Abstract
Four exterior beam-column joints reinforced by plain round bars designed according to pre-1970's codes were subjected to simulated seismic loading. Each of the test units was full size in scale. The main test variables were the manner in which the longitudinal beam bars were hooked in the joint core and the level of the axial load applied to the columns. The amount of transverse reinforcement in the beams, columns and joint cores was very small, as was typical of the pre-1970's. The tests on the units demonstrated that the available stiffness and strength was low when the axial load was zero. The final failure occurred due to opening of the beam bar hooks in tension and column bar buckling, irrespective of the hook details of the beam bars. The presence of axial compressive load of 0.25fc'Ag on the columns delayed the failure initiated by the beam bar hooks, leading to much improved stiffness and strength of the units. In one unit with beam bar hooks bent away from the joint core, the column regions adjacent to the joint core were jacketed with fibre-glass and the unit when tested showed much improved stiffness and strength. When compared with the results of similar units reinforced by deformed bars, the units reinforced by plain round bars showed less joint shear distortion but more opening of beam bar hooks in tension and column bar buckling. As a result, premature concrete tension cracking failure along the outer layer of column bars adjacent to the beam bar hooks was enhanced, and the attained stiffness and the force strength were significantly lower, especially the stiffness.
Highlights
There has been increasing emphasis in many countries on seismic assessment and retrofit of existing reinforced concrete structures designed to the pre-1970' s codes in recent years [I, 2, 7].A research program on Seismic Assessment and Retrofit of Existing Reinforced Concrete Structures has been under way at the University of Canterbury for several years
The general conclusions reached concerning the joint shearresisting mechanisms of exterior beam-column joints with alternative beam bar hook details in exterior columns were as follows: 1. The development of the postulated joint shear-resisting mechanisms in current seismic codes for exterior beamcolumn joints with the beam bar hooks bent into the joint cores necessitates prevention of the premature failure associated with the opening of the beam bar hooks
Use of plain round bar reinforcement makes more likely premature concrete tension cracking failure caused by opening of the beam bar hooks in tension and column bar buckling
Summary
There has been increasing emphasis in many countries on seismic assessment and retrofit of existing reinforced concrete structures designed to the pre-1970' s codes in recent years [I, 2, 7]. A research program on Seismic Assessment and Retrofit of Existing Reinforced Concrete Structures has been under way at the University of Canterbury for several years. A number of tests on other as-built and retrofitted reinforced concrete columns and beam-column joint assemblies [3, 4, 11, 12] have been conducted using simulated seismic loading. Very few tests were conducted on as-built reinforced concrete components reinforced by plain round bars. The bond strength of plain bar reinforcement is low, compared with deformed bar reinforcement, during cyclic loading. Plain round reinforcing bars when used for longitudinal reinforcement may lead to very different seismic performance, compared to that with deformed bar reinforcement
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