International Concrete Abstracts Portal

In structures subjected to lateral loading, slab reinforcement acting as effective tensile reinforcement of the beams has been found to increase significantly the beam flexural strength. The enhanced beam flexural strength has several effects on the structural behavior, including a shift in the ratio of strengths between the beams and other members. This may result in a failure mechanism different from that anticipated. The slab contribution depends on several variables, including the connection type (interior or exterior), lateral deformation level, and lateral load history (uniaxial or multiaxial). This paper summarizes general behavior observed during isolated and multiple beam-column-slab connection tests. An approximation is given for estimating the amount of slab reinforcement to be considered as effective tensile reinforcement of the beams.

Proposes mechanisms of the transfer of forces to beam-column joints, generated under typical seismic actions in cast-in-place reinforced concrete slabs. One of the main objectives of the paper is to review behavioral models that should assist designers in visualizing the flow of internal forces in beam-column-slab subassemblages. It is postulated that membrane forces play a dominant role and that contributions of other actions, such as bending in slabs and torsion in transverse beams, are relatively unimportant, particularly when significant ductility demands arise during seismic motions. Locations at which slab reinforcements transmit tensile forces by means of bond to the surrounding concrete are considered to be particularly important in the assessment of the enhancement of beam flexural strength. The description of these phenomena is related to observations made during the testing of isolated reinforced concrete beam-column subassemblages with slabs simulating one-and two-way cast-in-place floor systems. Subsequently, the findings are extended to describe the perceived behavior of continuous floor slabs supported by beams of multibay ductile frames. The relevance of the flexural strength enhancement of beams to the design of beam-column joints and columns is briefly examined. Design recommendations are made, particularly with respect to the effective width of the tension flanges.

Two series of experiments have been conducted to study the performance of wide beam-to-column subassemblages under lateral loadings. Results indicate that in moment-resisting frames: 1) the maximum effective beam width should be twice the column width, if all beam reinforcing bars are expected to yield within 2 percent of story deformation, and 2) the maximum amount of beam reinforcement not placed in the joint core should be limited in terms of resulting torsional stress in the outside beam portions. The torsional strength c åt = 24çb (by Kanoh and Yoshizaki) is a good design criteria for wide beam-column joints.

Geometrical configurations of reinforced concrete beam-column joints in actual building structures are quite varied because the configurations depend upon the number of structural members connecting the joints, the shapes of cross section of the members, the eccentricity among the axes of members, and so on. Focusing the interest mainly on the eccentricity from these factors, studies on seismic behavior of reinforced concrete interior beam-column joints in one-way frames with eccentricity are carried out with a classificatory examination, an investigation of a building destroyed by a strong earthquake, and a survey of previous studies and the authors' experiments. From the investigation of the destroyed building, it is suggested that the heavy eccentricity between columns and beams caused torsional moments in the columns and joints, causing severe damage. From the survey of three previous experiments in which one-sided eccentric joint specimens with wide columns and deep beams were subjected to lateral loading, it is shown that effective width and/or torsional moment should be considered for calculating the strength and stiffness of frames. Experimental results indicate that such eccentricities caused twisting of the columns and joints, resulting in reduction of the shear strength of the members. From the results of the classification examination and of the authors' tests in which five beam-column subassemblages with several types of eccentricity and beam width were subjected to cyclic lateral forces, it is observed that joints with one-sided eccentric beams suffer larger torsional moment around column axes, narrowing the effective joint width. Therefore, the shear cracking stress and the deformability of joints are reduced.

Cyclic loading tests of eight half-scale interior beam-column subassemblages using high-strength materials were carried out to investigate their seismic behavior under high joint shear stress vn ranging from 140 to 200 kg/cmý. Concretes with three nominal compressive strengths; 400, 600, and 800 kg/cmý was used. High-strength reinforcing bars with a yield strength of 4000 and 6000 kg/cmý were provided as beam longitudinal reinforcement. Reinforcing bars with a yield strength of 8700 kg/cmý were used for joint transverse reinforcement. To prevent premature shear failure in joints and significant slippage of beam bars through joints, four different types of joint detail were planned. They included high-strength bars for joint reinforcement, anchor plates attached to beam longitudinal bars in the joint, relocation of beam plastic hinges away from the joint, and joint reinforcement using steel plates. The beam-column joints using high-strength concrete of 600 kg/cmý or higher showed ductile behavior up to 5 percent story drift, even under conditions of high join-shear stress. No significant bar slippage or bond deterioration was observed, including the joints using high-strength beam main bars. The high-strength transverse reinforcement worked effectively as joint reinforcement, as indicated by considerably high strains measured in joint hoops. The relocation of beam plastic hinges away from the joint reduced damage of the beam-column joint. Based on the test results, guidelines for design of such reinforced concrete beam-column joints are presented.