International Concrete Abstracts Portal

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.

A set of shear-resistant actions is presented to analyze reinforced concrete interior beam-column joints in weak beam-strong column ductile frames. The proposed analysis explains the results of experiments of beam-column joints with and without bond at beam bars and with various horizontal shear reinforcement. Local bond strength at beam bars affects horizontal hoop stress. Under or up to the limit of enough bond, larger local bond strength demands larger horizontal hoop stress. Over this limit, larger local bond strength demands smaller horizontal hoop stress. Joint shear reinforcement improves anchorage of beam bars because horizontal hoop stress guarantees bond stress outside diagonal strut. This results in smaller compressive stresses of joint concrete, preventing compressive shear failure.

Bond and/or anchorage performances of longitudinal bars in reinforced concrete beam-column joints were outlined, based on the investigations performed in the United States, New Zealand, and Japan in the past 10 years. The effects of joint size-bar diameter ratio, development length, geometry of bent bar, column axial force, and transverse reinforcement were discussed. The bond deterioration caused such undesirable phenomena as pinching in force-story drift hysteresis curves, increasing the slip deformation at the beam-column interface, changing the shear transfer mechanism in the joint core, and decreasing the flexural strength of the adjoining members. Bars passing through an interior joint and bent bars in an exterior joint were treated separately to make the discussion clear.

Describes two types of prefabricated beam-column joints designed to save manpower requirements in construction work. The first type consists of making precast subassemblages with beam-column joints and integrated beams. Through-holes are provided in the vertical direction in the beam-column joint to accommodate column reinforcing bars (Type 1). The second type consists of precast subassemblages with beam-column joints and columns integrated. Through holes are provided in the horizontal direction in the beam-column joint to accommodate beam reinforcing bars (Type 2). Column or beam reinforcing bars are passed through the holes in these precast subassemblages; the parts are integrated by subsequent grouting of the holes with high-strength mortar. The earthquake resistance of these precast subassemblages was investigated with cyclic loading tests. The systems are intended for use in a 13-story reinforced concrete building, designed so that its collapse mechanism would be of the beam-yielding type. With Type 1 precast subassemblages, column reinforcing bars grouted and fixed inside sleeve-pipe holes are not subject to stresses extending into the plastic range. Therefore, by suitably designing the anchorage lengths of beam reinforcing bars inside the joints, there will be no slippage of the beam bars. A ductility of more than six times the yielding displacement may be attained. With Type 2 subassemblages, the beam reinforcing bars grouted and fixed inside sleeve-pipe holes are subjected to repeated stresses extending into the plastic range, such that bond deterioration occurs inside the joints. Strength declines at large deformations exceeding three times the yield displacement, and satisfactory ductility is not obtained. Taking test results into consideration, precast subassemblages of the first type are recommended for adoption in the 13-story building.

As part of a United States/New Zealand/Japan/China collaborative research project, interior and exterior beam-column joint subassemblages with floor slabs of prototype two-way and one-way reinforced concrete building frames were designed for earthquake resistance using the current New Zealand concrete design code, NZS 3101:1982. Three full-scale subassemblages as designed were constructed and tested under quasi-static cyclic loading which simulated severe earthquake actions. The overall performance of each subassemblage during the tests was satisfactory in terms of strength and ductility. The joint core and column remained essentially undamaged while plastic hinges formed in the beams. The strong column-weak beam behaviour sought in the design, desirable in tall ductile frames designed for earthquake resistance, was therefore achieved. Although the joint cores of the subassemblages remained in the elastic range, joint core shear deformations contributed significantly to the interstorey drifts. Also, a significant proportion of the slab bars in tension contributed to the negative moment flexural strength of the beams. The performance of the one-way joint was superior to the performance of the two way joints.