Behavior of Precast High-Strength Reinforced Concrete Columns

Double-curvature cyclic tests of large-scale columns were conducted to investigate the seismic performance of precast highstrength reinforced concrete columns. High-strength concrete and high-strength longitudinal and transverse reinforcement were used. The use of grouted coupler splices for the high-strength longitudinal reinforcement in the plastic hinge zone and the use of butt-welded splices for the high-strength transverse reinforcement were examined. Test results showed that precast columns with the grouted coupler splices exhibited comparable seismic performance with monolithic counterparts. The butt-welded splice had a negligible effect on the tensile behavior of the spliced bars. However, precast columns with such welded splices in transverse reinforcement showed smaller ultimate drift capacities than their counterparts with hooked transverse reinforcement. This was due to the reduced resistance of butt-welded transverse reinforcement to buckling of longitudinal reinforcement..

Panel Zone Shear Behavior of Through-Flange Connections for Steel Beams to Circular Concrete-Filled Steel Tubular Columns

AbstractThis research investigated the panel zone shear behavior of a proposed through-flange connection for steel beams to circular concrete-filled steel tubular (CFT) columns. Four exterior beam-column specimens were designed and tested using cyclic loading applied to the beam end. Three design variables were examined: concrete infill in the column tube, stiffeners to the column tube in the panel zone, and the width of the through-flange plates. Test results showed significant panel zone shear yielding for all specimens. Failure of the specimens was caused by fracture of the column tube near the through-flange plates and crushing of concrete at the stiffener side in the panel zone if concrete infill was present. The use of concrete infill, stiffeners, and 25% wider through-flange plates increased the peak applied load by 156, 7, and 14%, respectively. A model that considers crushing of concrete at the stiffener side as failure mode was proposed to calculate the panel zone shear strength contribution fro...

High-Strength Concrete and Reinforcing Steel in Beam-Column Connections

Based on test results made of normal-strength reinforced concrete, Joint ACI-ASCE Committee 352 has developed design recommendations for joints of special moment frames including requirements of moment strength ratio, shear strength, confinement, and development length. Due to the interests of the reinforced concrete industry in using high-strength reinforcement for special moment frames, ACI-ASCE Committee 352 appointed a task group to review available test data in the past 30 years and constructed an unified database of 357 beam-column joints made with normal-strength and high-strength concrete. Based on the database investigation and experimental verification, we concluded that current ACI 318 design provisions can be extended to include the use of high-strength reinforcement with modifications in the requirements of joint effective area, confinement, and development lengths. Provided information can be referred for further updating the design recommendations of beam-column joints for high-strength reinforcement.

Behavior and modeling of high-strength concrete tied columns under axial compression

ABSTRACT This article presents experimental and analytical results of 21 high-strength concrete tied columns under axial compression. Each 1500-mm-tall column had a 500 by 500 mm section reinforced with 12 D25 longitudinal bars enclosed by perimeter hoops only, or perimeter hoops plus typical crossties, or perimeter and intermediate hoops. The concrete strengths of cylinder tests ranged between 55 and 99 MPa. The column compression tests showed that the longitudinal bars could be laterally supported by hoop corners or 135-degree seismic hooks of crossties, but not restrained by 90-degree hooks, which lost effectiveness after spalling of cover concrete. The proposed analytical approach used the existing Mander model and the Euler equation to determine the confined concrete strength and the buckling strength of longitudinal bars, respectively. With rational assumptions of confinement effectiveness and unsupported lengths, the proposed analytical approach can well predict the complete load-deformation response of test columns.